A tall bike is an unusually tall bicycle, typically built for the purpose of fun and recreation, though with occasional practical use.
Modern tall bikes are most commonly constructed by individuals from spare parts. Two conventional bicycle frames are connected, by welding, brazing, or other means, one atop the other. The drive train is reconfigured to connect to the upper set of pedals, and the controls are moved to the upper handlebar area.
Alternatively, a bicycle can be built by inverting the frame, and inserting the forks from the 'wrong side', flipping the rear wheel, and adding a long gooseneck and tall handlebars, then welding a long seatpost tube to the 'bottom' (now the top) of the frame. This type of tall bike is made with only one bike frame, and is often called an upside-down bike rather than a tall bike, though the seat can be quite high, depending on the frame shape used. This type can be somewhat safer, as there is less tubing between the rider's legs and dismounting in a hurry can be easily accomplished.
Tall bikes are a popular mode of transportation for modern 'bicycle clubs' (SCUL, Rat Patrol, Zoobomb, Black Label Bike Club, The Winking Circle, Dead Baby Bikes, C.h.u.n.k. 666, Cyclecide, etc.) and activist groups. They are also a mainstay among builders of Clown bikes, art bikes, Clown alleys and parade groups. Bicycle modification is considered a fun and cheap hobby, and never fails to attract a lot of attention. Most modern cities contain large quantities of unused or abandoned bicycles that provide the raw materials for tall bikes and other mutant cycles.
Practical uses
Tall bikes can be used for general transportation and recreation, just like other bicycles. Regular tall-bike commuters note that both their increased visibility and the simple 'wow factor' give them a safety advantage in automobile traffic over 'short bikes.'
A Giraffe Lamplighter Bicycle, manufactured in 1898.
Historically, one of the first practical uses of the tall bike was as a late 1800s lamp lighting system, by which a worker would mount a specialized tall bicycle while equipped with a torch for lighting gas lamps. As the worker rode to each lamp, they would lean against the lamp post, light the lamp, and then ride to the next. Upon completing the circuit of lamps, an assistant would help the rider dismount.
In Bugbrooke, Northamptonshire, England The Clark Brothers in the 1950s built tall bikes to get to work when it flooded. These bikes they called 'Flood Bikes'. They have been on BBC TV and even did a Welsh language programme on them. They are currently in a museum somewhere.
Sporting
Tall bike jousting is a popular sport among bicycle hackers, and is commonly considered to have been introduced by Jake Houle and Lil' Bob of the Hard Times/Black Label Bike Club. Combatants arm themselves with lances, and attempt to score points by dislodging the other rider. Rules vary by area, and with the mood of the combatants. Like all jousting games, participants consider it a sport where honor plays a role and dishonorable wins are frowned upon.
Jousters create lances that vary from simple PVC pipe and foam devices that are flexible, soft, and relatively safe, up to wooden or metal lances that may be quite dangerous. Regional rules vary, some specifying flaming lances for effect, or glass containers attached to the end, the goal being to break the glass container in order to score points.
Design considerations
Tall bikes present some interesting design considerations, and different localities tend to have different methods of dealing with them.
One consistent issue is that the seat tends to end up in line with, or behind, the rear axle, which creates a powerful tendency to lift the front wheel of the bicycle on acceleration. Some bicycle builders simply accept this tendency, but others solve the problem by moving the seat post forward, lowering the handlebars, or by using a smaller wheel in front, typically a 24" instead of a 26".
Stability can also be negatively affected, and enhancements such as extended wheelbase by welding extensions on the front and rear dropouts can benefit stability. Contest holders often place restrictions on such modification to prevent unfair advantages.
See the bicycle and motorcycle geometry and bicycle and motorcycle dynamics articles for more on these issues.
2008年11月10日星期一
Electric motor
An electric motor uses electrical energy to produce mechanical energy. The reverse process, that of using mechanical energy to produce electrical energy, is accomplished by a generator or dynamo. Traction motors used on locomotives and some electric and hybrid automobiles often perform both tasks if the vehicle is equipped with dynamic brakes. Electric motors are found in household appliances such as fans, refrigerators, washing machines, pool pumps, floor vacuums, and fan-forced ovens. They are also found in many other devices such as computer equipment, in its disk drives, printers, and fans; and in some sound and video playing and recording equipment as DVD/CD players and recorders, tape players and recorders, and record players. Electric motors are also found in several kinds of toys such as some kinds of vehicles and robotic toys.
The principle of conversion of electrical energy into mechanical energy by electromagnetic means was demonstrated by the British scientist Michael Faraday in 1821 and consisted of a free-hanging wire dipping into a pool of mercury. A permanent magnet was placed in the middle of the pool of mercury. When a current was passed through the wire, the wire rotated around the magnet, showing that the current gave rise to a circular magnetic field around the wire. This motor is often demonstrated in school physics classes, but brine (salt water) is sometimes used in place of the toxic mercury. This is the simplest form of a class of electric motors called homopolar motors. A later refinement is the Barlow's Wheel. These were demonstration devices, unsuited to practical applications due to limited power.
The first real electric motor, using electromagnets for both stationary and rotating parts, was demonstrated by Ányos Jedlik in 1828 Hungary. He built an electric-motor propelled vehicle in 1828.
The first English commutator-type direct-current electric motor capable of a practical application was invented by the British scientist William Sturgeon in 1832. Following Sturgeon's work, a commutator-type direct-current electric motor made with the intention of commercial use was built by the American Thomas Davenport and patented in 1837. Although several of these motors were built and used to operate equipment such as a printing press, due to the high cost of primary battery power, the motors were commercially unsuccessful and Davenport went bankrupt. Several inventors followed Sturgeon in the development of DC motors but all encountered the same cost issues with primary battery power. No electricity distribution had been developed at the time. Like Sturgeon's motor, there was no practical commercial market for these motors.
The modern DC motor was invented by accident in 1873, when Zénobe Gramme connected the dynamo he had invented to a second similar unit, driving it as a motor. The Gramme machine was the first electric motor that was successful in the industry.
In 1888 Nikola Tesla invented the first practicable AC motor and with it the polyphase power transmission system. Tesla continued his work on the AC motor in the years to follow at the Westinghouse company.
The principle of conversion of electrical energy into mechanical energy by electromagnetic means was demonstrated by the British scientist Michael Faraday in 1821 and consisted of a free-hanging wire dipping into a pool of mercury. A permanent magnet was placed in the middle of the pool of mercury. When a current was passed through the wire, the wire rotated around the magnet, showing that the current gave rise to a circular magnetic field around the wire. This motor is often demonstrated in school physics classes, but brine (salt water) is sometimes used in place of the toxic mercury. This is the simplest form of a class of electric motors called homopolar motors. A later refinement is the Barlow's Wheel. These were demonstration devices, unsuited to practical applications due to limited power.
The first real electric motor, using electromagnets for both stationary and rotating parts, was demonstrated by Ányos Jedlik in 1828 Hungary. He built an electric-motor propelled vehicle in 1828.
The first English commutator-type direct-current electric motor capable of a practical application was invented by the British scientist William Sturgeon in 1832. Following Sturgeon's work, a commutator-type direct-current electric motor made with the intention of commercial use was built by the American Thomas Davenport and patented in 1837. Although several of these motors were built and used to operate equipment such as a printing press, due to the high cost of primary battery power, the motors were commercially unsuccessful and Davenport went bankrupt. Several inventors followed Sturgeon in the development of DC motors but all encountered the same cost issues with primary battery power. No electricity distribution had been developed at the time. Like Sturgeon's motor, there was no practical commercial market for these motors.
The modern DC motor was invented by accident in 1873, when Zénobe Gramme connected the dynamo he had invented to a second similar unit, driving it as a motor. The Gramme machine was the first electric motor that was successful in the industry.
In 1888 Nikola Tesla invented the first practicable AC motor and with it the polyphase power transmission system. Tesla continued his work on the AC motor in the years to follow at the Westinghouse company.
Small appliance
Small appliance refers to a class of home appliances that are portable or semi-portable or which are used on tabletops, countertops, or other platforms. Such items are contrasted with major appliances, which are typically fixtures that cannot be easily moved. All appliances are intended to perform, enable, or assist in performing a job or changing a status, such as the humidity of a room. In this way, they can be differentiated from other portable electrical items that provide only entertainment. Some items not typically considered appliances, such as lamps, can be used as appliances if they are used to cook or warm food.
Many small appliances are powered by electricity. The appliance may use a permanently attached cord which is plugged into a wall outlet or a detachable cord. The appliance may have a cord storage feature. A few hand-held appliances use batteries, which may be disposable or rechargeable. Some appliances consist of an electrical motor upon which is mounted various attachments so as to constitute several individual appliances, such as a blender, a food processor, or a juicer. Many stand mixers, while functioning primarily as a mixer, have attachments which can perform additional functions.
A few gas-powered appliances exist for use in situations where electricity is not expected to be available, but these are typically larger and not as portable as most small appliances. Items that perform the same function as small appliances but are hand powered are generally referred to as tools or gadgets, for example a hand-powered meat grinder.
Some small appliances perform the same or similar function as their larger counterparts. For example, a toaster oven is a small appliance that performs a similar function as an oven. Small appliances often have a home version and a commercial version. The commercial, or industrial, version is designed to be used nearly continuously in a restaurant or other similar setting. Commercial appliances are typically connected to a more powerful electrical outlet, are larger and stronger, have more user-serviceable parts, and cost significantly more.
Small appliances can be very inexpensive, such as a basic can opener or coffee maker which may cost only a few U.S. dollars, or very expensive, such as an elaborate espresso maker, which may cost several thousand U.S. dollars. Most homes contain several cheaper home appliances, with perhaps a few more expensive appliances, such as a high-end microwave oven or mixer.
Small appliances which are defective or improperly used or maintained may cause house fires and other property damage, or may harbor bacteria if not properly cleaned. It is important that users read the instructions carefully and that appliances that use a grounded cord be attached to a grounded outlet. Because of the risk of fire, some appliances have a short detachable cord that is connected to the appliance magnetically. If the appliance is moved further than the cord length from the wall, the cord will detach from the appliance.
Many small appliances are powered by electricity. The appliance may use a permanently attached cord which is plugged into a wall outlet or a detachable cord. The appliance may have a cord storage feature. A few hand-held appliances use batteries, which may be disposable or rechargeable. Some appliances consist of an electrical motor upon which is mounted various attachments so as to constitute several individual appliances, such as a blender, a food processor, or a juicer. Many stand mixers, while functioning primarily as a mixer, have attachments which can perform additional functions.
A few gas-powered appliances exist for use in situations where electricity is not expected to be available, but these are typically larger and not as portable as most small appliances. Items that perform the same function as small appliances but are hand powered are generally referred to as tools or gadgets, for example a hand-powered meat grinder.
Some small appliances perform the same or similar function as their larger counterparts. For example, a toaster oven is a small appliance that performs a similar function as an oven. Small appliances often have a home version and a commercial version. The commercial, or industrial, version is designed to be used nearly continuously in a restaurant or other similar setting. Commercial appliances are typically connected to a more powerful electrical outlet, are larger and stronger, have more user-serviceable parts, and cost significantly more.
Small appliances can be very inexpensive, such as a basic can opener or coffee maker which may cost only a few U.S. dollars, or very expensive, such as an elaborate espresso maker, which may cost several thousand U.S. dollars. Most homes contain several cheaper home appliances, with perhaps a few more expensive appliances, such as a high-end microwave oven or mixer.
Small appliances which are defective or improperly used or maintained may cause house fires and other property damage, or may harbor bacteria if not properly cleaned. It is important that users read the instructions carefully and that appliances that use a grounded cord be attached to a grounded outlet. Because of the risk of fire, some appliances have a short detachable cord that is connected to the appliance magnetically. If the appliance is moved further than the cord length from the wall, the cord will detach from the appliance.
Locomotive
A locomotive is a railway vehicle that provides the motive power for a train. The word originates from the Latin loco - "from a place", ablative of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine,. first used in the early 19th century to distinguish between mobile and stationary steam engines.
A locomotive has no payload capacity of its own, and its sole purpose is to move the train along the tracks. In contrast, some trains have self-propelled payload-carrying vehicles. These are not normally considered locomotives, and may be referred to as multiple units, motor coaches or railcars. The use of these self-propelled vehicles is increasingly common for passenger trains, but very rare for freight (see CargoSprinter). Vehicles which provide motive power to haul an unpowered train, but are not generally considered locomotives because they have payload space or are rarely detached from their trains, are known as power cars.
Traditionally, locomotives pull trains from the front. Increasingly common is push-pull operation, where a locomotive pulls the train in one direction and pushes it in the other, and is optionally controlled from a control cab at the opposite end of the train.
The first successful locomotives were built by Cornish inventor Richard Trevithick. In 1804 his unnamed steam locomotive hauled a train along the tramway of the Penydarren ironworks, near Merthyr Tydfil in Wales. Although the locomotive hauled a train of 10 tons of iron and 70 passengers in five wagons over nine miles (14 km), it was too heavy for the cast iron rails used at the time. The locomotive only ran three trips before it was abandoned. Trevithick built a series of locomotives after the Penydarren experiment, including one which ran at a colliery in Tyneside where it was seen by the young George Stephenson.
The first commercially successful steam locomotive was Matthew Murray's rack locomotive, The Salamanca, built for the narrow gauge Middleton Railway in 1812. This was followed in 1813 by the Puffing Billy built by Christopher Blackett and William Hedley for the Wylam Colliery Railway, the first successful locomotive running by adhesion only. Puffing Billy is now on display in the Science Museum in London, the oldest locomotive in existence.
In 1814 George Stephenson, inspired by the early locomotives of Trevithick and Hedley persuaded the manager of the Killingworth colliery where he worked to allow him to build a steam-powered machine. He built the Blücher, one of the first successful flanged-wheel adhesion locomotives. Stephenson played a pivotal role in the development and widespread adoption of steam locomotives. His designs improved on the work of the pioneers. In 1825 he built the Locomotion for the Stockton and Darlington Railway which became the first public steam railway. In 1829 he built The Rocket which was entered in and won the Rainhill Trials. This success led to Stephenson establishing his company as the pre-eminent builder of steam locomotives used on railways in the United Kingdom, the United States and much of Europe.
A locomotive has no payload capacity of its own, and its sole purpose is to move the train along the tracks. In contrast, some trains have self-propelled payload-carrying vehicles. These are not normally considered locomotives, and may be referred to as multiple units, motor coaches or railcars. The use of these self-propelled vehicles is increasingly common for passenger trains, but very rare for freight (see CargoSprinter). Vehicles which provide motive power to haul an unpowered train, but are not generally considered locomotives because they have payload space or are rarely detached from their trains, are known as power cars.
Traditionally, locomotives pull trains from the front. Increasingly common is push-pull operation, where a locomotive pulls the train in one direction and pushes it in the other, and is optionally controlled from a control cab at the opposite end of the train.
The first successful locomotives were built by Cornish inventor Richard Trevithick. In 1804 his unnamed steam locomotive hauled a train along the tramway of the Penydarren ironworks, near Merthyr Tydfil in Wales. Although the locomotive hauled a train of 10 tons of iron and 70 passengers in five wagons over nine miles (14 km), it was too heavy for the cast iron rails used at the time. The locomotive only ran three trips before it was abandoned. Trevithick built a series of locomotives after the Penydarren experiment, including one which ran at a colliery in Tyneside where it was seen by the young George Stephenson.
The first commercially successful steam locomotive was Matthew Murray's rack locomotive, The Salamanca, built for the narrow gauge Middleton Railway in 1812. This was followed in 1813 by the Puffing Billy built by Christopher Blackett and William Hedley for the Wylam Colliery Railway, the first successful locomotive running by adhesion only. Puffing Billy is now on display in the Science Museum in London, the oldest locomotive in existence.
In 1814 George Stephenson, inspired by the early locomotives of Trevithick and Hedley persuaded the manager of the Killingworth colliery where he worked to allow him to build a steam-powered machine. He built the Blücher, one of the first successful flanged-wheel adhesion locomotives. Stephenson played a pivotal role in the development and widespread adoption of steam locomotives. His designs improved on the work of the pioneers. In 1825 he built the Locomotion for the Stockton and Darlington Railway which became the first public steam railway. In 1829 he built The Rocket which was entered in and won the Rainhill Trials. This success led to Stephenson establishing his company as the pre-eminent builder of steam locomotives used on railways in the United Kingdom, the United States and much of Europe.
Iron
An iron is a small appliance used in ironing to remove wrinkles from fabric. Ironing works by loosening the ties between the long chains of molecules that exist in polymer fiber materials. With the heat and the weight of the ironing plate, the fibers are stretched and maintain its new shape when cooled. Some materials such as cotton require the use of water to loosen the intermolecular bonds. Many materials developed in the twentieth century are advertised as not needing or needing only a little ironing
Metal pans filled with charcoal were used for smoothing fabrics in China in the 1st century BC[citation needed]. From the 17th century, sadirons or sad irons (from an old word meaning solid) began to be used. They were thick slabs of cast iron, delta-shaped and with a handle, heated in a fire. These were also called flat irons. A later design consisted of an iron box which could be filled with hot coals, which had to be periodically aerated by attaching a bellows. In Kerala in India, burning coconut shells were used instead of charcoal, as they have a similar heating capacity. This method is still in use as a backup device since power outage is frequent. Other box irons had heated metal inserts instead of hot coals. Another solution was a cluster of solid irons that were heated from the single source: as the iron currently in use cools down, it can be quickly replaced by another one that is hot.
In the late nineteenth and early twentieth centuries, there were many irons in use which were heated by a fuel such as kerosene, alcohol, whale oil, natural gas, carbide gas (acetylene) as with carbide lamps, or even gasoline. Some houses were equipped with a system of pipes for distributing natural gas or carbide gas to different rooms in order to operate appliances such as irons, in addition to lights. Despite the risk of fire, liquid-fuel irons were sold in U.S. rural areas up through World War II.
In the industrialized world, these designs have been superseded by the electric iron, which uses resistive heating from an electric current. The hot plate, called the sole plate, is made of aluminium or stainless steel. The heating element is controlled by a thermostat which switches the current on and off to maintain the selected temperature. The invention of the resistively heated electric iron is credited to Henry W. Seely of New York in 1882. In the same year an iron heated by a carbon arc was introduced in France, but was too dangerous to be successful. The early electric irons had no easy way to control their temperature, and the first thermostatically controlled electric iron appeared in the 1920s. Later, steam was used to iron clothing. Credit for the invention of the steam iron goes to Thomas Sears.
Metal pans filled with charcoal were used for smoothing fabrics in China in the 1st century BC[citation needed]. From the 17th century, sadirons or sad irons (from an old word meaning solid) began to be used. They were thick slabs of cast iron, delta-shaped and with a handle, heated in a fire. These were also called flat irons. A later design consisted of an iron box which could be filled with hot coals, which had to be periodically aerated by attaching a bellows. In Kerala in India, burning coconut shells were used instead of charcoal, as they have a similar heating capacity. This method is still in use as a backup device since power outage is frequent. Other box irons had heated metal inserts instead of hot coals. Another solution was a cluster of solid irons that were heated from the single source: as the iron currently in use cools down, it can be quickly replaced by another one that is hot.
In the late nineteenth and early twentieth centuries, there were many irons in use which were heated by a fuel such as kerosene, alcohol, whale oil, natural gas, carbide gas (acetylene) as with carbide lamps, or even gasoline. Some houses were equipped with a system of pipes for distributing natural gas or carbide gas to different rooms in order to operate appliances such as irons, in addition to lights. Despite the risk of fire, liquid-fuel irons were sold in U.S. rural areas up through World War II.
In the industrialized world, these designs have been superseded by the electric iron, which uses resistive heating from an electric current. The hot plate, called the sole plate, is made of aluminium or stainless steel. The heating element is controlled by a thermostat which switches the current on and off to maintain the selected temperature. The invention of the resistively heated electric iron is credited to Henry W. Seely of New York in 1882. In the same year an iron heated by a carbon arc was introduced in France, but was too dangerous to be successful. The early electric irons had no easy way to control their temperature, and the first thermostatically controlled electric iron appeared in the 1920s. Later, steam was used to iron clothing. Credit for the invention of the steam iron goes to Thomas Sears.
Pylon transformer
Pylon transformers are normally located at a service drop, where wires run from a utility pole to a customer's premises. They are often used for the power supply of facilities outside settlements, such as isolated houses, farmyards or pumping stations at voltages below 30kV. Another application is the power supply of switch heatings from the overhead wire of railways electrified with AC. In this case single phase pylon transformers are used.
In North American utility practice, these devices are very commonly used in areas with overhead primary distribution lines. An alternative to a pole pig is used with underground distribution systems; a transformer in a locked steel case mounted on a concrete pad on the customer's premises. This is called a pad mount transformer. They are used in many urban areas and neighborhoods where the primary distribution lines run underground. Many large buildings have electric service provided at primary distribution voltage. These buildings have customer-owned transformers in the basement for step-down purposes.
High voltage hobbyists often use these transformers in reverse (step-up) by feeding 120 or 240 volts into the secondary and drawing the resulting high voltage off the primary bushings, using it to power devices like Jacob's Ladders and Tesla coils.
Connections
Both pole mount and pad mount transformers convert the high voltage of the 'primary' overhead or underground distribution lines to the lower voltage of the 'secondary' distribution wires inside the building. The primary wires supply power using the three phase system, so there are three wires, at one of a wide range of standard voltages from 4,600 to 33,000 volts but most commonly 7,200 or 14,400 volts.
Primary
The high voltage primary windings are brought out to bushings on the top of the case. Single phase transformers, generally used in the USA system, are attached to the overhead wires with two different types of connections:
If a primary neutral wire is available, a 'wye' or 'phase to neutral' transformer can be used. This usually has only one bushing on top, connected to one of the primary phases. The other end of the primary winding is 'grounded' to the transformer's case, which is connected to the neutral wire of the 3 phase system, and also earth ground. This type of distribution system, called 'grounded wye', is preferred because the pylon transformers present unbalanced loads on the line, causing currents in the neutral wire. With the 'delta' connection, this can cause variations in the voltages on the 3 phase wires.
If no neutral wire is available, a 'delta' or 'phase to phase' transformer must be used. This has two bushings on top which are connected to two of the three primary wires, so the voltage across the primary winding is the phase-to-phase voltage. This type is used on long distribution lines where it is uneconomical to run a fourth neutral wire.
Transformers providing three-phase secondary power, which are used for residential service in the European system, have three secondary windings and are attached to all three primary phase wires. The transformer is always connected to the primary distribution lines through protective fuses and disconnect switches. In the USA this usually takes the form of a 'fused cutout'. An electrical fault causes the fuse to melt, and the device drops open to give a visual indication of trouble. It can also be manually opened while the line is energized by lineworkers using insulated hot sticks.
Secondary
The low voltage secondary windings are attached to three terminals on the transformer's side.
In the USA and countries using its system, the secondary is most often the single phase 240/120 volt system. The 240 V secondary winding is center-tapped and the center neutral wire is grounded, making the two end conductors "hot" with respect to the center tap. These three wires run down the service drop to the electric meter and service panel inside the building. Connecting a load between either hot wire and the neutral gives 120 volts. Connecting between both hot wires gives 240 volts.
In Europe and countries using its system, the secondary is often the three phase 416Y/240 system. There are three 240 V secondary windings, each receiving power from a primary winding attached to one of the primary phases. The three secondary windings are connected together to a 'neutral' wire, which is grounded. The other end of the 3 secondary windings, along with the neutral, are brought down the service drop to the service panel. 240 V loads are connected between any of the three phase wires and the neutral.
Higher secondary voltages, such as 480 volts, are sometimes required for commercial and industrial uses. Some industrial customers require three-phase power at secondary voltages. To provide this, three-phase transformers can be used, or three identical single phase transformers can be wired in a transformer bank in either a wye or delta connection.
In North American utility practice, these devices are very commonly used in areas with overhead primary distribution lines. An alternative to a pole pig is used with underground distribution systems; a transformer in a locked steel case mounted on a concrete pad on the customer's premises. This is called a pad mount transformer. They are used in many urban areas and neighborhoods where the primary distribution lines run underground. Many large buildings have electric service provided at primary distribution voltage. These buildings have customer-owned transformers in the basement for step-down purposes.
High voltage hobbyists often use these transformers in reverse (step-up) by feeding 120 or 240 volts into the secondary and drawing the resulting high voltage off the primary bushings, using it to power devices like Jacob's Ladders and Tesla coils.
Connections
Both pole mount and pad mount transformers convert the high voltage of the 'primary' overhead or underground distribution lines to the lower voltage of the 'secondary' distribution wires inside the building. The primary wires supply power using the three phase system, so there are three wires, at one of a wide range of standard voltages from 4,600 to 33,000 volts but most commonly 7,200 or 14,400 volts.
Primary
The high voltage primary windings are brought out to bushings on the top of the case. Single phase transformers, generally used in the USA system, are attached to the overhead wires with two different types of connections:
If a primary neutral wire is available, a 'wye' or 'phase to neutral' transformer can be used. This usually has only one bushing on top, connected to one of the primary phases. The other end of the primary winding is 'grounded' to the transformer's case, which is connected to the neutral wire of the 3 phase system, and also earth ground. This type of distribution system, called 'grounded wye', is preferred because the pylon transformers present unbalanced loads on the line, causing currents in the neutral wire. With the 'delta' connection, this can cause variations in the voltages on the 3 phase wires.
If no neutral wire is available, a 'delta' or 'phase to phase' transformer must be used. This has two bushings on top which are connected to two of the three primary wires, so the voltage across the primary winding is the phase-to-phase voltage. This type is used on long distribution lines where it is uneconomical to run a fourth neutral wire.
Transformers providing three-phase secondary power, which are used for residential service in the European system, have three secondary windings and are attached to all three primary phase wires. The transformer is always connected to the primary distribution lines through protective fuses and disconnect switches. In the USA this usually takes the form of a 'fused cutout'. An electrical fault causes the fuse to melt, and the device drops open to give a visual indication of trouble. It can also be manually opened while the line is energized by lineworkers using insulated hot sticks.
Secondary
The low voltage secondary windings are attached to three terminals on the transformer's side.
In the USA and countries using its system, the secondary is most often the single phase 240/120 volt system. The 240 V secondary winding is center-tapped and the center neutral wire is grounded, making the two end conductors "hot" with respect to the center tap. These three wires run down the service drop to the electric meter and service panel inside the building. Connecting a load between either hot wire and the neutral gives 120 volts. Connecting between both hot wires gives 240 volts.
In Europe and countries using its system, the secondary is often the three phase 416Y/240 system. There are three 240 V secondary windings, each receiving power from a primary winding attached to one of the primary phases. The three secondary windings are connected together to a 'neutral' wire, which is grounded. The other end of the 3 secondary windings, along with the neutral, are brought down the service drop to the service panel. 240 V loads are connected between any of the three phase wires and the neutral.
Higher secondary voltages, such as 480 volts, are sometimes required for commercial and industrial uses. Some industrial customers require three-phase power at secondary voltages. To provide this, three-phase transformers can be used, or three identical single phase transformers can be wired in a transformer bank in either a wye or delta connection.
Electromagnetic interference
Integrated circuits are often a source of EMI, but they are never the "antenna". They must couple their energy to larger objects such as heatsinks, circuit board planes and cables to radiate significantly .
On integrated circuits, important means of reducing EMI are: the use of bypass or "decoupling" capacitors on each active device (connected across the power supply, as close to the device as possible), rise time control of high-speed signals using series resistors, and VCC filtering. Shielding is usually a last resort after other techniques have failed because of the added expense of RF gaskets and the like.
The efficiency of the radiation depends on the height above the ground or power plane (at RF one is as good as the other) and the length of the conductor in relation to the wavelength of the signal component (fundamental, harmonic or transient (overshoot, undershoot or ringing)). At lower frequencies, such as 133 MHz, radiation is almost exclusively via I/O cables; RF noise gets onto the power planes and is coupled to the line drivers via the VCC and ground pins. The RF is then coupled to the cable through the line driver as common-mode noise. Since the noise is common-mode, shielding has very little effect, even with differential pairs. The RF energy is capacitively coupled from the signal pair to the shield and the shield itself does the radiating. One cure for this is to use a braid-breaker or choke to reduce the common-mode signal.
At higher frequencies, usually above 500 MHz, traces get electrically longer and higher above the plane. Two techniques are used at these frequencies: wave shaping with series resistors and embedding the traces between the two planes. If all these measures still leave too much EMI, shielding such as RF gaskets and copper tape can be used. Most digital equipment is designed with metal, or conductive-coated plastic, cases.
Susceptibilities of different radio technologies
Interference tends to be more troublesome with older radio technologies such as analogue amplitude modulation, which have no way of distinguishing unwanted in-band signals from the intended signal, and the omnidirectional dipole antennas used with broadcast systems. Newer radio systems incorporate several improvements that improve the selectivity. In digital radio systems, such as Wi-Fi, error-correction techniques can be used. Spread-spectrum and frequency-hopping techniques can be used with both analogue and digital signalling to improve resistance to interference. A highly directional receiver, such as a parabolic antenna or a diversity receiver, can be used to select one signal in space to the exclusion of others.
The most extreme example of digital spread-spectrum signalling to date is ultra-wideband (UWB), which proposes the use of large sections of the radio spectrum at low amplitudes to transmit high-bandwidth digital data. UWB, if used exclusively, would enable very efficient use of the spectrum, but users of non-UWB technology are not yet prepared to share the spectrum with the new system because of the interference it would cause to their receivers. The regulatory implications of UWB are discussed in the Ultra-wideband article.
Interference to consumer devices
Complex electronic circuitry is found in all sorts of devices used in the home. This results in a vast interference potential that didn't exist in earlier, simpler decades. In the US, Public Law 97-259, enacted in 1982, gave the FCC the authority to regulate the susceptibility of consumer electronic equipment sold in the United States. The FCC, working with equipment manufacturers, decided to allow them to develop standards for EMI immunity and implement their own voluntary compliance programs.
Broadcast transmitters, two-way radio transmitters, paging transmitters, and cable TV are potential sources of RFI and EMI.Other possible sources of interference include a wide variety of devices, such as doorbell transformers, toaster ovens, electric blankets, ultrasonic pest control devices, electric bug zappers, heating pads, and touch controlled lamps. Multiple CRT computer monitors or televisions sitting too close to one another can sometimes cause a "shimmy" effect in each another, due to the electromagnetic nature of their picture tubes, especially when one of their de-gaussing coils is activated.
Switching inductive loads, such as electric motors, off causes interference, but it is easily suppressed by connecting a snubber network, a resistor in series with a capacitor, across the switch. Exact values can be optimised for each case, but 100 ohms in series with 100 nanofarads is usually satisfactory.
Switched-mode power supply can be a source of EMI, but have become less of a problem as design techniques have improved, such as integrated power factor correction.
Most countries have legal requirements that mandates electromagnetic compatibility: electronic and electrical hardware must still work correctly when subjected to certain amounts of EMI, and should not emit EMI which could interfere with other equipment (such as radios).
On integrated circuits, important means of reducing EMI are: the use of bypass or "decoupling" capacitors on each active device (connected across the power supply, as close to the device as possible), rise time control of high-speed signals using series resistors, and VCC filtering. Shielding is usually a last resort after other techniques have failed because of the added expense of RF gaskets and the like.
The efficiency of the radiation depends on the height above the ground or power plane (at RF one is as good as the other) and the length of the conductor in relation to the wavelength of the signal component (fundamental, harmonic or transient (overshoot, undershoot or ringing)). At lower frequencies, such as 133 MHz, radiation is almost exclusively via I/O cables; RF noise gets onto the power planes and is coupled to the line drivers via the VCC and ground pins. The RF is then coupled to the cable through the line driver as common-mode noise. Since the noise is common-mode, shielding has very little effect, even with differential pairs. The RF energy is capacitively coupled from the signal pair to the shield and the shield itself does the radiating. One cure for this is to use a braid-breaker or choke to reduce the common-mode signal.
At higher frequencies, usually above 500 MHz, traces get electrically longer and higher above the plane. Two techniques are used at these frequencies: wave shaping with series resistors and embedding the traces between the two planes. If all these measures still leave too much EMI, shielding such as RF gaskets and copper tape can be used. Most digital equipment is designed with metal, or conductive-coated plastic, cases.
Susceptibilities of different radio technologies
Interference tends to be more troublesome with older radio technologies such as analogue amplitude modulation, which have no way of distinguishing unwanted in-band signals from the intended signal, and the omnidirectional dipole antennas used with broadcast systems. Newer radio systems incorporate several improvements that improve the selectivity. In digital radio systems, such as Wi-Fi, error-correction techniques can be used. Spread-spectrum and frequency-hopping techniques can be used with both analogue and digital signalling to improve resistance to interference. A highly directional receiver, such as a parabolic antenna or a diversity receiver, can be used to select one signal in space to the exclusion of others.
The most extreme example of digital spread-spectrum signalling to date is ultra-wideband (UWB), which proposes the use of large sections of the radio spectrum at low amplitudes to transmit high-bandwidth digital data. UWB, if used exclusively, would enable very efficient use of the spectrum, but users of non-UWB technology are not yet prepared to share the spectrum with the new system because of the interference it would cause to their receivers. The regulatory implications of UWB are discussed in the Ultra-wideband article.
Interference to consumer devices
Complex electronic circuitry is found in all sorts of devices used in the home. This results in a vast interference potential that didn't exist in earlier, simpler decades. In the US, Public Law 97-259, enacted in 1982, gave the FCC the authority to regulate the susceptibility of consumer electronic equipment sold in the United States. The FCC, working with equipment manufacturers, decided to allow them to develop standards for EMI immunity and implement their own voluntary compliance programs.
Broadcast transmitters, two-way radio transmitters, paging transmitters, and cable TV are potential sources of RFI and EMI.Other possible sources of interference include a wide variety of devices, such as doorbell transformers, toaster ovens, electric blankets, ultrasonic pest control devices, electric bug zappers, heating pads, and touch controlled lamps. Multiple CRT computer monitors or televisions sitting too close to one another can sometimes cause a "shimmy" effect in each another, due to the electromagnetic nature of their picture tubes, especially when one of their de-gaussing coils is activated.
Switching inductive loads, such as electric motors, off causes interference, but it is easily suppressed by connecting a snubber network, a resistor in series with a capacitor, across the switch. Exact values can be optimised for each case, but 100 ohms in series with 100 nanofarads is usually satisfactory.
Switched-mode power supply can be a source of EMI, but have become less of a problem as design techniques have improved, such as integrated power factor correction.
Most countries have legal requirements that mandates electromagnetic compatibility: electronic and electrical hardware must still work correctly when subjected to certain amounts of EMI, and should not emit EMI which could interfere with other equipment (such as radios).
Heating pad
Electrical
Electric pads usually operate from household current and must have protection against overheating.
A moist heating pad is used dry on the user's skin. These pads register temperatures from 170 to 180 degrees Fahrenheit (76 to 82 °C) and are intended for deep tissue treatment and can be dangerous if left on unattended. Moist heating pads are used mainly by physical therapists but can be found for home use. A moist cloth can be added with a stupe cover to add more moisture to the treatment.
Chemical
Chemical pads employ a chemical heat reservoir or a one-time chemical reaction such as catalyzed rusting of iron.
A sodium acetate heat pad
A sodium acetate heat pad is a reusable heat reservoir. It contains a supersaturated solution of sodium acetate (CH3COONa). Crystallization is triggered by flexing a (patented ) small flat disc of notched ferrous metal embedded in the liquid. Pressing the disc releases very tiny adhered crystals of sodium acetate into the solution which then act as nucleation sites for the recrystallization of the remainder of the salt solution. Because the liquid is supersaturated, this makes the solution crystallize suddenly, thereby releasing the energy of the crystal lattice.
See sodium acetate for a more technical discussion.
The pad can be reused by placing it in boiling water for 10-15 minutes, which redissolves the sodium acetate in the contained water and recreates a supersaturated solution. Once the pad has returned to room temperature it can be triggered again. Triggering the pad before it has reached room temperature results in the pad reaching a lower peak temperature, as compared to waiting until it had completely cooled.
High specific-heat capacity materials
Heating packs can also be made by filling a container with a material that has a high specific heat capacity, which then gradually releases the heat over time. A hot water bottle is the most familiar example of this type of heating pad.
A microwavable heating pad is a heating pad that is warmed by placing it in a microwave oven before use. Microwavable heating pads are typically made out of a thick insulative fabric such as flannel and filled with grains such as buckwheat or flax seed. Due to their relative simplicity to make, they are frequently sewn by hand, often with a custom shape to fit the intended area of use. In rare instances, these types of pads have been known to ignite during or after the microwave process and cause fires.
Often, aromatic compounds will also be added to the filler mixture to create a pleasant or soothing smell when heated. The source of these can vary significantly, ranging from adding essential oils to ground up spices such as cloves and nutmeg, or even dried rose petals.
Electric pads usually operate from household current and must have protection against overheating.
A moist heating pad is used dry on the user's skin. These pads register temperatures from 170 to 180 degrees Fahrenheit (76 to 82 °C) and are intended for deep tissue treatment and can be dangerous if left on unattended. Moist heating pads are used mainly by physical therapists but can be found for home use. A moist cloth can be added with a stupe cover to add more moisture to the treatment.
Chemical
Chemical pads employ a chemical heat reservoir or a one-time chemical reaction such as catalyzed rusting of iron.
A sodium acetate heat pad
A sodium acetate heat pad is a reusable heat reservoir. It contains a supersaturated solution of sodium acetate (CH3COONa). Crystallization is triggered by flexing a (patented ) small flat disc of notched ferrous metal embedded in the liquid. Pressing the disc releases very tiny adhered crystals of sodium acetate into the solution which then act as nucleation sites for the recrystallization of the remainder of the salt solution. Because the liquid is supersaturated, this makes the solution crystallize suddenly, thereby releasing the energy of the crystal lattice.
See sodium acetate for a more technical discussion.
The pad can be reused by placing it in boiling water for 10-15 minutes, which redissolves the sodium acetate in the contained water and recreates a supersaturated solution. Once the pad has returned to room temperature it can be triggered again. Triggering the pad before it has reached room temperature results in the pad reaching a lower peak temperature, as compared to waiting until it had completely cooled.
High specific-heat capacity materials
Heating packs can also be made by filling a container with a material that has a high specific heat capacity, which then gradually releases the heat over time. A hot water bottle is the most familiar example of this type of heating pad.
A microwavable heating pad is a heating pad that is warmed by placing it in a microwave oven before use. Microwavable heating pads are typically made out of a thick insulative fabric such as flannel and filled with grains such as buckwheat or flax seed. Due to their relative simplicity to make, they are frequently sewn by hand, often with a custom shape to fit the intended area of use. In rare instances, these types of pads have been known to ignite during or after the microwave process and cause fires.
Often, aromatic compounds will also be added to the filler mixture to create a pleasant or soothing smell when heated. The source of these can vary significantly, ranging from adding essential oils to ground up spices such as cloves and nutmeg, or even dried rose petals.
Roller printing on textiles
Roller printing, also called cylinder printing or machine printing, on fabrics is a textile printing process patented by Thomas Bell of Scotland in 1783 in an attempt to reduce the cost of the earlier copperplate printing. This method was used in Lancashire fabric mills to produce cotton dress fabrics from the 1790s, most often reproducing small monochrome patterns characterized by striped motifs and tiny dotted patterns called "machine grounds".
Improvements in the technology resulted in more elaborate roller prints in bright, rich colours from the 1820s; Turkey Red and chrome yellow were particularly popular.Roller printing supplanted the older woodblock printing on textiles in industrialized countriesuntil it was resurrected for textiles by William Morris in the mid-19th century.
The printing of textiles from engraved copperplates was first practiced by Bell in 1770. It was entirely obsolete, as an industry, in England, by the end of the 19th century.
The presses first used were of the ordinary letterpress type, the engraved plate being fixed in the place of the type. In later improvements the well-known cylinder press was employed; the plate was inked mechanically and cleaned off by passing under a sharp blade of steel; and the cloth, instead of being laid on the plate, was passed round the pressure cylinder. The plate was raised into frictional contact with the cylinder and in passing under it transferred its ink to the cloth.
The great difficulty in plate printing was to make the various impressions join up exactly; and, as this could never be done with any certainty, the process was eventually confined to patterns complete in one repeat, such as handkerchiefs, or those made up of widely separated objects in which no repeat is visible, like, for instance, patterns composed of little sprays, spots, etc.
Improvements in the technology resulted in more elaborate roller prints in bright, rich colours from the 1820s; Turkey Red and chrome yellow were particularly popular.Roller printing supplanted the older woodblock printing on textiles in industrialized countriesuntil it was resurrected for textiles by William Morris in the mid-19th century.
The printing of textiles from engraved copperplates was first practiced by Bell in 1770. It was entirely obsolete, as an industry, in England, by the end of the 19th century.
The presses first used were of the ordinary letterpress type, the engraved plate being fixed in the place of the type. In later improvements the well-known cylinder press was employed; the plate was inked mechanically and cleaned off by passing under a sharp blade of steel; and the cloth, instead of being laid on the plate, was passed round the pressure cylinder. The plate was raised into frictional contact with the cylinder and in passing under it transferred its ink to the cloth.
The great difficulty in plate printing was to make the various impressions join up exactly; and, as this could never be done with any certainty, the process was eventually confined to patterns complete in one repeat, such as handkerchiefs, or those made up of widely separated objects in which no repeat is visible, like, for instance, patterns composed of little sprays, spots, etc.
Laser engraving
Laser engraving is the practice of using lasers to engrave or mark an object. The technique can be very technical and complex, and often a computer system is used to drive the movements of the laser head. Despite this complexity, very precise and clean engravings can be achieved at a high rate. The technique does not involve tool bits which contact the engraving surface and wear out. This is considered an advantage over alternative engraving technologies where bit heads have to be replaced regularly.
The impact of laser engraving has been more pronounced for specially-designed "laserable" materials. These include polymer and novel metal alloys.
In situations where physical alteration of a surface by engraving is undesirable, an alternative such as "marking" is available. This is a generic term that covers a broad spectrum of surfacing techniques including printing, hot-branding and laser bonding. In many instances, laser engraving machines are able to do marking that would have been done by other processes.
A laser engraving machine can be thought of as three main parts: a laser, a controller, and a surface. The laser is like a pencil - the beam emitted from it allows the controller to trace patterns onto the surface. The controller (usually a computer) controls the direction, intensity, speed of movement, and spread of the laser beam aimed at the surface. The surface is picked to match what the laser can act on.
There are three main genres of engraving machines: The most common is the X-Y table where, usually, the workpiece (surface) is stationary and the laser moves around in X and Y directions drawing vectors. Sometimes the laser is stationary and the workpiece moves. Sometimes the workpiece moves in the Y axis and the laser in the X axis. A second genre is for cylindrical workpieces (or flat workpieces mounted around a cylinder) where the laser effectively traverses a fine helix and on/off laser pulsing produces the desired image on a raster basis. In the third method, both the laser and workpiece are stationary and galvo mirrors move the laser beam over the workpiece surface. Laser engravers using this technology can work in either raster or vector mode.
The point where the laser (the terms "laser" and "laser beam" may be used interchangeably) touches the surface should be on the focal plane of the laser's optical system, and is usually synonymous with its focal point. This point is typically small, perhaps less than a fraction of a millimeter (depending on the optical wavelength). Only the area inside this focal point is significantly affected when the laser beam passes over the surface. The energy delivered by the laser changes the surface of the material under the focal point. It may heat up the surface and subsequently vaporize the material, or perhaps the material may fracture (known as "glass" or "glass up") and flake off the surface. This is how material is removed from the surface to create an engraving.
The impact of laser engraving has been more pronounced for specially-designed "laserable" materials. These include polymer and novel metal alloys.
In situations where physical alteration of a surface by engraving is undesirable, an alternative such as "marking" is available. This is a generic term that covers a broad spectrum of surfacing techniques including printing, hot-branding and laser bonding. In many instances, laser engraving machines are able to do marking that would have been done by other processes.
A laser engraving machine can be thought of as three main parts: a laser, a controller, and a surface. The laser is like a pencil - the beam emitted from it allows the controller to trace patterns onto the surface. The controller (usually a computer) controls the direction, intensity, speed of movement, and spread of the laser beam aimed at the surface. The surface is picked to match what the laser can act on.
There are three main genres of engraving machines: The most common is the X-Y table where, usually, the workpiece (surface) is stationary and the laser moves around in X and Y directions drawing vectors. Sometimes the laser is stationary and the workpiece moves. Sometimes the workpiece moves in the Y axis and the laser in the X axis. A second genre is for cylindrical workpieces (or flat workpieces mounted around a cylinder) where the laser effectively traverses a fine helix and on/off laser pulsing produces the desired image on a raster basis. In the third method, both the laser and workpiece are stationary and galvo mirrors move the laser beam over the workpiece surface. Laser engravers using this technology can work in either raster or vector mode.
The point where the laser (the terms "laser" and "laser beam" may be used interchangeably) touches the surface should be on the focal plane of the laser's optical system, and is usually synonymous with its focal point. This point is typically small, perhaps less than a fraction of a millimeter (depending on the optical wavelength). Only the area inside this focal point is significantly affected when the laser beam passes over the surface. The energy delivered by the laser changes the surface of the material under the focal point. It may heat up the surface and subsequently vaporize the material, or perhaps the material may fracture (known as "glass" or "glass up") and flake off the surface. This is how material is removed from the surface to create an engraving.
2008年11月7日星期五
Coaxial cable
Coaxial cable design choices affect physical size, frequency performance, attenuation, power handling capabilities, flexibility, and cost. The inner conductor might be solid or stranded; stranded is more flexible. To get better high-frequency performance, the inner conductor may be silver plated. Sometimes copper-plated iron wire is used as an inner conductor.
The insulator surrounding the inner conductor may be solid plastic, a foam plastic, or may be air with spacers supporting the inner wire. The properties of dielectric control some electrical properties of the cable. A common choice is a solid polyethylene (PE) insulator, used in lower-loss cables. Solid Teflon (PTFE) is also used as an insulator. Some coaxial lines use air (or some other gas) and have spacers to keep the inner conductor from touching the shield.
There is also a lot of variety in the shield. Conventional coaxial cable has braided copper wire forming the shield. This allows the cable to be flexible, but it also means there are gaps in the shield layer, and the inner dimension of the shield varies slightly because the braid cannot be flat. Sometimes the braid is silver plated. For better shield performance, some cables have a double-layer shield. The shield might be just two braids, but it is more common now to have a thin foil shield covered by a wire braid. Some cables may invest in more than two shield layers. Other shield designs sacrifice flexibility for better performance; some shields are a solid metal tube. Those cables cannot take sharp bends, as the shield will kink, causing losses in the cable. Many Cable television (CATV) distribution systems use such "hard line" cables, as they provide a lower signal loss.
The insulating jacket can be made from many materials. A common choice is PVC, but some applications may require fire-resistant materials. Outdoor applications may require the jacket to resist ultraviolet light and oxidation. For internal chassis connections the insulating jacket may be omitted.
Connections at the ends of coaxial cables are usually made with RF connectors.
The insulator surrounding the inner conductor may be solid plastic, a foam plastic, or may be air with spacers supporting the inner wire. The properties of dielectric control some electrical properties of the cable. A common choice is a solid polyethylene (PE) insulator, used in lower-loss cables. Solid Teflon (PTFE) is also used as an insulator. Some coaxial lines use air (or some other gas) and have spacers to keep the inner conductor from touching the shield.
There is also a lot of variety in the shield. Conventional coaxial cable has braided copper wire forming the shield. This allows the cable to be flexible, but it also means there are gaps in the shield layer, and the inner dimension of the shield varies slightly because the braid cannot be flat. Sometimes the braid is silver plated. For better shield performance, some cables have a double-layer shield. The shield might be just two braids, but it is more common now to have a thin foil shield covered by a wire braid. Some cables may invest in more than two shield layers. Other shield designs sacrifice flexibility for better performance; some shields are a solid metal tube. Those cables cannot take sharp bends, as the shield will kink, causing losses in the cable. Many Cable television (CATV) distribution systems use such "hard line" cables, as they provide a lower signal loss.
The insulating jacket can be made from many materials. A common choice is PVC, but some applications may require fire-resistant materials. Outdoor applications may require the jacket to resist ultraviolet light and oxidation. For internal chassis connections the insulating jacket may be omitted.
Connections at the ends of coaxial cables are usually made with RF connectors.
industrial control transformer
Motor controller
Domestic applications
Electric motors are used domestically in personal care products, small and large appliances, and residential heating and cooling equipment. In most domestic applications, the motor controller functions are built into the product. In some cases, such as bathroom ventilation fans, the motor is controlled by a switch on the wall. Some appliances have provisions for controlling the speed of the motor. Built-in circuit breakers protect some appliance motors, but most are unprotected except that the household fuse or circuit breaker panel disconnects the motor if it fails.
Office equipment, medical equipment etc.
There is a wide variety of motorized office equipment such as personal computers, computer peripherals, copy machines and fax machines as well as smaller items such as electric pencil sharpeners. Motor controllers for these types of equipment are built into the equipment. Some quite sophisticated motor controllers are used to control the motors in computer disc drives. Medical equipment may include very sophisticated motor controllers.
Commercial applications
Commercial buildings have larger heating ventilation and air conditioning (HVAC) equipment than that found in individual residences. In addition, motors are used for elevators, escalators and other applications. In commercial applications, the motor control functions are sometimes built into the motor-driven equipment and sometimes installed separately.
Industrial applications
Many industrial applications are dependent upon motors (or machines), which range from the size of one's thumb to the size of a railroad locomotive. The motor controllers can be built into the driven equipment, installed separately, installed in an enclosure along with other machine control equipment or installed in motor control centers. Motor control centers are multi-compartment steel enclosures designed to enclose many motor controllers. It is also common for more than one motor controller to operate a number of motors in the same application. In this case the controllers communicate with each other so they can work the motors together as a team.
Vehicle applications
All types of engine-driven vehicles from automobiles, airplanes, aircraft carriers and agricultural equipment to zambonis may have electric motors to perform a variety of functions. In electric vehicles, diesel-electric vehicles, and hybrid vehicles, electric motors are used to propel the vehicle. The motor controllers in vehicle applications are integrated into the vehicle.
Power tools
Power tools such as drills, saws and sanders are widely used by home owners, hobbyists, construction and repair trades people, and industrial workers. Both portable and stationary power tools usually have built in motor controllers and often include an adjustable speed feature.
Electric motors are used domestically in personal care products, small and large appliances, and residential heating and cooling equipment. In most domestic applications, the motor controller functions are built into the product. In some cases, such as bathroom ventilation fans, the motor is controlled by a switch on the wall. Some appliances have provisions for controlling the speed of the motor. Built-in circuit breakers protect some appliance motors, but most are unprotected except that the household fuse or circuit breaker panel disconnects the motor if it fails.
Office equipment, medical equipment etc.
There is a wide variety of motorized office equipment such as personal computers, computer peripherals, copy machines and fax machines as well as smaller items such as electric pencil sharpeners. Motor controllers for these types of equipment are built into the equipment. Some quite sophisticated motor controllers are used to control the motors in computer disc drives. Medical equipment may include very sophisticated motor controllers.
Commercial applications
Commercial buildings have larger heating ventilation and air conditioning (HVAC) equipment than that found in individual residences. In addition, motors are used for elevators, escalators and other applications. In commercial applications, the motor control functions are sometimes built into the motor-driven equipment and sometimes installed separately.
Industrial applications
Many industrial applications are dependent upon motors (or machines), which range from the size of one's thumb to the size of a railroad locomotive. The motor controllers can be built into the driven equipment, installed separately, installed in an enclosure along with other machine control equipment or installed in motor control centers. Motor control centers are multi-compartment steel enclosures designed to enclose many motor controllers. It is also common for more than one motor controller to operate a number of motors in the same application. In this case the controllers communicate with each other so they can work the motors together as a team.
Vehicle applications
All types of engine-driven vehicles from automobiles, airplanes, aircraft carriers and agricultural equipment to zambonis may have electric motors to perform a variety of functions. In electric vehicles, diesel-electric vehicles, and hybrid vehicles, electric motors are used to propel the vehicle. The motor controllers in vehicle applications are integrated into the vehicle.
Power tools
Power tools such as drills, saws and sanders are widely used by home owners, hobbyists, construction and repair trades people, and industrial workers. Both portable and stationary power tools usually have built in motor controllers and often include an adjustable speed feature.
industrial control transformer
Automatic fire suppression
According to the National Fire Protection Association, there were 1,602,000 fires reported in the United States in 2005. There were 3,675 civilian deaths, 17,925 civilian injuries, and $9.2 billion in property damage. A fire department responded to a fire every 20 seconds and a structure fire was reported every 62 seconds.
Although man has fought fire for centuries, it was not until Feb. 10, 1863 that the first fire extinguisher patent was issued to Alanson Crane of Virginia. The first fire sprinkler system was patented by H.W. Pratt in 1872. But the first practical automatic sprinkler system was invented in 1874 by Henry S. Parmalee of New Haven, CT. He installed the system in a piano factory he owned. Today there are numerous types of Automatic Fire Suppression Systems. Systems are as diverse as the many applications. In general, however, Automatic Fire Suppression Systems fall into two categories. These are engineered and pre-engineered systems.
Engineered Fire Suppression Systems are design specific. Engineered systems are usually for larger installations where the system is designed for the particular application. Examples include marine and land vehicle applications, computer clean rooms, public and private buildings, industrial paint lines, dip tanks and electrical switch rooms.
Pre-Engineered Fire Suppression Systems do not require the involvement of a design engineer beyond the original product design. Pre-engineered systems are comprised of pre-designed components. Examples of pre-engineered systems include commercial kitchen systems and industrial paint rooms and paint booths and industrial storage areas.
Pre-engineered systems most commonly use a simple wet or dry chemical agent, such as potassium carbonate or monoammonium phosphate (MAP). Engineered systems use a number of gaseous or solid agents. Many are specifically formulated. Some, such as 3M™ Novec™ 1230 Fire Protection Fluid, are stored as a liquid and discharged as a gas.
Although man has fought fire for centuries, it was not until Feb. 10, 1863 that the first fire extinguisher patent was issued to Alanson Crane of Virginia. The first fire sprinkler system was patented by H.W. Pratt in 1872. But the first practical automatic sprinkler system was invented in 1874 by Henry S. Parmalee of New Haven, CT. He installed the system in a piano factory he owned. Today there are numerous types of Automatic Fire Suppression Systems. Systems are as diverse as the many applications. In general, however, Automatic Fire Suppression Systems fall into two categories. These are engineered and pre-engineered systems.
Engineered Fire Suppression Systems are design specific. Engineered systems are usually for larger installations where the system is designed for the particular application. Examples include marine and land vehicle applications, computer clean rooms, public and private buildings, industrial paint lines, dip tanks and electrical switch rooms.
Pre-Engineered Fire Suppression Systems do not require the involvement of a design engineer beyond the original product design. Pre-engineered systems are comprised of pre-designed components. Examples of pre-engineered systems include commercial kitchen systems and industrial paint rooms and paint booths and industrial storage areas.
Pre-engineered systems most commonly use a simple wet or dry chemical agent, such as potassium carbonate or monoammonium phosphate (MAP). Engineered systems use a number of gaseous or solid agents. Many are specifically formulated. Some, such as 3M™ Novec™ 1230 Fire Protection Fluid, are stored as a liquid and discharged as a gas.
industrial control transformer
WEG Industries
The state of Santa Catarina in Brazil was subjected to German colonization (besides Italian and - much earlier - Portuguese), which must have influenced the founders (Werner Ricardo Voigt, Eggon João da Silva and Geraldo Werninghaus) in their decision to use the acronym WEG, joining the three first letters of their names.
The company started on the 16th of September 1961, when the three founded "Eletromotores Jaraguá". Years later, the company created by an electrician, an administrator and a mechanic would change its name to "Eletromotores WEG SA".
Initially producing electric motors, WEG started incrementing its activities during the eighties, with the production of electric components, products for industrial automation, power and distribution transformers, liquid and powder paints and electrical insulatins varnishes. More and more the company is consolidating itself not only as a motor manufacturer, but also as a complete, industrial, electrical systems supplier.
In 1989 the three founders created the company's Board of Directors and Décio da Silva was chosen as WEG's CEO. Two years later WEG's Quality and Productivity Program was implemented, consolidating the process of participative administration.
Products
WEG products are present at almost every electric engineering interest area: - Generation (Generators, transformers and switchgear) - Transmission (EPC, switchgears, transformers) - Distribution (EPC, transformers) - Electric products (motors, switchgears, frequency inverters, AC/DC converters, contactors, fuses, circuit breakers and servomotors, among others) - Automation (Hardware and Software) - Integration Engineering with products of different manufacturers
Figures
Counting on more than 19 thousand employees all over the world, WEG reached an annual turnover of R$ 4,552 billion in 2007, a growth of 29% in relation to 2006, R$ 3,527 billion. Exports were responsible for 30% of the company's turnover.
Production is distributed in manufacturing plants in Brazil (in the cities of São Paulo, São Bernardo do Campo, Hortolândia, Guarulhos, Manaus, Guaramirim, Itajaí, Blumenau and two plants in Jaraguá do Sul, two in Argentina, one in Mexico , one in Portugal and one in China. WEG also exports to over 100 countries and counts on branches and technical assistance in all five continents.
On May, 2008, WEG has announced a new factory in India.
A good part of these great results influence life in the city of Jaraguá do Sul directly. One of the most visible forms of this is the distribution of profits to the employees. Besides the injection of capital that the profit distribution provokes in all the sectors of the economy, WEG also participates directly in the increase in quality of life of the city. A great part of this success is due to participative administration, a concept applied from the factory floor up. The Circles of Quality Control (CCQ), implemented in 1982, are already part of the company culture. Through these groups, each employee is able to present suggestions on job safety, health and quality of life. Many suggestions result in new production processes and even in new machines, generating more savings and productivity.
industrial control transformer
Induction motor
An induction motor (IM) is a type of asynchronous AC motor where power is supplied to the rotating device by means of electromagnetic induction. Other commonly used name is squirrel cage motor due to the fact that the rotor bars with short circuit rings resemble a squirrel cage (hamster wheel).
An electric motor converts electrical power to mechanical power in its rotor (rotating part). There are several ways to supply power to the rotor. In a DC motor this power is supplied to the armature directly from a DC source, while in an AC motor this power is induced in the rotating device. An induction motor is sometimes called a rotating transformer because the stator (stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is the secondary side. Induction motors are widely used, especially polyphase induction motors, which are frequently used in industrial drives.
Induction motors are now the preferred choice for industrial motors due to their rugged construction, absence of brushes (which are required in most DC motors) and — thanks to modern power electronics — the ability to control the speed of the motor.
The induction motor with a wrapped rotor was invented by Nikola Tesla in 1882 in France but the initial patent was issued in 1888 after Tesla had moved to the United States. In his scientific work, Tesla laid the foundations for understanding the way the motor operates. The induction motor with a cage was invented by Mikhail Dolivo-Dobrovolsky about a year later in Europe. Technological development in the field has improved to where a 100 hp (73.6 kW) motor from 1976 takes the same volume as a 7.5 hp (5.5 kW) motor did in 1897. Currently, the most common induction motor is the cage rotor motor.
An electric motor converts electrical power to mechanical power in its rotor (rotating part). There are several ways to supply power to the rotor. In a DC motor this power is supplied to the armature directly from a DC source, while in an AC motor this power is induced in the rotating device. An induction motor is sometimes called a rotating transformer because the stator (stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is the secondary side. Induction motors are widely used, especially polyphase induction motors, which are frequently used in industrial drives.
Induction motors are now the preferred choice for industrial motors due to their rugged construction, absence of brushes (which are required in most DC motors) and — thanks to modern power electronics — the ability to control the speed of the motor.
The induction motor with a wrapped rotor was invented by Nikola Tesla in 1882 in France but the initial patent was issued in 1888 after Tesla had moved to the United States. In his scientific work, Tesla laid the foundations for understanding the way the motor operates. The induction motor with a cage was invented by Mikhail Dolivo-Dobrovolsky about a year later in Europe. Technological development in the field has improved to where a 100 hp (73.6 kW) motor from 1976 takes the same volume as a 7.5 hp (5.5 kW) motor did in 1897. Currently, the most common induction motor is the cage rotor motor.
industrial control transformer
2008年11月5日星期三
Portable Emissions Measurement System
A Portable Emissions Measurement System (PEMS) is essentially a lightweight ‘laboratory’ that is used to test and/or assess mobile source emissions (i.e. cars, trucks, buses, construction equipment, generators, trains, cranes, etc.) for the purposes of compliance, regulation, or decision-making. Governmental entities like the United States Environmental Protection Agency (USEPA), the European Union, as various states and private sector entities have begun to utilize PEMS in order to reduce both the costs and time of mobile emissions decisions. Various state, federal, and international agencies began referring to this shorthand term in early 2000, and the nickname became part of industry parlance.
Since the mid-1800’s, Dynamometers (or "dyno" for short) has been used to measure torque and rotational speed (rpm) from which power produced by an engine, motor or other rotating prime mover can then be calculated. A chassis dynamometer measures power from the engine through the wheels. The vehicle is parked on rollers which the car then turns and the output is measured. These dynos can be fixed or portable. Because of frictional and mechanical losses in the various drivetrain components, the measured horsepower is generally 15-20 percent less than the brake horsepower measured at the crankshaft or flywheel on an engine dynamometer . Historically though, dynamometer emission tests are very expensive, and have usually involved removing fleet vehicles from service for a long period of time. Also, the data derived from such testing is not representative of “real world” driving conditions, and cannot be deemed as quantifiable, especially due to the relatively low amount of repeatable tests at such a facility.
Portable systems began to be developed in the late 1990’s in order to better identify actual in-use performance of vehicles. PEMS are designed to measure emissions during the actual use of an internal-combustion engine vehicle or equipment in its regular daily operation, in a manner similar to operation on a chassis Dynamometer. This methodology and approach has been recognized by the USEPA
Many governmental entities (such as the USEPA and the United Nations Framework Convention on Climate Change or UNFCCC) have identified target mobile-source pollutants in various mobile standards as CO2, NOx, Particulate Matter (PM), Carbon Monoxide (CO), Hydrocarbons(HC), to ensure that emissions standards are being met. Further, these governing bodies have begun adopting in-use testing program for non-road diesel engines, as well as other types of internal combustion engines, and are requiring the use of PEMS testing. It is important to delineate the various classifications of the latest ‘transferable’ emissions testing equipment from PEMS equipment, in order to best understand the desire of portability in field-testing of emissions.
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Wallets And Purses
Cummins Engine Parts
nut bolt washer
Men's Outdoor Jacket
microscope cover glass
Boat trailer hardware
non-stick dutch oven
industrial coffee machine
vibration exercise machines
folding silk screen
jet fuel a1
latex bed sheets
automotive water pump
portable handsfree kit
diamond emerald necklace
Welding And Soldering
musical water globe
motorcycle wheel weights
Mini LED Torch
baby knee socks
av cables rca
ladies leather sandals
Plastic Pill Boxes
Since the mid-1800’s, Dynamometers (or "dyno" for short) has been used to measure torque and rotational speed (rpm) from which power produced by an engine, motor or other rotating prime mover can then be calculated. A chassis dynamometer measures power from the engine through the wheels. The vehicle is parked on rollers which the car then turns and the output is measured. These dynos can be fixed or portable. Because of frictional and mechanical losses in the various drivetrain components, the measured horsepower is generally 15-20 percent less than the brake horsepower measured at the crankshaft or flywheel on an engine dynamometer . Historically though, dynamometer emission tests are very expensive, and have usually involved removing fleet vehicles from service for a long period of time. Also, the data derived from such testing is not representative of “real world” driving conditions, and cannot be deemed as quantifiable, especially due to the relatively low amount of repeatable tests at such a facility.
Portable systems began to be developed in the late 1990’s in order to better identify actual in-use performance of vehicles. PEMS are designed to measure emissions during the actual use of an internal-combustion engine vehicle or equipment in its regular daily operation, in a manner similar to operation on a chassis Dynamometer. This methodology and approach has been recognized by the USEPA
Many governmental entities (such as the USEPA and the United Nations Framework Convention on Climate Change or UNFCCC) have identified target mobile-source pollutants in various mobile standards as CO2, NOx, Particulate Matter (PM), Carbon Monoxide (CO), Hydrocarbons(HC), to ensure that emissions standards are being met. Further, these governing bodies have begun adopting in-use testing program for non-road diesel engines, as well as other types of internal combustion engines, and are requiring the use of PEMS testing. It is important to delineate the various classifications of the latest ‘transferable’ emissions testing equipment from PEMS equipment, in order to best understand the desire of portability in field-testing of emissions.
Animal Print Rug
in wall aquarium
Wallets And Purses
Cummins Engine Parts
nut bolt washer
Men's Outdoor Jacket
microscope cover glass
Boat trailer hardware
non-stick dutch oven
industrial coffee machine
vibration exercise machines
folding silk screen
jet fuel a1
latex bed sheets
automotive water pump
portable handsfree kit
diamond emerald necklace
Welding And Soldering
musical water globe
motorcycle wheel weights
Mini LED Torch
baby knee socks
av cables rca
ladies leather sandals
Plastic Pill Boxes
Hoist
A hoist is a device used for lifting or lowering a load by means of a drum or lift-wheel around which rope or chain wraps. It may be manually operated, electrically or pneumatically driven and may use chain, fiber or wire rope as its lifting medium. The load is attached to the hoist by means of a lifting hook.
The basic hoist has two important characteristics to define it: Lifting medium and power type. The lifting medium is either wire rope, wrapped around a drum, or load-chain, raised by a pulley with a special profile to engage the chain. The power type can be either electric motor or air motor. Both the wire rope hoist and chain hoist have been in common use since the 1800s. A hoist can be built as one integral-package unit, designed for cost-effective purchasing and moderate use, or it can be built as a built-up custom unit, designed for durability and performance. The built-up hoist will be much more expensive, but will also be easier to repair and more durable. Package units are designed for light to moderate usage, while built-up units are designed for heavy to severe service. A machine shop or fabricating shop will use an integral-package hoist, while a Steel Mill or NASA would use a built-up unit to meet durabilty, performance, and repairability requirements.
Also known as a Man-Lift, Buckhoist, temporary elevator or construction elevator, this type of hoist is commonly used on large scale construction projects, such as high-rise buildings or major hospitals. The purpose being to carry personnel and materials quickly between the ground and higher floors, or between upper floors.
The construction hoist is made up of either one or cars (cages) which travel vertically along stacked mast tower sections. For controlled travel along the mast sections, most modern construction hoists utilize a motorized rack-and-pinion system mounted onto the mast sections.
While hoists have been predominantly produced the United States and Europe, China is emerging as a leading manufacturer of hoists.
In the US, General Contractors rent or lease hoists for a specific project. Rental companies provide erection, dismantling, and repair services to their hoists to provide General Contractors with turnkey services.
Animal Print Rug
in wall aquarium
Wallets And Purses
Cummins Engine Parts
nut bolt washer
Men's Outdoor Jacket
microscope cover glass
Boat trailer hardware
non-stick dutch oven
industrial coffee machine
vibration exercise machines
folding silk screen
jet fuel a1
latex bed sheets
automotive water pump
portable handsfree kit
diamond emerald necklace
Welding And Soldering
musical water globe
motorcycle wheel weights
Mini LED Torch
baby knee socks
av cables rca
ladies leather sandals
Plastic Pill Boxes
The basic hoist has two important characteristics to define it: Lifting medium and power type. The lifting medium is either wire rope, wrapped around a drum, or load-chain, raised by a pulley with a special profile to engage the chain. The power type can be either electric motor or air motor. Both the wire rope hoist and chain hoist have been in common use since the 1800s. A hoist can be built as one integral-package unit, designed for cost-effective purchasing and moderate use, or it can be built as a built-up custom unit, designed for durability and performance. The built-up hoist will be much more expensive, but will also be easier to repair and more durable. Package units are designed for light to moderate usage, while built-up units are designed for heavy to severe service. A machine shop or fabricating shop will use an integral-package hoist, while a Steel Mill or NASA would use a built-up unit to meet durabilty, performance, and repairability requirements.
Also known as a Man-Lift, Buckhoist, temporary elevator or construction elevator, this type of hoist is commonly used on large scale construction projects, such as high-rise buildings or major hospitals. The purpose being to carry personnel and materials quickly between the ground and higher floors, or between upper floors.
The construction hoist is made up of either one or cars (cages) which travel vertically along stacked mast tower sections. For controlled travel along the mast sections, most modern construction hoists utilize a motorized rack-and-pinion system mounted onto the mast sections.
While hoists have been predominantly produced the United States and Europe, China is emerging as a leading manufacturer of hoists.
In the US, General Contractors rent or lease hoists for a specific project. Rental companies provide erection, dismantling, and repair services to their hoists to provide General Contractors with turnkey services.
Animal Print Rug
in wall aquarium
Wallets And Purses
Cummins Engine Parts
nut bolt washer
Men's Outdoor Jacket
microscope cover glass
Boat trailer hardware
non-stick dutch oven
industrial coffee machine
vibration exercise machines
folding silk screen
jet fuel a1
latex bed sheets
automotive water pump
portable handsfree kit
diamond emerald necklace
Welding And Soldering
musical water globe
motorcycle wheel weights
Mini LED Torch
baby knee socks
av cables rca
ladies leather sandals
Plastic Pill Boxes
Crane (machine)
The first cranes were invented by the Ancient Greeks and were powered by men or beasts-of-burden, such as donkeys. These cranes were used for the construction of tall buildings. Larger cranes were later developed, employing the use of human treadwheels, permitting the lifting of heavier weights. In the High Middle Ages, harbour cranes were introduced to load and unload ships and assist with their construction – some were built into stone towers for extra strength and stability. The earliest cranes were constructed from wood, but cast iron and steel took over with the coming of the Industrial Revolution.
For many centuries, power was supplied by the physical exertion of men or animals, although hoists in watermills and windmills could be driven by the harnessed natural power. The first 'mechanical' power was provided by steam engines, the earliest steam crane being introduced in the 18th or 19th century, with many remaining in use well into the late 20th century. Modern cranes usually use internal combustion engines or electric motors and hydraulic systems to provide a much greater lifting capability than was previously possible, although manual cranes are still utilised where the provision of power would be uneconomic.
Cranes exist in an enormous variety of forms – each tailored to a specific use. Sizes range from the smallest jib cranes, used inside workshops, to the tallest tower cranes, used for constructing high buildings, and the largest floating cranes, used to build oil rigs and salvage sunken ships.
This article also covers lifting machines that do not strictly fit the above definition of a crane, but are generally known as cranes, such as stacker cranes and loader cranes.
The crane for lifting heavy loads was invented by the Ancient Greeks in the late 6th century BC.The archaeological record shows that no later than c.515 BC distinctive cuttings for both lifting tongs and lewis irons begin to appear on stone blocks of Greek temples. Since these holes point at the use of a lifting device, and since they are to be found either above the center of gravity of the block, or in pairs equidistant from a point over the center of gravity, they are regarded by archaeologists as the positive evidence required for the existence of the crane.
The introduction of the winch and pulley hoist soon lead to a widespread replacement of ramps as the main means of vertical motion. For the next two hundred years, Greek building sites witnessed a sharp drop in the weights handled, as the new lifting technique made the use of several smaller stones more practical than of fewer larger ones. In contrast to the archaic period with its tendency to ever-increasing block sizes, Greek temples of the classical age like the Parthenon invariably featured stone blocks weighing less than 15-20 tons. Also, the practice of erecting large monolithic columns was practically abandoned in favour of using several column drums.
Although the exact circumstances of the shift from the ramp to the crane technology remain unclear, it has been argued that the volatile social and political conditions of Greece were more suitable to the employment of small, professional construction teams than of large bodies of unskilled labour, making the crane more preferable to the Greek polis than the more labour-intensive ramp which had been the norm in the autocratic societies of Egypt or Assyria.The first unequivocal literary evidence for the existence of the compound pulley system appears in the Mechanical Problems (Mech. 18, 853a32-853b13) attributed to Aristotle (384-322 BC), but perhaps composed at a slightly later date. Around the same time, block sizes at Greek temples began to match their archaic predecessors again, indicating that the more sophisticated compound pulley must have found its way to Greek construction sites by then.
Animal Print Rug
in wall aquarium
Wallets And Purses
Cummins Engine Parts
nut bolt washer
Men's Outdoor Jacket
microscope cover glass
Boat trailer hardware
non-stick dutch oven
industrial coffee machine
vibration exercise machines
folding silk screen
jet fuel a1
latex bed sheets
automotive water pump
portable handsfree kit
diamond emerald necklace
Welding And Soldering
musical water globe
motorcycle wheel weights
Mini LED Torch
baby knee socks
av cables rca
ladies leather sandals
Plastic Pill Boxes
For many centuries, power was supplied by the physical exertion of men or animals, although hoists in watermills and windmills could be driven by the harnessed natural power. The first 'mechanical' power was provided by steam engines, the earliest steam crane being introduced in the 18th or 19th century, with many remaining in use well into the late 20th century. Modern cranes usually use internal combustion engines or electric motors and hydraulic systems to provide a much greater lifting capability than was previously possible, although manual cranes are still utilised where the provision of power would be uneconomic.
Cranes exist in an enormous variety of forms – each tailored to a specific use. Sizes range from the smallest jib cranes, used inside workshops, to the tallest tower cranes, used for constructing high buildings, and the largest floating cranes, used to build oil rigs and salvage sunken ships.
This article also covers lifting machines that do not strictly fit the above definition of a crane, but are generally known as cranes, such as stacker cranes and loader cranes.
The crane for lifting heavy loads was invented by the Ancient Greeks in the late 6th century BC.The archaeological record shows that no later than c.515 BC distinctive cuttings for both lifting tongs and lewis irons begin to appear on stone blocks of Greek temples. Since these holes point at the use of a lifting device, and since they are to be found either above the center of gravity of the block, or in pairs equidistant from a point over the center of gravity, they are regarded by archaeologists as the positive evidence required for the existence of the crane.
The introduction of the winch and pulley hoist soon lead to a widespread replacement of ramps as the main means of vertical motion. For the next two hundred years, Greek building sites witnessed a sharp drop in the weights handled, as the new lifting technique made the use of several smaller stones more practical than of fewer larger ones. In contrast to the archaic period with its tendency to ever-increasing block sizes, Greek temples of the classical age like the Parthenon invariably featured stone blocks weighing less than 15-20 tons. Also, the practice of erecting large monolithic columns was practically abandoned in favour of using several column drums.
Although the exact circumstances of the shift from the ramp to the crane technology remain unclear, it has been argued that the volatile social and political conditions of Greece were more suitable to the employment of small, professional construction teams than of large bodies of unskilled labour, making the crane more preferable to the Greek polis than the more labour-intensive ramp which had been the norm in the autocratic societies of Egypt or Assyria.The first unequivocal literary evidence for the existence of the compound pulley system appears in the Mechanical Problems (Mech. 18, 853a32-853b13) attributed to Aristotle (384-322 BC), but perhaps composed at a slightly later date. Around the same time, block sizes at Greek temples began to match their archaic predecessors again, indicating that the more sophisticated compound pulley must have found its way to Greek construction sites by then.
Animal Print Rug
in wall aquarium
Wallets And Purses
Cummins Engine Parts
nut bolt washer
Men's Outdoor Jacket
microscope cover glass
Boat trailer hardware
non-stick dutch oven
industrial coffee machine
vibration exercise machines
folding silk screen
jet fuel a1
latex bed sheets
automotive water pump
portable handsfree kit
diamond emerald necklace
Welding And Soldering
musical water globe
motorcycle wheel weights
Mini LED Torch
baby knee socks
av cables rca
ladies leather sandals
Plastic Pill Boxes
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