Sophisticated lifting devices such as the chain hoists of today have evolved from simple devices such as the block and tackle.
Used for thousands of years, the block and tackle owes its invention to the Greek philosopher Archimedes in the 3rd century BC, who designed a primitive system for raising ships out of the water. It was widely used on sailing ships where motorised lifting devices were not available, and is still used extensively in sailing today.
Block and tackle
Consisting of at least two pulleys (blocks) and a cable or rope running between them (tackle), with one pulley attached to a fixed point and the other to the object being lifted, the block and tackle works on the principle of mechanical advantage or the amplification of force with a mechanical device. A typical pulley with two lines gives a mechanical advantage of two (i.e., 50 kilos can be lifted using 25 kilos of tension).
The trade-off for such mechanical advantage is force for distance with the velocity ratio being two to one or, to raise the load at one metre per second, the rope must be hauled at two metres per second.
Friction is another problem one has to deal with in the block and tackle system, especially from the resistance of the ropes running through the pulleys. When more pulleys are added to a system, the weight of the load is effectively reduced but more rope is consequently required leading to friction that will interfere with the easy movement of the device.
Chain hoists evolved from the block and tackle and have been in common use since the 1800s. Key components of a chain hoist include a drum or lift wheel around which the chain is wrapped and a hook that attaches to the load.
A manual chain hoist consists of a closed chain loop, which is pulled, causing the larger of two pulleys in the hoist to draw in more chain than is released by the smaller pulley, thus lifting the load.
Electric chain hoists are powered by electric motors and are the most common type of hoist in use today. They can range in size from portable devices used to aid the disabled through to heavy-duty hoists used to lift large industrial loads.
Air chain hoists are operated using compressed air, and are more durable and weather-resistant than electric hoists, weighing up to half as much, but are also more difficult to control.
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