Counterweight
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A counterweight is a weight that balances a load. They are often used in traction lifts (elevators), cranes, and funfair rides.
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Simple_Crane_diagram..png
Image:Simple Crane diagram..png
If the counterweight were not there, the load would tend to tip the crane forwards [clockwise]. The counterweight, which is usually made of a dense material such as concrete or lead, provides an anticlockwise moment that balances the load on the crane.
What's more: When the load is lifted, the counterweight is lowered. This reduces the amount of work done.
A counterweight is also used in many rotating systems to reduce vibrations due to imbalances in the rotating assembly. A typical example is counterweights on crankshafts in piston engines.
In traction (non-hydraulic) elevators, a heavy counterweight counterbalances the load of the elevator car, so the motor only has to lift the difference between the car load and the counterweight load.