The Use Of Pulleys In Lifts

Pulleys are one of the most common forms of power transmission, and are used in a variety of applications. Perhaps their most obvious application however, is their use in lifts.

Lifts are found in most multi-storey buildings, and they are an essential feature for catering to the needs of the disabled.

The most common design is the roped lift, where the car is raised and lowered by traction steel ropes that are attached to the car and looped around a sheave. A sheave is a pulley with grooves around the circumference. These grooves cause it to grip with the steel rope, so that when the pulley moves, so too does the rope.

The pulley is connected to an electric motor, and when the motor turns one way, the rope moves in a way that raises the car. Consequently, when the motor turns the other way, the car is lowered. In gearless lifts, the motor rotates the pulley directly, while in geared lifts, the motor turns a gear train that then rotates the pulley. The pulley, motor, and control system are usually housed in a room above the shaft.

On the other side of the pulley hangs a counterweight, which weighs about the same as the lift when filled to 40% of its capacity. Therefore the counterweight and the car are perfectly balanced when the car reaches 40% capacity. This saves energy, as it will then only require a small amount of force to tip the balance either way, as the pulley only has to overcome friction, with the weight on the other side doing the rest of the work. The system can therefore be compared to a see-saw with two equal weights on either side.

Both the car and the counterweight ride on guide rails, which run along the sides of the elevator shaft. These rails prevent the counterweight and car from moving back and forth. They also work in combination with any safety system that may be in place, so that the car can stop in case of emergency.

Roped lifts are much more versatile than hydraulic lifts, and are also more efficient. They also have more safety systems that help prevent the car from plummeting to the ground in the event of something going wrong.

One of the safety systems is activated by a governor, which is activated when the lift moves too quickly. Most governor systems are built around a pulley at the top of the shaft. The governor rope is looped around the pulley and another weighted pulley at the bottom of the shaft. The rope is also connected to the car, meaning that it moves whenever the car goes up or down, and speeds up when the car speeds up.

Such safety systems mean that the Hollywood situation of a lift hurtling down a shaft, are actually quite unlikely.