Friction and elevators
R. David Whitby | TLT Worldwide November 2021
Elevator operation relies on friction in multiple ways to ensure safety and precision.
An elevator (also called a lift) is essentially the same as a children’s seesaw. It is much easier for each side of a seesaw to rise and fall if the children on it are of approximately equal weight. In an elevator, one end of the seesaw is the car with people in it and the other end is a counterweight. The car and the counterweight are joined by a wire rope, which is wound around a pulley at the top of the elevator shaft.
When the masses of both the car (and people) and the counterweight are approximately equal, the elevation of one will be, more or less, balanced by the descent of the other. The motor driving the pulley to raise either the car (and people) or the counterweight will need to do very little work to move the system. If there is nobody in the car or if the car is completely full of people, the motor will have to do much more work because of the differences in masses and the net change in the potential energy.
The cable that connects the car and the counterweight must be slightly longer than the height of the elevator shaft. (In very tall buildings, there are usually two or more elevator shafts—one for the lower floors, another for the middle floors and another for the upper floors.) When the elevator is at the bottom of the shaft, the counterweight will be at the top, and when the counterweight is at the bottom, the car will be at the top.
Unlike a crane, the cable does not just hang over the pulley. There must be sufficient friction between the cable and the pulley so that when the motor turns the pulley, the cable moves, too, to raise either the car or the counterweight. The pulley is a long cylinder, and the cable is wound around it several times to increase the amount of friction. Grooves are usually cut into the cylinder, and the cable lies in the grooves to further increase the friction. The motor can turn the pulley clockwise and anti-clockwise and at variable speeds. This will raise or lower the elevator car either faster or slower.
Most elevators do not rely on a single cable, even if a strong steel cable is able to hold up a full elevator car. Generally, modern elevators use four, six or eight cables. If one or even two were to break, the car would not plunge to the bottom of the shaft.
Even if all the cables holding the car were to break, it would still not plunge to the bottom of the shaft. Vertical rails on either side of the car run the full height of the shaft. Rollers on the car grip these rails, keeping the car perfectly vertical, even if all the passengers stand on one side of the car. Accelerometers in the rollers detect vibrations or wobbling and apply compensating forces to the elevator car. Elevator brakes also slide along the vertical rails. If the accelerometers detect any sudden downward movement of the car, the brakes are applied automatically to stop the car from falling.
The brakes use electromagnets that force the clamps open, but if power to the building is interrupted, the clamps will close on the rails automatically. Quite obviously, friction is required to stop the car from moving. Another safety feature is the use of a governor in the rotating pulley, with hooks that engage stationary ratchets to bring the pulley to a stop if it starts to rotate at too high a speed. The hooks and ratchets use centrifugal forces to become engaged at higher than normal speeds.
Newer models of elevators in tall buildings are designed to operate at higher speeds. However, high-speed elevators can generate a lot of friction heating when slowing or stopping a car. Earlier metal brakes have been replaced by extremely hard and heat-resistant ceramic materials.
A final use of friction in elevators is when the car needs to be stopped at exactly the right position in each floor of the building. Here, the brakes are applied gently as the car approaches the floor, and sensors in the car and on the rails ensure that the car stops precisely where needed. Who would have thought that people who work or live in tall buildings would need to rely on friction?