TERM 3
An elevator or lift is a single-pulley lifting machine, used to lift heavy loads. When you step into an elevator you walk through two doors and stand in a box called a car inside a vertical lift shaft. One door is in the walls of the floor level that you are on and the other door is part of the box itself.
Inside the shaft, hoisting cables are attached to the top of the box. The cables run over a pulley to the top of the shaft. The other ends of the cables are connected to a heavy steel weight called a counterweight. The box is guided up and down the lift shaft by guide rails with roller-like wheels attached to the elevator box.
The pulley is housed in a room above the shaft and is rather like a large drum connected to an electric motor. The counterweight reduces the power needed to raise and lower the elevator (lift) to a minimum. When the car goes up, the counterweight goes down. When the car goes down, the counterweight goes up.
Weight of counterweight = weight of the car + (about) 1 2 of its maximum passenger load
When the elevator operates, it only needs power to lift the weight of the extra passengers in the car. The rest of the weight is balanced by the counterweight.
The system that operates in a mine shaft is very similar. Figure 3 shows the part of the system that is above ground, known as the headgear. The large wheels at the top of the structure are the pulley wheels that the cable passes over. The cables can be seen stretching to the right of the picture where the motor that drives the system is housed. The cable is wound around a large drum and pulls the cage up and down the shaft. There is no counterweight in this lift. It is very similar to the ratchet winch used to pull a boat out of the sea.
Did you know?
Elevators in buildings travel about 2–3 metres per second.
Figure 2 An elevator uses a counterweight
Figure 3 Mine headgear lift system Practical Assessment Task 125
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