If a direct-current motor is used to drive the sheave, the motor becomes a brake, whenever it slows down or stops. The sheave also has a brake drum attached to it, and the brake is activated whenever electric power is shut off to the driving motor.
The brake clamps against the sheave thus slowing & eventually stooping the sheave cafe & car. These brakes are similar to automatic drum brakes, and they wear with use. They must be continually adjusted, and braking pads must be replaced.
The maximum car speed is set by a centrifugal governor. The governor is separate from the elevator machine, yet it controls the elevator machine. When the maximum car speed is reached, the governor shuts off electrical energy to the sheave-driving motor and activates save brake completing the first stage of stopping the car.
If the car speed exceeds a second setting. Or if the car is not stopped within a time period, the governor activates car safety rail clamps, which wedge between the car and the guide rails.
Whenever safety rail clamps are activated, their life is in use, and they must be replaced. These are designed to stop the fully loaded car in a short time period and in a relatively smooth stopping action.
There are two additional safety devices. Limit switches are placed above the highest floor to be serviced and below the lowest level. If the car was to strike either of these switches, the switch would stop energy flow to the sheave-driving motor; this also automatically sets the sheave brake. The final safety device is located in the pit; it is a spring or buffer designed to cushion the drop of the car into the pit.
Most building codes require a fully loaded car brake safety check about every five years. A service contract is usually used for these five year safety test inspections.
These safety devices cannot be checked to determine whether they will stop the car until a test or accident occurs. However, through maintenance, personnel should be able to determine whether they appear reasonably safe.
For example, these devices are located in the shaft area, and because of draft shaft conditions they wilt soil and get dirty in short time periods.
Proper preventive maintenance should require that these devices be cleaned and visually inspected for cracks, loose nuts, and proper electrical and mechanical connections. Any device that does not look right should be replaced.