Safety Devices, Governors, and Braking Systems
Domain 4 is the protective chain: overspeed governor, safeties, buffers and the machine brake. The exam asks for setting values, test conditions and what each device actually protects.
Governor and safeties
The governor is centrifugal, driven by a governor rope. When the car exceeds its set speed the governor grips the rope and the safety applies. Tripping speed is set at not less than 115% of rated speed, with an upper limit taken from the code table for that speed. Type A (instantaneous) safeties are limited to lower speeds; Type B (gradual) safeties are used at higher speeds to limit deceleration. Safeties must stop and hold the car with rated load, and the governor's electrical switch should operate before the mechanical grip.
Brakes
The machine brake is spring-applied and electrically released, and it must hold the car with 125% of rated load. Setting is on spring compression and air gap, with coil voltage confirmed. Required brake torque is set by the unbalanced load - loaded car minus counterweight - not by the total load.
Worked example
Car 3,000 lb, rated load 2,500 lb, counterweight 4,000 lb. Test load is 125% of rated load = 3,125 lb, so the loaded car weighs 6,125 lb. Net load the brake must hold = 6,125 − 4,000 = 2,125 lb. With a 30 in sheave (15 in radius) and 1:1 roping, required brake torque ≈ 2,125 × 15 = 31,875 lb-in, about 2,656 lb-ft.
Traps
Using total load instead of unbalanced load for brake torque; quoting 125% as the governor trip point; testing safeties without the required load; forgetting that slack-rope and broken-rope switches are separate protective devices.