Elevator and Escalator Mechanical Systems: Roping, Safeties, Buffers and Drives
Mechanical questions reward the candidate who reasons from rope, sheave and load relationships instead of memorizing part names. Expect traction and hydraulic arrangements, roping ratios, safeties and governors, buffers, door hardware, and escalator drive and step-chain components.
Traction machinery and roping
- Geared (worm gear) machines suit low- and mid-rise work; gearless machines with a large traction sheave suit high speed and high rise.
- Roping ratio: at 1:1 the car travels at rope speed; at 2:1 the rope travels twice as fast as the car while rope tension is halved, so a smaller, faster machine can be used and more sheaves are needed.
- The counterweight is normally car mass plus 40–50% of rated load, so the machine handles roughly half the payload in either direction and traction is preserved.
Safeties, governors and buffers
- Car safety gear is applied by the governor when the car overspeeds in the down direction. The governor tripping speed is set relative to rated speed under B44, and the adjustment is sealed.
- Safety gear types: instantaneous (Type A), instantaneous with buffered stop (Type B), and progressive (Type C). Progressive gear is required at higher rated speeds because it limits deceleration on the car structure and on passengers.
- Buffers in the pit absorb an overspeed or unintended movement at the terminal landings: spring or polyurethane types at low speed, oil buffers above the code threshold, with stroke related to speed.
Escalators
- Power flows motor → reducer → drive chain → step-chain sprockets, with the handrail driven from the same machine.
- The brake is spring-applied and electrically released; it is the emergency stopping means and must stop a fully loaded escalator running down.
- Safety devices include comb-impact, skirt-guard, handrail-entry, step-sag/missing-step, broken step-chain, non-reversal and handrail-speed switches.
Worked example
A gearless machine has a 760 mm traction sheave, 2:1 roping, and a rated car speed of 1.75 m/s. Find the sheave speed. Rope speed = 2 × 1.75 = 3.5 m/s. Sheave circumference = π × 0.76 = 2.388 m. Revolutions per second = 3.5 ÷ 2.388 = 1.465, so rpm ≈ 88. At 1:1 the same car would need only ≈ 44 rpm, which shows why 2:1 roping lets a smaller, faster motor be used.
Traps that cost marks
- Forgetting the ×2 when converting between car speed and rope speed.
- Confusing governor tripping speed with rated car speed.
- Assuming counterweight equals car weight, which wrecks balance and traction.
- Treating the escalator brake as a parking brake that does not need testing.