Hydraulic and Traction Drive Systems
Hydraulic and Traction Drive Systems
This domain tests how power is transmitted from the machine to the car and how drive-system faults show up in inspection and troubleshooting. Hydraulic elevators use a pump, valve, jack, and oil; traction elevators use a motor, sheave, ropes, and counterweight. You need to know the components, normal operating pressures and speeds, and which adjustments are permitted.
Hydraulic drive
- The power unit contains the pump, motor, check valve, pressure relief valve, and lowering valve. Pump flow sets car speed; the relief valve is a safety limit, not a speed control.
- The relief valve must open at not more than 2.5 times the full-load working pressure. A rupture valve or restrictor protects against a broken jack pipe.
- Hydraulic oil level, temperature, and contamination affect leveling and valve response.
Traction drive
- A geared machine uses a worm gear; a gearless machine couples the motor directly to the sheave. Rope traction depends on groove condition, rope tension equalization, and counterweight balance.
- Brakes must hold the car at rated load and be adjusted to the manufacturer's specification.
Worked example
A hydraulic elevator has a full-load working pressure of 280 psi. The maximum relief-valve setting is 2.5 × 280 = 700 psi. If the gauge shows the relief opening at 750 psi, the valve is set too high and must be corrected. Raising relief pressure to make the car lift a heavier load is wrong: the relief valve protects the system, and the real problem may be a worn pump, closed valve, or overload.
Traps
Using the relief valve to adjust speed; ignoring oil temperature effects; assuming all traction machines are gearless; and overlooking rope equalization after a rope replacement.