Heavy-Load Leveling Complaint on a Geared Traction Elevator
A ten-story hotel has a geared traction passenger elevator: 3,000 lb (1,361 kg) rated capacity, 250 ft/min (1.27 m/s), eight stops. The building reports that with a heavy morning load, roughly 2,300 lb of passengers and luggage, the car sometimes stops about 1 in. (25 mm) below the fourth-floor landing, holds for a second or two, then re-levels. The car is level at light loads. The route mechanic notes a hot brake drum after a series of runs, a heavy clack when the brake picks up, and dark dust around the brake. The linings were replaced nine months ago; the brake coil and rectifier are original. Machine-room temperature reaches 95 F (35 C) in summer. The mechanic's proposed fix is to widen the leveling deadband and increase leveling speed so the car corrects faster.
1. What is the most probable fault, and what measurements would you take to confirm it?
Read the worked answer
The thermal evidence points to a dragging brake, not a leveling-software problem. A hot drum after normal runs, heavy pick-up noise, and dust mean the brake is not fully releasing or is releasing late, so the lining drags on the drum. Dragging glazes and overheats the lining, which lowers its coefficient of friction. At 2,300 lb the car is about 77 percent loaded; when the brake sets, the glazed lining slips slightly, the car settles, and the leveling system then lifts it back after its delay. At light load the same weakness does not show.
Measurements that confirm or rule this out:
- Coil voltage at pick-up and while holding, measured at the coil, plus rectifier output under load. A failing rectifier or low voltage causes slow, noisy pick-up.
- Brake air gap at each side and plunger/guide-pin freedom against the manufacturer's values. Binding or an excessive gap delays release.
- Drum runout, surface condition, and any oil or grease on the lining.
- Spring condition and free length, and whether someone previously backed off the spring.
- Leveling sensor and encoder signals, to document that they are working and are not the cause.
The clack plus heat plus dust is a brake signature; the leveling system is only reporting the slip.
2. Evaluate the proposed fix of widening the deadband and increasing leveling speed.
Read the worked answer
It treats the symptom and leaves a brake that cannot reliably hold a loaded car. Widening the deadband means the car is allowed to sit further off level before correcting, and faster re-leveling just makes the correction more noticeable. The underlying brake torque is still marginal, and a marginal holding brake is exactly the condition the brake test is designed to catch.
The brake is a safety device; its spring provides the holding torque. Do not reduce spring force or open the gap to reduce drag, and do not compensate by tuning the drive. Correct the cause: repair or replace the rectifier/coil circuit if pick-up is slow, free and adjust the plunger and gap to manufacturer values, replace contaminated or glazed linings, and burnish new linings per the manufacturer. After any lining work, recheck stopping performance and heat after a run cycle. The machine room should also be brought back toward the manufacturer's ambient limit; sustained high temperature accelerates coil and lining degradation. Only after the brake is correct should normal leveling parameters be confirmed, and they should be left at the manufacturer's values unless the manufacturer directs otherwise.
3. What test must be completed before the car returns to service, and what is the acceptance criterion?
Read the worked answer
Perform the brake test required by A17.2 for the applicable test category, using 125 percent of rated load. For this car that is 3,750 lb (1,701 kg) of test load, placed in the car, with the car run and stopped in the down direction so the loaded car is overhauling the machine, which is the worst case for brake torque. Observe whether the brake stops and holds the car with no slip and record the stopping performance.
The acceptance criterion is that the brake alone stops and holds 125 percent of rated load without slipping or depending on the drive or leveling system to catch it. The brake must also hold the car at rest with that load. If there is any slip, the brake is not acceptable and must be repaired and the test repeated; a re-leveling correction during the test is a failure, not a pass. Also verify the brake picks up cleanly at the correct voltage, the drum is not dragging after a run, and the machine-room temperature is within the manufacturer's range. Document the test load, direction, result, brake adjustments, and the parts replaced, and have the AHJ witness the test if the jurisdiction requires it.