Hydraulic Car Settling After Leveling
A four-stop hydraulic elevator in a Michigan medical office has a 40 ft rise, 2,500 lb capacity, an 8-in single-stage jack, and a 25 hp pump. The car settles about 3/8 in below the floor after 10 minutes at the top landing. The pump runs about 4 seconds longer than normal on the up direction, and the oil temperature is 145 F. Maintenance replaced the down-valve coil, but the settling did not change. One mechanic wants to replace the cylinder packing; another says the check valve is leaking. The power unit is in a closet adjacent to the pit, and the shutoff valve between the cylinder and the power unit is accessible. The car is the only elevator in the building.
1. How would you distinguish an internally bypassing cylinder from a leaking valve?
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Use a controlled isolation test. Run the car to the top landing, block the car under the sling with rated blocking, and lock out the pump. Install a calibrated pressure gauge on the cylinder side and close the shutoff valve between the cylinder and the power unit. Relieve pressure on the power-unit side, then observe the cylinder pressure and car position over time.
If the cylinder pressure holds and the car does not drift with the valve closed, the cylinder and packing are probably holding; the leak is in the power unit, such as the check valve, down valve, or internal return path. If the cylinder pressure drops and the car drifts while isolated, the cylinder or packing is bypassing internally. Also inspect the oil tank level and look for oil at the packing; an internal bypass may return oil to the tank without an external leak. Do not rely only on the down-valve coil replacement.
2. What safety precautions are required before and during this diagnosis?
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Block the car mechanically before anyone works under it, and do not rely on the hydraulic valve to hold the car. Lock out and tag out the pump disconnect. Relieve hydraulic pressure before opening fittings, and wear eye protection because trapped oil can spray. Use a gauge rated for the system pressure and open the shutoff valve slowly.
Keep the car at the landing with the doors closed or guarded so no passenger can enter. Because this is the only elevator, coordinate with the building so the car is out of service and the lobby is barricaded. Do not stand in the pit under the car during the isolation test, and do not use the manual lowering valve as a test substitute.
3. If the cylinder is bypassing, what repair decision would you make before returning the car to service?
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If the isolation test shows the cylinder bypassing, repair or replace the cylinder and packing as required. On a single-stage jack, packing replacement may fix an external or minor internal leak, but a scored or corroded cylinder bore may require replacement. After repair, refill with the specified oil, purge air, and cycle the car several times.
Then retest for settling at the top landing, check the check valve and down valve for leakage, and verify the pump time returns to normal. Because the car was settling 3/8 in, confirm the final leveling and re-leveling operation and check that the car does not creep below the landing. Document the oil temperature and pressure readings. Do not return the car to service until it holds at the landing and the safety test is complete.