Hydraulic Car Drifting With Load and Jerky Lowering
A three-stop hydraulic elevator in a 1974 bank building: 2,100 lb (953 kg), 12 ft (3.7 m) travel, in-ground single-stage jack in a PVC casing, and a power unit in a closet beside the hoistway. The building reports the car sinks about 1/2 in. (13 mm) in 20 minutes when parked at the second floor with roughly 800 lb (363 kg) aboard, and that lowering is jerky on cold mornings. There is a small oil accumulation in the pit near the jack. Last year the pump was replaced and the relief valve was adjusted. The mechanic proposes to replace the lowering valve and check valve and to top off the tank with the oil already on the shelf, which is the same brand the building has used for years.
1. How do you determine whether the leak is in the jack/cylinder or in the valve block?
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You have to isolate the two halves of the system and watch which side loses the load. Put the car at the top landing with the load that reproduces the drift, install a gauge at the power unit, and close the shutoff valve at the jack if one is provided. If the car continues to sink with the jack isolated, the leakage is on the jack side of that valve: packing, fittings, or the cylinder itself. If the car holds, the leakage is in the power unit: lowering valve, check valve, manual lowering valve, or a relief valve that is not seating.
A pressure-decay check supports the result. With the jack isolated and the line pressurized, a gauge that falls confirms valve leakage; a gauge that holds while the car still sinks points to the jack side. If there is no jack shutoff, the test must be done with the car supported and the line safely capped, following a written procedure, because a loaded jack is stored energy. Record the drift over a measured time with a fixed reference before and after isolation. Also inspect the pit oil trail: oil at the jack head or packing supports a jack-side leak, while a dry jack with a sinking car and a holding gauge points to the valve block.
2. What do the cold-morning jerking and the pit oil indicate, and what is wrong with the proposed top-off?
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Jerky lowering when cold points to oil viscosity and valve behavior. Cold oil is thicker, so the lowering restriction and the valve respond differently; a sticky lowering valve, a partially clogged strainer or orifice, or oil that is not the manufacturer's specified grade will all make it worse. The first step is to identify the oil actually in the tank against the manufacturer's specification and check for contamination and water, not to assume any ISO 32 product is correct.
The oil in the pit near the jack is a leak indicator, not a maintenance nuisance. It could be packing or fitting leakage, or it could be the first sign of a cylinder problem. Topping off the tank masks the loss, changes the oil chemistry if the product on the shelf is not the specified fluid, can cause foaming and seal damage, and leaves an oil slip hazard in the pit. Fix the leak, clean the pit, dispose of the waste oil properly, and refill with the specified fluid. Also verify that the relief valve adjustment made last year is correct for the replacement pump per the manufacturer and code; a relief valve that is stuck partly open will cause both drift and heat, and adjusting it to cure drift or a weak pump is never acceptable.
3. What must be verified before the elevator goes back into service?
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Verify the repair actually eliminates the drift at every landing with rated load, observed over a period long enough to show no measurable movement, not just a quick look. Recheck the jack isolation result if the jack side was implicated, inspect the packing and fittings, and confirm no oil is reaching the pit.
Then verify system performance and protection:
- Correct oil type and level, clean strainer, and lowering speed and leveling within the manufacturer's values.
- Relief valve setting correct for the pump and code, verified with a gauge, with the valve proven not to be the leakage path.
- No jerking or sticking at cold and normal oil temperatures.
- If the cylinder or jack was repaired or replaced, the code-required pressure test and any AHJ inspection applicable to that work.
- Documentation of the drift measurement before and after, the isolation test, oil used, and parts replaced.
Because this is a repair to existing equipment, A17.3 requirements for the existing installation still apply, and the AHJ may require a periodic test after the work. Return the car to service only after the drift test and the safety checks pass.