A Hydraulic Car That Will Not Hold at the Top Landing
A 4-stop, 1985 hydraulic elevator (2,500 lb capacity, in-ground single-stage jack, submersible power unit in the tank) is drifting. Parked at the top landing with the doors closed, the car settles about 45 mm in 10 minutes, re-levels, and repeats; the building says the car is never at the floor in the morning. Parked at the bottom landing, the same 10-minute observation shows about 8 mm of movement. There is a thin film of oil on the pit floor near the jack and the tank level has dropped about 15 mm since the last quarterly visit. The lowering valve was cleaned eight months ago; there is no record of any cylinder or relief-valve test, and there is no shutoff valve between the jack and the power unit. The car is currently in service.
1. Does the top-versus-bottom difference localize the fault? Describe how you would prove where the oil is going.
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It does not localize anything. The static pressure on the oil is set by car weight over plunger area and is essentially the same at any landing, so the leakage rate is not meaningfully different. What differs is how far the car can travel before the plunger reaches the bottom of its stroke and the car lands on the stop ring or buffer. At the bottom landing only a few millimetres of travel are available, so 8 mm is the mechanical limit, not a lower leak rate.
To localize it, separate the jack from the power unit. With the car at the bottom and properly supported on stands, install a shutoff valve or blind the line at the jack head per the manufacturer, run the car up, park it at the top, and watch the car and a pressure gauge on the jack side together. Pressure holding while the car settles means oil is escaping past the plunger packing or through the cylinder. Pressure falling with the car means it is passing the check valve or lowering valve, or leaving through the piping. The oil film at the jack and the dropping tank level point at the packing, but a packing leak is often only the visible symptom - valve leakage is the more common cause of drift. Prove it before ordering parts.
2. What are the safety and code issues, and what must be true before the car goes back into service?
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A passenger car that moves down on its own at a landing is an entrapment and a fall hazard, and repeated re-level cycles wear the valve and hide the fault. Take the car out of service and post it while you investigate. The missing shutoff valve, the untested relief valve and the absent cylinder test history all belong in a conversation with the AHJ - a hydraulic unit with a 40-year-old in-ground jack and documented oil loss is a candidate for a cylinder condition evaluation, and if the jack is corroded the answer may be replacement rather than a repack.
Before return to service: repair the leak, not the oil level; verify the relief valve setting, verify the rupture valve or pipe check, confirm lowering speed and the stop at the bottom, and perform the periodic tests required by the edition of A17.1 in force plus any local requirement. Record the drift test, the gauge readings and the repair on the maintenance record and the test tag. Topping up oil and repacking the gland without proving the cause is how the same call comes back in three months.
3. The building manager says the car has done this for years and asks you to top up the oil and come back next quarter. How do you respond, and what goes on the ticket?
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Explain the mechanism in the manager's terms: the car is descending on its own with passengers aboard, the controller is compensating by re-levelling, and the amount of drift is a measure of a leak that will not improve. The observation that it has always done this makes it a long-standing defect, not an acceptable condition, and the drift rate is what you have to record so the next visit has a baseline.
On the ticket, write the measured drift at the top and bottom landings, the tank level, the oil found in the pit, the isolation test result and the parts or work required, plus the fact that no cylinder, relief-valve or shutoff-valve history exists. State clearly that the car is being returned to service or taken out of service, and why. If the manager declines the repair, document that the recommendation was made in writing and that the hazard was described. Do not record a repair you did not make, and do not leave the oil top-up as the only line item.