Hydraulic Elevator Systems

A hydraulic elevator is a car on a column of oil. A pump pushes oil into a jack to raise the car; a valve lets oil back to the tank to lower it. The three main designs (in-ground, holeless, and roped hydraulic) are three answers to one question: where do you put the piston?

Why it matters on the job

Hydraulics carry a huge share of low-rise service work, and their faults sort cleanly by direction and design. A mechanic who knows which jack layout is in front of them, and which components act in which direction of travel, walks into a callback with the diagnosis half done.

The power unit and the valve

The machine space holds the power unit: an oil tank, a submersible or dry motor-pump set, and the control valve. Going up, the pump runs and the valve routes oil to the jack. Going down, the pump stays off and the valve meters oil back to the tank, with the car descending under gravity. Speed in both directions is a valve function, which is why leveling and ride complaints on a hydro so often end at valve adjustment, oil condition, or oil temperature (cold oil is thick oil, and the first runs of a winter morning behave differently until the system warms).

Three places to put the piston

• In-ground (holed) jack: the cylinder is buried below the pit, and the piston pushes the car directly from underneath. Simple and compact in the hoistway, with travel limited by how deep you can drill, and with the cylinder living in the ground, where corrosion protection matters.
• Holeless: one or two jacks stand in the pit beside the car and push on the frame, with no drilled hole. Telescopic pistons stretch the travel. The trade-off is jack hardware in the hoistway, where at least you can see it.
• Roped hydraulic: the jack drives a sheave, and ropes from that sheave lift the car, typically doubling car travel and speed relative to piston movement. It is still a hydraulic: the jack is the power source, as the elevator-types lesson drew the line.

Worked example: pressure under the load

Pressure is load divided by piston area. Take an in-ground unit whose car, cab, and passengers put 8,000 lb on a 5 in diameter piston:

1. Piston area: A = πr² = 3.1416 × 2.5² = 3.1416 × 6.25 = 19.63 in²
2. Working pressure: P = 8,000 / 19.63 = 407.5 psi

One column of oil carries the whole car: 8,000 lb over 19.63 square inches is 407.5 psi

Run the same arithmetic with passengers off and you see why an empty car reads lower pressure at the gauge, and why a pressure reading against known car weight is a legitimate diagnostic.

The faults each design produces

• Car drifts down at the floor: oil is leaving the column, through valve leak-by, a jack packing leak you can see, or a buried cylinder leak you cannot. Drift plus no visible oil on an in-ground unit is a serious finding, not an adjustment item.
• Slow or no up, normal down: pump, motor, or valve up-circuit. Down is gravity, so it keeps working when the power side quits.
• Leveling changes through the day: oil temperature and viscosity. Chase the pattern before changing the setting.

Where it bites

• Low oil is a symptom, not a condition to top off. The oil went somewhere. Find where before refilling, especially on buried cylinders.
• Stored pressure is stored energy. The column of oil holds the car up. Valve work, jack work, and pit work under a hydraulic follow the trade’s stored-energy rules (Working Safely in the Hoistway), with the car supported before anything opens.
• Roped hydros carry traction-style parts. Ropes and a sheave on a hydraulic still need rope inspection habits from the traction lessons.