Hydraulic System Components and Circuit Layout

Overview: How a Hydraulic Elevator Works

A hydraulic elevator moves its car by pushing pressurized fluid into a cylinder that drives a piston (plunger) upward. When the car needs to descend, fluid is released back into the reservoir under controlled conditions. The heart of the system is the power unit, a self-contained assembly mounted in a machine room or pit. Understanding every component in that unit—and how fluid flows between them—is the foundation of safe, competent elevator work.

The Power Unit: Component by Component

Electric Motor

The motor converts electrical energy into mechanical rotation. It is coupled directly or through a coupling to the pump. Motor horsepower and speed are matched to the pump's flow-rate requirements and the elevator's load and speed rating. Before performing any inspection, verify that the motor is de-energized and locked out per your facility's lockout/tagout (LOTO) procedure.

Hydraulic Pump

The pump draws fluid from the reservoir and forces it into the circuit under pressure. Most elevator power units use a submersible pump mounted inside the reservoir, which keeps the pump primed, reduces noise, and contains any leaks. Pump output is measured in gallons per minute (GPM); this flow rate determines how fast the car rises.

Reservoir (Tank)

The reservoir stores the hydraulic fluid supply. It provides a place for fluid to return when the car descends, allows heat to dissipate, and gives entrained air a chance to separate. Reservoir capacity is sized to hold enough fluid to fill the cylinder plus a safety margin. Always check the fluid level only when the car is at the bottom landing and the system is de-energized.

Strainer (Suction Strainer)

Located at the pump inlet inside the reservoir, the strainer filters out particulate matter before fluid enters the pump. A clogged strainer restricts flow and can cavitate the pump. Strainers are rated by mesh size; service intervals are specified in the manufacturer's manual.

Check Valve

A check valve is a one-way valve that allows fluid to flow in one direction only. In a hydraulic elevator circuit, it sits downstream of the pump and prevents the loaded car from drifting down if the pump stops. The check valve is a critical safety device—never bypass or disable it.

Control Valve (Valve Unit)

The control valve is the brain of the hydraulic circuit. It regulates the rate of fluid flow (and therefore car speed) in both the up and down directions. Modern units use electronically controlled proportional valves for smooth, leveling-quality starts and stops. The valve unit typically contains several internal sub-valves, including those that manage acceleration and deceleration.

Pressure-Relief Valve

This valve sets the maximum system pressure. If pressure exceeds the preset limit—for example, if the car is overloaded or a valve sticks closed—the relief valve opens and bypasses fluid back to the reservoir, preventing damage to the pump, piping, and cylinder. The relief valve is not an operating control; it is a safety device. Its setting is specified by the manufacturer and must never be raised beyond the rated working pressure.

Manual Lowering Valve

The manual lowering valve allows maintenance personnel to lower the car to the bottom landing by hand—without electrical power—during an emergency or power failure. Opening this valve slowly meters fluid back to the reservoir. Training on proper use is essential: opening it too fast causes an uncontrolled descent.

Muffler

Hydraulic fluid moving at high velocity is noisy. The muffler (silencer) is installed in the return line or discharge line to dampen pressure pulsations and reduce operating noise transmitted through the piping.

Oil-Line Piping and Fittings

Schedule 40 or heavier steel pipe, along with flexible hydraulic hose at connection points, carries fluid between the power unit and the underground or holeless cylinder. Fittings must be rated for the system's maximum working pressure. Inspect all visible piping for corrosion, mechanical damage, and leaks during every static inspection.

The Fluid Path: Tracing the Circuit

Up travel (pump running): Fluid is drawn from the reservoir through the strainer → into the pump → past the check valve → through the control valve (up direction) → along the oil-line piping → into the cylinder, pushing the plunger and car upward.

Down travel (pump off): The weight of the car forces fluid back through the oil-line piping → through the control valve (down direction, metered to control speed) → past the muffler → into the reservoir. The check valve prevents unintended back-flow when the car is stopped.

Overpressure event: Excess pressure routes fluid from the pump outlet → through the relief valve → directly back to the reservoir.

Safety Points for Static Inspection

1. Lockout/Tagout first. Isolate and lock out the main disconnect before touching any component. Verify zero energy with a voltmeter.

2. PPE. Wear safety glasses and oil-resistant gloves. Hydraulic fluid under pressure can cause injection injuries.

3. No open flames. Hydraulic oil is flammable. Keep ignition sources away from the machine room.

4. Check for stored pressure. Even after the pump is off, the cylinder and lines may be under pressure. Follow the manufacturer's procedure for safely bleeding pressure before opening any fitting.

5. Fluid spills. Clean up any oil immediately—spills create slip hazards and are an environmental concern. Use absorbent materials and dispose of them properly.

6. Do not adjust the relief valve unless authorized and using a calibrated gauge.

Using the Schematic

A hydraulic schematic uses standardized symbols to represent each component. Lines represent pipes; symbols show pumps, valves, cylinders, and actuators. Reading a schematic lets you trace the fluid path, identify the function of each device, and diagnose problems logically. When labeling a schematic, start at the reservoir and follow fluid flow in the up-travel direction, naming each symbol as you encounter it.