Pressure supports the load. Flow moves it.
A hydraulic lift uses pressurised fluid acting on an actuator to produce lifting force. A motor-driven pump supplies fluid; valves control its path. The ram lifts the car directly or through a roped arrangement.
Pressure and flow do different jobs. Pressure acting over the ram's effective area produces force. For a fixed area, the volume of fluid entering or leaving each second determines ram speed. These relationships are explained in pressure and flow and cylinders and rams.
This lesson follows a conventional single-acting system: the pump supplies upward travel and gravity provides downward motion. Some designs use a different energy-management arrangement.
The lift controller also coordinates door conditions and position information. Hydraulic pressure alone does not establish that a journey is permitted or correctly controlled.
Up, stopped and down.
- Going up: electrical energy turns the pump. Fluid enters the cylinder and extends the ram.
- Stopped: the pump need not keep running. The closed hydraulic paths retain fluid to support the load.
- Going down: the car's weight pushes fluid back out. A controlled return path meters that flow to the tank.
The pump can therefore be stopped while the car descends. Stopping the motor alone does not stop fluid escaping from a damaged line or an open return path.
Follow the returning fluid.
- Cylinder and ram: the load pushes fluid out during descent.
- Rupture valve: excessive outward flow triggers closure at the cylinder.
- Pipe or hose: carries fluid between cylinder and power unit.
- Lowering valve: meters the return flow during a normal journey.
- Tank: receives the returning fluid.
Four valve functions to keep separate.
- Normal load holding
- A non-return valve prevents reverse flow through its path. Together with closed lowering paths, it helps retain fluid at a stop. This is part of normal operation.
- Controlled lowering
- The lowering and flow-control arrangement meters fluid leaving the cylinder to produce the requested descent.
- Pressure relief
- A pressure-relief valve limits excessive pressure in the circuit it protects by opening a relief path. It does not detect excessive descent simply because the car is moving too fast.
- Rupture protection
- A suitable rupture valve responds to excessive fluid flow out of the cylinder. It closes to stop the resulting descent. Its trigger is different from the pressure-relief function.
Why the cylinder connection matters.
A cylinder-side rupture valve, such as Blain's R10, places the stopping function between the stored load and the main cylinder line. If that line fails and fluid leaves too quickly, the valve closes the escape route at the cylinder.
A normal lowering valve at the other end of a broken pipe cannot close the break itself. This is why both what a device detects and where it acts matter.
Excessive-flow protection does not necessarily respond to a small leak or slow drift. Protection against creeping and unintended car movement with doors open has its own role. The ram, pipework and protective devices must suit the particular installation; the drawing above is one example, not a universal layout.
A closer look
A rupture valve's operating condition is fluid flow, while its purpose is to limit the resulting car movement. For a given effective ram area, faster fluid outflow means faster ram descent. In a roped hydraulic lift, car speed and ram speed differ according to the suspension arrangement.
Fluid temperature, leakage and load can change normal performance. Fluid and pipework also have some elasticity, so hydraulic behaviour is not perfectly rigid. These effects help explain why controlling a journey, holding at a landing and protecting against a failure are separate engineering questions.
Check your understanding
Must the pump always run to make a conventional hydraulic lift descend?
Reveal the explanation
No. In a conventional gravity-lowering arrangement, descent is controlled by returning fluid under the load.
Apply the idea
Pressure problem or excessive flow?
The pump is stopped, but a failed cylinder line allows a large flow of fluid out of the cylinder. Which function is relevant: pressure relief or rupture protection? Why does its location matter?
Show the reasoning
Rupture protection. A cylinder-side rupture valve can close the excessive-flow path before fluid reaches the failed line. Pump pressure relief addresses excessive pressure; it is not the stopping function for this event. A slow leak may not reach the rupture valve’s operating flow, so other provisions are needed for other hazards.
Further reading.
Optional reading to explore this lesson in more detail.
- Blain: R10 elevator rupture valve (PDF)
Component example: page 1 shows the cylinder-side position and describes closure following line failure or excessive descent. Page 2 distinguishes closing flow from pressure. Adjustment and testing sections are for qualified personnel.
- Blain: service manual (PDF)
Circuit example: compare the check, lowering and pressure-relief functions in the EV100 diagrams on pages 4–5. These are separate flow paths within one valve assembly.
- Bucher Hydraulics: lift cylinders
Design examples: compare plunger and telescopic cylinders. The actuator and its arrangement affect how ram movement becomes car movement.
- ELA: Safety of Existing Lifts (PDF)
Hazard overview: printed page 22 separates hydraulic free fall, excessive speed and creeping. It is European guidance on existing lifts, not a specification for an individual installation.