Hydraulic power unit: how is it built and how is it sized?
A hydraulic power unit produces the pressurised oil flow that drives cylinders and motors. At its core it consists of an electric motor and pump, a tank, a pressure relief valve, control valves, a filter and often a cooler. It is sized via pressure in bar, flow in l/min and motor power in kW.
View hydraulic power unitsWhich assemblies make up a hydraulic power unit?
A hydraulic power unit converts electrical into hydraulic energy: the electric motor drives the pump, which draws oil from the tank and delivers it under pressure into the system. Valves control pressure and flow direction, a filter keeps the oil clean, and a cooler limits the temperature.
The individual assemblies work together. The table below sums up the main components and their function.
| Component | Function | Note |
|---|---|---|
| Electric motor + pump | converts electricity into pressurised oil flow | power in kW, speed sets the flow rate |
| Tank / reservoir | stores oil, cools and de-aerates it | volume often 3 to 5 x pump delivery per minute |
| Pressure relief valve | limits system pressure, protects from overload | set in bar, routes excess back to the tank |
| Directional and control valves | set direction and amount of the oil flow | control cylinders and hydraulic motors |
| Filter | retains particles, secures oil cleanliness | cleanliness per ISO 4406, return or pressure filter |
| Cooler | removes heat, keeps oil temperature stable | air or water cooler depending on power loss |
Which key figures drive the sizing?
Three quantities lead to the sizing: the pressure in bar sets the achievable force, the flow in l/min sets the actuator speed, and from both follows the required motor power in kW. The starting point is always the task at the cylinder: required force and required speed.
From required force and piston area. Higher pressure allows smaller cylinders for the same force.
ViewA cylinder's force equals pressure times piston area, its speed equals flow divided by piston area. If you know the force and speed of the actuators, you can set pressure and flow and from these select pump and motor.
How do oil cleanliness and maintenance secure service life?
Most failures in hydraulics trace back to contaminated oil. The oil cleanliness per ISO 4406 describes the number of particles per size class with three code figures. A matching filter and regular maintenance keep these values within the range the component requires.
Maintenance includes changing filter elements, checking oil level and temperature, inspecting for leaks and regular oil analysis. See the matching article Hydraulic hose on lines and connections.
Frequently asked questions
What does a hydraulic power unit do?
A hydraulic power unit produces a pressurised oil flow. An electric motor drives a pump that draws oil from the tank and delivers it under pressure into the system. Valves, filter and cooler control and maintain this oil flow, which drives cylinders and hydraulic motors.
How do you size a hydraulic power unit?
The starting point is the required force and speed of the actuators. Force and piston area give the pressure in bar, area and speed give the flow in l/min. Pressure and flow give the motor power in kW, plus a reserve.
Why is oil cleanliness per ISO 4406 so important?
Contaminated oil causes most failures in hydraulics. ISO 4406 describes oil cleanliness with three code figures for the particle count per size class. A matching filter and regular maintenance keep the oil within the required cleanliness range and extend component service life.
The right hydraulic power unit for your application
Pressure, flow and motor power sized correctly - filter and cooler matched to the application. We advise you.
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