The basic principle.
Pressure is force per unit area. Flow is volume per unit time. A hydraulic circuit can have pressure with no flow, just as a supported stationary load can exert force without moving. Pressure may be expressed in pascals or bar; flow is often given in litres per minute.
A pump supplies flow. Pressure develops according to the load and resistance to that flow, within the capability and limits of the circuit. Calling a pump a source of a fixed pressure hides this relationship.
Consider a stationary cylinder.
A load can remain supported by trapped pressurised fluid even when the pump is stopped. Pressure alone does not mean the cylinder is moving.
Follow the relationship.
- PressureForce available per unit area.
- FlowVolume passing per unit time.
- MovementDepends on flow into or out of the actuator.
Connect it to the lift.
For a cylinder of fixed effective area, speed depends on flow and force depends on pressure. Restrictions and pipework also produce pressure losses when fluid moves. These relationships let an engineer separate a speed problem from a force problem without assuming that either has only one cause.
A closer look
One bar is 100,000 pascals. Pressure readings need context: gauge pressure is measured relative to the surrounding atmospheric pressure, whereas absolute pressure uses a vacuum reference.
Check your understanding
Can a hydraulic circuit have pressure while its flow is zero?
Reveal the explanation
Yes. Fluid trapped beneath a supported load can remain pressurised without moving.