Pressure and Flow Fundamentals in Hydraulics
Two quantities decide the performance of any hydraulic system: pressure sets the force at the cylinder, while flow sets the speed. Understanding both and how they interact with piston area and pump delivery lets you size drives safely and avoid overheating. This guide explains the fundamentals step by step.
View hydraulics basicsWhat exactly do pressure and flow do?
Pressure and flow are two separate quantities. Pressure (in bar) only builds up when the flow meets a resistance, such as a load on the cylinder. It creates the force. Flow (in l/min) is the volume of fluid delivered per unit of time and sets how fast the cylinder moves. A pump delivers flow, not pressure - the load sets the pressure.
How do I calculate force and speed?
The force at the cylinder is pressure times piston area: F = p x A. A piston with 50 cm2 of area at 250 bar (250 bar is about 25 N/mm2) delivers roughly 125 kN. The extend speed follows from flow divided by piston area: v = Q / A. If you want to move faster, you need more flow; if you need more force, you need more pressure or a larger area.
- Force: F = p x A - higher pressure or a larger piston area gives more force.
- Speed: v = Q / A - more flow for faster extension.
- Retract: the annular area is smaller, so the cylinder retracts faster but with less force.
- Power: P = p x Q - pressure and flow together give the hydraulic power.
What matters when sizing the system?
When sizing a system, pump, valve, cylinder and fluid all interact. The rated pressure of every component must match the highest working pressure, and the relief valve caps the peaks. The pump flow sets the cycle time. The oil viscosity (for example ISO VG 46) affects friction and efficiency - oil that is too thin increases leakage, oil that is too thick raises pressure loss.
The right viscosity grade keeps efficiency and wear under control.
To the fluid basicsFrequently asked questions
Does the pump create the pressure in a hydraulic system?
No. The pump delivers a flow. Pressure only builds up when that flow meets a resistance, for example the load on the cylinder or a closed valve. Without resistance the pressure stays low.
How do I make a hydraulic cylinder faster?
Speed depends on flow divided by piston area (v = Q / A). More flow from the pump makes the cylinder faster. A smaller piston area also raises speed, but it lowers the force.
Why is my hydraulic system overheating?
Usually hydraulic power is being turned into heat, for example through throttling, leakage or a relief valve that keeps opening. Wrong oil viscosity and an undersized tank also raise the temperature. Review the sizing and the cooling.
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