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Pressure or Flow?

When I teach hydraulic troubleshooting classes, I always ask my students, “If a cylinder does not move when I shift the directional valve, is it a pressure problem or a flow problem?”.

Most of the students will say it is a flow problem. Then I ask them, “But what if I don’t have enough pressure to move the load?”

If the actuator does not move, I always think to myself, “why don’t I have enough pressure to make it move?". Something is taking my flow elsewhere at a lower pressure. Or, if it’s moving slower than it should I think, "Why don’t I have enough flow? Where is part of my flow going where it shouldn’t?”.

In the circuit shown, it takes 800 PSI to lift the load at a 10 GPM rate. What would happen to the pressure if the cylinder developed an internal leak of 6 GPM past the piston? Would the pressure drop, stay the same or increase?


The pressure stays the same since the cylinder is receiving 10 GPM with 6 of it leaking past the piston, so 4 must be staying in the cylinder causing it to raise, which needs 800 PSI to do so. It just moves 60% slower.

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