Marine hydraulicsLesson 3 of 20
The basic hydraulic circuit: pump, relief valve, control valve and cylinder
The pump makes flow, the load makes pressure, the relief valve sets the limit and the control valve decides where the oil goes. Every hydraulic machine is built on these four parts.
7 minutes · For signed-up learners, free
Almost every hydraulic machine, from a provision crane to a factory press, is the same four parts doing the same four jobs. A pump pushes oil. A relief valve stops the pressure going too high. A control valve sends the oil one way or the other. A cylinder or motor turns it back into movement. Play with the lever and the sliders above, then tick a fault and watch what changes.
In simple terms
The pump makes flow, not pressure. An electric motor turns the pump, and every turn pushes out the same amount of oil. More flow means the cylinder moves faster.
The load makes the pressure. Oil only gets pushed as hard as it has to be to move what’s in the way. Lift a light load and the gauge reads low. Lift a heavy one and it reads high. Try it: change the load and watch the gauge, then change the pump flow and watch the speed. Each slider moves a different readout.
The relief valve is the safety limit. If the load needs more pressure than the relief valve allows, the valve opens and dumps the oil back to the tank. The machine stops moving, but nothing bursts. The cost is heat: all that oil is being forced through a small gap at full pressure, and that energy warms the oil. Hold the lever at the end of the stroke and watch the “heat” readout.
The control valve is the driver. Raise sends oil under the piston. Hold traps the oil in the cylinder so the load stays put, and lets the pump’s oil go straight back to the tank at low pressure. Lower sends oil above the piston and lets the oil underneath return to the tank.
The faults show two problems we see every week. A worn piston seal lets oil slip past inside the cylinder, so the load creeps down even with the valve in Hold. A worn pump leaks inside, so at high pressure it delivers less oil than it should and the machine gets slow and hot.
The engineer's explanation, the deep dive, key numbers and the quiz open with a free sign-up.
- Play with it: Lever, load, pump flow and relief setting, with a live gauge and the heat being made.
- Break it: a worn piston seal (the load drifts) and a worn pump (the lift slows).
Continue this lesson free
The rest of this lesson, the interactive Pump, relief, control valve, cylinder and the quiz are open to signed-up learners. It's free, takes a minute, and opens every lesson in all 14 courses.
Free awareness training. Al Safwan Marine is not an approved training provider, so no certificate is given, and these courses are not a qualification. They don't replace STCW training, manufacturer training or the certification technicians need for this work. The manufacturer's manual and your flag's rules always come first.
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This lesson is also published by our sister site hydraulicsuae.com.
