Marine hydraulicsLesson 17 of 20
Where hydraulic heat comes from, and why the oil overheats
Every kilowatt the motor puts in that doesn't do useful work ends up as heat in the oil. The oil settles at the temperature where the cooler and tank get rid of heat as fast as the system makes it.
7 minutes · For signed-up learners, free
Hydraulic systems are not very efficient. A typical machine turns between a third and two thirds of the power it takes from the supply into useful work. The rest becomes heat, and nearly all of that heat ends up in the oil. Whether the oil runs at a comfortable 50 °C or a damaging 85 °C depends on two things you can explore above: how much power is wasted, and how well the system gets rid of it.
In simple terms
Follow the band in the diagram from left to right. It starts as the electric power the motor takes. The motor wastes a little, which warms the air around it. Then each part of the hydraulic system takes its share:
- The pump loses some power to friction and internal leakage.
- The relief valve, if oil is going over it, turns that oil’s whole pressure into heat.
- Valves and pipes need some pressure just to push the oil through them, and that pressure is lost as heat too.
Whatever is left at the end is the useful work: lifting, turning, pressing.
Now try “Held at end of stroke”. The machine is doing nothing, but the motor is still working hard, because all the pump’s oil is being forced over the relief valve. Every kilowatt goes into the oil. That is the most common reason a machine overheats.
The oil warms up until the heat leaving it equals the heat going in. The tank walls get rid of a little. A cooler, with a fan blowing air through fins, gets rid of much more. When the air around the machine is 45 or 50 °C, as on a UAE summer afternoon, the cooler has less temperature difference to work with, so everything settles hotter. Dust packing the fins makes it worse again.
The engineer's explanation, the deep dive, key numbers and the quiz open with a free sign-up.
- Play with it: Pump flow, load pressure, relief setting, air temperature and tank size, with oil temperature over time.
- Break it: a leaking relief valve, cooler fins blocked with dust, and a low oil level.
Continue this lesson free
The rest of this lesson, the interactive Energy flow from motor to work 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.
