Marine hydraulicsLesson 6 of 20
How a hydraulic gear pump works, and why it wears
Two meshing gears carry oil round the outside of the housing in the spaces between their teeth, and the mesh in the middle squeezes it out. Every turn moves the same volume, so flow follows speed.
7 minutes · For signed-up learners, free
The external gear pump is the most common hydraulic pump in the world. It drives tipper bodies, small cranes, power packs, steering on plant and the auxiliaries on almost every vessel. It has two moving parts, it tolerates dirt better than most pumps, and when it wears it tells you in a predictable way. Spin it up in the animation, then raise the pressure and watch what happens to the flow.
In simple terms
Look at the orange and grey dots in the tooth spaces. As the gears turn, each space between two teeth opens up at the bottom, where the inlet is. Oil from the tank flows in to fill it. The gear then carries that pocket of oil round the outside of the housing, sealed in by the tight fit of the teeth against the wall, up to the outlet.
In the middle, where the two gears mesh, the teeth push into each other’s spaces. There is no room for the oil there, so it is squeezed out of the outlet. Oil never goes through the middle of the pump. It always goes round the outside.
Because every turn moves the same amount of oil, flow depends on how big the pump is and how fast it turns. Double the speed and you double the flow. Pressure is set by the load the oil is pushing, as in lesson 2, and the pump just has to be strong enough to deliver against it.
No pump is perfectly sealed. A little oil always slips back from the high-pressure outlet to the inlet across the sides of the gears. The higher the pressure, the more slips back. A new pump loses only a few percent. A worn one can lose a third of its flow at full pressure, which is why a machine with a worn gear pump works fine when it’s lightly loaded and goes slow when it works hard.
The engineer's explanation, the deep dive, key numbers and the quiz open with a free sign-up.
- Play with it: Spin the gears, change speed, displacement and pressure, and watch flow and efficiency.
- Break it: worn side plates (flow falls as pressure rises) and a clogged suction strainer.
Continue this lesson free
The rest of this lesson, the interactive External gear pump, cut open 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.
Sign in to save your progress.
This lesson is also published by our sister site hydraulicsuae.com.
