Three positive-displacement pumps, side by side. Use the view to compare how a gear pump, a vane pump and a piston pump each trap a fixed volume of fluid and force it out — and which job each one suits.
A gear pump, a vane pump and a piston pump are all positive-displacement pumps. Each one traps a set volume of fluid in a closed space and then forces it out, so its flow depends on how fast it turns, not on how hard the system pushes back. Just as important, none of them adds speed the way a centrifugal pump does — they displace fluid directly.
All three do the same job but with different strengths. A gear pump is simple and rugged, a vane pump is smooth and quiet, and a piston pump can reach very high pressures. Seeing them side by side makes it easy to match the pump to the job.
Two gears mesh inside a close-fitting casing. Fluid is caught in the gaps between the teeth and carried around the outside of each gear, from the inlet side to the outlet side, where the meshing teeth push it out.
Here a rotor sits off-centre inside the casing, with vanes that slide in and out of slots. As the rotor turns, the gap between rotor and casing grows and shrinks, so each vane sweeps a pocket of fluid from the inlet round to the outlet.
A crank drives a piston in a cylinder. On the stroke outward it opens the inlet valve and draws fluid in; on the stroke back it forces that fluid out through the outlet valve. The flow comes in pulses, but the pressure can be very high.
Because each traps a fixed volume, all three deliver roughly the same flow whatever the pressure, and all three can build pressure high enough to be dangerous if the outlet is blocked — so a relief valve is essential.
Gear pumps win where simplicity and cost matter, vane pumps where smooth and quiet flow is needed, and piston pumps where very high pressure is the priority.
All three are positive-displacement pumps. Each traps a set volume of fluid in a closed space, then forces it out through the outlet. That is why their flow depends on speed, not on how much pressure is pushing back.
A centrifugal pump is great for large flows of thin liquid at modest pressure, but its flow drops off as pressure rises. A positive-displacement pump keeps delivering nearly the same volume at any pressure, and can reach far higher pressures.
For most hydraulic systems a gear or vane pump is used. Gear pumps are simple, tough and cheap; vane pumps are smoother and quieter. Piston pumps are chosen when very high pressure is needed.
It only pushes fluid on half of each turn of the crank, so the output comes in pulses. Many piston pumps use several cylinders, or add a damper, to smooth the flow out.
Because it keeps trying to push the same fixed volume, the pressure can climb very high and something must give. That is why positive-displacement pumps need a relief valve to protect the pipework.