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 account.
For engineers
| Quantity | Formula | Example (20 cm³/rev, 1,500 rpm, 150 bar) |
|---|---|---|
| Theoretical flow | Q = V × n / 1000 | 20 × 1,500 / 1000 = 30 L/min |
| Delivered flow | Q × η(v) | 30 × 0.94 = 28.2 L/min |
| Hydraulic power | P = p × Q / 600 | 150 × 28.2 / 600 = 7.1 kW |
| Drive torque | T = V × p / (20π × η(m)) | 20 × 150 / 62.8 / 0.88 = 54 Nm |
| Drive power | P = T × n / 9549 | 54 × 1,500 / 9549 = 8.5 kW |
Internal leakage in a gear pump is a pressure-driven flow through fixed clearances: between the gear faces and the side (wear) plates, between the tooth tips and the housing, and across the mesh. To a first approximation it is proportional to pressure and nearly independent of speed. That has two practical consequences:
- Volumetric efficiency falls with pressure and rises with speed. At low speed the fixed leakage is a larger share of a smaller flow. Most gear pumps have a minimum speed (often 500 to 800 rpm) below which they can’t build rated pressure.
- Leakage is heat. Every litre that slips back has dropped the full pressure across a clearance. A worn pump losing 8 L/min at 180 bar puts 2.4 kW straight into the oil.
Modern pumps use pressure-balanced side plates: outlet pressure is fed behind the plates to push them against the gear faces, so the clearance stays small as pressure rises. When those plates wear, or their seals fail, the face clearance opens up and leakage rises sharply. That’s the “Break it” fault in the animation.
Deep dive
Testing a gear pump. The reliable test is a flow meter with a loading valve on the pump outlet. Record the flow at no load, then at 50%, 75% and 100% of working pressure, at normal operating temperature. A healthy pump loses perhaps 5 to 10% between no load and full pressure. A pump losing 20% or more is worn; above about 30% it will be hot, slow and getting worse. Testing cold oil flatters a worn pump because thick oil leaks less, so always test warm.
Wear patterns tell you why. When we strip a gear pump, the marks say what killed it:
- Scoring in the inlet half of the housing (the gears riding into the bore) is normal bedding-in on a new pump, but deep scoring all round means bearing wear or overloaded pressure.
- Worn side plates with a polished, dished pattern point to fine contamination: particles the size of the clearance being dragged through it, which is why cleanliness matters even for “dirt-tolerant” pumps (lesson 15).
- Pitted, eroded edges near the inlet mean cavitation: the tooth spaces were not filling.
Cavitation and the inlet. A gear pump can only deliver the oil it can draw in. The inlet must stay above roughly 0.8 bar absolute (about 0.2 bar below atmosphere) for most pumps. A clogged strainer, a collapsed suction hose, a closed suction valve, oil that is too thick on a cold start, or a pump turned too fast all drop the inlet pressure. Vapour bubbles form in the tooth spaces, then collapse violently as they reach the outlet. The sound is a rattle like gravel, and the collapse erodes metal. See lesson 18 and our noisy pump guide.
UAE conditions. High oil temperatures in summer thin the oil and increase leakage, so a pump that tests fine in January can be marginal in August. Dust ingress through tank breathers is the other local enemy. A desiccant or fine-filtered breather costs little and protects every clearance in the system.
Gear pump or piston pump? Gear pumps are cheaper to buy and to repair, tolerate dirt better and are quieter than vane pumps at low pressure. They are fixed-displacement, so they can’t vary their flow, and they are usually limited to about 250 to 280 bar. For higher pressures, variable flow or best efficiency, a piston pump is used (lesson 7).
Key numbers
| What | Typical value |
|---|---|
| Continuous pressure rating | 200 to 280 bar |
| Speed range | about 500 to 3,000 rpm |
| Volumetric efficiency, new, at rated pressure | 90 to 95% |
| Flow loss that means “worn” | more than 20% at working pressure |
| Minimum inlet pressure | about 0.8 bar absolute |
| Suction line velocity | 0.5 to 1.2 m/s |
Work out your own figures with the pump flow, power and torque calculator.
