How a Gear Pump Works: Positive Displacement Flow and Pressure Explained

Ask anyone on our shop floor what makes a hydraulic system move, and most people will point at the cylinder. That is fair, because the cylinder is where the visible work happens. But a cylinder is only a receiver. Something has to push the oil into it, and on a huge number of machines, from tractor loaders to small presses, that something is a gear pump. We have been building hydraulic cylinders since 1988, and we have spent just as long listening to customers describe the pumps sitting upstream of them. Here is how a gear pump works, in plain language.

The short answer first

A gear pump is a positive displacement pump. Two meshing gears trap a fixed volume of oil in the spaces between their teeth and the casing, carry that oil around the inside of the housing, and squeeze it out the delivery side. Because the volume moved per revolution is essentially fixed, flow depends on speed rather than on pressure. That one fact explains most of the pump's strengths and nearly all of its limitations.

What sits inside the housing

Strip a gear pump down and you find a surprisingly short list of parts. The simplicity is the reason these pumps survive decades of abuse in dirty, hot, vibrating machinery.

• Drive gear: keyed or splined to the input shaft and turned by the engine, motor or PTO.
• Idler gear: driven by the first gear and free to rotate on its own bearing.
• Pump body or casing: the bore that surrounds both gears and forms the sealing surface at the tooth tips.
• Front and rear covers: close the housing and often carry the bearings or bushings.
• Bearings or bushings: support the gear shafts and absorb the side load created by pressure.
• Seals: a shaft seal keeps oil in and dirt out, while O-rings seal the covers and port joints. Many units also carry an integrated relief valve.

How a gear pump works, step by step

Follow one pocket of oil through a single revolution and the whole cycle becomes easy to picture.

1. The inlet port is connected to a reservoir or a charge line, so oil is available at the pump face.
2. The gears turn. On the inlet side the teeth are unmeshing, so the space between them opens up and the pressure there drops below atmospheric. Oil is pushed in by the pressure in the tank.
3. Each tooth space is then closed off between the gear teeth and the casing wall, so the oil is trapped and cannot slide back towards the inlet.
4. The trapped oil travels around the outside of the gear pair, carried like a bucket on a wheel.
5. On the outlet side the teeth mesh again. The gap between them closes, the trapped volume shrinks, and the oil has nowhere to go but out through the delivery port.
6. The cycle repeats continuously, so delivery is smooth and almost pulse-free at normal speeds. If the outlet is blocked, the relief valve opens and routes oil back to tank instead of bursting a seal.

External and internal designs

Most industrial and mobile gear pumps are external, meaning both gears sit side by side and mesh at a single point between them. Internal gear pumps place a smaller gear inside a larger ring gear, with a crescent-shaped filler separating the two. The internal layout is quieter and handles thick fluids more gracefully, while the external layout is cheaper, easier to service and available almost anywhere.

In both cases the working principle is identical: trap, carry, squeeze. Only the geometry changes.

Displacement, flow and the effect of pressure

Every pump has a displacement figure, the volume it moves per revolution. Flow is roughly displacement multiplied by shaft speed, adjusted for losses. A pump rated at 20 cubic centimetres per revolution turning at 1,500 rpm moves about 30 litres per minute in theory, and a little less in practice.

The gap between theory and reality is called slip: pressurised oil squeezes back past the tooth tips, the gear faces and the bearings. Slip grows with pressure, with wear and with hot, thin oil. That is why a tired pump still turns quietly but delivers noticeably less flow, and why a cold start with heavy oil can briefly raise pressure far above normal working levels.

Where gear pumps sit in real hydraulic circuits

On construction equipment, a gear pump driven from the engine feeds control valves that sequence boom, bucket and travel circuits on a machine such as an excavator.

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On a farm, the same principle runs loader arms, steering and hitch lift, often through a simple tandem pump stack feeding cylinders that work in dust and temperature swings all season.

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Waste compaction is another classic duty: the pump builds pressure, the valve shifts, and the ram strokes smoothly through a heavy load before the relief valve takes over.

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In most of these circuits the oil ends up in a double-acting hydraulic cylinder, which is exactly where the pump's steady delivery turns into force and motion.

The limits worth respecting

A gear pump is tolerant, not indestructible. The usual failure routes are predictable.

• Pressure: pushing beyond the rated continuous pressure accelerates slip, heat and bearing wear.
• Contamination: hard particles score the gear faces and the bore, and slip rises permanently.
• Aeration: air travelling with the oil causes noise, erosion and erratic cylinder motion.
• Cavitation: a restricted or leaking inlet line starves the pump and damages the inlet side.
• Viscosity: too thick and the pump cannot fill; too thin and slip and internal leakage climb.

Keeping the pump and the circuit healthy

Most of the gear pump problems we hear about start somewhere other than the pump. A few habits go a long way.

• Keep the reservoir oil within the viscosity band the pump was sized for, and change it on schedule.
• Check the inlet strainer and the suction hose for cracks, loose clamps and collapsing walls.
• Watch the shaft seal weep hole; a persistent drip means the seal is failing, not that the pump is worn out.
• Confirm the relief valve setting with a gauge rather than by feel.
• Never run the pump dry, even briefly, and never start a cold machine at full throttle.

Wear usually shows up downstream as well, so inspect the rod and cylinder seal kits when you service the pump side of the circuit; oil that has been overheated or contaminated rarely leaves the cylinder untouched.

Matching the pump to the cylinder

A gear pump is a fixed-displacement device, so circuit speed is normally controlled by valves or by engine speed. That matters when you specify the cylinder: bore and rod diameter decide how much force you get for a given pressure, while stroke and mounting decide how the pump's flow is used. Get the pairing right and a small pump will move a large load all day without complaint.

If you are sizing a cylinder for an existing gear pump circuit, or replacing a worn unit on equipment that has been in service for years, send us the working pressure, required force and mounting details. We have been matching cylinders to real machines since 1988, and we are always glad to talk through the numbers before anything goes into production.