External Gear Pumps: Aluminium, Cast Iron and Tandem Units

How an External Gear Pump Works

Two identical gears sit side by side in a close-fitting housing, and the drive gear turns the idler gear. On the inlet side, the teeth move apart and draw oil in. The oil is carried around the outside of each gear, trapped between the teeth and the housing, then pushed out at the outlet as the teeth mesh again.

Pressure-loaded side plates or bushings press against the gear faces as pressure builds. This seals the gaps and keeps efficiency high, which is why a good external gear pump can work at up to about 250 bar. Most models run the gears in plain bushings rather than roller bearings, so clean oil and the right viscosity matter for long life.

How to Identify Your Old Pump

Most external gear pumps carry their key details on a label or stamped into the body. If the label is gone, these checks help:

• Inlet and outlet: the inlet port is usually the larger of the two.
• Rotation: look at the shaft end. If the inlet is on the right with the shaft pointing at you, the pump is usually clockwise, but always confirm with the maker's drawing.
• Displacement: often stamped as a number in cc/rev, or part of the model code.
• Flange and shaft: count the bolt holes and measure the shaft, or note the taper or spline.
• Body material: a magnet sticks to cast iron, but not to aluminium.

Send us photos of the label, flange, shaft and ports from a few angles. That is usually enough for our engineers to find the match.

Can You Change the Rotation?

On many external gear pumps, yes. Some designs can be converted by turning the body over or swapping the drive and idler gear positions, then fitting the correct seals. Others are built for one direction only and must be ordered the right way round.

There are also bi-rotational pumps with an external drain, often used where the drive can turn either way. Getting rotation wrong is one of the most common reasons a new gear pump fails within minutes, so we always confirm it before we ship.

Aluminium or Cast Iron for Your Machine?

Pick the body by duty, not just price:

• Aluminium suits tractors, forklifts and power packs that run in short bursts.
• Cast iron suits tippers, refuse trucks and machines that run hard for hours or see high heat.

If the original was cast iron, stay with cast iron. It handles heat, pressure spikes and PTO loads far better over time.

Popular Jobs for External Gear Pumps

• Tippers and hooklifts: cast iron pumps on the truck PTO, such as the Kayaba KP55 found on many tipping trucks.
• Tractors and farm machines: aluminium pumps on the engine drive for linkages, loaders and steering.
• Forklifts and access platforms: compact pumps for lifting, tilting and steering.
• DC power packs: small 12 or 24 volt hydraulic power units for tail lifts, dock levellers and trailers.
• Industrial power packs: AC motor-driven units for presses, clamps and general machinery.

Sizing a Gear Pump on a PTO

Getting the size right avoids slow cycle times or overheated oil. Use this quick sum:

1. Find the engine speed at your working rpm.
2. Multiply by the PTO ratio to get the pump shaft speed.
3. Multiply the pump shaft speed by the displacement in cc/rev, then divide by 1,000 to get flow in L/min.

For example, a 40 cc/rev pump turning at 1,200 rpm gives about 48 L/min. If that is more flow than your valves or cylinders need, a smaller pump will save heat and fuel. Need help with the numbers? Our pump selection and sizing service can check them for you.

What a Failed Pump Can Tell You

Before you throw the old pump away, look inside. The damage often points straight to the real problem:

• Scored housing and gears: dirt in the oil. Change the filters and flush the system.
• Pitted or eroded inlet side: cavitation from a blocked strainer, a small suction hose or cold oil.
• Broken teeth or a snapped shaft: pressure spikes or a sticking relief valve.
• Polished, worn bushings: oil that is too hot or too thin.
• Blown shaft seal: wrong rotation or a restricted suction line.

Fix the cause first, or the new pump will fail the same way.

External, Internal or Gerotor?

External gear pumps are the right pick for most mobile and general duties where cost and toughness matter most. If noise is a concern, such as on indoor machines, internal gear pumps run much quieter. For low-pressure lubrication or charge circuits, gerotor pumps are often the better fit.

Need a gear motor rather than a pump? They can look almost the same, but they are built differently inside. Our hydraulic motors range covers gear motors for fans, conveyors and augers.

Gear Pump Brands We Cross-Reference

Our engineers match external gear pumps from many makers every day, including Parker PGP models, European Group 1, 2 and 3 pumps, and common Japanese and American truck pumps. We tell you upfront whether we are offering a genuine pump or an equivalent. If your pump has no readable label, the flange, shaft and port measurements are usually enough for us to identify it.

Why Buy From TSA Hydraulics Engineering

As an external gear pump supplier, we focus on the details that decide fit: flange, shaft, ports and rotation. You get clear prices in AUD, fast answers from engineers who know mobile hydraulics, and help sourcing external gear pumps replacement parts such as seal kits and shaft seals.