Hydraulic Gear Pumps: External, Internal and Gerotor Designs

Why Gear Pumps Are Everywhere

Open the bonnet of almost any tractor, forklift or tipper and you will find a gear pump. There are good reasons for that:

• Simple design: only a few moving parts, so there is less to go wrong.
• Low cost: the most affordable way to make hydraulic flow.
• Dirt tolerant: they cope with tougher conditions than vane or piston pumps.
• Compact: a lot of flow from a small, light body.
• Easy to replace: standard flanges and shafts make swaps quick.

The trade-offs are fixed flow, more noise on external designs, and a steady drop in efficiency as the gears and side plates wear. For most mobile and general duties, those trade-offs are well worth it.

Three Gear Pump Types

External Gear Pumps

External gear pumps use two identical gears side by side inside a close-fitting housing. Oil travels around the outside of the gears and is pushed out where they mesh. They cover most mobile and general industrial jobs, with pressures up to about 250 bar on many models. Single, tandem and triple units are all common.

Internal Gear Pumps

Internal gear pumps place a small gear inside a larger ring gear, with a crescent-shaped seal between them. The result is very smooth, quiet flow with low pulsation. That makes them a top pick for indoor power units, machine tools and injection moulding machines, especially where noise limits apply.

Gerotor Pumps

Gerotor pumps use an inner rotor with one tooth fewer than the outer rotor. As they turn, the spaces between the teeth open and close to move oil. They work at lower pressures and suit lubrication systems, charge pumps, gearbox cooling and circulation duties.

Aluminium or Cast Iron?

External gear pumps come in two main body materials. Choosing the right one saves you trouble later:

• Aluminium bodies: light, compact and affordable. Ideal for mobile machines, small power packs and tractors, with pressures up to about 250 bar on many models.
• Cast iron bodies: heavier and tougher, with better heat and pressure handling. They suit continuous duty, high load work, and tipper or PTO pumps that run hard every day.

If your old pump was cast iron, replace it with cast iron. Swapping to aluminium to save money often shortens pump life on heavy-duty machines.

Single, Tandem and Multi Section Pumps

Many machines need more than one circuit. A tandem or multi-section gear pump stacks two or more gear sets on one shaft, so one drive can feed steering, lifting and auxiliary functions separately. Common examples include the Danfoss SNP range used on many mobile machines.

When you replace a multi-section pump, check:

1. The displacement of each section, front to back.
2. Whether the sections share one inlet or have separate inlets.
3. Which outlet feeds which circuit.
4. The rotation, as it applies to the whole pump.

Unusual multi-section or extra-compact designs often sit in our specialty hydraulic pumps range.

Gear Pump Specifications for a Perfect Fit

Gear pumps follow some common industry patterns. Check these details against your old pump:

• Size group: many European pumps are sorted into Group 1, 2 or 3 by body size.
• Displacement: from under 1 cc/rev to over 100 cc/rev, often stamped on the body.
• Mounting flange: European 4-bolt, SAE A or B, or a German pattern.
• Shaft: tapered 1:8 with a key, parallel keyed, or splined.
• Ports: flanged or threaded, and side or rear mounted.
• Rotation: clockwise or anticlockwise, viewed from the shaft end.
• Pressure and speed: continuous, peak and maximum rpm ratings.

A pump can match on displacement and still fail to fit because of the flange or shaft. We check every item before we quote.

Common Gear Pump Problems and Their Causes

Most gear pump failures trace back to the system, not the pump. Before you fit a new one, look for:

• Whining or a gravelly noise: cavitation from a blocked strainer, a small suction line, or cold, thick oil.
• Slow functions: worn side plates or gears letting oil slip back inside the pump.
• Shaft seal leaks: a blocked suction line or the wrong rotation putting pressure on the seal.
• Snapped shaft: a pressure spike, a jammed relief valve or a misaligned PTO.
• Hot oil: a relief valve blowing off for long periods.

Fix the cause, flush the system and fit new filters, and your new pump will last far longer.

Where Gear Pumps Work

Mobile users include tippers, tractors, forklifts, log splitters, cranes and truck PTOs, with Kayaba gear pumps a common sight on Japanese trucks and machines. Industrial users run gear pumps on hydraulic power units, lubrication systems and machine tools, where internal gear designs keep noise down.

Gear, Vane or Piston?

Gear pumps are not the answer to every job. Here is how to decide:

• Choose gear pumps for simple, low cost duties with fixed flow.
• Choose vane pumps when you want quieter running and variable flow at medium pressure.
• Choose piston pumps for high pressure, variable flow and the best efficiency.

On injection moulding machines, internal gear pumps driven by a servo motor are also popular. That setup sits in our servo hydraulic range. Still unsure? Our pump selection and sizing service will help you choose.

Why Buy Gear Pumps From TSA Hydraulics Engineering

As a gear pumps supplier, we know that a perfect match comes down to small details. Our engineers check the group size, flange, shaft, ports and rotation, and we tell you upfront whether we are offering a genuine pump or an equivalent. You get and help sourcing gear pumps replacement parts such as seal kits and shaft seals.