Internal Gear Pumps: The Complete Guide to Selection, Performance, and Real-World Applications

 Walk through any machine tool facility, visit any hydraulic press manufacturer, or open the power unit specification on any precision industrial machine — and the pump type you will find most consistently is not an axial piston pump, not an external gear pump, but a unit from the family of internal gear pumps. The reason is not brand loyalty or inertia. It is performance characteristics that match the demands of precision hydraulic circuits better than any other positive displacement pump type at the same pressure range and cost point. Internal gear pumps produce flow that is smooth enough to feed sensitive proportional valves without signal contamination, quiet enough to meet noise specifications in indoor manufacturing environments, and tolerant enough of varying viscosity to handle cold-start conditions without cavitation damage. This guide covers how they work, what distinguishes one model from another, where they outperform alternatives, and where their limitations become relevant to the selection decision.

The Mechanics Behind the Smooth Flow Characteristic

An internal gear pump uses two gears inside a single housing. The inner rotor — the smaller of the two — is mounted off-center relative to the outer ring gear. As the inner rotor turns, its teeth mesh with the teeth of the outer ring gear on one side. On the opposite side, the teeth are fully unmeshed, and the crescent-shaped divider between the two gears separates the expanding inlet volume from the shrinking outlet volume.

Because the fluid travels in a continuous arc from inlet to outlet — rather than being carried in discrete tooth pockets around the outside of two parallel gears as in an external design — the flow characteristic is inherently smoother. The number of teeth in mesh at any given moment is higher, the transition between tooth engagement is gradual, and the pressure pulse generated per revolution is smaller. The result is a pump that produces flow with very low ripple content — typically 1 to 3 percent peak-to-peak — compared to 5 to 10 percent for a comparable external gear pump.

This low ripple is not just a noise benefit. In servo-controlled hydraulic circuits, pressure ripple from the pump appears as noise in the controller's feedback signal. If the ripple amplitude is significant, the controller responds to it — generating small corrections that manifest as oscillation in actuator position or pressure. Internal gear pumps avoid this problem inherently, which is why they are the standard pump type in precision servo hydraulic applications.

Where Internal Gear Pumps Outperform Every Alternative

The clearest advantage of internal gear pumps over axial piston pumps is cost and simplicity at medium pressure levels. For applications in the 100 to 250 bar range with flow requirements below 200 liters per minute, an internal gear pump delivers comparable efficiency at significantly lower purchase price and with simpler maintenance requirements. There are no pistons, slippers, valve plates, or swash plates — the wear surfaces are the gear faces and the housing bore, and inspection is straightforward.

Compared to vane pumps, internal gear designs handle higher viscosity fluids more effectively. Vane pumps rely on centrifugal force and spring pressure to keep vanes in contact with the housing — at high viscosity or cold start, this becomes unreliable. Internal gear pumps have no vanes and are not dependent on centrifugal effects, making them significantly more reliable through cold-start cycles in outdoor or unheated environments.

The comparison with external gear pumps comes down to noise, flow quality, and the specific application. External gear pumps are cheaper and more contamination-tolerant, which makes them the right choice for cost-driven mobile applications where noise is not a specification requirement. Internal gear pumps are quieter, smoother, and more compact for equivalent displacement — which makes them the right choice whenever the machine builder or end user has a noise limit to meet or a servo control system to feed.

Matching Internal Gear Pump Specifications to a Hydraulic Servo System

When a hydraulic servo system is the power source, pump selection has consequences beyond flow rate and pressure rating. The servo controller adjusts motor speed continuously to maintain pressure and flow targets — and it does this by reading a pressure sensor signal in a closed loop. Any pump characteristic that introduces noise into that signal — pressure pulses, cavitation transients, or erratic flow at low speed — degrades the quality of the control loop and limits how tightly the system can regulate pressure and position. The internal gear pump's low ripple output means the servo pressure feedback signal is clean across the full speed range — from the 1500 to 3000 RPM range during peak demand phases down to the 100 to 400 RPM range during pressure-holding and dwell phases. This clean signal is what allows servo hydraulic systems to hold pressure to within 1 to 2 bar of setpoint continuously — a level of precision that is impossible with a noisier pump type feeding the same controller.

Viscosity Range and Fluid Compatibility

Internal gear pumps handle a wider viscosity range than most pump types. The standard operating range is 15 to 380 centistokes — which covers everything from thin hydraulic oils at high temperature to gear oils and biodegradable fluids at cold start. This range matters in two specific scenarios.

First, in outdoor or unheated environments where startup temperature can be well below zero, a pump that operates reliably at high viscosity avoids the cold-start cavitation damage that destroys axial piston pumps run before the oil has warmed. Second, in applications using fire-resistant hydraulic fluids — water-glycol, phosphate ester, or polyol ester types — internal gear pumps are compatible with modification to the seal and bearing materials, whereas some pump types require complete redesign for fluid compatibility.

Using a Linear Position Sensor for Hydraulic Cylinder Feedback With Internal Gear Pump Systems

When an internal gear pump feeds a cylinder that requires closed-loop position control, a linear position sensor for hydraulic cylinder feedback is the component that closes the loop between pump output and actuator position. The sensor measures cylinder rod position in real time and reports it to the machine controller, which commands the servo drive or proportional valve to adjust flow accordingly. The smooth, low-ripple output of the internal gear pump is directly beneficial here — because the position control loop is simpler to tune when the hydraulic input to the actuator is clean. A pump with high pressure ripple introduces disturbance into the position loop that the controller must filter or compensate for, which reduces bandwidth and limits achievable accuracy. With an internal gear pump as the source, position loops can be tuned to higher bandwidth — meaning faster, more accurate motion — without the instability that ripple-induced disturbance would otherwise create.

Displacement Selection and Speed Range Considerations

Internal gear pump displacement is typically expressed in cubic centimeters per revolution (cc/rev). To calculate the flow at a given speed: Flow (L/min) = Displacement (cc/rev) × Speed (RPM) / 1000, adjusted by volumetric efficiency (typically 92 to 96 percent at rated pressure).

For a servo motor driven hydraulic pump assembly, the displacement selection determines the speed range over which the system operates. Choosing a large displacement pump means the servo motor runs at lower speed for a given flow — which is better for noise and bearing life. Choosing a small displacement pump means higher motor speed — which may be acceptable for high-flow applications but increases noise and reduces motor efficiency at full load.

The optimal selection for most servo hydraulic applications is the largest displacement pump that keeps peak motor speed below 2500 RPM. This ensures the motor operates in its high-efficiency, low-noise speed range for the majority of the duty cycle. For a servo motor driven hydraulic pump assembly targeting peak flow of 80 liters per minute with a maximum motor speed of 2400 RPM, the calculation gives a minimum displacement of approximately 33 cc/rev — so a 40 or 45 cc/rev internal gear pump would be the appropriate selection, giving a useful speed buffer below maximum.

Why Buyers Searching for Hydraulic Motors for Sale Should Also Evaluate Pump Type

The hydraulic motor and pump in a circuit are not independent selections — they define the circuit's operating characteristics together, and a mismatch between pump flow quality and motor type can cause performance problems that appear unrelated to either component individually. Hydraulic motors for sale are frequently specified by displacement and pressure rating alone, without considering the pump's flow ripple characteristic and how it interacts with the motor's torque output. Low-speed high-torque motors — orbital and radial piston types — are particularly sensitive to inlet flow pulsation, which translates directly to torque ripple at the output shaft. If the application requires smooth rotation at low speed, specifying an internal gear pump as the circuit's pressure source reduces torque ripple significantly compared to an external gear pump, and the result is smoother, more consistent output shaft behavior without any changes to the motor itself.

FAQ

Q: Can internal gear pumps run in both directions of rotation?

Most internal gear pumps are designed for one specific rotation direction. Bidirectional models exist but are less common. Confirm rotation direction before ordering and match it to your drive arrangement.

Q: What causes an internal gear pump to become noisy after years of quiet operation?

The most common causes are increased internal clearance due to wear on the gear faces or housing bore, reduced oil viscosity from degraded fluid, or aeration caused by a low oil level or a leak on the pump inlet side.

Q: Are internal gear pumps suitable for high-water-content fluids?

Standard models are not suitable. For HFA and HFB water-based fluids, stainless steel internals and compatible bearing materials are required. Consult the manufacturer before using any non-mineral fluid.