Internal Gear Pumps vs External: Which One Actually Belongs in Your Hydraulic Circuit?
Gear pump selection is treated as a commodity decision in most hydraulic system specifications — two options are shortlisted on price and delivery, the cheaper one is ordered, and the conversation moves on to more apparently complex decisions. This approach works until it does not. The machine that runs noisier than the operator environment specification allowed. The proportional valve that never quite holds position as smoothly as the commissioning engineer expected. The fluid temperature that runs consistently above the thermal model's prediction despite correct cooler sizing. Each of these problems can be traced, through a methodical root cause investigation, to a pump selection that was made without adequate analysis of the application's actual requirements. The hydraulic internal gear pump and its external gear counterpart are not interchangeable alternatives at different price points. They are mechanically distinct technologies with genuinely different performance characteristics in noise, flow smoothness, viscosity tolerance, and bidirectionality — characteristics that are irrelevant in some applications and decisive in others. Understanding which characteristics matter for a given application is the engineering work that makes pump selection consequential rather than incidental.
The physical geometry of internal and external gear pump designs produces the differences that matter in practice. An internal gear pump uses an outer ring gear and a smaller inner pinion that rotate off-centre within it. Both gears rotate in the same direction. The fluid is drawn into the expanding space on the inlet side as the teeth unmesh, carried around to the outlet side, and expelled as the teeth re-engage. The single-direction rotation and the progressive, non-opposing tooth engagement geometry produces flow that is exceptionally smooth — near-free of the pressure pulsation that characterises pumps where opposing elements create periodic high-pressure events. An external gear pump drives two identical gears in opposing directions on parallel shafts. Fluid is trapped in the tooth spaces, carried around the outside of each gear from inlet to outlet, and squeezed out as the teeth mesh at the centre. Every tooth mesh event creates a small pressure pulse. At 1,500 RPM with a 12-tooth gear, that is 300 pressure pulses per second — a ripple that propagates through the circuit, sets hose assemblies into standing wave oscillation at certain frequencies, and challenges proportional valve spools that must maintain precise positions while their inlet pressure fluctuates rhythmically. In circuits without proportional valves and without noise-sensitive environments, this pulsation is a minor background characteristic. In circuits with proportional control, or in machines where the hydraulic power unit sits close to a precision measurement process, it is a genuine performance limitation.
Internal gear pumps represent the engineered choice — not the default choice — in applications where their specific characteristics provide functional value that external gear designs cannot deliver. Machine tool hydraulics is the highest-volume application where this distinction matters most. Precision machining centres, cylindrical grinders, jig borers, and EDM machines all specify internal gear pumps because the combination of low structural vibration and near-zero flow pulsation directly affects machining quality at the workpiece level. Vibration from the pump transfers through the machine base to the spindle and workpiece. Pressure pulsation in the hydraulic circuit drives positioning valve spools erratically, creating micro-velocity variations in table and spindle feed axes that appear on finished surfaces as periodic waviness correlated with the pump's pulsation frequency. These effects are small — measured in microns, not millimetres — but in surface grinding and precision boring where surface finish and roundness tolerances are specified in single-digit micrometres, they are not small enough to ignore. Lubrication systems are the second major application domain where internal gear pump characteristics are decisive. A lubrication pump must handle cold, very high-viscosity oil at startup — when the machine has been sitting overnight and the oil temperature is near ambient — and hot, low-viscosity oil at operating temperature. External gear pumps with tight clearances optimised for standard hydraulic oil viscosity at 40°C cannot self-prime reliably on oil at 5°C with viscosity ten times higher. Internal gear pumps handle this viscosity range in the same hardware, making cold-start reliability a designed-in characteristic rather than a field problem.
No pump selection is fully evaluated without understanding how the pump's characteristics affect the downstream circuit. Hydraulic pumps and motors function as a matched system — the pump's flow quality, pressure ripple amplitude, efficiency at part-load, and directional flexibility all affect how the hydraulic motors and cylinders downstream perform, and how the control valves in the circuit respond across the operating range. An internal gear pump driving a hydraulic motor in a closed-circuit drive benefits from the pump's near-zero pulsation — smooth flow input to the motor produces smooth rotational output, which matters in precision conveyor drives, winding machines, and positioning turntables where velocity regularity affects product quality. An internal gear pump feeding a proportional valve circuit gives the valve's electronic controller a cleaner pressure environment to work against — the proportional valve's spool positioning controller does not need to continuously compensate for inlet pressure variation, which would otherwise appear as small but measurable velocity oscillations in the downstream actuator. The hydraulic system integrator who understands these cross-component interactions — who specifies the pump not just for its pressure and flow rating but for how its operating characteristics mesh with every downstream component — produces systems that perform better in production than their individual component specifications would predict.