Working Principle and Application of Internal Gear Pumps

Meshing Principle: How an Internal Gear Pump Works

An internal gear pump consists of a driving gear (external tooth gear, placed inside) and a driven gear (internal tooth gear, placed outside). The two gears rotate in the same direction, with their tooth tips sealing against each other at a contact point to separate the suction side from the delivery side.

There are two main internal gear pump architectures. The crescent-type uses a crescent-shaped partition plate between the tooth tips to prevent leakage between the tips of the driving and driven gears. The crescentless type instead relies on direct contact between the tooth tip parts of both gears at one point to separate suction and delivery sections, with a very small clearance provided in practice to avoid locking.

The crescentless design offers a key efficiency advantage: because the volume between the teeth of the driving and driven gears is larger, the pump can achieve a larger theoretical displacement when tooth width, driving gear tip diameter and driven gear outer diameter are identical to a crescent-type pump. Alternatively, with the same displacement, the driven gear outer diameter can be smaller, which reduces friction torque from shearing viscous oil on gear sliding parts and improves mechanical efficiency — a major reason this type is widely used in automotive automatic transmissions, including CVTs.

Displacement Calculation for Beginners

There are two fundamental approaches to calculating internal gear pump displacement. The geometric method calculates the volume of the space enclosed by the teeth of the driving and driven gears. The theoretical torque method first calculates the pump's theoretical torque, then derives displacement from the principle that hydraulic energy given to the oil equals the power required to drive the pump shaft (disregarding energy loss).

For beginners, the practical flow calculation is:

Q = Vd × N

where Q is volumetric flow rate, Vd is displacement volume per revolution, and N is rotational speed.

The theoretical displacement method has historically been difficult to apply accurately because it requires integrating the volume between gear teeth at each minute change of rotational angle — requiring gear drawings for every rotational position. CAD tools have made this more practical, but accurate theoretical torque calculation remains valuable because it avoids the complexity of minute-by-minute volume integration.

In practice, gear pump displacement correlates with the square of gear module. Under the premise of constant pitch circle diameter, a larger module produces greater pump displacement.

Suction Pressure Limits

Suction performance directly affects whether a pump can operate reliably without cavitation. For internal gear pumps, the following limits apply:

General gear pump limit: Actual suction height should not exceed 0.5 m.

NBL-series specific limit: Self-priming height must not exceed 500 mm, equivalent to an inlet vacuum of no more than 0.03 MPa (0.3 bar). If using a forced-feed supply method, supply pressure must not exceed 0.5 MPa; otherwise, a pressure-resistant oil seal must be used.

Rexroth PGH series reference: Minimum suction pressure (load-free) is 0.8 bar; maximum suction pressure (load-free) is 2 bar.

Exceeding these limits risks cavitation, which causes gas precipitation, oil vaporization, noise, and accelerated wear on internal surfaces.

Typical Fault Diagnostics

Insufficient or No Suction

Common causes include: inner rotor not rotating (check drive system worm gear, worm wheel or gear, inner rotor fastening screws or locating pins for looseness); rotation direction mismatched with prime mover, causing inlet and outlet port reversal; discharge line blockage; inlet filter screen clogged; severe wear on inner and outer rotors preventing closed cavity formation; inlet pipe end face contacting oil tank bottom; air ingress at pump suction inlet; or oil level too low.

Large Pressure Fluctuation

This often results from: excessive eccentricity error or excessive clearance between outer rotor and pump body (eccentricity error should be within ±0.02 mm; outer rotor to pump body clearance within 0.04–0.06 mm); poor tooth profile accuracy of inner and outer rotors (common in powder metallurgy molded rotors); excessive radial and end-face runout; excessive tooth side clearance (should be within 0.07 mm); or air mixed into the pump.

Pressure Won't Rise

Check for: air ingress at pump suction; inner rotor speed too low (inspect power transmission connection from main shaft to inner rotor for looseness or slippage); partial suction port blockage; or poor worm gear/worm wheel or gear meshing causing fluctuating inner rotor speed.

Excessive Noise

Causes include: low oil level causing air ingestion, or partial filter screen blockage causing insufficient suction; severe component wear; or change in pump power transmission meshing point position.

External Oil Leakage

Check for: loose pump body fastening screws or connectors; damaged seals; poor discharge port flange sealing; or deformed/damaged pump body or cover.

NBL-Series Selection Notes

The NBL series is a straight-tooth conjugate internal gear pump designed for reliable self-priming performance. Key selection considerations for beginners:

Displacement and speed: Match pump displacement to system flow requirements at the intended operating speed. Internal gear pumps generally deliver smooth flow with low pulsation, making them suitable for applications requiring stable output.

Suction configuration: The NBL series requires self-priming height ≤500 mm (inlet vacuum ≤0.03 MPa). If using forced-feed supply, pressure must not exceed 0.5 MPa without upgrading to a pressure-resistant oil seal.

Mounting and shaft: Verify flange type (SAE or ISO standard) and shaft specification (keyed, splined) match your system. Standard SAE and ISO mounting interfaces are available on QT-series and similar internal gear pumps.

Application suitability: Internal gear pumps offer advantages in high-speed performance, low noise, and small flow pulsation, but feature complex tooth profiles and higher manufacturing cost than external gear pumps. They are preferred where smooth operation and quiet running are required.

Key Takeaways for Beginners

Internal gear pumps deliver smooth, low-pulsation flow by sealing the suction and delivery sides at a contact point between driving and driven gear tooth tips. Displacement is best calculated via theoretical torque principles for accuracy, with practical flow given by Q = Vd × N. Suction limits are strict — typically 0.5 m self-priming height and 0.03 MPa inlet vacuum for NBL-series pumps. Most field failures trace to air ingress, incorrect rotation direction, contaminated inlet screens, or excessive wear on rotor clearances. When selecting QT-series or NBL-series pumps, verify displacement, pressure rating, speed range, suction configuration and mounting interface against your system requirements.