A gear pump is a fixed-displacement hydraulic pump in which meshing gears carry oil from the suction port to the outlet. On injection molding machines it serves as the main pump on some smaller machines and in some auxiliary circuits, and, as an internal gear pump driven by a servo motor, on many servo-hydraulic machines. On the drawing it takes the same place as any other pump: bolted to the bell housing on top of the oil tank.
How a gear pump works
External gear pump
Two identical spur gears mesh inside a close-fitting housing; one is driven by the shaft and turns the other. Oil trapped in the spaces between the teeth and the housing is carried around the outside of both gears from the suction side to the pressure side, and the meshing teeth in the middle stop it from flowing back. Bushings or side plates seal the faces of the gears; in many pumps, outlet pressure presses them against the gears to keep the leakage low as pressure rises.
External gear pumps are simple, compact and tolerant of dirt, but they are louder and their flow pulses more than other types.
Internal gear pump
A small external gear (the pinion) drives a larger ring gear with internal teeth that turns in the same direction. A crescent-shaped filler piece fills the space between the two gears on one side and separates the suction zone from the pressure zone. Oil is carried in the tooth spaces of both gears past the crescent.
Internal gear pumps run quietly with little flow pulsation and can work at high pressure. Those qualities make them a common choice for servo pump systems, where the pump turns at whatever speed the cycle needs, from slow during hold pressure to full speed during fast movements.
Both types are fixed displacement: the flow depends only on the speed and on how much oil leaks inside the pump.
Where gear pumps are used on injection molding machines
- Main pump on some smaller machines, driven by a constant-speed motor.
- Auxiliary circuits on some machines, such as a separate oil filtration and cooling loop.
- Servo pump systems, where a servo motor drives the pump and controls its flow and pressure. See the servo pump system.
Our Sumitomo QT32 gear pump is an internal gear pump rated to 17.5 MPa, listed for Haitian MA series and Yizumi A5 series machines. For the other pump types and how to tell them apart, see hydraulic pump types.
Signs of a worn gear pump
- Slower movements at the same settings, and pressure that takes longer to build.
- On a servo machine, a higher motor speed than before during hold pressure, because the pump is making up for internal leakage.
- A louder whine, or a rattle that points to cavitation.
- The pump and the oil running hotter.
- Oil leaking at the shaft seal.
Why gear pumps wear
- Abrasive particles in the oil wear the gear tips, the housing and the side plates or bushings, which opens the gaps that seal the pump.
- Cavitation from a blocked suction filter, a low oil level or cold, thick oil erodes the gears and side plates.
- Pressure peaks above the pump's rating load the bearings and bushings.
- Side load on the shaft from a misaligned or worn pump coupling wears the bearings and the shaft seal.
- Hot or thin oil increases internal leakage and reduces lubrication; see hydraulic oil.
Inspecting and replacing a gear pump
- Record the pressure and the flow or movement times while the machine is healthy, so you have figures to compare with. On a servo machine, also note the motor speed during hold pressure.
- When wear is suspected, check the suction side first: oil level, suction filter, suction valve and suction line seals.
- Measure the outlet pressure with a gauge and compare it with the setting in the machine manual.
- To replace the pump, lock out the motor, close the suction valve if one is fitted, and relieve the pressure in the circuit.
- Check that the new pump matches the model code, direction of rotation, shaft, flange and ports, and fit a new coupling insert.
- Fill the pump with clean oil, open the suction valve, check the direction of rotation, and start it unloaded until the air is out.
- Change the filters, because the old pump's wear particles are still in the circuit.
Small external gear pumps are usually replaced as a unit. Larger internal gear pumps can be overhauled by a specialist if the housing and shafts are undamaged.
What to send for a replacement quote
The full model code from the pump nameplate, the machine model and serial number, and whether the pump is driven by a servo motor or a standard motor. If the plate cannot be read, add photos of the pump, the shaft, the flange, the ports and the direction-of-rotation arrow. Then request a quote, or see the other parts of the hydraulic system.
Updated by the IMSPARES technical team.