Gear Pump Operation

When a gear pump revolves, the drive gear will rotate the driven gear in the opposite direction and mesh with the other gear at a point inside a housing between the inlet port and the outlet port.

When the three gears revolve, the pressure of the fluid in the pump’s inlet will decrease (resulting in a vacuum in the inlet between the reservoir and the pump) allowing the fluid to flow from the reservoir into the pump inside gap A and then continue to flow between the gear box and the housing.

When the teeth of the gears mesh with each other, the fluid will be pushed through gap B into the pump’s outlet.  The tight mesh of the gear teeth provides a seal between the inlet and the outlet so that it eliminates slippage.

The oil tank is positioned on the upper surface of the pump so that the oil is forced into the pump’s inlet by the gravity or absorbtion.

The pump available is of fixed displacement type.  The output flow is variable with the pump’s speed.





Based on the principles of operation, gear pumps are classified into two (2) types, namely:

a).        Reversible pump

b).        Unidirectional pump

a).        Reversible pumps.

A reversible pump operates in two directions of rotation without changing the inner components.  The inlet and outlet ports are determined by the direction of the rotation.  The inlet and outlet ports have the opening of the same size.

b).  Unidirectional pumps.

This type of pump operates in one direction of rotation. It can be designed to rotate clockwise or anticlockwise.  However, once it has been designed for a specific direction, it will revolve in that direction.  The inlet port is larger than the outlet port.

A reversible gear pump is distinguished from a unidirectional one by:

  1. The flange.
  2. The isolation plate.
  3. The pressure plate.

The differences between a reversible pump and a unidirectional pump:

The pressure plate functions to seal the back sides of the gear to prevent leakages due to the high pressure oil flowing to the inlet.

This plate presses hydraulically the sides of the gear due to the discharged pressure.  The gaps between the plates marked with “A” is a trap that allows the flow to be trapped between the teeth of the gear and escape through its end.  This trap located next to the discharge pump is important in controlling the pump operation and noise.

The groove marked with “B” functions as a lubricating groove.  A small amount of the fluid is directed into each bore and downward into the bearing to lubricate the shaft.  The chamfer part with “C” mark carries the high pressure fluid to a proper point on the plate to gain the balanced load.

The scratches indicated by letter “D” exist due to the friction between the gear tip and the plate surface.

The suction side of the case, during manufacturing, is cut on the enclosure for accommodating the gear.  The groove is .008” deep and may not exceed .015” in depth.  This groove provides a small clearance on the gear tip and a high volumetric efficiency

The high pressure fluid from the discharge end is directed to the back side of the pressure plates to press the plates on to the gear tip.

A small part of the pumped oil flows to the lubricating groove on the pressure plate and enters each bearing.

This oil is accumulated at the gear tip and returns through the drill hole to the pump’s inlet.

The teeth of the gear mesh tighly with the pump’s case.  The gear pump creates a heavy side load on the bearing shaft, so that the damaged bearing is the major cause of the pump failure.  The damaged bearing leads to further problems including the damaged keys, broken shaft, worn pump case and pressure plate, even broken pump case.  The pump’s life span can be maintained by replacing the shaft bearing on a regular basis before wearing.

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