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Useful information on External Gear Pumps

A gear pump is a type of positive displacement (PD) pump. Gear pumps use the actions of rotating cogs or gears to transfer fluids.  The rotating gears develop a liquid seal with the pump casing and create a vacuum at the pump inlet.  Fluid, drawn into the pump, is enclosed within the cavities of the rotating gears and transferred to the discharge.  A gear pump delivers a smooth pulse-free flow proportional to the rotational speed of its gears.

There are two basic designs of gear pump: internal and external (Figure 1).  An internal gear pump has two interlocking gears of different sizes with one rotating inside the other.  An external gear pump consists of two identical, interlocking gears supported by separate shafts.  Generally, one gear is driven by a motor and this drives the other gear (the idler).  In some cases, both shafts may be driven by motors.  The shafts are supported by bearings on each side of the casing.

This article describes plastic gear pump in more detail.

There are three stages in an internal gear pump’s working cycle: filling, transfer and delivery (Figure 2).

As the gears come out of mesh on the inlet side of the pump, they create an expanded volume.  Liquid flows into the cavities and is trapped by the gear teeth as the gears continue to rotate against the pump casing.

The trapped fluid is moved from the inlet, to the discharge, around the casing.

As the teeth of the gears become interlocked on the discharge side of the pump, the volume is reduced and the fluid is forced out under pressure.

No fluid is transferred back through the centre, between the gears, because they are interlocked.  Close tolerances between the gears and the casing allow the pump to develop suction at the inlet and prevent fluid from leaking back from the discharge side (although leakage is more likely with low viscosity liquids).

External gear pump designs can utilise spur, helical or herringbone gears (Figure 3).  A helical gear design can reduce pump noise and vibration because the teeth engage and disengage gradually throughout the rotation.  However, it is important to balance axial forces resulting from the helical gear teeth and this can be achieved by mounting two sets of ‘mirrored’ helical gears together or by using a v-shaped, herringbone pattern.  With this design, the axial forces produced by each half of the gear cancel out.  Spur gears have the advantage that they can be run at very high speed and are easier to manufacture.

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