Know Your Pumps: External Gear Pumps

Recently, I took you through the workings of an internal gear pump. Today I’m going to guide you through the inner workings of an external gear pump.

How do They Work?

External gear pumps use two identical gears rotating against each other to drive fluid from the suction port to the discharge port. Each gear is supported by a shaft with bearings on both sides of the gear. Typically, all four bearings operate in the pumped liquid.

Because the gears are supported on both sides, external gear pumps are often used for high pressure applications. Usually, small external gear pumps operate at 1,750 or 3,450 rpm and larger versions operate at speeds up to 640 rpm.

The design of an external gear pump allows them to be made to closer tolerances than internal gear pumps. The pump is not very forgiving of particulate in the pumped liquid. Since there are clearances at both ends of the gears, there is no end clearance adjustment for wear. When an external gear pump wears, it must be rebuilt or replaced.

External gear pumps handle viscous and watery-type liquids. Thicker liquids require careful setting of the pump speed because gear teeth come out of mesh for a short time, and viscous liquids need more time to fill the spaces between the gear teeth than thinner liquids.

The pump does not perform well under critical suction conditions. Volatile liquids tend to vaporize locally as gear teeth spaces expand rapidly. When the viscosity of pumped liquids rises, torque requirements also rise, and pump shaft strength may not be adequate. We supply torque limit information when it is a factor and advise against external gear pumps for applications when the torque requirements to pump a given liquid are beyond the tolerances of a given pump.

What Do We Use Them For?

We see external gear pumps used to pump fuel oils and lube oils, chemical additives, on hydraulics and low volume transfer applications. It is also common to use an external gear pump for chemical mixing and blending.

Score Card

E = Excellent, G = Good, A = Average, P = Poor