When an external gear pump wears, it must be rebuilt or replaced. External gear pumps handle viscous and watery-type liquids, but speed must be properly set for thick liquids. Gear teeth come out of mesh a short time, and viscous liquids need time to fill the spaces between gear teeth. As a result, pump speed must be slowed down considerably when pumping viscous 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. Pump manufacturers supply torque limit information when it is a factor. Lobes in lobe pumps do not make contact, because they are driven by external timing gears. This design handles low-viscosity liquids. Lobe contact is prevented by external timing gears.