Geared oblique-shaft variable piston pumps are self-priming and can be raised dry, although their starting characteristics will increase if the gears are wet. Gears need to be lubricated with the pumped liquid and should not dry for long periods. Some gear pump designs can operate in either direction, so for example the same pump can be used to load and unload containers.
The tight tolerances between the gear and the sleeve mean that these types of pumps are prone to wear, especially when used with abrasive fluids or feeds containing entrained solids. However, some designs of oblique-shaft variable displacement plunger pumps are specifically internal variants that allow handling of solids. The external gear pump has four bearings in the pumping medium and tight tolerances, so it is not suitable for handling abrasive fluids. The internal gear pump is solid and has one bearing (sometimes two) running in the fluid. Gear pumps should always have filters installed on the suction side to protect them from large, potentially harmful solids.
Generally, if the pump is expected to handle abrasive solids, it is recommended to choose a pump with a larger capacity so that it can run at a lower speed to reduce wear. However, it should be kept in mind that the volumetric efficiency of a gear pump decreases at lower speeds and flow rates. The operating distance of the gear pump should not exceed the selected speed. For high temperature applications, it is important that the operating temperature range of the tube is compatible with the pump specifications. Thermal expansion of the housing and gear reduces the clearances inside the pump, which can also lead to increased wear and, in some cases, pump failure.