Under conditions, the volumetric efficiency of the gear pump. When fluid is pumped at a fixed pressure and variable volume, there is always a loss due to the gap between the gear teeth, the gear face and the housing. Therefore, in order for the gear pump to operate at its rated speed, the volumetric efficiency remains low and results in insufficient flow. The gear pump is divided into external parts, and the internal gear in the internal gear pump is used to pressurize the fluid. The gear is turned by using the pump's power to force fluid through the pump outlet. The internal gear pump is a compact unit, so it can be used as a portable unit. Regarding the external gear pump, the external gear is used to pressurize fluid through the pump outlet, and the external gear is also rotated using a power source from the pump. Since this happens on the inlet side of the pump, the pressure forces the oil in the chamber and as the space decreases, the liquid is forced to drain from the outlet. Considering the various stages of the pump, they are divided into single stage, double stage, third stage, and fourth stage. For high pressure systems, alloys and polymers are used, while for low pressure systems, inexpensive and low pressure pumps can be used. It is always practical to consider the operating conditions and requirements of the machine before purchasing an oil pump motor unit . There are many manufacturers to choose from. These hydraulic power units are individually and customized to perform functions in mobile and industrial applications. Both latter designs provide an overflow pump suction line. This allows the positive fluid head to fill the suction port of the pump and protect the pump from cavitation. Reducing cavitation causes the pump to provide rated system life.