For understanding role of transmission gear in any system, it is important to discuss about transmission. By transmission generally it is referred to transmission of power generated by an engine to other parts of the system where that power is utilized to get desired job done. Now this transmission of power is done through transmission gears for so many reasons, but basic objective is to get the desired force in a specific direction. Most common example of transmission gear is gears that are used in automobiles. In case of automobiles fitted with manual transmission system, shift of gear will drive a car in various speeds while moving forward and it can also move the vehicle backward if required. All these are done by the transmission gears. In case of manual transmission, it had to be done by the driver to adjust the speed of the vehicle. The advanced automatic transmission gears adjust the proper gear itself according to the speed. But the basic mechanism behind any transmission gear remains the same.

In a hydraulic system gear motor has comprises of two gears. The first one is the driven gear which attached to an output shaft with a key. The other one is the idler gear. Engine oil is ported in high pressure that moves around one side of the gear. This oil is meant to stay between the gear pins and walls of the gear box. The gears then mesh to each other preventing the pressurized oil to flow to the inlet side of the gear. However, very small amount of this engine oil is used by the gear motors for lubricating the bearings and gear pins to prevent friction and loss. The oil used for such purpose returns to the low-pressure side of the gear through drain port meant for this specific purpose. This intricate mechanism within the gear box prevents regular wearing off the gear pins and extends the life of gears effectively.