The spur, helical, and herringbone design gears are used in gear-type hydraulic motors. The motors use external-type gears, as discussed in the chapter on Pumps.
The operation of a gear-type motor is shown in Figure 6-10. Both gears are driven by gears. However, only one is connected to the output shaft. As pressurized fluid enters chamber A, it takes the path of least resistance and flows around the inside surface of the housing, forcing the gears to rotate as indicated. The flow continues through the outlet port to the return line. This rotary motion of the gears is transmitted through the attached shaft to the work unit.
The motor shown in Figure 6-10 is operating in one direction, but can be operated in either direction. To reverse the direction of rotation, the ports may be alternated as inlet and outlet. When fluid is directed through the outlet port into chamber B, the gears rotate in the opposite direction.