The most efficient and versatile hydraulic motors are piston type but they are also the most expensive. Inline or axial and bent axis types operate smoothly from 10 to 2000 rpm with high torque throughout their speed range. Radial piston types go as low as 1 rpm but usually not higher than 400 rpm. Their main use is very high-torque/low-speed applications.

The cutaway in Figure 15-27 shows typical construction of inline fixed- and variable-displacement hydraulic motors. Low-displacement variable motors may be controlled manually while larger motors need pistons to change displacement. An inline hydraulic motor shaft rotates as fluid pushes against a piston, forcing its shoe to slide up the angled swash plate. A small amount of pressure fluid goes through an orifice in the piston and behind the shoe to keep it from rubbing metal-to-metal while it is producing torque.

With the swash plate at a steep angle, torque is high while speed is usually low. A shallow swash plate angle gives high speed but less torque. Most manufacturers recommend a minimum swash plate angle of 15° to 17° for best results. A maximum angle of 40° to 45° gives good torque and long motor life.

The bent-axis piston motor in Figure 15-28 has the same operating characteristics as the inline motor but is more rugged and capable of higher operating pressures. Since there is no sliding action of piston shoes there is less friction and higher torque for a given energy input. The angle of the cylinder block to the input shaft determines torque and speed ranges. The greater the angle, the higher the torque and the lower the speed. Kidney-shaped openings in both inline and bent-axis motors port fluid to and from pistons as they rotate. Internal leakage is sent to tank through the case drain. Variable-displacement bent-axis motors are available but not commonplace due to expense and size.








The radial piston motor shown in Figure 15-29 uses pistons pushing against an eccentric to produce rotary motion. These motors usually have five or seven pistons with rods and shoes, with half of them pushing against the eccentric while the other half return oil to tank. The shoes have high pressure fluid fed to them from the piston through the rod to keep them from rubbing the eccentric during the power stroke. A rotary valve attached to the output shaft feeds and exhausts fluid to and from the pistons as they turn the eccentric.

Some radial piston motors are made with a moveable eccentric that allows different offset amounts. Usually the offset is full or one-half so a motor with this feature can have higher speed at lower torque for fast movement. Eccentric-type radial piston motors are one type of motor that cannot function as a pump without special inlet considerations.

Other radial piston motor designs are similar in action and torque output, but arrange the pistons in different configurations. One design has the pistons facing in and pushing outward against a cam-shaped housing. The shaft is connected to the machine and the housing rotates. It was originally designed as a wheel motor.