Figure 10-15.-Typical vane type of hydraulic motor.

converted into kinetic energy in the form of rotary motion and force. The air at reduced pressure is exhausted to the atmosphere. The shaft of the motor is connected to the unit to be actuated.

Many vane-type motors are capable of providing rotation in either direction. A motor of this design is illustrated in figure 10-15, view B. The principle of operation is the same as that of the vane type of motor previously described. The two ports may be alternately used as inlet and outlet, thus providing rotation in either direction. Note the springs in the slots of the rotors. Their purpose is to hold the vanes against the housing during the initial starting of the motor, since no centrifugal force exists until the rotor begins to rotate. Springs are not required in vane-type pumps because the drive shaft provides the initial centrifugal force.

PISTON TYPE. - Like piston (reciprocating) type pumps, the most common design of the piston type of motor is the axial. This type of motor is the most commonly used in hydraulic systems.

Although some piston-type motors are controlled by directional control valves, they are often used in combination with variable displacement pumps. This pump-motor combination (hydraulic transmission) is used to provide a transfer of power between a driving element (for example, an electric motor or gasoline engine) and a driven element. Some of the applications for which hydraulic transmissions may be used are speed reducer, variable speed drive, constant speed or constant torque drive, and torque converter. Some advantages of hydraulic transmission over mechanical transmission of power are as follows:

1. Quick easy speed adjustment over a wide range while the power source is operating at constant (most efficient) speed. Rapid, smooth accelera- tion or deceleration.

2. Control over maximum torque and power.

3. Cushioning effect to reduce shock loads.

4. Smoother reversal of motion.

While you are studying the description of the piston type of motor in the paragraphs below, it may be necessary to refer back to the piston type of pump for a review of the operation and particularly the parts nomenclature.

The operation of the axial-piston motor (fig. 10-16) is similar to that of a radial piston motor. Fluid from the system flows through one of the ports in the valve plate and enters the bores of the cylinder block that are open

Figure 10-16.-Example of a piston type of hydraulic motor.