A typical vane-type air motor is shown in
figure 10-13. This particular motor provides
rotation in only one direction. The rotating
element is a slotted rotor which is mounted on
a drive shaft. Each slot of the rotor is fitted with
a freely sliding rectangular vane. The rotor and
vanes are enclosed in the housing, the inner
surface of which is offset from the drive shaft axis.
When the rotor is in motion, the vanes tend to
slide outward due to centrifugal force. The
distance the vanes slide is limited by the shape of
the rotor housing.
This motor operates on the principle of
differential areas. When compressed air is directed
into the inlet port, its pressure is exerted equally
in all directions. Since area A (fig. 10-13) is greater
than area B, the rotor will turn counterclockwise.
Each vane, in turn, assumes the No. 1 and No.
2 positions and the rotor turns continuously. The
potential energy of the compressed air is thus
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 shown in figure 10-14. This motor
operates on the same principle as the vane motor
shown in figure 10-13. 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 rotor. Their purpose is to hold the
vanes against the housing during the initial
Figure 10-13.Vane-type air motor.
Figure 10-14.Vane-type motor.
starting of the motor, since centrifugal force does
not exist until the rotor begins to rotate.
Piston-type motors are the most commonly
used in hydraulic systems. They are basically the
same as hydraulic pumps except they are used to
convert hydraulic energy into mechanical (rotary)
The most commonly used hydraulic motor is
the fixed-displacement piston type. Some
equipment uses a variable-displacement piston
motor where very wide speed ranges are desired.
Although some piston-type motors are
controlled by directional control valves, they
are often used in combination with variable-
displacement pumps. This pump-motor combina-
tion is used to provide a transfer of power between
a driving element and a driven element. Some
applications for which hydraulic transmissions
may be used are speed reducers, variable speed
drives, constant speed or constant torque drives,
and torque converters. Some advantages of
hydraulic transmission of power over mechanical
transmission of power are as follows:
Quick, easy speed adjustment over a wide
range while the power source is operating
at a constant (most efficient) speed. Rapid,
smooth acceleration or deceleration.
Control over maximum torque and power.
Cushioning effect to reduce shock loads.
Smoother reversal of motion.