controlled by either a four-way directional control
valve or a variable-displacement pump.
A fluid power motor is a device that converts
fluid power energy to rotary motion and force.
The function of a motor is opposite that of a
pump. However, the design and operation of
fluid power motors are very similar to pumps.
Therefore, a thorough knowledge of the pumps
described in chapter 4 will help you understand
the operation of fluid power motors.
Motors have many uses in fluid power
systems. In hydraulic power drives, pumps and
motors are combined with suitable lines and valves
to form hydraulic transmissions. The pump,
commonly referred to as the A-end, is driven by
some outside source, such as an electric motor.
The pump delivers fluid to the motor. The motor,
referred to as the B-end, is actuated by this flow,
and through mechanical linkage conveys rotary
motion and force to the work. This type of power
drive is used to operate (train and elevate) many
of the Navys guns and rocket launchers.
Hydraulic motors are commonly used to operate
the wing flaps, radomes, and radar equipment in
aircraft. Air motors are used to drive pneumatic
tools. Air motors are also used in missiles to
convert the kinetic energy of compressed gas into
electrical power, or to drive the pump of a
Fluid motors may be either fixed or variable
displacement. Fixed-displacement motors provide
constant torque and variable speed. The speed is
varied by controlling the amount of input flow.
Variable-displacement motors are constructed so
that the working relationship of the internal parts
can be varied to change displacement. The
majority of the motors used in fluid power
systems are the fixed-displacement type.
Although most fluid power motors are capable
of providing rotary motion in either direction,
some applications require rotation in only one
direction. In these applications, one port of the
motor is connnected to the system pressure line and
the other port to the return line or exhausted to
the atmosphere. The flow of fluid to the motor
is controlled by a flow control valve, a two-way
directional control valve, or by starting and
stopping the power supply. The speed of the
motor may be controlled by varying the rate of
fluid flow to it.
In most fluid power systems, the motor is
required to provide actuation power in either
direction. In these applications the ports are
referred to as working ports, alternating as inlet
and outlet ports. The flow to the motor is usually
Fluid motors are usually classified according
to the type of internal element, which is directly
actuated by the flow. The most common types of
elements are the gear, the vane, and the piston,
AU three of these types are adaptable for hydraulic
systems, while only the vane type is used in
The spur, helical, and herringbone design
gears are used in gear-type motors. The motors
use external-type gears, as discussed in chapter 4.
The operation of a gear-type motor is shown
in figure 10-12. Both gears are driven gears;
however, only one is connected to the output
shaft. As fluid under pressure 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.
This rotary motion of the gears is transmitted
through the attached shaft to the work unit.
The motor shown in figure 10-12 is operating
in one direction; however, the gear-type motor is
capable of providing rotary motion 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 (fig. 10-12)
into chamber B, the gears rotate in the opposite
Figure 10-12.Gear-type motor.