inch. In a 2,000-psi system, pressure acting against the
blank side of the piston creates a force of 6,000 pounds
(2,000 x 3). When the pressure is applied to the rod side
of the piston, the 2,000 psi acts on 2 square inches (the
cross-sectional area of the piston less the cross-sectional
area of the rod) and creates a force of 4,000 pounds
(2,000 x 2). For this reason, this type of cylinder is
normally installed in such a manner that the blank side
of the piston carries the greater load; that is, the cylinder
carries the greater load during the piston rod extension
A four-way directional control valve is normally
used to control the operation of this type of cylinder. The
valve can be positioned to direct fluid under pressure to
either end of the cylinder and allow the displaced fluid
to flow from the opposite end of the cylinder through
the control valve to return/exhaust.
A fluid power motor is a device that converts fluid
power to rotary motion and force. Basically, the function
of a motor is just the opposite as that of a pump;
however, the design and operation of fluid power motors
are very similar to pumps. In fact, some hydraulic pumps
can be used as motors with little or no modifications;
therefore, your having a thorough knowledge of the
pumps will be extremely helpful to you in understanding
the operation of fluid power motors.
Motors serve many applications in fluid power
systems. In hydraulic power drives, pumps and motors
are combined with suitable lines and valves to form
hydraulic systems. The pump, commonly referred to as
the A-end, is driven by some outside source, such as a
diesel or gasoline engine. 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.
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, vane, and piston. All three of these types are
adaptable for hydraulic systems, while only the vane
type is used in pneumatic systems.
GEAR TYPE. The gears of the gear-type motor
are of the external type and may be of the spur, helical,
or herringbone design. These designs are the same as
those used in gear pumps.
The operation of a gear-type motor is illustrated in
figure 10-14. 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 return. This
rotary motion of the gears is conveyed through the
attached shaft to the work unit.
Although the motor illustrated in figure 10-14
shows operation in only one direction, the gear-type
motor is capable of providing rotary motion in either
direction. The ports alternate as inlet and outlet, To
reverse the direction of rotation, the fluid is directed
through the port-labeled outlet, into chamber B. The
flow through the motor rotates the gears in the
opposite direction, thus actuating the work unit
VANE TYPE. A typical vane-type air motor is
illustrated in figure 10-15, view A. This particular motor
provides rotation in only one direction. The rotating
element is a slotted rotor mounted on a drive shaft. Each
slot of the rotor is fitted with a freely sliding rectangular
vane. The rotor and vane are enclosed in the housingthe
inner surface of which is offset with the drive shaft axis.
When the rotor is in motion, the vanes tend to slide
outward because of 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 is greater than area B, the rotor will turn
counterclockwise. Each vane, in turn, assumes the No.
1 and No. 2 position and the rotor turns continuous y.
The potential energy of the compressed air is thus
Figure 10-14.-Example of a gear-type of hydraulic motor.