often used in combination with variable
of the pump is only half as long as the stroke of the
displacement pumps. This pump-motor
m o t o r, i t w i l l r e q u i r e t wo f u l l s t r o ke s o f t h e
combination (hydraulic transmission) is used to
discharge piston to produce one full stroke of the
provide a transfer of power between a driving
motor. In this position of the plate, the motor will
element (for example, an electric motor or gasoline
revolve only half as fast as the pump. If there is no
e n g i n e ) a n d a d r ive n e l e m e n t . S o m e o f t h e
angle on the pump's tilting plate, the pumping
applications for which hydraulic transmissions may
pistons will not move axially, and liquid will not be
be used are speed reducers, variable speed drives,
delivered to the motor. Thus, the motor will not
constant speed or constant torque drives, and torque
c o nve r t e r s . S o m e a d va n t a g e s o f h y d r a u l i c
If the angle of the tilting plate is reversed, the
transmission of power over mechanical transmission
direction of flow is reversed. Then, liquid enters the
of power are as follows:
motor through the port by which it was formally
· Quick, easy speed adjustment over a wide range
discharged, reversing the direction of rotation of the
while the power source is operating at constant
( m o s t e ffi c i e n t ) s p e e d . R a p i d , s m o o t h
A n a d d i t i o n a l b e n e fi t t o t h i s a x i a l p i s t o n
pump/axial piston motor configuration is the dynamic
· Control over maximum torque and power.
braking effect created when the motor, in a coasting
situation, in effect, becomes a pump itself and attempts
to reverse-rotate the hydraulic pump. The pump then
· Smoother reversal of motion.
becomes a motor and attempts to reverse-rotate the
prime mover. The degree of reverse angle on the tilting
You may find it useful to refer back to the
plate in the pump determines the effectiveness of the
piston-type pump shown in figure 8-17 to review its
dynamic braking effect. This type of drive motor is
operation and component parts.
used in the propulsion systems of A/S32A-32 tow
tractors (spotting dollies).
The operation of the axial piston motor shown in
figure 8-42 is similar to that of an axial piston pump.
Detailed descriptions and illustrations of different
Fluid from the system flows through one of the ports in
types of hydraulic motors can be found in Fluid Power,
the valve plate and enters the bores of the cylinder
block that are open to the inlet port. The fluid acting on
Q8-16. The most common type of elements found in a
the pistons in those bores forces the pistons to move
hydraulic motor is gears and pistons?
away from the valve plate. Since the pistons are held by
connecting rods at a fixed distance from the output
shaft flange, they can move away from the valve plate
only by moving in a rotary direction. This causes the
Q8-17. What is the most common type of motor used
drive shaft and cylinder block to rotate. While some of
in a hydraulic system?
the pistons are being driven by liquid flow from the
A gear type
A spur type
A piston type
This type of motor may be operated in either
A vein type
direction of rotation. The direction of rotation is
controlled by the direction of flow to the valve plate.
The direction of flow may be instantly reversed
without damage to the motor.
LEARNING OBJECTIVE: Identify the
The speed of rotation of the motor is controlled
components of a hydraulic system.
by the length of the piston stroke in the pump. When
the pump is set to allow a full stroke of each piston,
In spite of the great variety of support equipment,
each piston of the motor must move an equal
all hydraulic systems, from the simplest to the most
distance. In this condition, the speed of the motor
complex, operate according to basic principles.
equals that of the pump. However, if the tilting plate
of the pump (sometimes called a swashplate or
As an AS, you are responsible for analyzing the
hanger assembly) is changed so that the piston stroke
malfunctions of hydraulic equipment ranging from the