to the inlet port. (For example, in a nine piston motor,
four cylinder bores are receiving fluid while four are
discharging.) The fluid acting on the pistons in those
bores forces the pistons to move 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. The pistons move in this direction to a
point around the shaft axis, which is the greatest distance
from the valve plate. Therefore, driving the pistons
axially causes them to rotate the drive shaft and cylinder
block. While some of the pistons are being driven by
liquid flow from the system, others are discharging flow
from the outlet port.
This type of motor may be operated in either
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. This design is found mainly on
construction equipment as an auxiliary drive motor.
The speed of the rotation of the motor is controlled
by the length of the piston stroke in the pump. When the
pump is set to allow a full stroke of each piston, each
piston of the motor must move an equal distance. In this
condition, the speed of the motor will equal that of the
pump. If the tilting plate of the pump (normally called a
swash plate or hanger assembly) is changed so that the
piston stroke of the pump is only one half as long as the
stroke of the motor, it will require the discharge piston
one full stroke; therefore, in this position of the plate,
the motor will revolve just one half as fast as the pump.
If there is no angle on the tilting plate of the pump, the
pumping pistons will not move axially, and liquid will
not be delivered to the motor; therefore, the motor will
deliver no power.
If the angle of the tilting plate is reversed, the
direction of flow is reversed. Liquid enters the motor
through the port by which it was formally discharged.
This reverses the direction of rotation of the motor.
An additional benefit to this axial-piston
pump/axial-piston motor configuration is the dynamic
braking effect created when the motor, in a coasting
situation, in effect, becomes a pump itself and attempts
to reverse-rotate the hydraulic pump. In this situation
the pump now becomes a motor and attempts to
reverse-rotate the prime mover. The degree of reverse
angle on the tilting plate in the pump determines the
effectiveness of the dynamic braking.
Once the pump has begun to move the fluid in a
hydraulic system, valves are usually required to control,
monitor, and regulate the operation of the system. While
the pump is recognized as the heart of the system, the
valves are the most important devices for providing
flexibility in todays complex hydraulic systems.
Valves are included in a hydraulic system to control
primarily (1) the direction of fluid flow, (2) the volume
of fluid going to various parts of the system, and (3) the
pressure of the fluid at different points in the system.
It is beyond the scope of this training manual to
cover all of the many different valves in use today;
however, since most of these valves are almost always
combinations and elaborations of basic types, an
understanding of their operation can be obtained by a
review of the basic types.
The basic valves are those designed to do one of the
primary functions mentioned above; that is, control
direction of flow, control volume, and regulate fluid
Valves, like pumps, are precision made. Usually, no
packing is used between the valve element and the valve
seat since leakage is reduced to a minimum by machined
clearances. (Packing is required around valve stems,
between lands of spool valves, etc.) Here again is an
important reason for preventing system contamination.
Even the most minute particle of dirt, dust, and lint can
do considerable damage to hydraulic valves.
A relief valve is a simple pressure-limiting device.
It is incorporated in most hydraulic systems and acts as
a safety valve, used to prevent damage to the system in
case of overpressurization.
A simple two-port relief valve is shown in figure
10-17. An adjustment is provided so that the valve may
be regulated to any given pressure; therefore, it can be
used on a variety of systems. Before the system pressure
can become high enough to rupture the tubing or damage
the system units, it exceeds the pressure required to
overcome the relief valve spring setting. This pushes the
ball off its seat and bypasses excess fluid to the reservoir.
If the system pressure decreases, the spring setting
reseats the ball; the ball then remains seated until the
pressure again reaches the predetermined maximum.