Figure 10-16.Exploded view of a axial-piston hydraulic speed gear.
to the suction pistons of the pump. The pump is
constantly putting pressure on one side of the
motor and receiving hydraulic fluid from the other
side. The fluid is merely circulated from pump
to motor and back again.
Both of the axial-piston motors described in
this section may be operated in either direction.
The direction of rotation is controlled by the
direction of fluid flow to the valve plate. The
direction of flow may be instantly reversed
without damage to the motor.
Turbines are used in pneumatic systems to
convert kinetic energy of gases to mechanical
energy. Turbines are used to drive electric
generators, to convert mechanical energy into
electrical energy, and to drive pumps to supply
fluid flow in hydraulic systems.
The basic parts of a turbine are the rotor,
which has blades projecting radially from its
periphery; and nozzles, through which the gas is
expanded and directed. The conversion of kinetic
energy to mechanical energy occurs on the blades.
The basic distinction between types of turbines
is the manner in which the gas causes the turbine
rotor to move. When the rotor is moved by a
direct push or impulse from the gas impinging
upon the blades, the turbine is said to be an
impulse turbine. When the rotor is moved by force
of reaction, the turbine is said to be a reaction
Although the distinction between impulse
turbines and reaction turbines is a useful one,
it should not be considered as an absolute
distinction in real turbines. An impulse turbine
uses both the impulse of the gas jet and,
to a lesser extent, the reactive force that results
when the curved blades cause the gas to change
direction. A reaction turbine is moved primarily
by reactive force, but some motion of the rotor
is caused by the impact of the gas against the
The impulse turbine consists essentially of a
rotor mounted on a shaft that is free to rotate in
a set of bearings. The outer rim of the rotor carries
a set of curved blades, and the whole assembly
is enclosed in an airtight case. Nozzles direct the