hydraulic motors & variable
displacement pumps
Hydraulic Motor
A hydraulic motor is a device that
generates rotary motion directly from the hydraulic system. An example
of this is a 4-wheel drive system.
To change the hydraulic pump into a
hydraulic motor, several changes need to be made. One of these changes
is putting a gear box on the shear shaft.
Hydraulic motors give a steady,
continuous torque. They are small and compact.
Vickers Hydraulic
Pump
When the hydraulic pump is connected to a
selector valve, the hydraulic oil coming into the pump pushes down on the
pistons, causing the whole piston assembly to rotate. If this assembly is
connected to a shaft, the shaft will rotate with great rotational speed.
The Vickers
Hydraulic Motor has a gear
reduction box which is attached to the pump to reduce the rotational
speed to a useable range. When a system's energy is conserved, input
torque x input rotational speed = output torque x output rotational
speed. If there are no energy losses in the pump and we decrease the
rotational speed, we increase the torque generated.
This type of motor is used in gun
turrets to get azimuth directional control. The speed of movement
of the turret depends on the rate of hydraulic fluid flow, which can be
controlled by restrictors (see section 1.16 -
Flow Restrictors).
It is also used in operating a radar, since it does not introduce electronic interference due to
the movement of radar.
The hydraulic pump may be used for starting engines, but it is difficult.
Also, it may be used in wing sweep
(a large gear moved by smaller gears attached to the motor shaft).
It is used in winching systems;
for example, to haul up cargo, personnel, casualty baskets and the
like, on helicopters (US Coast Guard rescue helicopters, US Navy Sea
Stallions, US Marines, and US Army Hueys and Chinooks)
The rotation of the motor
lowers the winch
It may be used to spin up the
aircraft's wheels before landing, to get the wheel speed up to the
touchdown speed of the aircraft. This technique may be used to
increase the life of the aircraft's tires.
The motor can be used with variable
displacement pumps for automatic transmissions and could be
operated by a metering selector valve for movement of gun turrets.
Variable Displacement Pumps
Variable displacement pumps work by
having the pump side at an angle to the horizontal. The movement of the
pistons within the pump will draw hydraulic fluid into (or expel fluid
out of) the pump. The amount of fluid drawn into (or
expelled out of) the pump will depend upon the angle the pump makes with
the horizontal. The pump could be controlled by a yoke that moves the
pump to different angles, thus varying the pump displacement.
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