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Section 1.19 - 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 (see section 1.8-Aircraft Hydraulic System Power Pumps).

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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Updated: March 12, 2004