T.S. Patriot State Engineering Manual

Patriot State was the training ship of the Massachusetts Maritime Academy from 1986 to 1998.

Compensated Hydraulic Pump Unit

The compensated hydraulic pump unit consists of a Vickers Vane type hydraulic pump directly driven by a 3/4 HP motor, sump, check valve, variable flow control and pressure valve. A separate motor controller, with low voltage and overload protection is provided for starting the pump motor.

The function of the compensated hydraulic pump unit is to provide hydraulic oil pressure for operating the linear hydraulic power units .

The pump is a rotary, single stage unit consisting of sliding vanes, compensating cam ring, rotor and housing.

The rotor has a series of slots into which are fitted sliding vanes. As the rotor turns, the vanes are thrown outward by a combination of hydraulic pressure and centrifugal force which holds the vanes in contact with the casing or cam ring. The cam ring is offset by the pressure of the governor piston which is a spring type pressure compensator used to control maximum system pressure.

The cam ring is allowed to move only in the horizontal plane. An aligning block is provided to insure smooth movement and prevent binding of the cam. Figure A on the previous page shows the cam ring in the maximum flow position. The cam ring being offset, causes the vanes on one side to slide out further, thereby, pulling hydraulic oil from the sump into the inlet port and into the pump. Notice in figure A that the pressure is near minimum and this low pressure causes the pressure control or compensator spring to stroke the cam ring to full flow position and maximum oil flow is now provided.

As load is placed on the pump several actions take place simultaneously. As system load increases, oil pressure will increase, while there is a tendency for flow to decrease (figure B). As pressure (load) is increased, the pressure of the oil within the surrounding cam ring builds up on the side opposite the governor piston. As oil begins to flow in this area, the pressure starts to decreases, but is still greater than the force exerted by the piston spring. Oil flow around the rotor decreases. The cam ring will continue to move laterally in the direction of the governor piston until the pressure within the pump system is opposed by an equal force, derived from the governor piston spring (compensator spring.) At this time, a maximum pressure exists within the system with a minimum oil flow (figure C) Note that the cam ring is as a nearly neutral position, supplying only leakage losses at the set pressure.

The spring-type pressure compensator or governor spring is used to control maximum system pressure by adjusting the tension on the spring. Pump displacement is changed automatically to supply the exact rate of flow required by system. If the pump displacement changes, system pressure remains nearly constant at the value selected by the pressure control setting. Any flow rate from zero to maximum is automatically delivered to the system to match the circuit demands precisely, by the balance of reaction pressure and compensator spring force. This reduces horsepower consumption as flow rate is reduced. By-passing of pressure oil does not occur and excess heat is not generated which are important factors in the efficiency of the circuit.

The volume control is adjusted to afford a minimum of flow. This control (figure A) which when positioned in or out, controls the amount of hydraulic oil flow. A check valve of the spring loaded type and is located in the return line from the power unit. The check valve maintains the required back pressure of 50-100 psi necessary for operation of the directional and bypass valves. Located in the linear hydraulic power units.

The motor driving the Vickers Vane pump is a drip-proof marine underdeck type operating at 850 RPM and directly coupled to the pump.

A 10 gallon sump tank on which the motor and pump are mounted contains suitable baffles and is designed to eliminate oil spillage under conditions of maximum roll. Clean out doors are provided on either end of the sump to facilitate access for cleaning. An oil level indicator is provided on the side of the sump and two magnets are suspended in the hydraulic fluid to separate metallic particles from the hydraulic fluid. When renewing the hydraulic oil use premium grade hydraulic oil 200-250 seconds, Saybolt Universal 100° F minimum viscosity index of 90.

Direct comments to William Haynes whaynes@maritime.edu
Mon, Jul 1, 1996
TSPS Engineering Manual ©1995 Massachusetts Maritime Academy