Hydraulic systems

Although the hydraulic systems used aboard vary widely. They may, however, be grouped into the three basic set ups.

Constant flow

For a constant flow pump it is possible to vary the flow to a load by varying the outlet from the load.

The major components are the fixed delivery pump, unloading valve, and accumulator.

The accumulator consists of a steel pressure vessel within which is a rubber bag. The space between the bag and the steel walls is filled with nitrogen at a set pressure. When the constant delivery pump is started, oil is supplied to the accumulator inflating the bag with oil against the nitrogen pressure.

When the set system pressure is reached the unloader valve will open maintaining system pressure at a set amount. The check valve maintains the pressure in the accumulator giving a ready reserve on demand. An alternative to this is to have a pressure cut out switch for the pump with a suitable dead band to prevent cycling.

When a throttle or control valve is opened the accumulator supplies oil to the load, the pressure above the check valve drops, the valve opens, the unloading valve begins to close and oil is supplied through the check valve.

A common use for this system is for valve opening/closing.

Constant pressure

Uses one or more variable delivery pumps which supply oil at nearly constant pressure to either a system of multiple loads or a single load such as a hydraulic elevator hoist.

When the pumping capacity exceeds load requirements, the system pressure increases above a set value, at which point the pressure compensator acts to take the pump off stroke. A relief valve is fitted in case of malfunction of the compensator.

Fluid flow to the load may be controlled by a variety of methods one of which is the simple three position valve shown.

This system suits an installation containing several high demand units such as deck winch hydraulics

Shown is a pump typical of the service described. The Flow Limiter acts to restrict the maximum movement of the Swash plate.The maximum pressure adjustement acts on the set point spring for the swash plate positioning control. The Torque limiter takes both the Pressure and Flow (Swash Plate Position)signals in calculating torque

Demand system

The demand hydraulic system is a closed loop system particularly adapted to meet the precise demands of varying loads such as steering gear and automatic weapons handling equipment. The variable and reversible delivery pump does not take suction from a sump tank, as a loop of oil is maintained between the main pump and the load. The closed loop eliminates pressure drop which would exist if the load control were achieved by a servo valve located in the main hydraulic piping. A servo pump and replenishing pump are driven off the same shaft as the main pump, provide actuating fluid to stroke the main pump and to provide make up fluid to the closed loop, respectively. Control of the main pump is accomplished by command and feedback signals. The command and feedback signals may be composed of a combination of mechanical, electrical, or hydraulic devices.

This system is able to deliver a reasonably precise flow of oil which makes it suitable for equipment where finite speed control is essential. An example other than those given above could be an anchor handling windlass.