hydraulic systems.doc

8/10/2019 Hydraulic systems.doc

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Hydraulic systemsAlthough the hydraulic systems used aboard vary widely. They may, however, begrouped into the three basic set ups.

Constant flow

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

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


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

When the set system pressure is reached the unloader valve will openmaintaining system pressure at a set amount. The check valve maintains the

pressure in the accumulator giving a ready reserve on demand. An alternative tothis 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 suppliesoil to the load, the pressure above the check valve drops, the valve opens, theunloading valve begins to close and oil is supplied through the check valve.

A common use for this system is for valve openingclosing.

Constant pressure


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!ses one or more variable delivery pumps which supply oil at nearlyconstant pressure to either a system of multiple loads or a single load such as ahydraulic elevator hoist.

When the pumping capacity exceeds load re"uirements, the systempressure increases above a set value, at which point the pressure compensatoracts 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 unitssuch as deck winch hydraulics

Demand system

The demand hydraulicsystem is a closed loop system particularly adapted to meet the precise demands


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of varying loads such as steering gear and automatic weapons handlinge"uipment. The variable and reversible delivery pump does not take suction froma 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 controlwere achieved by a servo valve located in the main hydraulic piping. A servopump 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 tothe closed loop, respectively. #ontrol of the main pump is accomplished by

command and feedback signals. The command and feedback signals may becomposed of a combination of mechanical, electrical, or hydraulic devices.

This system is able to deliver a reasonably precise flow of oil whichmakes it suitable for e"uipment where finite speed control is essential. Anexample other than those given above could be an anchor handling windlass.

hydraulic systems.doc

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