Primary Hydrolic Power Supply

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SECTION 2

CHAPTER 1

PRIMARY HYDRAULIC POWER SUPPLIES

Contents

Para

1 Introduction6 Self idling power circuit7 Non-self idling power circuit

Hydraulic power supply components8 Self-idling pump (variable delivery)10 Non self-idling (constant delivery) pump11 Automatic cut-out valve (ACOV)

12 ACOV operation14 Accumulator17 Reservoirs18 Unpressurised reservoirs19 Pressurised reservoirs20 Gas pressurised reservoirs21 Hydraulic system pressurised reservoirs22 Pressure relief valve24 Pressure maintaining valve26 Non-return valve (NRV)27 Filters29 Low pressure filter30 High pressure filter31 By-pass filters32 Ground test connections34 Pressure relief valve35 Power supply circuits36 Two pump systems

Figure Page

1 Self-idling pump and hydraulic symbol 4-2-1-42 Non-self idling (constant delivery) pump and hydraulic symbol 4-2-1-53 Automatic cut-out valve (ACOV) and hydraulic symbol 4-2-1-64 ACOV operation 4-2-1-75 Accumulator and hydraulic symbol 4-2-1-86 Unpressurised reservoir and hydraulic symbol 4-2-1-97 Pressurised reservoir hydraulic symbol 4-2-1-98 Gas pressurised reservoir 4-2-1-109 Hydraulically pressurised reservoir 4-2-1-1010 Pressure relief valve (PRV) and hydraulic symbol 4-2-1-1111 Pressure maintaining valve and hydraulic symbol 4-2-1-1212 NRV and hydraulic symbol 4-2-1-13

13 Filter hydraulic symbol 4-2-1-1314 By-pass filter and hydraulic symbol 4-2-1-1415 Ground test connection (Push type) and hydraulic symbol 4-2-1-1516 Pressure relief valve hydraulic symbol 4-2-1-15


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Contents (continued)Figure Page

17 Pressure switch and hydraulic symbol 4-2-1-1618 Single pump self-idling power circuit 4-2-1-1719 Single pump non self-idling power circuit 4-2-1-18


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PRIMARY HYDRAULIC POWER SUPPLIES

INTRODUCTION

1. A power supply circuit controls and governs the hydraulic flows and pressuresrequired to operate the individual hydraulic operating circuits (services) installed on

the aircraft.

2. It would be impractical due to the high flow rates (speed of operation), highpressures required and physical effort needed, to operate the hydraulic systems inflight using a hand pump. Hand pumps fitted to aircraft are normally for groundmaintenance or emergency use. Therefore powered hydraulic pumps are utilised.

3. Aircraft hydraulic systems may have one or more engine driven pumps(EDPs), which are driven by the aircraft engine accessory gearbox. Pumps aresimply fluid movers; they generate a flow of fluid. Pressure is generated when thereis a restriction to flow.

4. A hydraulic pump is a device for the conversion of mechanical energy intohydraulic energy. The mechanical action of the pump traps the fluid within thepumping cavities, transports it through the pump, and forces a flow of fluid into thehydraulic system. By repeating this operation a continuous flow of hydraulic fluid isproduced.

5. Supply circuits fall into two main categories, which are dictated by the type ofpump used:

Self-idling (variable delivery).

Non-self idling (constant delivery) - pumps constantly provide a flow offluid.

Self-idling power circuit

6. In a self-idling power circuit, the pump is self-regulating and provides variableflow at constant pressure to meet the requirements of the system without the needfor an external pressure-regulating device.

Non self-idling power circuit

7. In a non self-idling power circuit, the pump is not self-regulating and provides

a continuous flow of fluid to the pressure line. To prevent over pressurisation, anautomatic cut-out valve (ACOV) is fitted in the circuit to off-load the pump when thepre-determined system pressure has been built up. An accumulator is also fitted inthe circuit to provide a backpressure to ensure that the ACOV operates efficiently.


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HYDRAULIC POWER SUPPLY COMPONENTSSelf-idl ing pump (variable delivery) (figure 1)

8. The most common pumps used in modern aircraft hydraulic systems are ofthe piston type with an integral self-idling facility. This type of pump consists of amultiple bore cylinder block with the cylinder bores parallel to, and symmetrically

spaced around, the pump axis. The cylinder block containing the pistons and thedrive-shaft rotate together. As the cylinder block rotates, the pistons move aroundthe angled yoke and are drawn in and out once per revolution. This action displacesfluid into the system. The angle of the yoke can be varied to increase or decreasepump stroke, so that constant pressure is maintained in the system. This type ofpump is often referred to as a 'swash plate pump'.

9. When system pressure is low, which is the case when a service has beenselected, spring pressure on the control piston moves the yoke to its maximumangle. The pistons move to full stroke and maximum output is delivered to thesystem. As system pressure increases, pressure acting against the control piston

moves the yoke back towards the minimum stroke angle. In this position a smallflow through the pump is maintained to lubricate the mechanical parts, to helpdissipate heat and to ensure a quick response time when a service is operated.

Outlet

Inlet

Figure 1 - Self-idling pump and hydraulic symbol


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Non self-idling (constant delivery) pump (figure 2)

10. The non self-idling pump is similar in design and operation to the self-idlingpump, except that it does not incorporate a control piston assembly. The pumpdisplaces or delivers a constant fluid output for any rotational speed, e.g. 45 litres offluid per minute, (10 gallons/min) at a pump speed of 2800 RPM. As long as the

pump maintains that RPM. it will deliver at that rate, regardless of the pressure in thesystem

Inlet

Outlet

Cylinder

blockEnd view of pump

Fixed angle

swash plate

Drive

shaft

Piston

Figure 2 - Non-self idling (constant delivery) pump and hydraulic symbol


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Automat ic cut -out valve (ACOV) (figure 3)

11 This component is used in power supply circuits, which employ non-self idlingpumps. The ACOV regulates system pressure by offload flow from the pump toreturn when there is no demand from the operating services. The ACOV is sensitiveto system pressure. When system operating pressure has been attained the ACOV

will operate, trapping fluid downstream of the ACOV, and offloading flow back to thereservoir.

ToServices

Relief to Reservoir

From Pump

Flow

NRV

Figure 3 - Automatic cut-out valve (ACOV) and hydraulic symbol


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12. ACOV operation (figure 4) The ACOV operates as follows:

a. Fluid flow from the EDP opens the NRV (B) and flows into the circuit.

b. Springs (F) and (G) keep the spindle valve (D) seated until systempressure is attained.

c. When system (cut-out) pressure is attained, the spindle valve (D)moves to the right, compressing springs (F) and (G).

d. As pressure within the ACOV drops, the NRV (B) closes, aided byspring pressure and the accumulator.

e. Accumulator pressure continues to move the spindle (D) andcompresses spring (G).

f. Pressure acts on the flutter plate (E) displacing it against the spring (F).

g. Fluid then passes to the reservoir via the holes in the flutter plate, thusoffloading flow.

13. As the accumulator pressure falls during initial operation of services, spring(G) pushes the spindle valve (D) towards its seat. When the cut-in pressure isreached, the spindle valve (D) engages its seat; spring (F) returns the flutter plate (E)on to the spindle valve (D), NRV (B) opens and fluid can flow directly from the pumpinto the system.

E

FG

D

CB

Reservoir

EDP

Accumulator

To system

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Figure 4 - ACOV operation


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Accumulator (figure 5)

14. An accumulator is a cylinder and floating piston assembly with openings oneach end. One chamber contains hydraulic fluid at system pressure and the otherchamber contains gas under pressure. Hydraulic system fluid pressure enters oneport and forces the piston against the gas charge.

Note - An accumulator is alwaysfitted in a non-self idling power circuit.

15. As hydraulic system pressure increases, an increase in fluid within theaccumulator acts to move the piston and compress the gas. This will continue untilgas and hydraulic pressures are equal. As hydraulic system pressure decreases,the gas will expand forcing fluid back into the system

16. The purposes of an accumulator include:

Providing an emergency store of hydraulic energy.

Giving initial impetus to hydraulically operated components.

Damping out pump pulsations.

Compensating for temporary drop in supply pressure on selection of aservice.

Preventing 'chatter' in the ACOV.

Compensating for thermal expansion of the fluid.

Gauge

NitrogenConnection Piston

FluidConnections

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Figure 5 Accumulator and hydraulic symbol


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Reservoirs

17. Reservoirs can be divided into two main groups:

Unpressurised.

Pressurised.

18. Unpressurised reservoirs (Figure 6). Unpressurised reservoirs arenormally fitted vertically and higher than the EDPs, relying on gravity to feed thepumps. This type of reservoir is often fitted to transport aircraft and helicopters.

Figure 6 - Unpressurised reservoir and hydraulic symbol

19. Pressurised reservoirs (figure 7) Pressurised reservoirs subject the storedfluid to pressure. This ensures that the pump, regardless of aircraft attitude oraltitude, is always being positively supplied with fluid. Reservoirs are pressurised byone of two methods:

Gas pressure.

Hydraulic system pressure taken from the power supply circuit.

Hydraulic symbol

Figure 7 - Pressurised reservoir hydraulic symbol


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20. Gas pressurised reservoi rs (figure 8). In a gas-pressurised reservoir, onemethod of pressurisation is accomplished using air bled from the engine. An airpressure regulator valve is used to control the engine bleed air to a desired range.Another method involves pre-charging the reservoir with pressurised nitrogen on theground.

Figure 8 - Gas pressurised reservoir

21. Hydraulic system pressurised reservoir (figure 9). The reservoir isconstructed with a piston rod assembly, with two pistons of differing surface areas.System pressure acts upon the small piston in a chamber separated from the mainreservoir body. This pressure generates a force, which is transmitted to the largerpiston within the main reservoir body. The transmitted force acts on the surface areaof the larger piston producing a low pressure within the fluid. As system pressure is

applied to the small piston, the piston assembly tries to move. However, as hydraulicfluid is virtually incompressible, this movement will be small. As the piston assemblymoves it pressurises the fluid in the reservoir. Pressure ratios between the small(inlet) piston and the larger reservoir piston are usually in the order of 60:1. Thismeans that a 3000-psi inlet pressure is reduced to about 50 psi in the main reservoir.

SystemPressure

Return

Supply8145(77

Figure 9 - Hydraulically pressurised reservoir


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Pressure maintaining valve (figure 11)

24. The function of the pressure maintaining valve (also referred to as a pressureregulator or priority valve) is to cut off the supply of fluid to those services deemedto be non-essential and to ensure that the available fluid pressure is directed tothose services deemed to be essential in the event of hydraulic pressure failure.

25. With normal system pressure available, the spring loading is overcome andthe valve opens to allow pressure to all services. If system pressure drops below apredetermined level, the pressure-maintaining valve closes under spring pressure toisolate the non-essential services, thus directing all available pressure to theessential services.

Normal system pressure Low system pressure

Non - Essential

Essential

Figure 11 - Pressure maintaining valve and hydraulic symbol


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30. High pressure fil ter. In a high pressure filter, fluid is forced through the filterelement and any foreign matter will be removed. The effectiveness of a filter isdetermined by the size of the smallest particle that it can remove and is referred toas the filter rating. Particle size is measured in microns and the manufacturerdetermines the filter rating for a particular system. Filters, which have this ability areoften referred to as 'micronic filters'

31. By-pass fil ters (figure 14). In the event of a filter element becoming blockedno fluid would be able to enter the system. To prevent this, many filtersincorporate a relief valve that opens in the event of a blockage to allowunfiltered fluid to by-pass the element. To indicate to the groundcrew that thefilter has become blocked, a tell tale indicator is activated. This indicatortakes the form of a red warning button, which protrudes, from the filterassembly.

FilterHousing

Telltale indicator

Filter element

Figure 14 By-pass filter and hydraulic symbol


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Ground test connections (figure 15)

32. To enable hydraulic system testing without using the engine driven pumps, aground maintenance trolley can be connected to the aircraft hydraulic systemthrough ground test connections. The ground test connections are composed of twohalf couplings; one integral to the aircraft hydraulic system and the other on the

ground maintenance trolley hose.

33. These connections are self-sealing couplings, which allow the maintenancetrolley to be connected quickly and easily. Each coupling is in effect a shut-off valve,which is opened as the male and female portions are mated together. They come ina variety of forms, one of which is illustrated in figure 15.

Aircraft half coupling

Maintenance trolley hose

half coupling

Figure 15 - Ground test connection (Push type) and hydraulic symbol

Pressure release valve (figure 16)34. Before carrying out certain hydraulic maintenance tasks or procedures, it isessential to ensure that system pressure is dissipated. To facilitate this, someaircraft are fitted with pressure release valves. Pressure release valves allowsystem pressure to be dissipated to return and are manually operated.

Figure 16 - Pressure release valve hydraulic symbol


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POWER SUPPLY CIRCUITS

36. All of the components covered so far form the basis of a hydraulic powersupply circuit. A single pump self-idlingpower circuit is shown at figure 18 and asingle pump non self idlingpower circuit is shown at figure 19.

1

2

34 5

6 7

8 9

10

EssentialServices

11

1716

15

14

13

Non-essentialServices

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1

2

3

4

5

6

7

8

9

Reservoir (pressurised)

Ground test connection

Pump (self-idling)

Non return valve

Filter


Pressure switch

Accumulator

Accumulator pressure gauge

10

11

12

13

14

15

16

17

Accumulator charging point

Pressure maintaining valve

Pressure release valve

Pressure relief valve

By-pass filter

Hand pump

Non return valve

Filter

Figure 18 Single pump self-idling power circuit


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1

16

23 4

5

6

1817

1415

13

12

11 10 9

87

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EssentialServices

Non-essentialServices

1

2

3

4

5

67

8

9

Reservoir (pressurised)

Pressure switch

Accumulator

Accumulator pressure gauge

Accumulator charging point

Pressure maintaining valvePressure release valve


Automatic cut out valve (ACOV)

10

11

12

13

14

1516

17

18

Filter

Non return valve

Pump (non-self idling)


Pressure relief valve

By-pass filterHand pump

Non return valve

Filter

Figure 19 Single pump non self-idling power circuit


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36. Two pump systems. It is common for two pumps to be fitted into a circuit, toprovide back up in the event of a pump failure. Under normal conditions both pumpswill operate with a combined flow and a common regulated pressure. However, ifone pump fails:

Pressure remains the same, governed by the ACOV in a non-self idling

system or by the control piston in the remaining pump in a self-idlingsystem.

Flow rate is reduced in a self-idling circuit or halved in a non-self idlingcircuit.

Services operating times may be extended.

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