hydraulic pumps

10/2/2014 Pump, Hydraulic Description

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Pump, Hydraulic Description

All hydraulic systems require a source of hydraulic power. In most applications,the source of hydraulic power is a variable delivery axial piston pump. Variabledisplacement means that pump outlet flow varies according to system flowdemands (as more sources actuator, motors, etc. are operating, the pumpwill increase output to maintain maximum pump outlet pressure). Other sourcesof hydraulic power are vane or gear pumps (see Motors, Hydraulic Descriptionfor description of vane or gear rotating pumps/motors), or an accumulator (seeAccumulators, Hydraulic - Description). An aerospace vehicles main hydraulicpumps are usually mounted on the engine and connected to the engine rotatingshaft through a gearbox. Pumps may also be driven by an electric motor, APU,ram air turbine, or second hydraulic system (using a hydraulic motor and pumpcombination).

To understand how a variable delivery axial piston pump operates refer to thepump cross sectional view shown in Figure 1. The figure shows pumpcomponents and also how pump outlet pressure is controlled through acompensator valve and control piston arrangement. The key element in controlof pump outlet flow is control of the swashplate angle, , which in turn controlspiston relative displacement and hence pump flow. Ideally, the pump deliverszero flow when there are no flow demands and the required flow when requiredwhile maintaining system design pressure at all times. Variable displacementpumps dont obtain this ideal goal, but they do come close when flow demandsare within their design flow range.

Prior to the introduction of variable delivery pumps 50-60 years ago, pumps werefixed delivery which means they always delivered the same amount of fluidirregardless of system flow needs. Unused fluid was then ported back to thereservoir though a pressure relief valve. Fixed delivery pumps wasted a lot ofenergy through heat. A fixed delivery pump has a fixed swashplate angle andtherefore no compensator valve or control piston (refer to Figure 1). Fixeddelivery pumps may be found today in very specialized applications where thefixed delivery (or flow) is tailored to the application to minimize wasted energy.The benefit in this case is a lighter, less expensive pump. An example might be astandalone hydraulic system to power a large cargo door where flow demandsare constant while the door is moving and the pump would only be on duringdoor movement. A pressure relief valve should also be installed in thisapplication.

Figure 1 Simplified Pump Schematic

Referring again to Figure 1, pump flow rate is determined by the swashplateangle, , which in turn controls pump piston displacement. Swashplate angle iscontrolled by the compensator valve and control piston. The compensator valvesets the no flow outlet pressure of the pump (e.g., 3000 50 psi) and metersflow to the compensator piston. The compensator is fed hydraulic pressure fromthe pump outlet and is positioned based on a force balance between



compensator chamber pressure acting on the piston and spring force (plus, to alesser extent, friction and flow forces). The housing is fixed and does not rotate.The housing must remain fixed so that inlet/outlet ports, compensator valve,solenoids, and other equipment can be mounted to the pump. All other partsrotate at the pump speed.

At the compensator pressure setting (e.g., 3000 50 psi for a 3000 psi system),the swashplate angle, , is zero. As the pump flow increases, the pump outletpressure decreases. As the outlet pressure decreases, the compensator movestowards the closed position and the pressure on the control piston is reduced. Asthe control piston pressure is reduced, the control piston spring pushes thecontrol piston so that the swashplate angle, , increases, resulting in greaterpiston stoke and increased flow rate.

Figure 2 shows 3-dimensional view of a piston hydraulic pump. This figureprovides a better view of how a pump is built and how the pistons andswashplate operate. Not shown in the figure is a compensator valve and controlpiston, so this is more representative of a fixed delivery pump. The pistons areattached to the swashplate via a spherical bearing arrangement. The swashplatedoes not rotate. As the cylinder rotates the piston is at the lower end of thecylinder during one part of a revolution (intake) and at the top end of the cylinderduring the other of the rotation (high pressure outlet side of the pump). During1 revolution of the swashplate, each piston will pull in fluid and push out highpressure fluid once. For a nine piston pump, this will lead to 9 pressurepulsations per 1 revolution of the swashplate.

Figure 2 Pump Cross Section

The relationship of outlet flow to outlet pressure for a variable delivery pump isshown in Figure 3. This plot can be used to estimate pump flow for a given outletpressure. The plot also shows the flow rate where pump outlet pressure starts todrop off dramatically. It is important to note that Figure 3 represents pumpcharacteristics for a fixed pump rotational speed (RPM) or displacement(in3/rev). Normally these curves are provided for the rated pump speed, but inaircraft engine speeds vary - hence pump speed varies and maximum flowvaries also. Therefore, the curve shown in Figure 3 will shift down for lowerpump speeds and shift up for higher pump speeds up to the maximum flow ofthe pump.

Figure 3 Typical Flow vs Outlet Pressure Plot (for a given pump rotational speed)

The response of the pump to a change in outlet flow is on the order of 50milliseconds.

Figure 4 shows the relationship between pump flow, efficiency and outletpressure. The drop off in flow occurs due to hydraulic fluid leakage at higherpump outlet pressures (higher delta pressure across the piston seals), which isequivalent to volumetric efficiency. As volumetric efficiency drops off, pumpoutlet flow drops by the same amount. The other curve in Figure 4 is the overallefficiency curve (overall efficiency = volumetric efficiency x mechanical



efficiency). Pump horsepower increases linearly with pump speed.

Figure 4 Typical Performance Curves for a Variable Delivery Piston Pump

Pump Design Considerations

The most important characteristics for a hydraulic pump are listed below. Theseparameters assume a variable delivery constant pressure pump.

Rated Pressure this is the nominal pressure setting of the pump and must becompatible with the design operating pressure for the system (e.g., 3000 50 psifor 3000 psi system or 5000 50 psi for a 5000 nominal psi system).

Rated Speed this is the nominal speed rating of the pump. The gearboxconnecting the pump to the driver (engine, APU, etc) will need to be compatiblewith the drive unit speed and the rated pump speed. The minimum RPM of thepump may also need to be considered

Design Displacement this is the flow per revolution of the pump (in3/rev) thatthe pump is capable of achieving without a significant reduction in outletpressure.

Flow vs Pressure Curve this is a plot that has flow rate on the y-axis and pumpoutlet pressure on the x-axis. This plot shows the drop off in pressure at a givenflow conditions and shows where the knee in the flow curve lies (see Figure 4).This graph is required for a simulation model. Nominally, this graph is providedat the rated speed of the pump. If available, this graph for various pump speedswould be helpful - otherwise the flow can be ratioed using the designdisplacement for different pump speeds.

Temperature Rating the pump must be rated for the temperature extremes thatit will see in operation, such as engine nacelles. Pump seals are the most criticalcomponent when considering temperature.

Case Drain Pressure this is the nominal pressure that would be in the pumpcase. All pumps have a case drain line to provide a flow path to the reservoir forhydraulic fluid that flows by the piston seals and fluid that flows through thecompensator. Without a case drain the pressure would blow out a case or shaftseal. The case drain pressure needs to be greater than the reservoir pressure(and line resistance) to assure drainage from the case to the reservoir.

Inlet Line Size Standard pumps will have an inlet port sized by themanufacturer. The connecting inlet line/hose will need to be compatible with theport size and type.

Outlet Line Size - Standard pumps will have an outlet port sized by themanufacturer. The connecting outlet line/hose will need to be compatible with theport size and type.

Case Drain Line Size - Standard pumps will have a case drain port sized by themanufacturer. The connecting line/hose will need to be compatible with the portsize and type.

Recommended Inlet Pressure to operate properly a pump must be suppliedsufficient hydraulic fluid at a pressure level sufficient to fill the piston cylinders asthe pump rotates. A pump manufacturer will provide recommended inletpressures and the reservoir and reservoir to pump hydraulic lines need todesigned/sized to meet this requirement. In some instances, a boost pump maybe required to achieve desired inlet pressures and flows.

Number of Pump Pistons most aircraft piston pumps have 9 pistons. An oddnumber of pistons have been shown to have smaller output pressure fluctuationsthan an even number of pistons and experience has shown 9 pistons to be anoptimum number for performance.

Power Requirements what horsepower is required to drive the pump at itsmaximum operating condition. Horsepower is the product of flow and deltapressure across the pump, divided by the overall efficiency of the pump.



Maximum Fluid Viscosity Pump manufacturers will provide a recommendationfor the maximum recommended fluid viscosity. If fluid viscosity is greater thanthe recommended value, then the pump may start to cavitate with the pump inletat the recommended inlet pressure. Maintaining the fluid within the necessaryviscosity range will affect the pump inlet system design.

Seals Seals must be compatible with the specific type of hydraulic fluid used inthe pump. Specifically, seal material for carbon based fluids and synthetic fluidsare different.

Filtration Requirements A pump manufacturer will provide recommended fluidcleanliness requirements to help ensure reliable pump operation. This filtrationrequirement needs to be taken into consideration when designing/sizing thereturn filters in the hydraulic system.

Weight Is always a concern on aircraft. A oversized pump is not desirable fromboth a weight and cost standpoint.

Envelope engine nacelles and APU installations usually have limited spaceavailable and the pump installation must be compatible with available volumes

Shaft Type Shafts are usually splined and the spline characteristics must bedefined to ensure a proper interface to the

Direction of Rotation Pumps can rotate either clockwise or counterclockwiseand may be of a concern in certain installations.

Mounting The bolt flange mounting of the pump must be compatible with theattachment on the gearbox or other attaching plate.

Relief Valve A relief valve should be installed in the outlet (high pressure) linedownstream of the pump. This relief valve will provide protection againsthydraulic shock loads, thermal expansion and any possible overpressurecondition. The relief valve setting should be 5-10% greater than maximum pumppressure.

Pump Installation Considerations

Considerations for the mounting/installation of pumps include vibration,temperature, alignment of drive motor to pump, spline matching and torquerequirements. In some applications, a means of quick installation and removal isrequired. Quick installation devices must uphold rigidity of the pump installationand maintain alignment of rotating shaft.

Vibration Need to consider vibration from power source such as the engine orAPU, vibration characteristics from the pump, g-loading and possibly flutter.Mounting should be sufficient to withstand these loads from both a stress andfatigue standpoint. Testing to appropriate levels from RTCA-DO160 or MIL-STD-810, Method 510 should be conducted.

Temperature Due to the high speed and compression of fluid, pumps operateat high temperatures. Additionally, the power source for the pump, such as theengine, is at high temperature. Temperature considerations should include pumpseals, fluid temperature, mounting pads, connecting hoses or tubes, etc.

Pump/Motor Alignment Alignment of pump to drive motor splines needs to heldto tight tolerances. Considerations are tolerance stickups, relative motionbetween drive motor and pump, possible angular displacements on installations,spline teeth dimensions, etc. Improper alignment can cause excessive vibration(leading to premature failure), or failure of the pump shaft seal.

Splines Beyond alignment, spline wear is an important consideration. Usuallysome lubrication (grease) is applied to the splines to minimize wear. Selection oflubrication should include temperature, corrosion inhibiting and reasonable life ofthe grease before breakdown occurs.

Torque Requirements Both start-up torque and running torque should beconsidered. Start-up torque is higher than running torque. Start-up torqueaccelerates the mass/inertia of the pump and fluid, leading to temporary highstresses within the mounting hardware and pump. This is more of a concern onAPU and RAT installations. Obviously, the speed of the drive motor must matchthe manufacturers recommended speed for the pump (usually accomplishedthrough a gear box).

Axial and Radial Shaft Load Capability Ensures pump shaft and splines areadequately sized for static and fatigue loads that the pump will see over itsoperating life.

Case Drain Line A case drain line is installed to drain pump leakage flow backto the reservoir. Case drain back pressures affect seals and bearings (via loadbalance across them), balance and loading of the pump rotating hardware, andpiston leakage characteristics. In most aircraft installations, the case drain lineback pressure is equal to the reservoir pressure (which is approximately thepump inlet pressure). This minimizes leakage on the intake side of the pump.Normally, back pressure from the case drain line is not an issue, but if back



pressures are abnormally high (such as clogged filter), the effects on the pumpshould be looked at more closely. Case drain pressure < 150 psi is a rule ofthumb for good pump life. The case drain line must be large enough to cover themaximum case drain flow at the nominal case drain pressure or even a slightlylower pressure. The case drain line will normally flow back to return through thereturn line filters. In some applications, a separate filter is used on the case drainline.

Inlet Line The inlet line to the pump is designed as part of the pump inletsystem. The pump inlet system consist of reservoir and reservoir pressurizationand tubes/hoses from the pressure to the pump. This system should bedesigned to ensure fluid is provided to the pump inlet within the inlet pressurerange and viscosity recommended by the manufacturer. In sizing the inlet line,the length of the tubing, bends, height fluid is pumped, reservoir pressurization,additional components (such as a heat exchanger) in the line and other factorsshould be taken into account. Sizing of a pump inlet line uses basic pipe flowequations and reservoir (or supply) pressurization (see Reservoir, Hydraulic Description).

Outlet Line The pump outlet line should be sized to system pressure droprequirements and to minimize affects of pump pressure pulsations. Primaryconsiderations in design of the outlet line are pump pressure pulsations,accumulators (see Accumulator, Hydraulic - Description), pump system responseand parallel pump installation.

Regarding pressure fluctuations, hydraulic fluid has mass and is compressible.Hence the oil in the pump downstream tubing behaves like a very stiff spring,with variable stiffness as the downstream configuration changes. Pulsations area result of each piston within the pump transferring a discrete amount of fluid tothe system, leading to a pulsed input in the hydraulic system. The flow pulsesdecay over time from the damping provided by internal flow friction in thedownstream tubing and components. The pulsation frequencies for a oddnumbered piston pump are

Example: For a pump running at 2700 rpm with 9 pistons

For aircraft systems, both pulsation frequencies are usually above the responsefrequency of the downstream components, however, in some cases the effectsmay need to be analyzed. The lower frequency is usually more dominant withpump noise, but both should be analyzed.

System Interaction

System interaction occurs when the natural frequency of the pump compensatoris at or near the natural frequency of a downstream component (such as a servovalve or actuator). Generally there is sufficient difference between the naturalfrequencies so that system interactions do not occur.

Another source of interaction occurs when pumps are connected in a parallelarrangement. This interaction can be stopped by installation of check valves inthe outlet lines of each pump.

Pump Pulsation Damping

Several methods exist to dampen the effect of pressure pulsations from a pump:

1. Change configuration (geometry, parts, characteristics, etc.)

2. Increase volumes in pump outlet line

3. Install accumulator close to the pump. Some research shows that for theaccumulator to be effective, it should be installed with 0.3 meters of the pump, andthe supply line between the main line and accumulator should be between 5 and 10centimeters in length. Also, the volume of the gas accumulator should be sufficientso that its resonance (response) frequency is less than the pump pulsationfrequency.



4. Install a hose at the pump outlet, or downstream plumbing.

5. Install a Helmholtz resonator (H-filter) in the pump outlet line. A H-filterconsists of two lines in series, of different volumes, that branch away from themain line. By properly selecting the lengths and cross-sectioal area of bothlines, the H-filter can be tuned to a specific frequency.

6. Install a Quincke tube in the main line. The Quincke tube is a side line withareas based on main line area and lengths sized for a specific frequency.

Pump Qualification

See Qualification - Hydraulic Components for discussion on hydraulic pumpqualification and required certification testing.

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