All hydraulic systems require a source of hydraulic power. In most applications, the source of hydraulic power is a variable delivery axial piston pump. Variable displacement means that pump outlet flow varies according to system flow demands (as more sources – actuator, motors, etc. – are operating, the pump will increase output to maintain maximum pump outlet pressure). Other sources of hydraulic power are vane or gear pumps (see Motors, Hydraulic – Description for description of vane or gear rotating pumps/motors), or an accumulator (see Accumulators, Hydraulic - Description). An aerospace vehicle’s main hydraulic pumps are usually mounted on the engine and connected to the engine rotating shaft 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 pump combination).
To understand how a variable delivery axial piston pump operates refer to the pump cross sectional view shown in Figure 1. The figure shows pump components and also how pump outlet pressure is controlled through a compensator valve and control piston arrangement. The key element in control of pump outlet flow is control of the swashplate angle, α, which in turn controls piston relative displacement and hence pump flow. Ideally, the pump delivers zero flow when there are no flow demands and the required flow when required while maintaining system design pressure at all times. Variable displacement pumps don’t obtain this ideal goal, but they do come close when flow demands are within their design flow range.
Prior to the introduction of variable delivery pumps 50-60 years ago, pumps were fixed delivery which means they always delivered the same amount of fluid irregardless of system flow needs. Unused fluid was then ported back to the reservoir though a pressure relief valve. Fixed delivery pumps wasted a lot of energy through heat. A fixed delivery pump has a fixed swashplate angle and therefore no compensator valve or control piston (refer to Figure 1). Fixed delivery pumps may be found today in very specialized applications where the fixed 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 a standalone hydraulic system to power a large cargo door where flow demands are constant while the door is moving and the pump would only be on during door movement. A pressure relief valve should also be installed in this application.
Figure 1 Simplified Pump Schematic
Referring again to Figure 1, pump flow rate is determined by the swashplate angle, α, which in turn controls pump piston displacement. Swashplate angle is controlled by the compensator valve and control piston. The compensator valve sets the no flow outlet pressure of the pump (e.g., 3000 ± 50 psi) and meters flow to the compensator piston. The compensator is fed hydraulic pressure from the pump outlet and is positioned based on a force balance between compensator chamber pressure acting on the piston and spring force (plus, to a lesser 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 parts rotate at the pump speed.
At the compensator pressure setting (e.g., 3000 ± 50 psi for a 3000 psi system), the swashplate angle, α, is zero. As the pump flow increases, the pump outlet pressure decreases. As the outlet pressure decreases, the compensator moves towards the closed position and the pressure on the control piston is reduced. As the control piston pressure is reduced, the control piston spring pushes the control piston so that the swashplate angle, α, increases, resulting in greater piston stoke and increased flow rate.
Figure 2 shows 3-dimensional view of a piston hydraulic pump. This figure provides a better view of how a pump is built and how the pistons and swashplate operate. Not shown in the figure is a compensator valve and control piston, so this is more representative of a fixed delivery pump. The pistons are attached to the swashplate via a spherical bearing arrangement. The swashplate does not rotate. As the cylinder rotates the piston is at the lower end of the cylinder during one part of a revolution (intake) and at the top end of the cylinder during the other ½ of the rotation (high pressure outlet side of the pump). During 1 revolution of the swashplate, each piston will pull in fluid and push out high pressure fluid once. For a nine piston pump, this will lead to 9 pressure pulsations per 1 revolution of the swashplate.
Figure 2 Pump Cross Section
The relationship of outlet flow to outlet pressure for a variable delivery pump is shown in Figure 3. This plot can be used to estimate pump flow for a given outlet pressure. The plot also shows the flow rate where pump outlet pressure starts to drop off dramatically. It is important to note that Figure 3 represents pump characteristics for a fixed pump rotational speed (RPM) or displacement (in3/rev). Normally these curves are provided for the rated pump speed, but in aircraft engine speeds vary - hence pump speed varies and maximum flow varies also. Therefore, the curve shown in Figure 3 will shift down for lower pump speeds and shift up for higher pump speeds up to the maximum flow of the 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 50 milliseconds.
Figure 4 shows the relationship between pump flow, efficiency and outlet pressure. The drop off in flow occurs due to hydraulic fluid leakage at higher pump outlet pressures (higher delta pressure across the piston seals), which is equivalent to volumetric efficiency. As volumetric efficiency drops off, pump outlet flow drops by the same amount. The other curve in Figure 4 is the overall efficiency 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. These parameters assume a variable delivery constant pressure pump.
Rated Pressure – this is the nominal pressure setting of the pump and must be compatible with the design operating pressure for the system (e.g., 3000 ± 50 psi for 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 gearbox connecting the pump to the driver (engine, APU, etc) will need to be compatible with the drive unit speed and the rated pump speed. The minimum RPM of the pump may also need to be considered
Design Displacement – this is the flow per revolution of the pump (in3/rev) that the pump is capable of achieving without a significant reduction in outlet pressure.
Flow vs Pressure Curve – this is a plot that has flow rate on the y-axis and pump outlet pressure on the x-axis. This plot shows the drop off in pressure at a given flow 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 provided at the rated speed of the pump. If available, this graph for various pump speeds would be helpful - otherwise the flow can be ratioed using the design displacement for different pump speeds.
Temperature Rating – the pump must be rated for the temperature extremes that it will see in operation, such as engine nacelles. Pump seals are the most critical component when considering temperature.
Case Drain Pressure – this is the nominal pressure that would be in the pump case. All pumps have a case drain line to provide a flow path to the reservoir for hydraulic fluid that flows by the piston seals and fluid that flows through the compensator. Without a case drain the pressure would blow out a case or shaft seal. 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 the manufacturer. The connecting inlet line/hose will need to be compatible with the port size and type.
Outlet Line Size - Standard pumps will have an outlet port sized by the manufacturer. The connecting outlet line/hose will need to be compatible with the port size and type.
Case Drain Line Size - Standard pumps will have a case drain port sized by the manufacturer. The connecting line/hose will need to be compatible with the port size and type.
Recommended Inlet Pressure – to operate properly a pump must be supplied sufficient hydraulic fluid at a pressure level sufficient to fill the piston cylinders as the pump rotates. A pump manufacturer will provide recommended inlet pressures and the reservoir and reservoir to pump hydraulic lines need to designed/sized to meet this requirement. In some instances, a boost pump may be required to achieve desired inlet pressures and flows.
Number of Pump Pistons – most aircraft piston pumps have 9 pistons. An odd number of pistons have been shown to have smaller output pressure fluctuations than an even number of pistons and experience has shown 9 pistons to be an optimum number for performance.
Power Requirements – what horsepower is required to drive the pump at it’s maximum operating condition. Horsepower is the product of flow and delta pressure across the pump, divided by the overall efficiency of the pump.
Maximum Fluid Viscosity – Pump manufacturers will provide a recommendation for the maximum recommended fluid viscosity. If fluid viscosity is greater than the recommended value, then the pump may start to cavitate with the pump inlet at the recommended inlet pressure. Maintaining the fluid within the necessary viscosity range will affect the pump inlet system design.
Seals – Seals must be compatible with the specific type of hydraulic fluid used in the pump. Specifically, seal material for carbon based fluids and synthetic fluids are different.
Filtration Requirements – A pump manufacturer will provide recommended fluid cleanliness requirements to help ensure reliable pump operation. This filtration requirement needs to be taken into consideration when designing/sizing the return filters in the hydraulic system.
Weight – Is always a concern on aircraft. A oversized pump is not desirable from both a weight and cost standpoint.
Envelope – engine nacelles and APU installations usually have limited space available and the pump installation must be compatible with available volumes
Shaft Type – Shafts are usually splined and the spline characteristics must be defined to ensure a proper interface to the
Direction of Rotation – Pumps can rotate either clockwise or counterclockwise and may be of a concern in certain installations.
Mounting – The bolt flange mounting of the pump must be compatible with the attachment on the gearbox or other attaching plate.
Relief Valve – A relief valve should be installed in the outlet (high pressure) line downstream of the pump. This relief valve will provide protection against hydraulic shock loads, thermal expansion and any possible overpressure condition. The relief valve setting should be 5-10% greater than maximum pump pressure.
Pump Installation Considerations
Considerations for the mounting/installation of pumps include vibration, temperature, alignment of drive motor to pump, spline matching and torque requirements. In some applications, a means of quick installation and removal is required. Quick installation devices must uphold rigidity of the pump installation and maintain alignment of rotating shaft.
Vibration – Need to consider vibration from power source such as the engine or APU, vibration characteristics from the pump, g-loading and possibly flutter. Mounting should be sufficient to withstand these loads from both a stress and fatigue 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 operate at high temperatures. Additionally, the power source for the pump, such as the engine, is at high temperature. Temperature considerations should include pump seals, fluid temperature, mounting pads, connecting hoses or tubes, etc.
Pump/Motor Alignment – Alignment of pump to drive motor splines needs to held to tight tolerances. Considerations are tolerance stickups, relative motion between 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. Usually some lubrication (grease) is applied to the splines to minimize wear. Selection of lubrication should include temperature, corrosion inhibiting and reasonable life of the grease before breakdown occurs.
Torque Requirements – Both start-up torque and running torque should be considered. Start-up torque is higher than running torque. Start-up torque accelerates the mass/inertia of the pump and fluid, leading to temporary high stresses within the mounting hardware and pump. This is more of a concern on APU and RAT installations. Obviously, the speed of the drive motor must match the manufacturer’s recommended speed for the pump (usually accomplished through a gear box).
Axial and Radial Shaft Load Capability – Ensures pump shaft and splines are adequately sized for static and fatigue loads that the pump will see over it’s operating life.
Case Drain Line – A case drain line is installed to drain pump leakage flow back to the reservoir. Case drain back pressures affect seals and bearings (via load balance across them), balance and loading of the pump rotating hardware, and piston leakage characteristics. In most aircraft installations, the case drain line back pressure is equal to the reservoir pressure (which is approximately the pump 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 pump should be looked at more closely. Case drain pressure < 150 psi is a rule of thumb for good pump life. The case drain line must be large enough to cover the maximum case drain flow at the nominal case drain pressure or even a slightly lower pressure. The case drain line will normally flow back to return through the return line filters. In some applications, a separate filter is used on the case drain line.
Inlet Line – The inlet line to the pump is designed as part of the pump inlet “system”. The pump inlet system consist of reservoir and reservoir pressurization and tubes/hoses from the pressure to the pump. This system should be designed to ensure fluid is provided to the pump inlet within the inlet pressure range 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 factors should be taken into account. Sizing of a pump inlet line uses basic pipe flow equations and reservoir (or supply) pressurization (see Reservoir, Hydraulic – Description).
Outlet Line – The pump outlet line should be sized to system pressure drop requirements and to minimize affects of pump pressure pulsations. Primary considerations in design of the outlet line are pump pressure pulsations, accumulators (see Accumulator, Hydraulic - Description), pump system response and 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 are a result of each piston within the pump transferring a discrete amount of fluid to the system, leading to a pulsed input in the hydraulic system. The flow pulses decay over time from the damping provided by internal flow friction in the downstream tubing and components. The pulsation frequencies for a odd numbered piston pump are
Example: For a pump running at 2700 rpm with 9 pistons
For aircraft systems, both pulsation frequencies are usually above the response frequency of the downstream components, however, in some cases the effects may need to be analyzed. The lower frequency is usually more dominant with pump noise, but both should be analyzed.
System interaction occurs when the natural frequency of the pump compensator is at or near the natural frequency of a downstream component (such as a servo valve or actuator). Generally there is sufficient difference between the natural frequencies so that system interactions do not occur.
Another source of interaction occurs when pumps are connected in a parallel arrangement. This interaction can be stopped by installation of check valves in the outlet lines of each pump.
Pump Pulsation Damping
Several methods exist to dampen the effect of pressure pulsations from a pump:
Change configuration (geometry, parts, characteristics, etc.)
Increase volumes in pump outlet line
Install accumulator close to the pump. Some research shows that for the accumulator to be effective, it should be installed with 0.3 meters of the pump, and the supply line between the main line and accumulator should be between 5 and 10 centimeters in length. Also, the volume of the gas accumulator should be sufficient so that it’s resonance (response) frequency is less than the pump pulsation frequency.
Install a hose at the pump outlet, or downstream plumbing.
Install a Helmholtz resonator (H-filter) in the pump outlet line. A H-filter consists of two lines in series, of different volumes, that branch away from the main line. By properly selecting the lengths and cross-sectioal area of both lines, the H-filter can be tuned to a specific frequency.
Install a Quincke tube in the main line. The Quincke tube is a side line with areas based on main line area and lengths sized for a specific frequency.
See Qualification - Hydraulic Components for discussion on hydraulic pump qualification and required certification testing.