A primer for Hydraulic Pump Overhaul
By Greg Napert
Hydraulic pumps can be one of the longest lasting and trouble-free components on an aircraft. The longevity of these pumps comes from the fact that they are self-lubricating and self-cooling. Given a clean supply of hydraulic oil, they can run for about 20,000 hours or more between overhauls.
Maintenance for hydraulic pumps, from the relatively light-duty general aviation pumps to air carrier capacity heavyweights are what is referred to as "on-condition." As long as they produce the desired pressure, they are allowed to continue operating.
According to John Schuerman, Technical Support manager for Eaton Aerospace's Vickers Fluid Systems, in Jackson, MS, "A small portion of the pumps that we manufacture are returned prematurely - and even then, 5 or 10 percent of those are removed in error, meaning that the pump is actually OK. Those pumps are run on our test bench to verify operation and then returned to the customer. Vickers pumps actually have no life-limiting components, all components are repaired as necessary." As further testimony to the longevity of the pumps, Schuerman says that all of its products are warranted for three years. "The average commercial aircraft flies around 3,500 hours a year," he says.
Schuerman says for the pumps that need repair, "One of the items that we are concerned with during the overhaul process is signs of contamination that the customer didn't know existed. It's important for us to inform the customer of this contamination so they can address the aircraft system. For example, we found a small piece of wire in the intake of a pump recently and told the airline about it. It turns out that the wire was from the wire mesh surrounding one of the hydraulic system filters. The airline investigated and it turns out that the hydraulic filter was coming apart. If we had not told them about this, they could have damaged the replacement pump and other components in the system or had a more serious failure."
Vickers has an in-house, FAA-approved repair station from which it offers aftermarket repair and overhaul services to its customers. Schuerman says that one of the biggest challenges that it faces with pump repair is that the problem is not well defined by the customer. "Often, we have to run the unit through a series of tests to determine what needs to be done," explains Schuerman. "It sure helps if the failure mode and operating parameters are defined on the receiving paperwork. This can also eliminate costs related to unnecessary testing."
Regardless, Tim Bartholet, Vickers Service Center manager, says, "We still test the majority of the pumps (75 percent or more) when they first come in, to establish a baseline for how the pump or unit is running. The pump came off for a reason. If the customer says there is low pressure, we like to verify it has low pressure and that the gauge in the cockpit is not the problem. There are some units that we don't place on the test bench such as those that are obviously defective or if we suspect they have been contaminated we don't want to contaminate our equipment."
Schuerman explains, "All of our test benches are run by computers that access a database for the particular unit being tested. The pump is then run according to the test specifications and the performance of the pump is recorded on the computer. Necessary adjustments are made to the unit while it is on the bench. Typical run time on the bench is around two hours."
Mike McKay, test bench technician for Vickers, says, "The test bench is invaluable as a troubleshooting tool. Many parameters are monitored that you could not monitor in the field. For instance, case pressure is monitored. Case pressure can impact the output pressure of the pump because the pump compensator references the case pressure in order to provide the proper pump output. If you have a blockage of your case drain filter, for example, and the case pressure increases, that will feed back to the compensator and give you a higher pressure indication. In this instance, the case filter is the problem."
Once the condition of the pump is determined, the next step is to make decisions as to how to approach a repair. Schuerman says that every customer has different requirements and they always communicate the various options regarding the repair. For instance, many customers like to overhaul a unit, but typically, it's more economical to repair a pump as opposed to overhaul. "We often can repair a pump cheaper than overhauling it."
"We make it a point to communicate with the customer relative to what type of inspection or overhaul we are going to perform on their unit. In some cases, the repair will cost as much as an overhaul does. In that case, it may be worthwhile to spend a few more dollars to get the product overhauled," he says.
As a repair station that exists at the manufacturing facility, there are some interesting twists regarding how the Vickers Service Center is regulated.
"In the case of air carriers," explains Schuerman, "when we work on their components as a vendor, we have to answer to the air carrier and comply with their operating specifications relative to what we do to the product. Even though we are the manufacturer, the air carrier is the authority. We make recommendations on what should be done, but the air carrier makes the final decision."
PV3-240-18 The latest in a line of hydraulic pump improvements
Of the few problems that develop related to hydraulic pumps, one of the most persistent is leakage of fluids from different mating flanges and openings.
In order to reduce leakage possibilities and simplify their line of pumps, Vickers has re-designed the PV3-240-18, used on Boeing 737-600 to 900, to reduce the number of mating surfaces on the high pressure end of the pump.
The result is less leakage and more reliable service. The unit still operates the same as its predecessors, however.
Normal pumping mode
The pressure compensator is a spool valve that is held in the closed position by an adjustable spring load. When pump outlet pressure (system pressure) exceeds the pressure setting, 3,025 psi (209 bar), the spool moves to admit fluid from the pump outlet into the actuator piston. (In the schematic, the pressure compensator is shown at cracking pressure; i.e., pump outlet pressure just high enough to move the spool to begin to admit fluid to the actuator piston.)
The yoke is supported inside the pump housing on two bearings. At pump outlet pressures below 3,025 psi, it is held at its maximum angle - in relation with the drive-shaft centerline - by the force of the yoke return spring. Decreasing system flow demand causes outlet pressure to become high enough to crack the compensator valve open and admit fluid to the actuator piston. This control pressure overcomes the yoke return spring force and strokes the pump yoke to a reduced angle. The reduced angle of the yoke results in a shorter stroke for the pistons and reduced displacement.
The lower displacement results in a corresponding reduction in pump flow. The pump delivers only that flow required to maintain the desired pressure in the system. When there is no demand for flow from the system, the yoke angle decreases to nearly zero degrees stroke angle. In this mode, the unit pumps only its own internal leakage.
Thus, at pump outlet pressures above 3,025 psi, pump displacement decreases as outlet pressure rises. At system pressures below this level, no fluid is admitted through the pressure compensator valve to the actuator piston and the pump remains at full displacement, delivering full flow. Pressure is then determined by the system demand.
When the solenoid valve is energized, outlet fluid is ported to the EDV control piston on the end of the compensator. The high pressure fluid pushes the compensator spool beyond its normal metering position. This removes the compensator from the circuit and connects the actuator piston directly to the pump outlet.
Outlet fluid is also ported to the blocking valve spring chamber. This equalizes pressure on both sides of its plungers, and the blocking valve closes due to the force of the blocking valve spring and isolates the pump from the external hydraulic system. The pump strokes itself to zero delivery at an outlet pressure equal to the pressure required on the actuator piston to reduce the yoke angle to nearly zero. This depressurization and blocking feature can be used to reduce the load on the engine during starting and, in a multiple pump system, to isolate one system for checkout purposes.
When is an overhaul, an overhaul?
In 1999, Vickers produced a service letter to clarify the overhaul definition and criteria in hopes of ensuring there is uniformity of maintenance within the industry.
According to Vickers' service letter, there are many repair facilities around the world performing maintenance of Vickers components; however, there is a big disparity in the quality of repairs between one repair center to another. Much of this disparity is the result of differing terminology from one repair center to another. Due to the broad interpretations in terminology, it is difficult for operators to determine what each repair center performs during their maintenance actions.
Vickers interprets the following terminologies as describing an overhauled component: Zero-Timed, Factory Rebuilt, Overhauled to Zero Time, Like New Condition, and Reconditioned.
These are all interpreted as overhaul; however, many maintenance facility's overhauls fall short of "Overhauled" as defined by Vickers. Vickers believes many maintenance actions identified as "Overhauls" should be classified as repairs.
Definition of Repair
There are various terminologies used to describe repair, such as, Repair as Necessary, Repair for Continued Time, or Repair to Serviceable Condition. Repair is defined by Vickers as "the limited actions necessary to restore the component to compliance with pertinent performance requirements without necessarily implementing any or all applicable service bulletins, (S/Bs)." This normally covers the actions necessary to correct the reason for removal and other maintenance actions necessary to meet the performance requirements defined in the technical data (Component Maintenance Manual (CMM), Service Bulletins (S/Bs), or production specifications).
Definition of Overhaul (Hydraulic Rotating Components)
A Vickers overhaul for hydraulic rotating components applies to all hydraulic pumps, hydraulic motors, power transfer units, and Ram Air Turbine (RAT) pumps.
Overhaul of a hydraulic rotating component includes all maintenance actions to effectively restore the component to an "As New" condition. An overhaul includes the replacement of all bearings, all springs (including wave washers) and all soft goods (packings, face seals, gaskets, and backup rings). Overhaul kits listed on page 4 of this service letter contain the required replacement parts.
All replacement parts will be in compliance with the current issue of the approved technical data (CMM, S/Bs, or product specifications). Unserviceable parts will be replaced with new, serviceable used or certified recondition item parts.
Overhauled components are tested in accordance with the current issue of the component maintenance manual or the production acceptance test procedure.
Overhauled components are returned with a full new unit standard warranty.
Definition of Overhaul (Electrical Rotating Components)
A Vickers overhaul for electrical rotating components applies to electric motors and generators integral to Vickers hydraulic rotating equipment.
Overhaul of electrical rotating components includes all repair actions to effectively restore the component to an "As New'' condition. An overhaul includes the replacement of all bearings, brushes (where applicable), all springs (including wave washers), and all soft goods (packings, face seals, gaskets, and backup rings).
All replacement parts will be in compliance with the current issue of the approved technical data (CMM, S/Bs, or product specifications). Unserviceable parts will be replaced with new, serviceable used, or certified recondition item parts.
Overhauled components are tested in accordance with the current issue of the component maintenance manual (CMM) or the production acceptance test procedure.
Overhauled components are returned with a full new component standard warranty (normally 3 years).
Vickers is continuously improving their components through the issuance of product improvement service bulletins. Vickers incorporates many production product improvements during an overhaul and recommends the implementation of unincorporated, non-production service bulletins when overhauling the component.
Time Between Overhaul (TBO)
The majority of Vickers components are maintained as "On Condition" without established TBO intervals; however, some operators have established TBO intervals for these components due to identified cost benefits of hard time overhauls. Due to variables in utilization, route structure, system conditions, maintenance practices, component configuration, etc.
TBO intervals differ significantly within the industry. Vickers will assist Operators in the establishment of practical TBO intervals.
For more information, contact: Eaton Aerospace Vickers Fluid Systems Jackson, Mississippi Tel: (601) 981-2811 Fax: (601) 987-5255