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."
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