A manufacturer of electrical power generators for commercial and general aviation aircraft required a custom scavenge pump for their new aircraft generator. The pump would scavenge lubrication oil from the generator sump thereby creating a dry sump, which allows the generator to operate at maximum efficiency. The high-speed, gerotor style pump assembly scavenges an air/oil mixture from two locations within the case the generator.
The two main design challenges in this application are: A) Ensuring that the pump does not cavitate while operating under extremely low ambient pressure conditions (2.4 psia) associated with high altitude. B) Maintaining high volumetric efficiency throughout the operating temperature range.
The first challenge is addressed by designing very small diameter gerotor elements that operate at high speed (11,400 rpm) and low pressure. Two gerotor elements are stacked axially on a single shaft that is then inserted into an aluminum “cartridge” housing creating two independent scavenge circuits, each with independent inlet ports but with a common discharge port. The pump cartridge is then installed into a close fitting bore in the generator housing, with inlet and discharge ports that align with cavities in the generator housing. The pump cartridge relies on a close fit between the two for sealing. The arrangement minimizes the volume and mass of the pump, but also maximizes reliability and performance by reducing elements such as dynamic seals and fittings that could potentially fail.
The challenge related to loss of performance during extreme thermal conditions is managed by paying close attention to the materials of construction and through the mechanical arrangement of the various components. Construction materials consist of steel gerotors housed in cast iron eccentric rings, with aluminum port plates on each side of the gerotor. The cast iron eccentric rings provide for consistent axial gerotor clearance throughout the operating temperature range. Differences in thermal expansion between the pump stack and the aluminum cartridge housing are accommodated by two Bellville washers. The gerotors are mounted on a hardened steel shaft supported by journal bearings that operate hydrodynamically over the specified range of operating conditions.
Fluid: Synthetic lubricating oil
Inlet Pressure: 2.4 psia
1st Scavenge: 4 GPM @ 80psia
2nd Scavenge: 2 GPM @ 80psia
Ambient Temp Range: -55 F to +110 F
Operating Speed: 20,000 rpm
Input Power: less than 0.88 hp
Weight: 1.45 lbs
Life: 30,000 flight hours