Kalamazoo, MI —Although aircraft manufacturers Airbus and Boeing permit a certain amount of oil leakage—up to a drop per minute—in aircraft hydraulic components, the design engineers at these companies don't like any leakage at all. "For one thing, the oil can damage the airplane's exterior paint and other coatings over time," says Greg Warner, an engineer and product manager of engine-driven pumps at Parker-Hannifin Corp.
Parker-Hannifin supplies gearbox-mounted engine-driven pumps to Airbus for the manufacturer's A320/330/340 series aircraft. Boeing uses similar pumps on the B717 through 777 series aircraft. The pumps include shaft seals sub-contracted to Qualiseal Technology, a division of Quality Control Corp. (Harwood Heights, IL). "In the past, we made them ourselves, but now we outsource the seals," says Warner.
The pumps have a maximum allowable seal leakage rate of 3 ml per hour, or approximately one drop per minute. Parker-Hannifin encountered difficulty when primary sealing faces and secondary sealing o-rings deteriorated, thereby causing excessive leakage of five drops or more per minute. Part of the problem is that when the aircraft's engines are turned off, heat dissipates from the engines through the seals, deforming and deteriorating them, according to Warner.
"With the aid of FEA, we incorporated new face materials and hydraulic balancing into a new design that minimizes seal leakage and allows the seal to run cooler," says Bob Viloria, a senior design engineer at Qualiseal Technology. The company uses a combination of FEA software packages for analyzing both contacting and non-contacting seals. FEA analysis is performed in 2D and 3D, linear and non-linear, depending on the application involved and the outputs required. FEA also allows analysis of seal transient behavior in hydraulic pumps, starter units, and other components subjected to rapidly changing start conditions.
Qualiseal Technology's seal designers evaluate seal performance by plotting stress from hydraulic and temperature deflection for the primary seal components. They use FEA-generated output plots of leakage as a function of pressure vs. temperature and pressure vs. speed. FEA also allows them to evaluate seal misalignment with dynamic tracking analysis.
The new seal is spring loaded and rotates with the shaft. "Rotating seals are generally recommended for seal applications where speeds do not exceed 5,000 rpm due to vibration and dynamic balancing limitations," says Viloria. A laser interferometer detects surface imperfections on critical sealing faces.
"During initial acceptance test procedures for endurance and qualification of the hydraulic pumps using the new seal design, no measurable seal leakage was encountered," says Viloria. In addition to reduced leakage, the new seal design improved the pressure-velocity capability of the seal and increased the reliability of the pump.
Additional applications for the company's seals include mixers, agitators, compressors, engine turbines, starter units, and fuel pumps in the aerospace, commercial, and processing industries.
For more information about seals from Qualiseal Technology: Enter 539