F117–PW–100 Hybrid Ball Bearing Ceramic Technology Insertion

Abstract

Results of an Advanced Research Projects Agency (ARPA) sponsored project to demonstrate the operational benefits of incorporating advanced structural ceramic ball elements into the F117–PW–100 aircraft gas turbine engine high rotor thrust bearings is described. This program consists of design, fabrication and experimental evaluation of candidate hybrid ball bearing designs in Pratt & Whitney and MRC Bearings test facilities. The bearing design criteria and development test conditions utilized for the project are compatible with the requirements of the F117–PW–100 engine system application. Two hybrid bearing designs were produced by analytically varying internal geometry features such as M–50 race curvatures and contact angles to optimize for the modulus of elasticity of the ceramic balls. One–and–one–eighth inch size CERBEC grade NBD 200 silicon nitride ceramic balls demonstrated integrity and a quadruple rolling contact fatigue life improvement versus state–of–the–art M–50 steel balls in single ball test rigs. Thermal performance data obtained in full scale bearing rig performance testing with one–hundred–seventy–eight millimeter size hybrid and all–steel baseline bearings shows comparable characteristics. The hybrid bearing displayed a distinct survivability benefit in bearing liquid lubricant starvation testing. Two dozen hybrid bearings will be fabricated for full–scale bearing rig endurance tests to be conducted in 1995–1996 as a prerequisite to validation in operating F117–PW–100 engines in 1996–1997.

Critical stresses and strains at the spall edge of a case hardened bearing due to ball impact

“…Ceramic silicon nitride balls are being increasingly used in high performance bearings due to their high hardness, low density, and performance in oil-out conditions [2][3][4][5][6]. Bearings that use ceramic balls have shown better spall propagation characteristics than those that use heavier steel balls [23].…”

“…The worldwide annual economic activity represented by the rolling-element bearing industry is roughly $50 billion [1]. Demands for increased speed, corrosion resistance, and oil-out survivability of aircraft engine bearings have led to the development of hybrid bearings with case hardened steel raceways and silicon nitride ceramic balls [2][3][4][5][6]. Although the performance, load capacity, and useful life of bearings have increased significantly in the past 70 years, both surface and subsurface initiated fatigue failures continue to be a problem.…”

An Experimental and Theoretical Study of Hybrid Bearing Micropitting Performance under Reduced Lubrication

“…Furthermore, restoration of oil flow after an oil shutoff event could restore (undamaged) hybrid bearings to the normal running temperature, whereas full-steel bearings developed permanent damage (Miner,et al (10)). Miner, et al (13) performed a very effective demonstration of hybrid bearing performance under reduced lubrication by reproducing an F117-TW-100 main engine bearing (178-mm bore with 28.6-mm-diameter silicon nitride balls) under oil-off conditions. The test was performed under maximum load (takeoff power) and speed (2.1 million DN).…”

An investigation of partial cone cracks in silicon nitride balls

“…The performance of hybrid silicon nitride ball/steel raceway bearings has been shown to be much superior to steel bearings (Miner et al, 1996 andTanimoto et al, 2000). However, silicon nitride balls are sensitive to surface defects and can fail from fatigue spalls emanating from pre-existing c-cracks, due to crack growth driven by RCF (Hadfield et al, 1993b).…”