This research presents fatigue life test results for several versions of 208-size (40-mm bore) Pyrowear 675 angular contact bearings. Results with different heat treatments of the Pyrowear 675 materials in a hybrid configuration (silicon nitride rolling elements) are compared to all-metal AISI M50, hybrid AISI M50, and hybrid AISI M50NiL configurations. Bearings were tested for rolling contact fatigue life at an applied thrust load of 22,250 N resulting in maximum Hertzian contact stress values of 3.10 (all metal) and 3.55 (hybrid) GPa. Rolling contact fatigue life testing was conducted at 128°C using a gas turbine engine lubricant conforming to MIL-PRF-23699G. All versions of hybrid Pyrowear 675 bearings showed significant improvement in fatigue life compared to baseline M50 and M50NiL bearing materials. After testing, selected bearings were analyzed for tribofilms using Auger electron spectroscopy. Auger electron spectroscopy showed phosphorus-rich tribofilm formation on the Pyrowear 675 bearing contact zones. The results suggest that the tricresyl phosphate antiwear additive used in current gas turbine lubricant formulations positively reacts with Pyrowear 675 surfaces and shows no detrimental effect on fatigue life at the test conditions studied here.

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This work details the heat generation analysis of a turbine aero-engine main-shaft bearing using the computer program Advanced Dynamics Of Rolling Elements (ADORE). The empirical models used for traction and churning heat generation are detailed. The predictions of ADORE are shown to demonstrate the differing contributions of traction and churning to total heat generation at different load/speed regimes. These results are then compared with experimental results. In addition, the results of ADORE are also compared with results from the well-known bearing analysis program SHABERTH (Shaft Bearing Thermal Analysis). The comparisons showed good agreement between ADORE and the experimental results for loads between 13.35 and 53.40 kN and speeds between 1.8 and 2.2 MDN. The results also showed under prediction of heat generation by SHABERTH in this regime. Limitations of both programs were identified and speculated to include limitations in the empirical models due to the lack of available experimental traction data at high speeds/loads. Finally, recommendations for future research are provided which will likely provide significant improvements in the ability to predict bearing heat generation in turbine aero-engine applications.

Wright Patterson AFB OH 45433133 mm bore ball bearings with metal rolling elements were tested at the following conditions: speeds from 1.5 × 10 6 to 2.6 × 10 6 DN; thrust loads from 13,350 to 53,400 N; oil delivery temperatures from 66 to 121 • C; and oil flow rates from 7.3 to 11.4 L/min. The resulting bearing outer race temperature, oil exit temperature, and power loss determined from the shaft torque and power loss determined from the oil temperature rise are reported. Experimental power loss values are compared to the analytical results obtained with the computer code SHABERTH.The experimental data are also fitted to an empirical equation to predict the total bearing power loss. The results indicate that bearing operating temperature is a challenge for next-generation engines, primarily driven by limits of polyolester lubricants used in gas turbine engines. The results also indicate that the computer code SHABERTH underpredicts the bearing lower loss at high load conditions. A new empirical model was able to reasonably predict the bearing power loss over the conditions studied.

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Foil bearings (FB) are one type of hydrodynamic air/gas bearings but with a compliant bearing surface supported by structural material that provides stiffness and damping to the bearing. The hybrid foil bearing (HFB) in this paper is a combination of a traditional hydrodynamic foil bearing with externally pressurized air/gas supply system to enhance load capacity during the start and to improve thermal stability of the bearing. The HFB is more suitable for relatively large and heavy rotors where rotor weight is comparable to the load capacity of the bearing at full speed and extra air/gas supply system is not a major added cost. With 4448–22,240 N thrust class turbine aircraft engines in mind, the test rotor is supported by HFB in one end and duplex rolling element bearings (REB) in the other end. This paper presents experimental work on HFB with diameter of 102 mm performed at the U.S. Air force Research Laboratory (AFRL). Experimental works include: measurement of impulse response of the bearing to the external load corresponding to rotor's lateral acceleration of 5.55 g, forced response to external subsynchronous excitation, and high-speed imbalance response. A nonlinear rotordynamic simulation model was also applied to predict the impulse response and forced subsynchronous response. The simulation results agree well with the experimental results. Based on the experimental results and subsequent simulations, an improved HFB design is also suggested for higher impulse load capability up to 10 g and rotordynamics stability up to 30,000 rpm under subsynchronous excitation.

Foil bearings are one type of hydrodynamic air/gas bearings but with a compliant bearing surface supported by structural material that provides stiffness and damping to the bearing. The hybrid foil bearing (HFB) in this paper is a combination of a traditional hydrodynamic foil bearing with externally-pressurized air/gas supply system to enhance load capacity during the start and to improve thermal stability of the bearing. The HFB is more suitable for relatively large and heavy rotors where rotor weight is comparable to the load capacity of the bearing at full speed and extra air/gas supply system is not a major added cost. With 4,448N∼22,240N thrust class turbine aircraft engines in mind, the test rotor is supported by HFB in one end and duplex rolling element bearings in the other end. This paper presents experimental work on HFB with diameter of 102mm performed at the US Air force Research Laboratory. Experimental works include: measurement of impulse response of the bearing to the external load corresponding to rotor’s lateral acceleration of 5.55g, forced response to external subsynchronous excitation, and high speed imbalance response. A non-linear rotordynamic simulation model was also applied to predict the impulse response and forced subsynchronous response. The simulation results agree well with experimental results. Based on the experimental results and subsequent simulations, an improved HFB design is also suggested for higher impulse load capability up to 10g and rotordynamics stability up to 30,000rpm under subsynchronous excitation.

Polyol-ester lubricants have been used and developed for aviation gas turbine engines for many decades. The newest MIL-PRF-23699 lubricant class, called enhanced ester (EE), provides the best combination of thermal stability, load carrying capability, boundary lubrication and compatibility with fluoroelastomer O-rings. Two candidate EE Class formulations and one high thermal stability class formulation conforming to MIL-PRF-23699G were evaluated for oil degradation with up to 3000 h of bearing operation. Lubricant degradation was studied using VIM VAR M50 bearings with M50 and silicon nitride balls under two operating conditions using two bearing test rigs. Oil degradation in terms of oxidation time, total acid number and viscosity was studied as a function of time with varying results for the three lubricants.