High-speed turbomachines such as air cycle machines (ACM), auxiliary power units (APU) and turbogenerators as used in aerospace systems operate in hot and harsh environments. Such operating conditions often dictate the application of compliant foil bearings to provide reliable, low power loss performance. Many such turbomachines in operation today were developed, validated and commissioned into service more than 30 years ago. While these machines are still valuable and highly effective, in some cases premature failures have been observed by the end user. In particular, equipment that has seen use in environments with high concentrations of airborne particulates such as sand, dust and other debris (i.e. Middle East) appear to be particularly vulnerable. Exponential gains in load carrying capacity, damping characteristics and temperature limitations have been demonstrated from compliant foil bearings over the past 30 years and retro-fitting these higher performance complaint foil bearings into legacy turbomachines can provide drastic improvements in system performance, increase mean time between service and reduce, and reduce failure rates.
In this paper the authors will present results from testing of an ACM which was developed and initially commissioned into service more than 30 years ago for use in the F/A-18 but remains active in aerospace and military service today [1]. Testing of the fully refurbished ACM and its new cantilever leaf style foil gas bearing components was conducted under normal and abusive conditions such as might be encountered in operational use. The ACM foil bearings came with a Teflon coating. The performance of the cantilever leaf style compliant foil bearings was compared a tension dominated or compliant bump foil style bearing with a Korolon™ polyimide type coating. During component level testing, the compliant bump foil bearings demonstrated lower starting torque, temperature rise, and more consistent and reliable performance than the cantilevered leaf foil bearings under the same operating conditions. System testing of the ACM with both types of bearings under identical conditions was also conducted. Under baseline conditions, cantilevered-leaf bearings were observed to operate at temperatures of 72°C higher than those of the compliant bump foil bearings. The lower operating temperature of the compliant bump foil design indicated lower power loss with compliant bump foil bearings as confirmed by 10% lower turbine pressure and flow required to reach the same speed condition. In dust ingestion testing, the cantilevered-leaf bearings showed a temperature spike due ingestion of dust, leading to failure of the thrust bearings, whereas, the compliant bump foil bearings were unaffected by dust ingestion and operated reliably with minimal signs of wear or damage.
