Design, Commissioning and Testing of a 230 mm Gas Foil Thrust Bearing Test Rig

A major growth market has been based around sub-500 kW turbomachinery with direct drive motors and power electronics, operating at high speeds. Not just eliminating gearboxes, these machines enable very high speeds and a next-generation aero-performance cycle. Along with the development of these high speed direct drive motors, oil-free gas foil bearings have evolved from limited commercial use in the 1970’s to many new applications in turbomachinery sizes from a few kW’s to hundreds of kW’s. This ideal match of technologies enable meaningful efficiency increases with mass production of thousands of air blowers, microturbines, and HVAC compressors operating in Freon, among other applications. On the forefront of this technology, the authors have developed a family of Gas Foil Journal Bearings (GFJB) ranging from 15.5 mm to 80 mm journals (approximately .6 to 3.2 inches), and matched Gas Foil Thrust Bearings (GFTB) from 26 mm to 160 mm disk OD (approximately 1 to 6.3 inches). This range is growing to now include a new customer HVAC application. A much larger 230 mm GFTB matched with a 130 mm GFJB have been designed and manufactured (the 130 mm GFJB is not part of this paper). This is the largest size foil bearing set known in the literature. This GFTB retains the mass production oriented features of the smaller bearings, including a no-weld assembly, and only a single, simple, manual forming step. The 230 mm GFTB can be applied in a high temperature environment up to 500°C or a cryogenic environment and maintain a near-infinite fatigue life. A GFTB test rig has been designed, built and operated for this larger family of GFTB’s. It is based on the motor, rotor and journal bearings of a mass production blower. The rotor has been modified to add an appropriate thrust disk and a thrust piston, while removing unnecessary blower hardware (ie. compressor stages). The commissioning of the test rig is based on running two generations of 160 mm GFTB’s already used in applications to verify differences are measurable and matching experience, which is the scope of this paper. With the next phase, a new 230 mm GFTB will be tested. The primary function of the thrust bearing test rig is to provide a Load-Loss curve that permits real and useful design data for subsequent integration with the turbomachinery. Over the development programs of many applications, the authors have identified that the additional features of many GFTB test rigs in the literature offer limited practical value. For example, measuring film thickness, temperatures within the gas or foils, or using high or low temperatures do not affect the system design of the product machine. The reasons for which will be discussed in a later section, but overall these values get used in modeling efforts whereas real applications must be so robust as to be insensitive to the micro-level changes. The test rig design will be reviewed, along with the gas foil thrust bearing features, and the resulting test results. It is hoped that other GFTB developers can value the simplicity of the test rig concept that is described to enable quick and high-value testing, and developed from off-the-shelf products.