MEASUREMENTS OF DRAG TORQUE, LIFT OFF SPEED, AND STRUCTURAL PARAMETERS IN A 1ST GENERATION FLOATING GAS FOIL BEARING

Abstract

Gas foil bearings (GFBs) in oil-free microturbomachincry enable low maintenance and near frictionless performance, thus reducing operating cost and increasing overall system efficiency. Empirical characterization of foil bearing performance is still relevant in lieu of the limitations of analytical GFB models to predict reliable data, in particular for lift-off speed, power loss and ultimate load capacity. Presently, varying rotor speed tests with increasing external loads applied on a 1st generation GFB serve to identify the bearing lift off speed and drag torque during recurrent start and shut down events. The test rig comprises of a commercial turbocharger air turbine driving a test journal (36.512 mm in OD) and floating gas foil hearing (weight = 10.27 N, length=38.1 mm) mounted on the compressor side. A thermoplastic resin-bonded PTFE coating (16 um thick) on the journal ensures low friction, albeit its durability is not suited for extended operation. Upon completion of the test series, visual inspection reveals the rotor OD is denuded of its coating with the top foil showing polished wear marks. The GFB drag torque during dry friction sliding operation (startup and shut down) increases with the applied load, max 53 N. The bearing drag torque during rotor shut down is similar to 20% lesser than the break away torque during start up. Once airborne, typically at 12 krpm and above, the GFB drag torque is small and remains fairly constant -regardless of the applied load- to the largest operating speed of 45 krpm. Prior to bearing lift off, the drag power loss increases rapidly with rotor speed. Once airborne, the bearing power loss remains relatively constant even as the journal speed increases. Large static loads also cause larger power losses (75 W on average). Other tests conducted with an electro magnetic shaker aim to determine the foil bearing structural stiffness and damping coefficients, both frequency and amplitude dependent. The structural loss factor, best to describe the mechanical energy dissipation characteristics of a foil bearing, decreases with frequency, and its magnitude at just 0.3 and lesser is typical for a first generation foil hearing.