A reader asks whether sleeve bearings can take the place of more expensive tilting pad bearings in a high-speed compressor application.
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EXPERT NO. 1 The answer is that every application needs to be separately analyzed. In real machinery, steady-state conditions do not exist. There is relative motion between the fluid film bearing and the shaft as in starting, in stopping or changing the bearing load suddenly. Changes in bearing loads may result from meshing gears, engaging clutches, etc., or may be periodic as in reciprocating machinery. In addition, rotating loads superimposed on unidirectional loads result from shaft unbalance. The other issue is that whirl of the shaft can result in half-frequency whirl and loss of the hydrodynamic film thickness separating the shaft from the bearing and failure of the operating system.
My guess is that the original design using the tilting pad bearing was based on inhibiting whirl in the bearing system as a real or anticipated problem. My recommendation is if the system is working and not causing a problem by using the tilting pad bearing, continue to use it. However, if a straight sleeve bearing is to be substituted in the application, the entire system needs to be analyzed including a heat balance if the bearing behavior is to be predicted with any accuracy. In summary, you cannot substitute one bearing type for another without performing an engineering design analysis and parametrically testing the new design.
