INCLUDING PIVOT FRICTION IN PAD MOTION FOR A TILTING PAD JOURNAL BEARING WITH BALL-SOCKET PIVOTS

Tilting pad journal bearing has its wide application in turbomachinery. For years, frictionless contact has been assumed for all the contact types in bearing coefficients prediction of tilting pad bearings. Existing experimental data demonstrates contact friction does present in pivots and can prevent pads from effective tilting, which is especially true for the surface contact pivot, e.g. ball-socket pivot. However, no open literature is available to discuss individual pad motion with consideration of pivot contact friction and document the effects on the dynamic behaviors of tilting pad bearings. This paper tries to improve the understanding in this area by discussing the pad motion of a tilting pad bearing with ball socket pivots subject to pivot friction. A model, which couples the journal motion, hydrodynamic pressure from oil film and contact pressure within the pivots, is established. Instead of Hertz contact, a conformal contact model is applied for the ball socket contact. Through the coupled transient analyses, the effects of contact friction coefficients, bearing dynamic and static loading, and pivot size on pads motion of a 5-pads load on-pad bearing are considered and compared. The results show that the presence of friction within ball socket pivots does not guarantee "locked up" pads motion, but only limits pads from effective tilting for a given range of friction coefficients. Larger dynamic load increases pad tilting amplitude; this does not mean that enlarged tilting magnitudes reduce pivot friction moments for the pads. With varying bearing loads, the pivot friction could result in non-synchronous motion of pads tilting and pivots deformation and bring these non-synchronous components into the journal vibration. Size reduced pivots are found to diminish the effects of pivot friction and offer preferred pads dynamics and journal vibration.