Since the decreasing pad preload, i

Since the decreasing pad preload, increasing effective damping trend is a typical characteristic of many rotor bear-ing systems, the temptation to decrease tilting pad preload to near zero to improve machine stability is strong. However, there are several major disadvantages to low preload pads. First, there may be a drastic decrease in horizontal stiffness and damping (Kxx and Cxx) as the pad preload becomes nega-tive (Figure 2-35). This trend is shown analytically and dis-cussed in detail in Nicholas [21] for a centrifugal compressor on 5-pad tilting pad bearings with on pivot loading where the negative preload results from a rather generous machining tolerance range. If zero preload is desired, the tolerance range on the journal diameter, the pad radius of curvature and the assembled bearing clearance can all contribute to producing a negative preload.