Page 78 - Steam Turbines Design, Applications, and Rerating

Bearings for Mechanical Drive Turbines 59

bearings are being designed with offset pivots, typically around 60 per-
cent. Offset pivots increase load capacity and thereby reduce bearing
operating temperatures.
Possibly the most important tilting-pad bearing parameter that the
designer must consider is pad preload. Tilting-pad bearing preload is
defined as:
m = 1 − (C b /C p )

where m = pad preload
C b = bearing assembled clearance, mils
C p = pad clearance, mils
The zero preload case is illustrated in Fig. 3.8, where the pad’s radius
of curvature equals the pivot radius (R p = R b ) and thus the pad clear-
ance equals the bearing clearance (C p = C b ).
A preloaded pad is illustrated in Fig. 3.9. The pad clearance is
greater than the bearing clearance. Typical preload values for steam
turbine bearings range from 0.2 to 0.5. When a pad is preloaded, a con-
verging film section always exists and the pad will produce a hydrody-
namic force even if the bearing load approaches zero.
In many cases, the biggest advantage of light preloaded pads for
steam turbine applications is that bearing damping tends to increase
while bearing stiffness remains approximately constant. Both of these
trends help to increase bearing effective damping (the amount of bear-
ing damping that is effective in rotor vibration suppression) and
decrease rotor vibration levels.
One disadvantage to decreased pad preload is that the top pads
become unloaded and may be susceptible to pad flutter. Fluttering

Figure 3.8 Zero preloaded pad.
(RMT, Wellsville, N.Y.)