Page 116 - Improving Machinery Reliability

88 Improving Machinery Reliability

NOWIZBD CRITICAL SPEED HAP

MODE 9 2!3XPp
MODE 4 iexpr
MODE 3 9XFr
MODE a 4x17,

YODB 1 1XFp

8
BBARRINC? SUPPORT STIFFNESS LBS/IN
Figure 3-3. Normalized critical speed map.

of the frequencies for these modes compared to the first critical speed for rigid bear-
ings is 2.27 and 6.25.
Bearing Stiffness and Damping. The dynamic stiffness and damping coefficients
of bearings can be adequately simulated using eight linear coefficients (Kxx, Kyy,
Kxy, Kyx, Cxx, Cyy, Cxy, Cyx) (Figure 3-4). This information along with the lubri-
cant minimum film thickness, flow, power loss, and temperature rise at operating con-
ditions are needed to evaluate the bearing design. The bearing stiffness and damping
coefficients are calculated as functions of the bearing type, length, diameter, viscosi-
ty, load, speed, clearance and the Sommerfeld number which is defined as:

Y

b JOURNAL

cxx
-x
K xx

Figure 3-4. Bearing stiffness and
KYY CYY damping coefficients.