Page 136 - Improving Machinery Reliability

108 Improving Machinery Reliability

Torsional natural frequencies are a function of the torsional masses and the tor-
sional stiffnesses between the masses. The natural frequencies and mode shapes are
generally calculated by the Holzer method or by eigenvalue-eigenvector procedures.
Either of the methods can give accurate results. It is desired that the torsional natural
frequencies have a 10% margin away from all potential excitation mechanisms.
An example of the mass-elastic diagram of a torsional system is given in Figure
3-22. The natural frequencies and mode shapes associated with the first four natural
frequencies are given in Figure 3-23. The mode shapes can be used to determine the
most influential springs and masses in the system. This information is important if
encroachment is calculated and system changes must be made to detune the systems.
Parametric analyses should be made of the coupling stiffness if changes are neces-
sary, since most torsional problems can be solved by coupling changes.

US6 X(1E-6) PTATIOY
USS/ELASTIC DIAORAN n. In-lb-82 ln-lb/rbd DESCBIPTXOq
....... 1 891.48 1051.65 sm 1 pl~t
....... 2 048.73 506.31 STO 2 vI[I
.......... a 43.20 257.39 RIBOST DSI
.......... 4 42.97 17.50 =-ea4 sua
..........
5
F 26.12 165.80 811-604 WB
A...m.o..
am
BULL
238.03 1000.00
6
.. 7 es.71 106.48 PIllrox
... 8 6.61 10.80 JUI-454 WB
... a 6.42 37.24 B1(-454 WB
..... 10 .38 370.86 SLEEVE
.... 11 8.74 180.13 STO 1 IXP
.... 12 2.75 220.02 STO 2 1-
GAS TURBINE SPEED 6670 RPM
..... 13 .13 em. oa LArJrnIXTx
COMPRESSOR SPEED 10762 RPH
.... 14 2.41 352.03 STO 3 I€@
..... 15 .21 8T0.78 DIV LARY
..... 16 .21 352.03 DIV LABI
..... 17 1.71 ezo.02 STO 6 1XP
..... 18 1.75 163.T4 STO S IXP
..... 18 2.02 57.87 STO I IUP
l..... 1.60 . 00 BAL PISTOY
20

Figure 3-22. Torsional mass-elastic data for gas turbine-compressor train.