Page 183 - Steam Turbines Design, Applications, and Rerating

162 Chapter Nine

9.3 Maintenance
Lubrication with oil or grease reduces friction in a gear tooth coupling. At
higher speeds and outputs it is difficult to maintain a reasonable degree
of balance with a grease-filled coupling, and the surrounding air does not
provide adequate cooling to remove the heat generated by friction.
Much better results are obtained with continuous oil flow lubrica-
tion. However, oil impurities may be centrifuged in the sleeve and for
that reason only very clean, dehydrated oil ensures safe and reliable
operation. At very high speeds there is again a balancing problem, and
shaft alignment to a high degree of accuracy is required.
A rigid coupling has no maintenance requirements. Neither does a
diaphragm coupling, although it requires careful protection against
overstretching and/or accidental scratching of the relatively thin
diaphragms.

9.4 Influence on the Critical Speeds
The rigid coupling has a marked influence on the critical speed of the
individual shafts; in fact, it increases the critical speeds. Calculating
the critical speed of shaft strings with several bearings is easily accom-
plished with a routine computer program. However, it is most impor-
tant to know such important parameters as stiffness and damping
constants of bearing pedestals and various other factors.

9.5 Differential Expansions
Expansion and contraction of steam turbine components must be
taken into account whenever rigid couplings are applied. This is easily
accomplished at the design stage but could prove difficult if rigid cou-
plings are chosen as an afterthought.
With a gear type coupling the cumulative effect of differential expan-
sions is avoided, since the shafts can slide axially within the sleeve of
the coupling. However, they slide under the force effect of the torque
acting on the connected shafts. This axial force can be expressed as:

Tμ
F =
r

where T = torque
r = distance from shaft center to gear pitch line
μ= coefficient of sliding friction

On inadequately lubricated gear couplings, μ has been observed to
approach values as high as 0.25.