Page 92 - Steam Turbines--Design, Applications, and Rerating by Heinz-Bloch, Murari-Singh

Bearings for Mechanical Drive Turbines 73

Experience shows, however, that manufacturing tolerances of bear-
ing pads as well as minor deformations of the thrust bearing housing
effect different loading of the individual pads. This can result in possi-
ble temperature differences of the hottest spots of the individual pads
of approximately ±20°C (36°F). A temperature limit of 90°C (194°F) is
thus assigned to the hottest spot of the babbitt lining of thrust bearings
used by such experienced manufacturers as Siemens. The permissible
load is often defined for this conservative temperature.
The design specification for thrust bearings is therefore aimed at
keeping the maximum temperature of the babbitt lining as low as pos-
sible. Also, the bearing design must be so rigid to keep any deforma-
tions leading to uneven load distribution as small as possible.
The temperature rise of the oil in the thrust bearing, i.e., the differ-
ence in temperature of the oil at inlet and outlet should be approxi-
mately 15°C (27°F). Considerable friction is generated at high speeds
and high-axial forces and a large oil flow quantity is thus required.
When operating at lower speeds a reduced flow of oil is adequate for
the temperature difference of 15°C (27°F). The oil flow required to pro-
duce this temperature rise is obtained by plugging an appropriate
number of oil drain holes. Too low a temperature rise results in un-
necessarily high friction due to steeply increasing oil viscosity at low
temperatures. Oil that is too hot causes excessive babbitt metal tem-
peratures. Exact dimensioning of the axial clearance between the bear-
ing collar and the two tilting-pad rings is also extremely important: Too
much clearance will incorrectly locate the shaft and can produce axial
vibration of the rotor upon thrust reversal.
Too little clearance, on the other hand, produces both unnecessarily
high friction and thrust bearing loads. If a double-acting thrust bearing
is operated without an axial force present, the oil films between the
bearing collar and the two pad rings have the same thickness. The
bearing collar locates itself exactly in the center of the bearing clear-
ance. The oil films, however, already generate hydrodynamic forces
against the two faces of the bearing collar. These forces are equal and
balance each other. They already constitute a load for the axial bearing
and frictional heat is generated. The smaller the axial clearance
selected, the smaller the thickness of the oil film becomes at no-load
operation and high loads act on the bearing pads already at no-load
operation. In that case the no-load friction heat is also very high. If one
of the two rings of pads is now subjected to an external axial load, the
thickness of the oil film on the loaded side is decreased. This causes the
hydrodynamic axial forces to increase until equilibrium with the exter-
nal axial force is achieved.
Simultaneously the thickness of the oil film at the unloaded side
increases (thickness of oil film = axial clearance − oil film thickness of