Page 91 - Steam Turbines Design, Applications, and Rerating

72 Chapter Three

P z

Permissible Mean Load

➀ Thickness of oil film = The slope of ➀ is a function of
constant = min. thickness bearing size
➁ Babbitt metal temperature U = Circumferential speed of
= constant = max. permissible bearing collar
babbitt metal temperature P z = Permissible mean load per
➂ Load per unit area = unit area of thrust bearing
constant = max. permissible
load per unit area
Figure 3.25 Typical operating parameters for turbomachinery thrust bearings.
(Siemens Power Corporation, Milwaukee, Wis. and Erlangen, Germany)

The thickness of the oil film increases with rising circumferential
speed, since the permissible load is now below that for constant oil film
thickness.
For thrust bearings with mean diameter of 250 mm and up, the load
permitted for constant oil film thickness reaches the load limit set by
the fatigue limit of the babbitt metal prior to approaching the temper-
ature limit. Therefore, the load must be maintained constant with ris-
ing circumferential speed (limit curve 3 in Fig. 3.25). The thickness of
the oil film will now increase and the temperature rises. At point A of
the diagram the temperature limit is again reached. From here on
limit curve 2 applies again and the load decreases to a value below the
fatigue limit.
In theory, the babbitt surface can stand temperatures of up to
approximately 145°C (293°F). If a maximum operating temperature of
110°C (230°F) is selected, the safety margin up to 145°C (293°F) would
correspond to a further 100 percent load increase.