Page 88 - Steam Turbines Design, Applications, and Rerating

Bearings for Mechanical Drive Turbines 69

ing set bore radius and journal radius is the bearing set clearance. The
set clearance is the same as the running clearance, which is often spec-
ified as a clearance ratio of mils (milli-inches) per inch of journal diam-
eter. Some typical values of clearance ratio are between 1.5 and 2.0
mils/in. Obviously, there are some applications where these values do
not apply. The manufacturer will specify the recommended clearances
for the particular bearing application.
Slenderness ratio is also referred to as L/D ratio. This is the ratio of
the bearing length to the shaft diameter. This ratio typically varies
between 0.2 and 1.0. However, some plain journal bearings have slen-
derness ratios above 1.0. The bearing length affects the stiffness and
damping characteristics of the bearing. In the selection of a bearing
length, one must consider the bearing unit loading. The unit load is the
bearing load divided by the product of the bearing length and the shaft
diameter; therefore, the units are psi. Typical values of unit loading are
2
between 150 and 250 psi (10 to 17 kg/cm ).

3.3 Thrust Bearings for Turbomachinery
The thrust bearing has two functions in a turbomachine: It constitutes
the axial reference point for locating the rotor in the casing and it car-
ries the axial thrust (Fig. 3.23).
The axial thrust can originate from steam thrust generated by the
rotor parts subject to steam pressure or from the thrust forces devel-
oped in flexible couplings (gear tooth or diaphragm couplings).
A coupling thrust can always be expected when two shafts, each of
which is located by an axial bearing, are connected via a flexible cou-
pling. If one or both the rotors change their length because of tempera-
ture changes, forces are developed in the coupling that counteract the
thermal movement. With gear tooth couplings the friction between the
teeth has to be absorbed; with diaphragm couplings the spring force of
the deflecting diaphragm comes into play.
The thrust bearing of many modern industrial turbines consists of
the bearing collar (integral part of the shaft) and two rings of thrust
bearing pads, each of which is provided with a tilting edge (Fig. 3.24).
By tilting the pads a wedge-shaped gap can form between the bearing
collar and the bearing pads. The space between bearing collar and pads
is filled with oil.
Because of its viscosity the oil is forced from the surface of the rotat-
ing collar into the wedge gap.
Because oil is practically incompressible, the decreasing flow section
in the wedge gap must effect an increase of the oil pressure in the
wedge gap. This oil pressure is balanced by the axial force of the rotor
via the bearing collar.