Page 92 - The Tribology Handbook

Profiled pad thrust bearings A16

BEARING TYPE AND DESCRIPTION
W
The bearing comprises a ring of sector-shaped pads.
Each pad is profiled so as to provide a convergent lubricant
film which is necessary for the hydrodynamic generation
of pressure within the film. Lubricant access to feed the
pads is provided by oil-ways which separate the individual
pads. Rotation of the thrust runner in the direction of de-
creasing film thickness establishes the load-carrying film.
For bi-directional operation a convergent-divergent pro-
file-must be used (see later). The geometrical arrangement
is shown in Fig. 16.1

I -- I FILM THICKNESS AND PAD PROFILE

In order to achieve useful load capacity the film thick-
ness has to be small and is usually in the range 0.005 mm
(0.0002 in) for small bearings to 0.05 mm (0.002 in) for
large bearings. For optimum operation the pad rise should
be of the same order of magnitude. Guidance on suitable
values of pad rise is given in Table 16.1.
The exact form of the pad surface profile is not especially
important. However, a flat land at the end of the tapered
section is necessary to avoid excessive local contact stress
under start-up conditions. The land should extend across
the entire radial width of the pad and should occupy about
15-20% of pad circumferential length.

Table 16.1 Guidance on suitable values
of pad rise
Bearing inner Pad rise
diameter d
mm inch mm inch
~~
25 1 0.015-0.025 0.00064.001
O'LWd APPROX 50 2 0.025-0.04 0.001 -0.0016
75 3 0.038-0.06 0.00I5-0.0025
~ ~~ ~
100 4 0.05 -0.08 0.002 -0.0032
150 6 0.075-0.12 0.003 -0.0048
- 200 8 0.10 -0.16 0.004 -0.0064

RUNNER
I 250 10 0.12 -0.20 0.005 4.008
OILWAY
I
It is important that the lands of ail pads should lie in
the same plane to within close tolerances; departure by
more than 10% of pad rise will significantly affect per-
formance (high pads will overheat, low pads will carry
PROFILE ALONG PAD -UNI-DIRECTIONAL little load). Good alignment of bearing and runner to the
axis of runner rotation (to within 1 in lo4) is necessary.
Poorly aligned bearings are prone to failure by overheating
of individual pads.
Fig. 16.1 Bearing and pad geometry

A16.1