Page 238 - Tribology in Machine Design

Sliding-element bearings 223

righting couple is thus set up which tends to correct the misalignment
by taking up some of the available clearance in the adjacent journal
bearings. Though it is obviously desirable to avoid misalignment, the
bearing will automatically reduce its ill effects;
(v) the fixed pad bearing is a simple, effective and compact device capable
of functioning under the severe conditions associated with stopping
and starting under load. It is particularly useful where axial length has
to be kept to a minimum.

5.10.1. Tilting-pad bearing characteristics
The tilting-pad bearing is a complex arrangement because of the intricate
interplay between a number of design features. The conventional form
consists of a ring of pads, each supported on pivots, which may be either at
the optimum point, 0.4 of the pad width from the trailing edge, or, if rotation
in both directions has to be allowed for, at the centre of the pad. Better still,
at the cost of some design complication the pads may be supported on some
form of mechanical or hydrostatic articulation system with a view to
equalizing the loads on them.
For some 50 years after the original Michell bearing was invented it was
assumed that the pads tilted so as to adopt something like an ideal angle of
inclination with respect to the thrust collar, and thus to induce the
formation of effective hydrodynamic lubrication. During this period the
limiting specific loading on the thrust bearings for steam turbines, vertical
hydroelectric machines and similar plant remained around 0.021 MPa.
Little or no attempt was made to improve this, so as to reduce the large size,
weight, cost and power losses of these bearings. When these problems were
investigated, some very interesting facts were established. First of all it was
found that under typical current conditions of load and speed the pads did
not tilt and their hydrodynamic action is due to thermal and mechanical
distortion of the surfaces. The load shearing between pads is often
extremely poor, the ratio of the highest to the lowest load being as high as 7
or 8 in a typical installation. Even with extreme care in fitting the pads to
gauge room standards of accuracy it is between 2 and 4. This and the very
thin oil film accounted for the failure of many such bearings in service.
Experiments with alternate pairs of pads removed resulted in substantially
increasing the permissible loading. For example, whereas with eight pads
seizure of at least one of them occurred at an overall nominal specific load of
roughly 0.07 MPa, with only two pads this figure became at least 0.28 MPa
in the most favourable speed range (1000 to 1750 r.p.m.) and over 0.21 MPa
at all speeds between 500 and 3000 r.p.m. Reducing the number of pads
increased the chances of load sharing, proving that one or more of the pads
in the full bearing were almost certainly carrying more than the overall
average of 0.07 MPa specific load. The conventional thrust bearing is
lubricated and cooled by pumping oil into the housing at a low point and
allowing it to flow out from somewhere near the top. The whole assembly
thus becomes a fluid brake resulting in heat generation and hence the need