Page 240 - Tribology in Machine Design

Sliding-element bearings 225

applications. Adequately designed and made, the latter is likely to carry
larger loads while occupying less space. With oil fed from an inner annular
30 per cent of those in a conventional flooded tilting-pad assembly.
The kind of application to which the tilting pad type is well suited is in a
large, vertical hydroelectric machine, where each pad can be separately
supported on a jack for adjustment of height. With inserted thermocouples
or pressure transducers in the most heavily loaded area of each pad, the
readings obtained can be used to monitor the loading on them so that
appropriate adjustments can be made. Such conditions permitted one
particular machine to be fitted with pads of much reduced area so that the
mean specific load was increased from 0.021 MPa to about 0.07 MPa and
the performance was reported as satisfactory for several years afterwards.

5.10.2. Design features of hydrostatic thrust bearings
This type of bearing depends for its operation on a balance between the
pressure in one or more cavities in the thrust block and the applied load.
The fluid escapes through an annular gap between the block and the thrust
collar. It is the variation of these gaps with the load which automatically
adjusts the hydraulic pressure over the whole area of the bearing. The load
limit is set by the maximum pressure the feed pump can maintain at small
delivery rates, i.e. at the highest thrust load, when the gaps are reduced
towards zero. Obviously, a failure or sudden decrease of fluid supply will
allow the peripheral lands of the thrust block to come into contact with the
collar, and rapid scoring or seizure is to be expected. Safety, depends
therefore on the reliability of the oil supply.
In one important application safety was ensured by duplicating the
pump, the feed being through simple non-return valves, so that, if the flow
from one pump stopped, that from the other took over. In view of the need
for complete reliability, a third pump was also provided, driven by a motor
drawing its power from a separate emergency supply.
The internal losses of a well-designed hydrostatic thrust bearing can be
very small compared with those of any hydrodynamic bearing with pads.
The only area of high shear is in the peripheral gaps and these can be made
quite narrow radially and very smooth. The fluid can be an oil of low
viscosity so that churning losses are negligible with adequate drainage. The
heat to be removed from the central zone of the bearing is therefore
significantly reduced and much of the work in supporting the thrust load is
transferred outside, to the pumps, where it can much more easily be dealt
with and where a breakdown can more readily be attended to.
The type of bearing with a single annular cavity has no self-aligning
properties, but by substituting a series of separate cavities, each fed through
a suitable restriction, it is possible to design for almost any righting
moment. However, the greater this effect, the greater also will be the total
losses in the system.
A final caution; the designer should not line one side or the other of the
gaps with white metal, in the hope of reducing the bad effects of metal-metal