Page 239 - Tribology in Machine Design

224 Tribology in machine design

for a copious flow of oil. If only to reduce the losses, something in the nature
of a dry sump lubrication system is obviously best. In order to feed oil to the
individual pads in a dry sump system the oil is sprayed in high velocity jets
radially outwards and obliquely onto the surface of the thrust collar
between pairs of pads. This supplies an adequate film to the downstream
pads while quickly draining the hot oil emerging from the upstream pads.
The scrubbing action on the collar surface removes much of the heat carried
here. The total oil flow required is much reduced, the pad surface
temperature is lower and in a typical bearing, losses are reduced to some 30
per cent of those with flooded lubrication. Theoretically, both circumferen-
tial and radial tilting of the pads is desirable which suggests support by a
hydraulic capsule under each pad. By linking the individual capsules
hydraulically and keeping the fluid-filled volume as small as possible and
absolutely free from air a workable articulating system can be arranged.
The main design problem involved is to accommodate the relatively high
pressure in the capsules. Such a system can improve the load ratio to about 2.
A mechanical method of achieving this goal, the Kingsbury lever system,
seems to be insufficiently sensitive and ineffective in practice, while rubber-
bonded capsules have stress limitations which reduce their usefulness.
Before setting out to design a tilting-pad thrust bearing, it is useful to
consider some of the more general factors which may determine its capacity
limits under various conditions.
First, it is not true, particularly with a large, heavily loaded bearing, that
it can accept a larger load at moderate speeds. On the contrary, in any
device depending on hydrodynamic lubrication the oil film pressure
decreases with the speed. Power losses plotted against specific loads at
various speeds, show clearly the limiting loads at each speed. At the lowest
speeds, the load is limited by the reduction of hydrodynamic pressure and
therefore oil film thickness. At the highest speeds, performance is limited by
the high rate of shear in the oil film, which generates high temperatures and
thus reduces the oil viscosity. The best performance is thus obtained at
intermediate speeds and the limit is determined by straightforward seizure
and wiping of the bearing surfaces initiated by a very thin, hot oil film.
Summarizing, the tilting-pad thrust bearing has some adverse features; it
is relatively complex and heavy and requires more axial space than the
simpler forms. It is difficult to ensure that the heights of the pad surfaces are
uniform and the internal losses are very high unless special steps are taken
to provide directed lubrication and dry sump drainage. Self-aligning
properties are rather similar to those of a fixed-pad bearing. In the view of
the evidence that the pads do not tilt, at least with conventional design and
under normal conditions, it would seem pointless to design as though they
did. However, some form of flexible support for the individual pads should
be helpful towards achieving reasonably good load sharing. Allowance
should be made however, for the development of such a system.
The Michell type bearing is traditionally selected for marine thrust
blocks and large steam turbines, but designers may well give serious
consideration to the simpler fixed-pad bearing for these and similar