Page 237 - Tribology in Machine Design

222 Tribology in machine design

A practical point needs to be watched, since failure to appreciate it has
resulted in expensive damage to quite a few main bearings. The radial width
of the thrust face on the shaft must always be greater than that of the thrust
ring. It is easy for a design which was originally satisfactory to become
dangerous when, at some later stage, the width of the thrust ring is
increased. The edge of the much harder thrust face will then bite into the soft
bearing material, forming a step which severely restricts the outward flow of
oil. The result is overheating and usually the complete failure of the main
bearing also with possible damage to an expensive shaft. It is therefore
prudent to allow ample radial width of the thrust faces in case the need is felt
later, as a result of service experience, to increase the bearing area. It is
obvious too, that differential thermal expansion must be estimated in order
to prevent endwise tightening of the assembly. In the design of thrust
bearings for more precisely defined conditions, especially where the loads
are very heavy, the performance required and the disposal of the heat
generated, are likely to be the controlling considerations. The plain flat
thrust bearing with radial oil grooves can carry surprisingly high loads.
Although the thrust face is machined flat, pressure-viscosity effects in the oil
film, combined with small thermal and mechanical deflections of the pads
between the oil grooves, enable an effective oil film to be built up in
accordance with hydrodynamic theory.
Apart from this simple configuration there are three other types of thrust
bearing. The fixed-pad type is the plain, flat, grooved thrust washer, but
with the pads inclined to form ramps to promote the development of the
hydrodynamic oil film. The tilting-pad bearings have pads supported on a
central or offset step or pivot, or on some articulating device, to improve the
load-sharing between pads. The hydrostatic bearing prevents contact and
hence excessive friction and wear between the thrust collar and the bearing
block by applying a static fluid pressure to one or more annular cavities in
the bearing block. It is usual to supply the fluid by constant-volume pumps,
so that the peripheral gaps through which it leaks to the drains vary
according to the applied load, and the pressure in the cavity is thereby
adjusted to balance the load.
The characteristics of these three types of thrust bearing are known
mainly from experiments carried out on full scale bearings loaded with a
wide range of loads. Of particular interest to the designer is that:
(i) the load capacity is enormously influenced by the slope of the ramps;
(ii) ideally the slope should be very small; in practice and with commonly
used dimensions, a slope of 0.025 to 0.050 mm over the pad width gives
acceptable results while remaining within attainable manufacturing
standards;
(iii) with suitably designed pads, the load capacity increases rapidly with
speed, even from zero. Starting or stopping under load is therefore not
a serious problem;
(iv) under conditions of misalignment, the pads in the more heavily loaded
arc of circumference operate with smaller clearance than those in the
opposite arc and therefore develop higher hydrodynamic pressures. A