Page 233 - Tribology in Machine Design

218 Tribology in machine design

dictated by the housing into which it fits. This implies that if the housing
contains errors or irregularities, these will be reflected in the assembled
bearing and, therefore, have to be contained within tight limits.

5.9.1. Bearing fit
To ensure conformity of the bearing shell to its housing, an accurate
interference fit has to be provided between the two, thereby restricting
manufacturing tolerances of peripheral lengths of both the housing and the
bearing. The interference fit is derived from an excess peripheral length in
each half bearing which has to be closely defined to enable bearings of the
same part number to be directly interchangeable. On assembly, the excess
peripheral length, or so called crush, creates a hoop or circumferential stress
around the bearing and a radial contact pressure between the bearing back
and the housing bore. This contact pressure resists relative movement
between the bearing housing and the bearing back thus preventing fretting.
Unfortunately there is a theoretically correct level - housings with a great
flexibility require a higher contact pressure than stiffer ones. On early
engines, having thin-wall bearings, a contact pressure as low as 2 MPa was
usually sufficient to resist fretting, but as engine ratings increase, and
housing stress analysis becomes more sophisticated, higher pressures are
necessary, often reaching 8-10 MPa today. In these very high interference
fit assemblies, particular care has to be taken to ensure that the joint face
clamping bolts have sufficient capacity to assemble the bearing, yet with
sufficient reserve to resist the dynamic separating forces from engine
operation. As the contact pressure is increased for any given bearing size,
the hoop stress increases to the point where the steel backing begins to yield,
adjacent to the joint face, and of course, this must be avoided. Knowing the
combined effect of bearing steel yield strength and the friction force for
bearing assembly, a wall thickness can be determined which will avoid
yield. It is worth noting that the yield strength of the bearing back, in
finished form, varies considerably with the method of manufacture. For
instance, a bearing which is roll formed at some stage, but not fully
annealed, will have a considerably greater yield strength than that of the
raw steel.
An increased contact pressure requires a greater bolt tension for fitting
bearing caps to their opposite half-housings. Proper bolt preload is very
important because if it is insufficient, the housing joints will separate
dynamically, giving rise to a high dynamic loading and to probable fatigue
cracking of the bolts.
To reduce the tendency to fretting, even though the majority of engine
bearings suffer fretting to some degree, it is recommended that the housing
bore surface finish should not exceed 1.6/im c.l.a. Bearing backs are
typically 0.8 jum surface finish or better and in highly loaded zones should
always be supported. Cyclic variation of the hydrodynamic oil pressure on
the bearing surface will attempt to make the bearing back conform to the
housing and if for example there are grooves or oil holes behind the plain