Page 86 - Tribology in Machine Design

Elements of contact mechanics 73

sions are non-metallic particles that are formed in, and not eliminated from,
the melt in the refining process. They may be formed during the
deoxidization of steel or by a reaction with the refractory of the container.
The inclusion does not bond with the metal, so that essentially a cavity is
present with a concentration of stress. The usual way to detect inclusions is
by a magnetic particle method. A crack, starting at the inclusion, may
propagate through the subsurface region for some distance, or the crack
may head for the surface. If cracks on the surface form, further propagation
may be by hydraulic action, with a final appearance similar to that from a
point-surface origin. The damaged area is often large. It is well known that
bearings made from vacuum-melted steel, and therefore a cleaner, more
oxide-free steel, are less likely to fail and may be given higher load ratings.
There are three other types of failure which usually occur in heavily
loaded roller bearings in test rigs. Geometric stress concentration occurs at
the ends of a rectangular contact area, where the material is weaker without
side support. Slight misalignment, shaft slope or taper error will move much
of the load to one of the ends. In peeling, fatigue cracks propagate over large
areas but at depths of 0.005 to 0.01 mm. This has been attributed to loss of
hydrodynamic oil film, particularly when the surface finish has many
asperities which are greater than the film thickness under the conditions of
service. Subcase fatigue occurs on carburized elements where the loads are
heavy, the core is weak and the case is thin, relative to the radii of curvature
in contact. Cracks initiate and propagate below the effective case depth, and
cracks break through to the surface at several places, probably from a
crushing of the case due to lack of support.

3.6. Design values and Previous investigations, some of which are published, have not produced a
procedures common basis on which materials, properties, component configuration,
operating conditions and theory may be combined to determine dimen-
sions for a satisfactory life of concentrated contacts. The investigations
indicate that much progress is being made, and they do furnish a guide to
conditions and changes for improvement. Most surface-contact com-
ponents operate satisfactorily, and their selection is often based on a
nominal Hertz pressure determined from experience with a particular
component and material, or a selection is made from the manufacturers
tables based on tests and experience with their components. The various
types of stresses, failures and their postulated causes, including those of
subsurface origin, are all closely related to the maximum contact pressure
calculated by the Hertz equations. If an allowable maximum Hertz pressure
seems large compared with other physical properties of the particular
material, it is because it is a compressive stress and the other two principal
stresses are compressive. The shear stresses and tensile stresses that may
initiate failures are much smaller. Also, the materials used are often
hardened for maximum strength. Suggestions for changes in contact-stress
components by which their load or life may be increased are:
1. larger radii or material of a lower modulus of elasticity to give larger
contact area and lower stress;