Page 85 - Tribology in Machine Design

72 Tribology in machine design

contacts, consists of fatigue cracks which progress into and under the
surface, and particles which then fall out of the surface. The holes resulting
from this process are called pits or spalls. This pitting occurs on convex
surfaces, such as gear teeth, rolling element bearings and cams. It is a well-
established fact that the maximum shearing stress occurs below the surfaces
of bodies which are in contact. Hence, at one time, it was strongly held that
the crack forming a pit started at this point of maximum shear stress, then
progressed outwards. Data from pure rolling tests disclosed, however, that
the cracks commonly started at the surface and progressed only in the
presence of oil. A good penetrant, filling any fine cracks present, acted as a
hydraulic wedge. Experiments also revealed that only cracks with their lips
facing the approaching load would progress to failure.
In Fig. 3.4, a crack, 1, filled with oil, approaches the loading zone and has
its lip sealed off. As the full length of the crack comes under the load, oil in
the crack cannot escape, and high hydraulic pressure results. After repeated
occurrence of this process, high stress from stress concentration along the
root of the crack to spread by fatigue. Eventually, the crack will progress
towards the surface, favouring the most highly stressed regions. Then, a
Figure 3.4
particle will fall out, exposing a pit with the typical lines of progressive
cracking, radiating from the pointed lip. The pit may look much as though
it were moulded from a tiny sea shell, with an arrowhead point of origin. Pit
depths may vary from a few microns to about 1 mm, with lengths from two
to four times their depths.
Cracks facing away from the approaching zone of loading, such as crack
2 (Fig. 3.4), will not develop into pits. The root of the crack first reaches the
loaded area and the oil in the crack is squeezed out by the time its lip is
sealed off. A more viscous oil reduces or eliminates pitting, either by not
penetrating into fine cracks, or by forming an oil film thick enough to
prevent contact between asperities.
There are several possible causes for the initial surface cracks, which only
need to be microscopic or even submacroscopic. Machining and grinding
are known to leave fine surface cracks, either from a tearing action or from
thermal stresses. Polishing inhibits pitting, presumably by the removal of
these cracks. Along the edges of spherical and elliptical contact areas a
small tensile stress is present under static and pure-rolling conditions.
Tangential forces caused by sliding combined with rolling, as on gear teeth,
add tensile stresses to the above and to the rectangular contact area of
cylinders. Surface inclusions at the tensile areas create stress concentrations
and add to the chance that the repeated tensile stresses will initiate cracks.
Sometimes a piece that has dropped out of a pit passes through the contact
zone, making a shallow indentation probably with edge cracks. Sometimes
the breaking out of material continues rapidly in a direction away from the
arrowhead point of origin, increasing in width and length. It is then called
spalling. Spalling occurs more often in rolling-element bearings than in
gears, sometimes covering more than half the width of a bearing race.
Propagation of the crack from the surface is called a point-surface origin
mode of failure. There might be so-called inclusion-origin failure. Inclu-