Page 425 - Failure Analysis Case Studies II

Failure Analysis Case Studies II
D.R.H. Jones (Editor)
0 2001 Elsevier Science Ltd. All rights reserved 409

CONTACT FATIGUE IN ROLLING-ELEMENT BEARINGS

P. J. L. FERNANDES
Advanced Engineering and Testing Services, CSIR, Private Bag X28 Auckland Park 2006. South Africa
(Received 15 January 1997)

1. INTRODUCTION

In the paper entitled “Surface contact fatigue failures in gears”, Fernandes and McDuling [ 11 discuss
the mechanism of contact fatigue damage frequently encountered on the active flanks of gear teeth.
This mode of failure operates not only on counterformal surfaces in contact, as in matching gear
teeth, but also on conformal surfaces (Fig. I). The latter are found in ball bearings in contact with
the inner and outer raceways, in roller or nccdlc bearings in contact with the outer raceway. and in
shafts in contact with sliding bearings [2]. In the case of gears, three types of contact fatigue damage
were identified, depending on the relative movement of the contacting bodies, and the resulting
stress distribution in the surface and near-surface material [l]. The characteristics of each type of
failure were discussed in detail in [l].
Rolling-element bearings consist of balls or rollers positioned between raceways which conform
to the shape of the rolling element. Depending on the bearing design, the loads acting on the bearing
may be radial, angular or axial [3]. These loads lead to elastic deformation at the points of contact
between the rolling elements and the raceways. The stress distribution in the surface and near-
surface material under these conditions depends on the loads and the curvature and relative
movement between the contacting bodies.

2. ROLLING AND ROLLING-SLIDING CONTACT FATIGUE

When bearing operation leads to pure rolling contact between the rolling elements and the
raceway, the maximum shear stress occurs at some distance below the surface. This situation is
similar to that encountered along the pitch-line of gear teeth [ 11. In the early stages of damage, pure
rolling forms a highly polished surface, as shown in the case of a bearing cup from a large thrust

(a) (b)
Fig. 1. Schematic illustration of counterformal (a) and conformal (b) surfaces in contact.

Reprinted from Engineering Failure Analysis 4 (2), 155-160 (1997)