Contact Fatigue

CONTACT FATIGUE is a surface-pitting-type failure commonly found in ball or roller bearings. This type of failure can also be found in gears, cams, valves, rails, and gear couplings. Contact fatigue has been identified in metal alloys (both ferrous and nonferrous) and in ceramics and cermets. Contact fatigue differs from classic structural fatigue (bending or torsional) in that it results from a contact or Hertzian stress state. This localized stress state results when curved surfaces are in contact under a normal load. Generally, one surface moves over the other in a rolling motion as in a ball rolling over a race in a ball bearing. The contact geometry and the motion of the rolling elements produces an alternating subsurface shear stress. Subsurface plastic strain builds up with increasing cycles until a crack is generated. The crack then propagates until a pit is formed. Once surface pitting has initiated, the bearing becomes noisy and rough running. If allowed to continue, fracture of the rolling element and catastrophic failure occurs. Fractured races can result from fatigue spalling and high hoop stresses. Rolling contact components have a fatigue life (number of cycles to develop a noticeable fatigue spall). However, unlike structural fatigue, contact fatigue has no endurance limit. If one compares the fatigue lives of cyclic torsion with rolling contact, the latter are seven orders of magnitude greater (Ref 1). Rolling contact life involves ten to hundreds of millions of cycles. the direction of ball travel. Not all spalls in ballbearing races are of the shape shown in Fig. 2. Figure 3 shows a fatigue spall near the race shoulder of a deep-groove ball bearing. The spall appears to have been formed by the joining of several pits. The fact that the spall occurred close to the race shoulder may have distorted the contact state of stress, causing a multiple origin. Fatigue in roller bearings may differ from ballbeating contact fatigue. Quite often the pitting occurs in the inner race at the contact zone of the roller ends. In some cases, contact stress peaks at the roller ends and pitting originates in these locations. Roller-end pitting can be a sign of misalignment. Cams and Gears. Valve lifter cams and rollers are subject to contact fatigue. An example is shown in Fig. 4 (Ref 3). The character of the damage is very similar to that found in rolling contact bearings. The example shown in Fig. 4 was found in both cam nose and lifters during automobile engine tests (Ref 3). Lifters were nodular iron, and cams were flake graphite cast iron. Fatigue cracks were associated with cracked carbides, graphite flakes, and hard inclusions. Contact fatigue occurs in gears along the pitch line. The geometry of tooth mesh is such that rolling occurs at the pitch line while sliding occurs at the addendum as the gears come out of mesh. An example of pitch line contact fatigue is shown in Fig. 5 (Ref4). The pits seen on the teeth will grow in size and depth, ultimately resulting in tooth fracture. Another form of contact fatigue, known as micropitting, occurs in bearings. An example is shown in Fig. 6. This feature can show up over the entire raceway surface. It is often the result of too thin a lubricant film or excessive surface roughness and sometimes heavy loading. In gears, micropitting is termed frosting and in the present ANSIdAGNA standard it is considered a form of contact fatigue. For bearings,