Bearing maintenance takes know-how

Understanding what triggers and propagates different types of damage to bearing surfaces can lead to methods that reduce the incidence or severity of such damage.

Over-rolling of solid particles can produce surface indentations of raceways in rolling/sliding lubricated contact areas. These indentations are known to increase the risk of contact failure. The failure process may be influenced by the amount of sliding present at the contact.

For low sliding conditions, typical for rolling bearings, the failure process is dominated by film thickness reduction and surface distress around the shoulders of the indentation.

Keep in mind that indentations in rolling/sliding lubricated contacts repre-sent a significant risk for the service life of bearings. Bearing deterioration and consequent failure can have many forms and several degrees of severity.

In an in-depth technical paper on bearing failure modes and mechanisms, SKF Canada’s Jennifer Moritz presented a comprehensive overview at an education workshop last fall, convened by the Hamilton, ON, section of the Society of Tribologists and Lubrication Engineers (STLE).

According to Moritz, failure modes can be classified into failures resulting from fatigue, wear, corrosion, electrical erosion, plastic deformation, and fracture and cracking.

Fatigue is manifested visibly as a flaking of particles from the surface. It is caused by repeated stresses developed in the contacts between the rolling elements and the raceways. Wear is the progressive removal of material resulting from the interaction of the asperities of two sliding or rolling/sliding contacting surfaces during service. Wear can be abrasive or adhesive.

Corrosion (rust) is a chemical reaction on metal surfaces. It can be caused by moisture or by friction. The latter is manifested by fretting or false brinelling.

Frictional corrosion is a chemical reaction activated by relative micro movements between mating surfaces under certain conditions. These micro movements lead to oxidation of the surfaces and become visible as powdery rust and/or loss of material from one or both mating surfaces.

Electrical erosion is the removal of material from the contact surfaces caused by the passage of electric current – either excessive voltage or current leakage. Excessive voltage creates craters of up to one-tenth of a millimetre. Current leakage creates smaller and more closely positioned craters. There is usually a dark grey discolouration.

Plastic deformation occurs when the yield strength of the bearing material is exceeded. Typically this happens in two different ways: on a macro scale, where the contact load between a rolling element and the raceway causes yielding over a substantial portion of the contact footprint; or on a micro scale, where a foreign object (debris) is over-rolled between a rolling element and the raceway, and yielding occurs only over a small part of the contact footprint.

Fracture and cracking happens when there is a stress concentration in excess of the material’s tensile strength. Fracture is the result of a crack propagating to the point of complete separation of a part. Cracking, which normally precedes fracture, is the incomplete separation of material under stress conditions.

To increase the understanding of the failure process and to develop preventive measures, SKF engineers carried out a detailed analysis of the dynamic behaviour of indentations in rolling/sliding heavily loaded lubricated contacts. They have created advances to bearing theory models that agree with experimental results. Their work also points out the importance of cleanliness, proper bearing mounting, and appropriate lubrication to reduce the risk of surface damage.

Senior scientists of the SKF Engineering & Research Centre in Holland suggest three cardinal measures that reduce the risk of generating surface indentation:

1. Keep the lubricant as clean as possible and check for damaged seals.

2. Follow the manufacturer’s mounting recommendations.

3. Always maintain a good lubricating film by selecting the proper viscosity for the operating conditions. This minimizes surface distress and extends bearing life.

This article was compiled from information provided by SKF AB, Gothenburg, Sweden, and SKF Canada Ltd., Toronto, by Steve Gahbauer, who is an engineer, a freelance writer, and a regular contributor of technical articles to MRO Magazine. For more information on this subject, visit SKF Canada’s website at www.skf.ca.