False brinelling
False brinelling is damage caused by fretting, with or without corrosion, that causes imprints that look similar to brinelling, but are caused by a different mechanism. Brinell damage is characterized by permanent material deformation (without loss of material) and occurs during one load event, whereas false brinelling is characterized by material wear or removal and occurs over an extended time from vibration and light loads.
The basic cause of false brinelling is that the design of the bearing does not have a method for redistribution of lubricant without large rotational movement of all bearing surfaces in the raceway. Lubricant is pushed out of a loaded region during small oscillatory movements and vibration where the bearings surfaces repeatedly do not move very far. Without lubricant, wear is increased when the small oscillatory movements occur again. It is possible for the resulting wear debris to oxidize and form an abrasive compound which further accelerates wear.
Mechanism of action
In normal operation, a rolling-element bearing has the rollers and races separated by a thin layer of lubricant such as grease or oil. Although these lubricants normally appear liquid (not solid), under high pressure they act as solids and keep the bearing and race from touching.
If the lubricant is removed, the bearings and races can touch directly. While bearings and races appear smooth to the eye, they are microscopically rough. Thus, high points of each surface can touch, but "valleys" do not. The bearing load is thus spread over much less area increasing the contact stress, causing pieces of each surface to break off or to become pressure-welded then break off when the bearing rolls on.
The broken-off pieces are also called wear debris. Wear debris is bad because it is relatively large compared to the surrounding surface finish and thus creates more regions of high contact stress. Worse, the steel in ordinary bearings can oxidize (rust), producing a more abrasive compound which accelerates wear.
Examples
The discovery of false brinelling is unclear, but one story describes how, in the 1930s, new automobiles were loaded onto trains for delivery; when they were unloaded, some would show severe wheel bearing damage. On further inspection, it turned out that many wheel bearings were slightly damaged. The damage was eventually traced to rocking of the autos and the regular impact every time a railroad car wheel passed a track joint. These conditions led to false brinelling.
Although the auto-delivery problem has been solved, there are many modern examples. For example, generators or pumps may fail or need service, so it is common to have a nearby spare unit which is left off most of the time but brought into service when needed. Surprisingly, however, vibration from the operating unit can cause bearing failure in the unit which is switched off. When that unit is turned on, the bearings may be noisy due to damage, and may fail completely within a few days or weeks even though the unit and its bearings are otherwise new. Common solutions include: keeping the spare unit at a distance from the one which is on and vibrating; manually rotating shafts of the spare units on a regular (for example, weekly) basis; or regularly switching between the units so that both are in regular (for example, weekly) operation.
Until recently, bicycle headsets tended to suffer from false brinelling in the "straight ahead" steering position, due to small movements caused by flexing of the fork. Good modern headsets incorporate a plain bearing to accommodate this flexing, leaving the ball race to provide pure rotational movement.
External links
- Emerson Bearing picture of false Brinelling
- Precision Spindle explanation of false Brinelling
- MRO Today on rotating spares
- US Motors service bulletin case study, explanation, and pictures (PDF)