Tuesday, October 27, 2009

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.
The basic cause of false brinelling is that lubricant is pushed out of a loaded region. Without lubricant, wear is increased. It is possible for the resulting wear debris to oxidize and form an abrasive compound which further accelerates wear.
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 solids), 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.
In normal operation, bearings remain lubricated. However, if a bearing is stationary but subject to a very small oscillating or vibrating load, lubricant may be pushed out of the loaded area. Since the bearing is rolling only small distances (less than roller spacing), there is no action or movement that replaces the displaced lubricant.
With the lubricant gone, the damage described above creates wear debris; and since there is a small motion in the bearing, the damage is ongoing, creating more and more wear debris; and the wear debris acts as an abrasive to further damage the bearing surfaces. Typically the races are most damaged by this action and the appearance is similar to that of brinelling (overload) damage. Thus, the damage is often described as false brinelling.
The discovery of false brinelling is unclear but one story describes how, in the 1930s, new automobiles were loaded on to 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 in to 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.
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.

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