Figure 3-23 lists the troubles that may be encountered with all types of (antifriction frictionless) bearings.

Since dirty bearings will have a very short serv-ice life, every possible precaution must be taken to prevent the entry of foreign matter into bear-ings. Dirt in a bearing which has been improperly or insufficiently cleaned may be detected by noise when the bearing is rotated, by difficulty in rotating, or by visual inspection. Do not discard an antifriction bearing until you have definitely established that something in addition to dirt has caused the trouble. You may determine this by properly cleaning the bearing.

Spalled or pitted rollers or races may be first recognized by the noisy operation of the bearing. Upon removal and after a very thorough clean-ing, the bearing will still be noisy when rotated by hand. (Never spin a frictionless bearing with compressed air.) Roughness may indicate spall-ing at one point on the raceway.

Pay particular attention to the inner surface of the inner race, since it is here that most sur-face disintegration first occurs. Since pits may be covered with rust, any sign of rust on the rollers or contact surfaces of the races is a probable indication that the bearing is ruined.

Figure 3-23.Antifriction bearing troubles and their causes.

Brinelled or dented races are most easily recognized by inspection after a thorough clean-ing. Brinelling receives its name from its similarity to the Brinnell hardness test, in which a hardened ball is pressed into the material. The diameter of the indentation is used to indicate the hardness of the material. Bearing races may be brinelled by excessive and undue pressures during installa-tion or removal, or by vibration from other machinery while the bearing is inoperative. If heavy shafts supported by frictionless bearings are allowed to stand motionless for a long time, and if the equipment is subject to considerable vibra-tion, brinelling may occur. This is due to the peen-ing action of the rollers or balls on the races.

Brinelled bearings must not be placed back in service. Steps can be taken to prevent brinelling. Proper maintenance will help a great deal, and the best insurance against brinelling caused by vibration is to rotate the shafts supported by the frictionless bearings at regular intervals (at least once a day) during periods of idleness. These actions will prevent the rollers from resting too long upon the same portion of the races. Separator failure may become apparent by noisy operation. Inspection of the bearings may reveal loose rivets, failure of a spot weld, or crack-ing and distortion of the separator. Failure of separators can usually be avoided if proper installation and removal procedure are followed, and steps are taken to exclude the entry of dirt. Abrasion (scoring, wiping, burnishing) on the external surface of a race indicates that relative motion has occurred between the race and the bearing housing or shaft surface. The race adjacent to the stationary member is usually made a push fit so that some creep will occur. Creep is a very gradual rotation of the race. This extremely slow rotation is desirable as it prevents repeated stressing of the same portion of the stationary race. Wear resulting from the proper creep is negligible and no damaging abrasion occurs. However, abrasion caused by locked bearings or the improper fit of the races must be prevented.

Cracked races will usually be recognized by a definite thump or clicking noise in the bearing during operation. Cleaning and inspection is the best means of determining if cracks exist. Cracks usually form parallel to the axis of the race. The cracking of bearing races seldom occurs if proper installation and removal procedures are followed.

Excessive looseness may occur on rare occasions even though no surface disintegration is apparent. Since many frictionless bearings appear to be loose, even when new, looseness is not always a sign of wear. The best check for excessive looseness is to compare the suspected bearing with a new one.

Wear of bearings, which cause looseness without apparent surface disintegration, is generally caused by the presence of fine abrasives in the lubricant. Taking steps to exclude abrasives and keeping lubricating oil filters and strainers in good condition is the best way to prevent this type of trouble.

Most of the troubles listed in figure 3-23 require the replacement of an antifriction bearing. The cause of damage must be determined and eliminated so that similar damage to the replacement bearing may be prevented.

Dirty bearings may be made serviceable with a proper cleaning, providing other damage does not exist. In some cases, races abraded on the external surfaces can be made serviceable, but it is generally advisable to replace abraded bearings. Dirty frictionless bearings must be thoroughly cleaned before being rotated or inspected.