The better bearing battle Spotting trouble early can extend bearing life, cut downtime by Jeff Calvin Regardless of how sophisticated your machinery might be, one bad bearing can shut down an entire production line.  A good predictive maintenance program, however, can prevent costly bearing problems. Most bearings never reach their fatigue life.  The finest materials, the best finishes and the most sophisticated engineering cannot overcome poor maintenance. It is important to spot the signs of trouble early and take corrective action.   Careful troubleshooting uncovers problems that can be corrected, extending bearing life. Reduced downtime and lower maintenance costs through proper bearing mounting, lubrication and maintenance yields big savings. Here are some warning signs: Normal fatigue spalling The limiting criterion for fatigue damage is largely application dependent.  For the purpose of establishing bearing dynamic ratings, it has been arbitrarily set as a spall size of 0.01 square inches in area. Bearings may continue to function and, as such, demonstrate a "useful life" well beyond this point. Abrasive wear Foreign material can enter the bearing through a damaged or worn seal or during cleaning, when water or other cleaning fluids contaminate the lubricant.  This causes premature wear of the bearing.  If the wear is mild (i.e., light surface bruising or denting), bearings may be reused after the housing and bearings are thoroughly cleaned and adjusted. Fine dust, metal particles, grinding dust or airborne dirt, common in a factory, cause abrasive wear.  Metal components in contact, such as gear teeth, create fine metal particles in the lubricant that lap the roller end and rib face as well as the roller body and races.  If the foreign material particles are fine enough, polished or mirror finish surfaces result. Any amount of wear changes the bearing adjustment.  This wear reduces fatigue life and, in extreme cases, can cause misalignment in the bearing, which affects other components in the application. Bearings with excessive wear from foreign material should be replaced.  Indications of a need for replacement include a worn rib face or roller ends and a sharp corner on the roller radius. Pitting and bruising Larger metal clips or particles of dirt entering the bearing can cause pitting and bruising.  The number and depth of the indentations caused by the particles determine whether to replace the bearing.  Any visible indentation could cause premature fatigue damage. Etching and corrosion Moisture in the bearing results in etching or corrosion.  Causes include condensation from a humid environment or moisture passing through a damaged or worn seal. Bearings removed for inspection are also susceptible to corrosion damage.  Proper cleaning and drying are important.  If you store a bearing, it should immediately be coated with oil or a good rust preventative, wrapped in protective paper and stored in a dry area. Often, corrosion is a surface stain. If the stain is removed with a fine emery cloth or crocus cloth, the bearing may be reused.  If there are pits to a depth that cannot be cleaned with light polishing, the bearing should be discarded.  Pits lead to premature spalling. Scoring Inadequate lubrication of the rib face and roller end results in metal-to-metal contact and scoring. Scoring on the roller ends is easily noticed in a bearing assembly.  If the roller ends are scored, then the rib face is scored.  Scoring damage normally results in a change in the geometry of the end of the roller and the rib face. Continued operation of a scored bearing will have serious consequences.  The rollers can skew under the cage bridges and become distorted and broken.  Rollers may also become welded to the races and possibly the rib, seizing the bearing and severely damaging other expensive components. Heat damage is evident on the rib outside diameter and on the roller ends.  It appears as a dark blue spot or, in extreme cases, dark black.  Heat of this magnitude changes the hardness of the bearing. Causes of heat damage include: absence of lubricant in the bearing, not enough lubricant to sustain a lubricant film, wrong oil viscosity, wrong lubrication system for the speed and load involved, excessive bearing preload, or deterioration of the lubricant due to contamination. Cage breakage and peeling Cage breakage doesn't happen often.  It can occur as a result of vibration, orbital rotation (in plant idler bearings), or rapid acceleration or deceleration of the bearing.   Other factors are marginal lubrication systems or excessive end play in the bearings.  The risk of cage breakage may be reduced by eliminating or minimizing vibration, careful control of bearing adjustment and improving the lubrication system. Insufficient lubrication can also result in peeling, which produces a superficial frosted appearance on the surface, usually at a depth of less than 0.0001 inches. Thin film conditions may also occur at elevated temperatures.  Heat lowers the lubricant viscosity, reducing lubricant film thickness. This condition also may occur under heavily loaded conditions or applications where high temperatures exist with oil mist lubrication. Bearing adjustment Bearing adjustment is a critical component of successful bearing operation.  With excessive end play in the bearing, rollers are very loose out of the load zone, resulting in skidding and sliding and cause scalloped wear on the cup race. Excessive preload can also cause damage similar in appearance to inadequate lubrication, thus making it difficult to tell the difference. A lubricant that is suitable for non-preloaded applications may not be suitable for preloaded applications.  Under heavy preload, lubricants may break down because of heavy loads and high temperatures, resulting in scored ribs and roller ends.  On low-speed applications, heavy pre-load may show up as premature fatigue spalling. Misalignment Bearing misalignment results in reduced bearing life, depending on the amount of misalignments as well as the loads and speeds involved.  If misalignment exceeds set limits, the load on the bearing is concentrated in one area on the bearing races, resulting in high edge stresses and subsequent fatigue spalling. High spots or burrs A bearing cup conforms to the surface of the cup seat housing where it is mounted.   Any damage occurring to the cup seat when removing old bearings or assembling new bearings automatically transfers to the cup race. If the cup seat surface is gouged by a hardened tool, it leaves a raised area around the gouge.  If these high spots are not removed before the new cup is installed, they transfer to the cup race.  As a result, a roller hitting the high spot causes stress and early fatigue of the cup race. False brinnelling/impact damage False brinelling is bearing surface wear.  It occurs when the rollers slide axially back and forth on the race while the bearing is essentially stationary but subject to vibration.  A groove is worn into the race by the roller's sliding action. This condition, while caused by vibration, may be exacerbated or inhibited by other operating environmental factors such as loading, lubrication, bearing setting, etc.   Vehicles shipped by rail, truck or boat over long distances are susceptible to false brinelling.  A bearing in this condition is very rough and noisy in operation because of the roller-spaced wear on the races. Extremely heavy impact loads can also result in brinelling of the bearing races.  These heavy loads also can fracture the races or rollers.  This is true metal deformation and not wear, like seen in false brinelling. Electric current When electric current passes through a bearing, arcing or burning occurs at the point of contact between the races and rollers.  This may cause a single burn or appear as a line of small burns, called "fluting," along the line of contact of the rollers and the race. Jeff Calvin is the principal service engineer for The Timken Company of Canton, Ohio. This article appeared in the April/May 1999 issue of MRO Today magazine.  Copyright 1999. Back to top Back to Uptime archives