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Creep is less common than premature fatigue, In bearings, it is caused by excessive clearance between bore and shaft that allows the bore to rotate on the shaft, Creep can be expensive because it causes damage to other components in addition to the bearing.
0ther more likely creep indicators are scratches, scuff marks, or discoloration to shaft and bore, To prevent creep damage, the bearing housing and shaft fittings should be visually checked.
Misalignment is related to creep in that it is mounting related, If races are misaligned or cocked, The balls track in a noncircumferencial path, The problem is incorrect mounting or tolerancing,or insufficient squareness of the bearing mounting site, Misalignment of more than 1/4•can cause an early failure.
Contaminated lubricant is often more difficult to detect than misalignment or creep, Contamination shows as premature wear, Solid contaminants become an abrasive in the lubricant, In addition。insufficient lubrication between ball and retainer wears and weakens the retainer, In this situation, lubrication is critical if the retainer is a fully machined type, Ribbon or crown retainers ,in contrast, allow lubricants to more easily reach all surfaces.
Rust is a form of moisture contamination and often indicates the wrong material for the application, If the material checks out for the job, the easiest way to prevent rust is to keep bearings in their packaging, until just before installation.
2 Avoiding failures
The best way to handle bearing failures is to avoid them, This can be done in the selection process by recognizing critical performance characteristics, These include noise, starting and running torque, stiffness, nonrepetitive runout, and radial and axial play, In some applications, these items are so critical that specifying an ABEC level alone is not sufficient.
Torque requirements are determined by the lubricant, retainer, raceway quality(roundness cross curvature and surface finish),and whether seals or shields are used, Lubricant viscosity must be selected carefully because inappropriate lubricant, especially in miniature bearings, causes excessive torque, Also, different lubricants have varying noise characteristics that should be matched to the application. For example, greases produce more noise than oil.
Nonrepetitive runout(NRR)occurs during rotation as a random eccentricity between the inner and outer races, much like a cam action.
NRR can be caused by retainer tolerance or eccentricities of the raceways and balls, Unlike repetitive runout, no compensation can be made for NRR. NRR is reflected in the cost of the bearing, It is common in the industry to provide different bearing types and grades for specific applications, For example, a bearing with an NRR of less than 0.3um is used when minimal runout is needed, such as in disk—drive spindle motors, Similarly, machine—tool spindles tolerate only minimal deflections to maintain precision cuts.
Consequently, bearings are manufactured with low NRR just for machine-tool applications, Contamination is unavoidable in many industrial products, and shields and seals are commonly used to protect bearings from dust and dirt, However, a perfect bearing seal is not possible because of the movement between inner and outer races, Consequently, lubrication migration and contamination are always problems/
Once a bearing is contaminated, its lubricant deteriorates and operation becomes noisier, If it overheats, the bearing can seize, At the very least, contamination causes wear as it works between balls and the raceway, becoming imbedded in the races and acting as an abrasive between metal surfaces, Fending off dirt with seals and shields illustrates some methods for controlling contamination.
Noise is as an indicator of bearing quality, Various noise grades have been developed to classify bearing performance capabilities.
Noise analysis is done with an Anderonmeter, which is used for quality control in bearing production and also when failed bearings are returned for analysis. A transducer is attached to the outer ring and the inner race is turned at 1,800rpm on an air spindle. Noise is measured in andirons, which represent ball displacement in μm/rad.