Machinery's Handbook, 31st Edition
2472 MOUNTING BEARINGS outer races in housings which deform significantly under load or after appreciable service wear. Though many such “fixes” are offered, the only sure solution is to press the race into a housing of sufficient stiffness with the heaviest fit consistent with the installed and operating clearances. In many cases, inserts, or liners of cast iron or steel, are provided to maintain the desired fit and increase useful life of both bearing and housing. Quiet or Vibration-Free Mountings.— In seeming contradiction is the approach to bear ing mountings in which all shaft or rotating element excursions must be isolated from the frame, housing, or supporting structure. Here bearing outer races are often supported on elastomeric or metallic springs. Fundamentally, this is an isolation problem and must be approached with caution to insure solution of the primary bearing objective — location and restraint of the rotating body, as well as the reduction or elimination of the dynamic problem. Again, the danger of skidding rolling elements must be considered and reference to the resident engineers or sales engineers of the numerous bearing companies is recom mended, as this problem generally develops requirements for special, or non-catalog-type bearings. General Mounting Precautions.— Since the last operations involving the bearing appli cation — mounting and closing — have such important effects on bearing performance, durability, and reliability, it must be cautioned that more bearings are abused or “killed” in this early stage of their life than wear out or “die” under conditions for which they were designed. Hammer and chisel “mechanics” invariably handle bearings as though no blow could be too hard, no dirt too abrasive, and no misalignment of any consequence. Proper tools, fixtures, and techniques are a must for rolling bearing application, and it is the responsibility of the design engineer to provide for this in his design, advisory notes, mounting instructions, and service manuals. Nicks, dents, scores, scratches, corrosion staining, and dirt must be avoided if reliability, long life, and smooth running are to be expected of rolling bearings. All manufacturers have pertinent service instructions avail able for the bearing user. These should be followed for best performance. In a later section, methods for inspecting bearings and descriptions of most common bearing deficiencies will be given. Seating Fits for Bearings.— Anti-Friction Bearing Manufacturers Association (AFBMA) standard shaft and housing bearing seat tolerances are given in Table 13 through Table 18, inclusive. Clamping and Retaining Methods.— Various methods of clamping bearings to prevent axial movement on the shaft are employed, one of the most common being a nut screwed on the end of the shaft and held in place by a tongued lock washer (see Table 23). The shaft thread for the clamping nut (see Table 24) should be cut in accurate relation to bearing seats and shoulders if bearing stresses are to be avoided. The threads used are of Ameri- can National Form, Class 3; special diameters and data for these are given in Table 25 and Table 26. Where somewhat closer than average accuracy is required, the washers and locknut faces may be obtained ground for closer alignment with the threads. For a high degree of accuracy the shaft threads are ground and a more precise clamping means is employed. Where a bearing inner ring is to be clamped, it is important to provide a suffi - ciently high shoulder on the shaft to locate the bearing positively and accurately. If the dif- ference between bearing bore and maximum shaft diameter gives a low shoulder which would enter the corner of the radius of the bearing, a shoulder ring that extends above the shoulder and well into the shaft corner is employed. A shoulder ring with snap wire fitting into a groove in the shaft is sometimes used where no locating shaft shoulder is present. A snap ring fitting into a groove is frequently employed to prevent endwise movement of the bearing away from the locating shoulder where tight clamping is not required. Such a retaining ring should not be used where a slot in the shaft surface might lead to fatigue failure. Snap rings are also used to locate the outer bearing ring in the housing. Dimen- sions of snap rings used for this latter purpose are given in AFBMA and ANSI standards.
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