Sổ tay tiêu chuẩn thiết kế máy P32

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Sổ tay tiêu chuẩn thiết kế máy P32 CHAPTER 27 ROLLING-CONTACT BEARINGS Charles R. Mischke, Ph.D., RE. Professor Emeritus of Mechanical Engineering Iowa State University Ames, Iowa21A INTRODUCTION / 27.227.2 LOAD-LIFE RELATION FOR CONSTANT RELIABILITY / 27.727.3 SURVIVAL RELATION AT STEADY LOAD / 27.827.4 RELATING LOAD, LIFE, AND RELIABILITY GOAL / 27.927.5 COMBINED RADIAL AND THRUST LOADINGS / 27.1227.6 APPLICATION FACTORS / 27.1327.7 VARIABLE LOADING / 27.1327.8 MISALIGNMENT / 27.16REFERENCES / 27.17GLOSSARY OF SYMBOLS a Exponents; a = 3 for ball bearings; a = 10A for roller bearings AF Application factor b Weibull shape parameter C5 Static load rating C10 Basic load rating or basic dynamic load rating / Fraction F Load Fa Axial load Feq Equivalent radial load F1 /th equivalent radial load Fr Radial load / Integral L Life measure, r or h LD Desired or design life measure LR Rating life measure L10 Life measure exceeded by 90 percent of bearings tested n Design factor nD Desired or design rotative speed, r/min HI Application or design factor at /th level nR Rating rotative speed, r/min R Reliability V Rotation factor; inner ring rotations, V = I ; outer ring, V = 1.20 x Life measure in Weibull survival equation Jc0 Weibull guaranteed life parameter X Radial factor for equivalent load prediction Y Thrust factor for equivalent load prediction 0 Weibull characteristic life parameter, rotation angle $ Period of cyclic variation, rad27.7 INTRODUCTIONFigures 27.1 to 27.12 illustrate something of the terminology and the wide variety ofrolling-contact bearings available to the designer. Catalogs and engineering manualscan be obtained from bearing manufacturers, and these are very comprehensive andof excellent quality. In addition, most manufacturers are anxious to advise designerson specific applications. For this reason the material in this chapter is concernedmostly with providing the designer an independent viewpoint. FIGURE 27.1 Photograph of a deep-groove preci- sion ball bearing with metal two-piece cage and dual seals to illustrate rolling-bearing terminology. (The Barden Corporation.) FIGURE 27.2 Photograph of a precision ball bearing of the type generally used in machine-tool applications to illustrate terminology. (Bearings Divi- sion, TRW Industrial Products Group.)FIGURE 27.3 Rolling bearing with spherical FIGURE 27.4 A heavy-duty cage-guided nee-rolling elements to permit misalignment up to dle roller bearing with machined race. Note the±3° with an unsealed design. The sealed bearing, absence of an inner ring, but standard innershown above, permits misalignment to ±2°. rings can be obtained. (McGiIl Manufacturing(McGiIl Manufacturing Company, Inc.) Company, Inc.)FIGURE 27.5 A spherical roller bearing with two FIGURE 27.6 Shielded, flanged, deep-grooverows of rollers running on a common sphered race- ball bearing. Shields serve as dirt barriers; flangeway. These bearings are self-aligning to permit mis- facilitates mounting the bearing in a through-alignment resulting from either mounting or shaft bored hole. (The Barden Corporation.)deflection under load. (SKF Industries, Inc.) FIGURE 27.7 Ball thrust bearing. (The Tor- FIGURE 27.8 Spherical roller thrust bearing. rington Company.) (The Torrington Company.)FIGURE 27.9 Tapered-roller thrust bearing.(The Torrington Company.) FIGURE 27.10 Tapered-roller bearing; for axial loads, thrust loads, or combined axial and thrust loads. (The Timken Company.) FIGURE 27.11 Basic principle of a tapered-roller bearing with nomenclature. (The Timken Company.) FIGURE 27.12 Force analysis of a Timken bearing. (The Timken Company.) TABLE 27.1 Coefficients of Friction Bearing type Coefficient of friction n Self-aligning ball 0.0010 Cylindrical roller with flange-guided short rollers 0.0011 Ball thrust 0.0013 Single-row ball 0.0015 Spherical roller 0.0018 Tapered roller 0.0018 SOURCE: Ref. [27.1]. Rolling-contact bearings use balls and rollers to exploit the small coefficients offriction when hard bodies roll on each other. The balls and rollers are kept separatedand equally spaced by a separator (cage, or retainer). This device, which is essentialto proper bearing functioning, is responsible for additional friction. Table 27.1 givesfriction coefficients for several types of bearings [27.1]. Consult a manufacturerscatalog for equations for estimating friction torque as a function of bearing meandiameter, load, basic load rating, and lubrication detail. See also Chap. 25. Permissible speeds are influenced by bearing size, properties, lubrication detail,and operating temperatures. The speed varies inversely with mean bearing diameter.For additional details, consult any manufacturers catalog. Some of the guidelines for selecting bearings, which a ...