Machinery's Handbook, 31st Edition
LIMITS AND FITS 651 between its minimum and maximum value is small to maintain the resulting pressures within reasonable limits. These fits are described as follows: FN 1 Light drive fits are those requiring light assembly pressures and produce more or less permanent assemblies. They are suitable for thin sections or long fits, or in cast-iron external members. FN 2 Medium drive fits are suitable for ordinary steel parts or for shrink fits on light sec tions. They are about the tightest fits that can be used with high-grade cast-iron external members. FN 3 Heavy drive fits are suitable for heavier steel parts or for shrink fits in medium sections. FN 4 and FN 5 Force fits are suitable for parts that can be highly stressed or for shrink fits where the heavy pressing forces required are impractical. Graphical Representation of Limits and Fits.— A visual comparison of the hole and shaft tolerances and the clearances or interferences provided by the various types and classes of fits can be obtained from the diagrams on page 652. These diagrams have been drawn to scale for a nominal diameter of 1 inch. Use of Standard Fit Tables.— Example 1: A Class RC 1 fit is to be used in assembling a mating hole and shaft of 2-inch nominal diameter. This class of fit was selected because the application required accurate location of the parts with no perceptible play (see De- scription of Fits , RC 1 close sliding fits). From the data in Table 8a, establish the limits of size and clearance of the hole and shaft. Maximum hole = 2 + 0.0005 = 2.0005; minimum hole = 2 inches Maximum shaft = 2 – 0.0004 = 1.9996; minimum shaft = 2 – 0.0007 = 1.9993 inches Minimum clearance = 0.0004; maximum clearance = 0.0012 inch Modified Standard Fits.— Fits having the same limits of clearance or interference as those shown in Table 8a to Table 12 may sometimes have to be produced by using holes or shafts having limits of size other than those shown in these tables. These modifications may be accomplished by using either a Bilateral Hole System ( Symbol B ) or a Basic Shaft System ( Symbol S ). Both methods will result in nonstandard holes and shafts. Bilateral Hole Fits (Symbol B): The common situation is where holes are produced with fixed tools such as drills or reamers; to provide a longer wear life for such tools, a bilateral tolerance is desired. The symbols used for these fits are identical with those used for standard fits except that they are followed by the letter B. Thus, LC 4B is a clearance locational fit Class 4, except that it is produced with a bilateral hole. The limits of clearance or interference are identical with those shown in Table 8a to Table 12 for the corresponding fits. The hole tolerance, however, is changed so that the plus limit is that for one grade finer than the value shown in the tables and the minus limit equals the amount by which the plus limit was lowered. The shaft limits are both lowered by the same amount as the lower limit of size of the hole. The finer grade of tolerance required to make these modifications may be obtained from Table 6 . For example, an LC 4B fit for a 6-inch diameter hole would have tolerance limits of + 4.0, – 2.0 ( + 0.0040 inch, – 0.0020 inch); the shaft would have toler ance limits of – 2.0, – 6.0 ( – 0.0020 inch, – 0.0060 inch). Basic Shaft Fits (Symbol S) For these fits, the maximum size of the shaft is basic. The limits of clearance or interference are identical with those shown in Table 8a to Table 12 for the corresponding fits, and the symbols used for these fits are identical with those used for stan dard fits except that they are followed by the letter S. Thus, LC 4S is a clearance locational fit Class 4, except that it is produced on a basic shaft basis.
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