Machinery's Handbook, 31st Edition
689
British Standard Preferred Numbers and Sizes British Standard Preferred Numbers and Preferred Sizes.— This British Standard, PD 6481:1977 1983, gives recommendations for the use of preferred numbers and pre ferred sizes for functional characteristics and dimensions of various products. The preferred number system is internationally standardized in ISO 3. It is also referred to as the Renard, or R, series (see American National Standard for Preferred Numbers on page 687). The series in the preferred number system are geometric series, that is, there is a constant ratio between each figure and the succeeding one, within a decimal framework. Thus, the R5 series has five steps between 1 and 10, the R10 series has 10 steps between 1 and 10, the R20 series, 20 steps, and the R40 series, 40 steps, giving increases between steps of approximately 60, 25, 12, and 6 percent, respectively. The preferred size series have been developed from the preferred number series by rounding off the inconvenient numbers in the basic series and adjusting for linear measurement in millimeters. These series are shown in Table 2. After taking all normal considerations into account, it is recommended that (a) for ranges of values of the primary functional characteristics (outputs and capacities) of a series of products, the preferred number series R5 to R40 (see page 687) should be used, and (b) whenever linear sizes are concerned, the preferred sizes as given in the following table should be used. The presentation of preferred sizes gives designers and users a logical selection and the benefits of rational variety reduction. The second-choice size given should only be used when it is not possible to use the first choice, and the third choice should be applied only if a size from the second choice cannot be selected. With this procedure, common usage will tend to be concentrated on a limited range of sizes, and a contribution is thus made to variety reduction. However, the decision to use a particular size cannot be taken on the basis that one is first choice and the other not. Account must be taken of the effect on the design, the availability of tools, and other relevant factors. Table 2. British Standard Preferred Sizes, PD 6481: 1977 (1983) Choice Choice Choice Choice Choice Choice 1st 2nd 3rd 1st 2nd 3rd 1st 2nd 3rd 1st 2nd 3rd 1st 2nd 3rd 1st 2nd 3rd 1 5.2 23 65 122 188 1.1 5.5 24 66 125 190 1.2 5.8 25 68 128 192 1.3 6 26 70 130 195 1.4 6.2 28 72 132 198 1.5 6.5 30 74 135 200 1.6 6.8 32 75 138 205 1.7 7 34 76 140 210 1.8 7.5 35 78 142 215 1.9 8 36 80 145 220 2 8.5 38 82 148 225 2.1 9 40 85 150 230 2.2 9.5 42 88 152 235 2.4 10 44 90 155 240 2.5 11 45 92 158 245 2.6 12 46 95 160 250 2.8 13 48 98 162 255 3 14 50 100 165 260 3.2 15 52 102 168 265 3.5 16 54 105 170 270 3.8 17 55 108 172 275 4 18 56 110 175 280 4.2 19 58 112 178 285 4.5 20 60 115 180 290 4.8 21 62 118 182 295 5 22 64 120 185 300 For dimensions above 300, each series continues in a similar manner, i.e., the intervals between each series number are the same as between 200 and 300.
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