CSA Z234.3-89 pdf free download.Guide for the Selection and Use of Preferred Numbers.
a given characteristic, the same series should if possible be used for the complete range; however, it may
be necessary to consider the effect on other parameters. For example, the concept can be used for variety
reduction when applied to the diameters of tube. However, the cost per length of tube is a function of its
mass, or of the square of the diameter Therefore, In the larger sizes. it may be necessary to change from an
R20 to an R40 series to avoid excessively large cost steps. See Examples 7 and 10 In Appendix C.
6.4.2
Numbers should not be picked out of various series at random merely because they happen to be preferred numbers.
6.4.3
If it is necessary to select a derived series, preference should be given to a series that inch.rdes the number 1.
6.4.4
Series derived from the R40 series should be avoided as far as possible. Series derived from the R80 series are not used.
6.4.5
A shifted series should be used only when It cannot be avoided; for example, when it IS imperative that a particular size be included that is not part of a basic series.
6.5 Selection of Degree of Rounding
6.5.1
The selection of numbers may be from the R, R’, or R” series, as appropriate for the specific Characteristic.
The R series of numbers are, in general, more applicable to nominal sizes and to primary functional characteristics, such as safe working loads for wire ropes, the nominal capacities of containers, or the lifting capacities of cranes. For example, the fact that a crane has a lifting capacity of 6.3 t is advantageous in the context of a logical range of capacities, and It presents no design or production problem that would discourage the use of the preferred number.
The R’ and R” series, on the other hand, may be more suitable for physical dimensions, where such dimensions as 6 mm or 3 m may be more convenient than the corresponding preferred numbers 6.3 mm or 3.15 m.
6.5.2
It may be desirable, or even necessary, to depart from preferred numbers in such cases as the following
(a) when whole numbers are needed, such as 32 instead of 31 .5 for the number of teeth on a gearwheel;
(b) when the number of significant figures gives an impression of precision that is not justified: for
example. 1/30 s instead of 1/31.5 s for a camera shutter speed (see Example 12, Appendix C);CSA Z234.3-89 pdf free download.