Limits, Tolerances, and Fits 223
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d:\N-Design\Des15-1.pm5
15.3.2.2 Formulae for calculating fundamental hole deviation
The fundamental deviation for holes are derived from the formulae, corresponding to the shafts,
with the following modifications :
(i) As a general rule, all the deviations for the types of holes mentioned in (ii) and (iii)
below, are identical with the shaft deviation of the same symbol, i.e., letter and grade but
disposed on the other side of the zero line. For example, the lower deviation EI for the hole is
equal to the upper deviation es of the shaft of the same letter symbol but of opposite sign.
(ii) For the holes of sizes above 3 mm and of type N and of grade 9 and above, the upper
deviation, ES is 0.
(iii) For the holes of size above 3 mm of types J, K, M and N of grades upto and inclusive
of 8 and for the types P to ZC of grades upto and inclusive of 7, the upper deviation ES is equal
to the lower deviation ei of the shaft of same letter symbol but one grade finer (less in number)
and of opposite sign, increased by the difference between the tolerances of the two grades in
question.
Example 2 Calculate the fundamental deviations for the shaft sizes given below :
(a) 30 e8 (b) 50 g6 (c) 40 m6.
From Table 15.4, the deviations for shafts are obtained :
(a) The upper deviation es for the shaft e
= – 11 D0.41
The value for D = 18 30× = 23.24 mm.
Hence, es = – 40 microns (tallies with the value in Table 15.2).
(b) The upper deviation es for the shaft g
= – 2.5 D0.34
The value for D = 30 50× = 38.73 mm.
Hence, es = – 9 microns (tallies with the value in Table 15.2)
(c) The lower deviation ei for the shaft m
= + (IT 7 – IT 6)
From the Table 15.1, the size 40 is in the range 30 and 50 and hence the mean diameter
D, is 38.73 mm
Tolerance unit i = 0.45^3 D + 0.001 D
= 1.58 microns
The fundamental tolerance for grade 7, from the Table 15.1 is 16i, i.e., 25 microns.
The fundamental tolerance for grade 6 is 10i or 16 microns.
Hence, ei = 25 (IT 7) – 16 (IT 6) = + 9 microns (tallies with the value in Table 15.2).
Example 3 Calculate the fundamental deviations for the hole sizes given below :
(a) 40 D9 (b) 65 F8.
From Table 15.4, the deviations for holes also can be obtained (article 15.3.2.2).
(a) The lower deviation EI for the hole D is given by
EI = + 16 D0.44, where D = 30 50× = 38.73 mm
Thus, EI = 80 microns (tallies with the value in Table 15.3).
