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It should be noted that according to the law of parity of
tangential stresses the formula (5.8) determines the tangential
stress in the plane of the cross section and at the same time in the
perpendicular plane to the diametric longitudinal cross section of
the rod.
It is easy to see, that each element of the material is in stressed
state of pure shear.

5.4 Calculation for strength

The condition of rod strength (circular or ring cross-section) in
torsion is as follows
M
  x     .
max (5.10)
W


If the mechanical characteristics of the rod material in pure
shear ( and  ) are unknown, the allowable tangential stresses
h m
are    taken on the basis of the theories of strength:


- for the plastic material   0,5 0,6     ;

- for the fragile material   1 1,2     , or the theory of
t
Mohr     1      .
t
For circular cross-section
 D 3
W  . (5.11)

16
The ring section (with internal D and external d diameters)
 D 3
W  1 c 4  , (5.12)

16
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