72
Chapter 2
The kb, greater than 0.5, is for steel and only serves as a guide to other
materials. When d_<0.30 many materials fall into the range of spring
diameters where ultimate and endurance limit strengths [2.9] are stated
as a function of wire diameter and the kb is greater than one. This also applys
for constant material thickness.
3. Reliability, kc
The kc value corrects ~re~ for an 8% standard deviation when no other data are
available. In Fig. 2.15 the ~rr-am curve can be developed; however, the solid
line is the average or mean of all data. The reliability of a design using any
point on the solid line is 0.50. Tests have been conducted [2.27] where
the dotted lines A and B represent data spread of 4- 3a derived from several
tests along the curve. Eq. (2.61)-(2.64)
The k¢ values [2.51] are as follows:
R(0.50) = 1.00
(2.6l)
R(0.90) = 0.897
(2.62)
R(0.95) = 0.868
(2.63)
R(0.99) = 0.814
(2.64)
Note: this correction is used when attempting to derive a design line to be
\\\’~
0" m
Figure 2.15 ~rr -- am with material property variations.