18 1. INTRODUCTION TO RELIABILITY IN MECHANICAL DESIGN
• From Table 1.5, both Cases #1 and #2 have the same factor of safety: 1.25. e same
value of the factor of safety implies that both Case #1 and Case #2 will have the
same measure of the status of safety. However, when the reliability approach is used
to check the status of safety of Case #1 and Case #2, the reliabilities of these two
designs are quite different, as shown in Table 1.5. Case #1 has a reliability 0.9977,
and Case #2 has a reliability 0.8340 only. e cause for this inconsistent result is
due to the uncertainty of design parameters. e traditional design approach with
the factor of safety cannot quantitively consider the effect of uncertainty. e design
approach with reliability does consider the effects of uncertainty.
• From Table 1.5, the factor of safety in the design Case #3 is 2 and is larger than the
factor of safety 1.25 of the design Case #1 from the table. According to the meaning of
factor of safety, this indicates that the components from the design Case #3 should be
relatively safer than the components from the design Case #1. However, the reliability
of the components for the design Case #3 is much less than the reliability of the
component for the design Case #1 from Table 1.5. e cause for these contradictory
conclusions is mainly due to the uncertainty of design parameters. So, a higher factor
of safety does not guarantee a much safer component. However, higher reliability will
certainly guarantee just that.
• From Table 1.4, the simple information about the design parameters, that is, deter-
ministic values, are required when the design approach with a factor of safety is used
for component design. However, when the design approach with reliability is used
for component design, a large amount of information about design parameters are
needed because the type of distributions and corresponding distribution parameters
are required.
In summary, both the factor of safety and the reliability are the measure of the status of
safety of a component. Both are successfully used for mechanical component design. e ad-
vantages of the factor of safety are simple and do not require much information about design
parameters. e disadvantages are: (1) it cannot be used to explain possible component failure;
(2) the higher the factor of safety of components does not guarantee that it will be much safer;
and (3) it cannot include the effects of uncertainty of the design parameters. e advantages
of reliability are: (1) it not only indicates the probability of safe components, but also indicates
the probability of component failure; (2) the higher reliability of a component certainly guar-
antees that it is much safer; and (3) the approach with reliability can fully consider the effects
of uncertainties of design parameters. e main disadvantage of the design approach with reli-
ability is that much more information or a large amount of data about uncertainties of design
parameters are required. Without reliable descriptions of uncertainties of design parameters, the
implementation of the design approach with reliability is meaningless.