Page 292 - Marks Calculation for Machine Design

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STRENGTH OF MACHINES
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over very long periods of time. Unknowingly, the designers of these machines used such
large factors-of-safety that the stress levels were kept below a value that allowed an infinite
life of the machine. This was clearly the source of the first law of machine design: “When
in doubt, make it stout!”
7.2 REVERSED LOADING
The first type of dynamic loading to be presented is called reversed loading, where the load
on the machine element varies from some positive value to the same value but negative, and
back. The cycle repeats itself some number of times, or cycles, denoted (N). Three square
wave cycles of this type of loading are shown in Fig. 7.1, where the mean stress (σ m ) is zero
and the amplitude stress (σ a ) is the magnitude of the stress (σ) produced by the reversed
loading.

s a Square wave

Stress (s) s m 0 1 2 3 N

cycles

-s a

FIGURE 7.1 Reversed loading (square wave).

The reversed loading could also be represented by a saw tooth wave form like that
shown in Fig. 7.2, or as the sinusoidal wave form shown in Fig. 7.3, which is the most
common form, and is the basis for determining the critical level of stress for a safe design
under dynamic conditions. However, whatever wave form the reversed loading represents,

Saw tooth wave
s a
Stress (s) s m 0 1 2 3 cycles

N

-s a

FIGURE 7.2 Reversed loading (saw tooth wave).