Shafts are often subjected to fluctuating loads in practice rather than constant loads. To design shafts like line shafts and counter shafts that experience fluctuating torque and bending moments, combined shock and fatigue factors must be accounted for in calculating the twisting moment and bending moment. These equivalent moments are calculated using combined shock and fatigue factors for bending (Km) and torsion (Kt), with recommended values for Km and Kt provided in a table.

1. Design of Shaft
Shafts Subjected to Fluctuating Loads

2. Shafts Subjected to Fluctuating Loads
In the previous articles we have assumed that the shaft is subjected to constant torque and
bending moment. But in actual practice, the shafts are subjected to fluctuating torque and
bending moments. In order to design such shafts like line shafts and counter shafts, the
combined shock and fatigue factors must be taken into account for the computed twisting
moment (T ) and bending moment (M ). Thus for a shaftsubjected to combined bending
and torsion, the equivalent twisting moment, and equivalent bending moment,
where Km = Combined shock and fatigue factor for bending, and
Kt = Combined shock and fatigue factor for torsion.
The following table shows the recommended values for Km and Kt.