Most power transmission shafts experience two types of combination of stresses:
- the combination of alternating stresses with mean stresses
Use the Soderburg relationship for ductile materials. This covers the majority of cases.
Direct Stresses:
f m/f y + f alt/f e = 1
Shear Stresses:
Tm/Ty + Talt/Te = 1
- the combination of direct stresses with shear stresses
In most power transmission shafts both shear stresses and direct stresses are fluctuating, often in phase with one another since they are the result of a single system element (eg: belt ).
Use an equivalent static stress, for both direct and shear stresses, as an estimate of their combined effect. This has the same effect as the combined effect of the mean and alternating stresses.
Static Direct Stress:
f s = f m + c.f alt (where c = f y/f e)
Static Shear Stress:
Ts = Tm + c'.Talt (where c' = Ty/Te)
Using these equivalents the maximum shear stress in the (ductile) material may be found in the conventional manner. Refer to a standard text for this purpose.