Shaft Under Torsion

Consider the steel shaft or sample problem N. 5 of Sect. 5.3 for which the S-N diagram of Fig. 5.26 has been derived under fuUy reversed torsional fatigue limit with if and at 1,000 cycles, corrected to take into consideration factors affecting fatigue, equal to (see sample problem N. 5 of Sect. 5.3) 

5 Stress-Based Fatigue Analysis High Cycle Fatigue 

The maximum stress Xmax will be equal to 2-t = 2-t = 2 t / = 358 MPa. In case (b), 7 = 0.5, we shall assume that the fatigue limit is equal to the stress amplitude )/2 with equal to 

As to 3 it remains exactly that found under fully reversed torsion since at 1,000 cycles the mean stress has absolutely no effect at all 

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Most materials scientists at an early stage in their university courses learn some elementary aspects of what is still miscalled strength of materials . This field incorporates elementary treatments of problems such as the elastic response of beams to continuous or localised loading, the distribution of torque across a shaft under torsion, or the elastic stresses in the components of a simple girder. Materials come into it only insofar as the specific elastic properties of a particular metal or timber determine the numerical values for some of the symbols in the algebraic treatment. This kind of simple theory is an example of continuum mechanics, and its derivation does not require any knowledge of the crystal structure or crystal properties of simple materials or of the microstructure of more complex materials. The specific aim is to design simple structures that will not exceed their elastic limit under load. 

Fig. 5.50 S-N diagrams fm high strength machined finish shaft under torsional fatigue with different R ratio equal to -1, 0, 0.5 and 0.8, respectively...

Belts should not be tightened more than necessary, otherwise the drive and the driven shafts will come under torsion and excessive bending moment. The bearings would also be subjected to excessive stresses. 

Leger, J. Fatigue life testing of crane drive shaft under crane-typical torsional and rotary bending loads. Schenck Hydropuls Mag. 1(89), 8-11 (1989)... 

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