What is torsion of shafts?

Torsion is caused by bending moment along the longitudinal direction (polar direction). Torsion is nothing but the twisting of shaft.

Is there any difference between bending moment and twisting moment ?

Yes there are some differences between bending moment and twisting moment. These differences are as follows:-

Twisting Moment

1) The moment is taken about longitudinal axis. I.e along the length of the shaft.
2) Due to twisting moment, only shear stress is produced.

Bending Moment

1) The moment is about transverse axis. I.e not along the axis of the shaft.

2) Only normal stresses are produced in bending.

What are the assumptions in pure torsion theory?
  • The material should be isotropic, homogenous and linear elastic such that hook's law is valid.
  • Twisting should be uniform throughout the shaft.
  • Torque applied should be along the axis of shaft / along longitudinal direction.
  • The cross section should be circular and prismatic. No warping of cross section should take place even after application of twisting moment.

What are the effects of torsion?
  • Development of shear stresses in two mutual perpendicular planes:-
  • Along circumference (cross section)
  • Perpendicular to plane of cross section i.e along the length of shaft.

In radial direction there is no shear stress.
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