Five Questions on Torsion
Question.1. Differentiate between torque and torsion.
Answer.
Torque : The product of tangential force acting at the circumferential causing twist and the radius of the shaft is called torque or twisting moment. Denoted by T.
Torsion: A shaft is said to be in torsion when equal and opposite forces are applied at the two ends of the shaft.
Question .2. Write the formula for power transmitted by the shaft.
Answer.
Question.3. Write torsion equation.
Answer.
Question.4. Define torsional rigidity of a shaft.
Answer.
Torsional rigidity of a shaft is defined as the torque required to produce a twist of one radian per unit length of the shaft.
Question.5. Derive torsion equation.
Answer.
Fig.
Consider a solid shaft fixed at the end A and free at end B as shown in fig.(a).
Now, after the application of the torque, a line AC originally parallel to the axis of the shaft becomes 
. The angle 
in radians is the shearing strain at all points along the surface of the shaft.
We know that the shear strain is the deformation per unit length.
But from fig. (b) arc 
From equation (i) and (ii)
From the above relation:
For a given shaft and for a given torque, G (modulus of rigidity of the shaft material), l (length of the shaft) and 
(angle of twist) are constant,
Hence shear stress at any section in a shaft is directly proportional to the distance of the section from the axis of the shaft. Hence the maximum shear stress will be at the outer surface and minimum shear stress (i.e. zero) will be at the axis of the shaft as shown in fig. (b)
(a) X-section of a shaft (b) Shear stress diagram
Fig. (b)
Consider an elementary circular ring of thickness ‘dr’ at a distance ‘r’ from the centre of the shaft as shown in fig. (b).
Turning moment of shear stress offered by the elementary ring,
From equation (iii) and (iv)
The above equation is known as Torsion Equation.