TABLE OF CONTENTS
Torsion Constant
The torsion constant also known as the polar moment of inertia (or second polar moment of area), is a geometric property of an object representing its resistance to twisting in response to an applied torque. The units are length^4. The torsion constant is the second moment of area calculated along the z axis. The torsion constant is dependant on the shape (see Torsion Constant for Common Shapes).
Consider the cylinder below where the x axis and y axis are a cross section of the cylinder making a circle and z axis is going through the centre of the cylinder.
The torsion constant is calculated by adding the Second Moment of Area about the x and y axis as follows.
For a circle the second moment of area in both the x and y axis is given by:
Hence the torsion constant of a cylinder is:
The torsion constant along with the Shear Modulus can be used to calculate the angle of rotation given a torque applied at a distance from a fixed point.
See Also
Cross Arm Parameter - Torsion Constant