Skip to main content
Henry Newman
Water resource engineer
Asked a question 2 years ago

How do you calculate twisting moment?

how to calculate twisting moment?

Where am I?

In TheConstructor you can ask and answer questions and share your experience with others!

Timmothy Ferguson
Assistant Project Manager
How do you calculate twisting moment?


Torque, T = F x r x sin (theta), where r=distance between the rotation axis and the point of force applied, F  =force applied and theta= angle between F and r.

Twisting moment,

T = (shear stress X polar moment of inertia) / r.

Torsion is the twisting of a beam under the action of a torque (twisting moment). It is systematically applied to screws, nuts, axles, drive shafts etc, and is also generated more randomly under service conditions in car bodies, boat hulls, aircraft fuselages, bridges, springs and many other structures and components. A torque, T , has the same units (N m) as a bending moment, M . Both are the product of a force and a distance. In the case of a torque, the force is tangential and the distance is the radial distance between this tangent and the axis of rotation.

All torsion problems can be solved using the following formula:

T/J = shear stress/ r = (G * angle)/ L


T = torque or twisting moment, [N×m, lb×in]
J = polar moment of inertia or polar second moment of area about shaft axis, [m4, in4]
τ = shear stress at outer fibre, [Pa, psi]
r = radius of the shaft, [m, in]
G = modulus of rigidity (PanGlobal and Reed’s) or shear modulus (everybody else), [Pa, psi]
θ = angle of twist, [rad]
L = length of the shaft, [m, in]

Calculation of twisting moment:

When we try to rotate a steel bar then that moment is nothing but the bending moment.

The twisting moment is a special case of a bending moment.

The twisting moment is also called a torsional moment or torque

When we twist the end of the bar either clockwise or counterclockwise then bending moment will form.


T= (G x angle ) x J/ L


T – Torque

J- polar moment of inertia

L – length

G – Modulus of rigidity

Thank You.