Derivation of Torsion Equation

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Created by: Team Physics -, Last Updated: May 21, 2024

Derivation of Torsion Equation

The derivation of the torsion equation is a fundamental concept in the study of mechanical engineering and materials science. It describes the relationship between the torque applied to a cylindrical shaft and the resulting shear stress and angle of twist along its length. The equation is derived from the principles of equilibrium, compatibility, and material behavior, typically using polar coordinates. This derivation helps in understanding how shafts transmit mechanical power in machines, ensuring their design can withstand applied loads without failure, which is critical for the safety and efficiency of mechanical systems.

Torsion Equation Derivation

The torsion equation describes the relationship between the applied torque (T), the polar moment of inertia (J), the shear stress (τ), the radius of the shaft (r), and the angle of twist (θ) per unit length (L) in a cylindrical shaft. Here’s a step-by-step derivation of the torsion equation:

  1. Assumptions:
  • Homogeneous, isotropic material
  • Plane cross-sections remain plane
  • Angle of twist uniform along the length
  • Material obeys Hooke’s law in shear

Polar Moment of Inertia (J):
The polar moment of inertia for a circular shaft of radius r is given by:

J = πr⁴/2

Shear Strain (γ):
Consider a shaft of length 𝐿L subjected to a torque 𝑇T. The shear strain 𝛾γ at a radius 𝑟r is given by the angle of twist per unit length 𝜃𝐿

γ= r ⋅ θ/L

Shear Stress (τ):
According to Hooke’s law for shear, shear stress τ is proportional to shear strain γ:

τ = Gγ = G ( r ⋅ θ/L )

Infinitesimal Torque (dT):
dT = τ ⋅ r ⋅ dA

Total Torque (T):
Integrate 𝑑𝑇 over the entire cross-sectional area to get the total torque:

T = ∫A τ ⋅ r ⋅ dA

𝑇 = ∫𝐴 ( 𝐺 ⋅ 𝑟 ⋅ 𝜃/𝐿)⋅𝑟⋅𝑑𝐴

𝑇 = Gθ/L ∫𝐴 r²dA

𝑇 = 𝐴 r²dA = J

𝑇 = GθJ/L

Torsion Equation:

Rearranging the above equation gives the torsion equation:

T/J = Gθ/L = τ/r

What is Torsion?

Torsion refers to the twisting of an object due to an applied torque or rotational force. This type of deformation occurs when a moment or couple is applied to a structural member, such as a shaft, causing it to twist around its longitudinal axis. In mechanical and structural engineering, torsion is a critical consideration for the design and analysis of components like drive shafts, axles, and beams subjected to twisting loads.

Key points about torsion:

  1. Torque (T): The applied twisting force that causes torsion.
  2. Angle of Twist (θ): The measure of the rotational deformation of the object.
  3. Shear Stress (τ): The internal stress developed within the material due to the applied torque.
  4. Polar Moment of Inertia (J): A geometric property of the cross-section that affects its resistance to torsion.
  5. Shear Modulus (G): A material property that relates shear stress to shear strain.

The torsion equation, which relates these factors, is crucial for ensuring that components can withstand the applied loads without failure.

What is Torsion Constant?

The torsion constant, denoted as ( J ), measures a cross-section’s resistance to twisting or torsion. For circular shafts, it equals the polar moment of inertia, ( J=2πr⁴/2 ), where ( r ) is the radius. For non-circular sections, ( J ) varies based on shape and dimensions. It influences the relationship between applied torque, shear stress, angle of twist, shear modulus, and length in the torsion equation: (T/J) =Gθ/L = τ/r. The torsion constant is crucial for assessing torsional strength and rigidity in structural and mechanical components.

Torsion Units

Torque (T): The twisting force applied to an object in torsion analysis is measured in Newton-meters (N·m) in the SI system and Pound-feet (lb·ft) in the Imperial system.

Angle of Twist (θ): The angle of twist measures rotational deformation, typically in radians (rad) in the SI system, though degrees (°) can also be used for some applications.

Shear Stress (τ): The internal stress resulting from applied torque is measured in Pascals (Pa) or Newtons per square meter (N/m²) in the SI system, and in Pounds per square inch (psi) in the Imperial system.

Polar Moment of Inertia (J): This geometric property of the cross-section is measured in meters to the fourth power (m⁴) in the SI system and in inches to the fourth power (in⁴) in the Imperial system.

Shear Modulus (G): The shear modulus, relating shear stress to shear strain, is measured in Pascals (Pa) or Newtons per square meter (N/m²) in the SI system, and in Pounds per square inch (psi) in the Imperial system.

Moment of Resistance

The moment of resistance is a crucial concept in structural engineering and mechanics, representing the capacity of a structural member to withstand bending moments. It is the internal moment that balances the external applied moment, ensuring the structure remains stable and does not fail under load.

Definition: The moment of resistance is the moment generated by internal stresses within a structural member, such as a beam or column, that opposes the applied bending moment. It is crucial for maintaining equilibrium and preventing structural failure.

Calculation: The moment of resistance is calculated using the formula:
Mᵣ = f⋅Z
where ( M_R ) is the moment of resistance, ( f ) is the allowable stress (such as yield stress for ductile materials or ultimate stress for brittle materials), and ( Z ) is the section modulus, a geometric property of the cross-section.

Section Modulus (Z): The section modulus is defined as:
Z =I/Y
where ( I ) is the second moment of area (or moment of inertia) of the cross-section, and ( y ) is the distance from the neutral axis to the outermost fiber of the section.

Units: The moment of resistance is typically measured in Newton-meters (N·m) or Pound-feet (lb·ft), depending on the unit system used.

Understanding and calculating the moment of resistance is essential for designing safe and efficient structures, ensuring they can resist applied loads without excessive deformation or failure.

Uses of Torsion Equation Derivation

The derivation of the torsion equation is fundamental in several areas of engineering and materials science. Here are some key uses:

Mechanical Design: The torsion equation helps in designing shafts, axles, and other components subjected to torsional loads, ensuring they can withstand the applied torque without failure.

Material Testing: By deriving the torsion equation, engineers can determine the shear modulus of materials through torsion tests, which is essential for characterizing material properties.

Power Transmission: The torsion equation is critical in the analysis and design of power transmission systems, such as those in automotive drivetrains and industrial machinery, where shafts transmit rotational power.

Structural Analysis: In civil engineering, the torsion equation aids in analyzing and designing structural elements like beams and columns that may experience torsional effects due to asymmetric loading or complex geometries.

Failure Analysis: Understanding the torsion equation allows engineers to predict failure modes in components subjected to torsional loads, contributing to the development of safer and more reliable designs.

Examples of Torsion Equation Derivation

Determining Safe Torque Limits:
One practical application of the torsion equation is determining the safe torque limits for cylindrical shafts. By using the derived equation, engineers can calculate the maximum torque that a shaft can withstand before yielding or failing. For example, in designing a steel drive shaft, the torsion equation helps ascertain the maximum torque that can be applied without exceeding the material’s yield strength, ensuring the shaft’s durability and safety in operation.

Designing Twist Drills:
In manufacturing, the torsion equation is used to design twist drills. These tools must withstand high torsional loads while maintaining their structural integrity. By applying the torsion equation, engineers can optimize the drill’s diameter and material properties to resist twisting forces, preventing failure during drilling operations and enhancing the tool’s lifespan.

Analyzing Torsional Vibrations:
In rotating machinery, torsional vibrations can lead to mechanical failures. The torsion equation helps in analyzing these vibrations and designing components to mitigate them. For instance, in turbine shafts, the equation is used to determine natural frequencies and design damping systems, ensuring smooth operation and reducing the risk of fatigue failure.

Evaluating Torsional Rigidity:
The torsion equation is vital for evaluating the torsional rigidity of various structural elements. In aerospace engineering, for example, the equation helps assess the rigidity of aircraft wings and fuselage sections. By determining the angle of twist under specific loads, engineers can ensure that these components maintain their structural integrity under operational stresses.

Optimizing Material Selection:
In material science, the torsion equation assists in optimizing material selection for components subjected to torsional loads. For instance, in the design of a high-performance automotive axle, engineers use the equation to compare different materials, such as steel and aluminum, to find the best combination of strength, weight, and cost. This optimization ensures that the component performs reliably under torsional stress while meeting other design criteria.


Can the torsion equation be applied to non-circular cross-sections?

Yes, but the calculation of the polar moment of inertia 𝐽J is more complex for non-circular cross-sections.

What is the significance of the radius (r) in the torsion equation?

The radius determines the distribution of shear stress, with maximum stress occurring at the outer surface.

How does the torsion equation help in mechanical design?

It aids in designing shafts and other components to ensure they can withstand applied torques without failure.

Why is understanding the torsion equation important in engineering?

It ensures safe and efficient design of mechanical and structural components subjected to torsional loads.

How does the torsion equation apply to power transmission?

It helps in designing shafts that efficiently transmit rotational power without excessive twist or failure.

How is the torsion equation used in material testing?

It is used to determine the shear modulus of materials through torsion tests.

What is the derivation process for the torsion equation?

It involves relating torque to shear stress through equilibrium, compatibility, and material behavior principles.

Can the torsion equation predict failure modes?

Yes, it helps predict potential failure modes such as yielding or buckling under torsional loads.

What is shear strain (γ) in the context of torsion?

Shear strain is the angular deformation per unit length due to applied torque.

What assumptions are made in deriving the torsion equation?

Assumptions include homogeneous, isotropic material, plane cross-sections remaining plane, and small deformations.

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