In a shaft design, increasing the length of the shaft primarily affects what aspect of its physical properties?

• A. Tensile strength
• B. Torsional rigidity
• C. Bending stress
• D. Weight

Answer: (B)

Increasing the length of a shaft primarily affects its torsional rigidity. Torsional rigidity refers to the resistance of a shaft to twisting or torsion. It is calculated using the modulus of rigidity and the shaft's polar moment of inertia. When the shaft length increases, the polar moment of inertia remains unchanged for a uniform cross-section, but the overall structure's ability to resist twisting does change in relation to other design factors.

For instance, a longer shaft, given the same material and diameter, will generally exhibit greater deformation under a given torsional load compared to a shorter shaft. This means that while the torsional rigidity may be effectively altered by other factors such as cross-sectional area or material properties, the increase in length contributes to a decreased capability of that shaft to resist twisting, which can lead to failure under torsional stresses if not properly accounted for in the design.

Tensile strength, bending stress, and weight are influenced by factors like material properties and geometric changes, but in terms of pure length increase, torsional rigidity is the physical property that sees the most direct correlation.