Can a hollow shaft be compared with a solid shaft by outside diameter alone?

The short answer

No. The material distribution across a shaft section affects torsional stiffness and stress, not just outside diameter. A hollow and solid shaft of equal outside size therefore need different assessments. This matters when comparing wind-generator shaft variants, even before their materials or fatigue details are considered.

Use the appropriate section property

For an ideal uniform circular shaft, the polar second moment is proportional to the outside diameter to the fourth power minus the inside diameter to the fourth power. A solid section has zero inside diameter. This area property has length-to-the-fourth units; it is different from the rotor’s mass moment of inertia used in acceleration and kinetic-energy calculations.

Compare an explicitly hypothetical pair

If inside diameter is 60% of outside diameter, the ideal hollow section retains 1 - 0.6 to the fourth power = 0.8704, or 87.04%, of the solid section’s polar second moment. Its cross-sectional area is 64% of the solid section’s area. These ratios illustrate geometric distribution only. They do not establish allowable torque, fatigue strength, buckling resistance or a safe wall thickness for any actual shaft.

Request the complete shaft comparison

Ask for outside/inside diameters, length, material, connection details and the relevant torque/bending/load cases. Keyways, shoulders and manufacturing features can govern local stresses beyond the uniform-shaft model. Ringfeder’s selection resources separately assess hollow-shaft connections, reinforcing that bore geometry matters at the interface. No MARSTON shaft construction is asserted. Do not substitute a hollow shaft from this ratio or infer capacity from weight reduction; use the designer’s actual structural and connection assessment. At a connection, a hollow shaft’s wall can also deform under radial contact pressure. A torsional section comparison therefore does not settle the suitability of a shrink fit or keyless locking device on that shaft.