Why does keyway presence reduce allowable shaft stress by 25%?

Keyways create sharp internal corners and cut through the shaft's outermost fibers where bending and torsional stresses are highest. This geometric disruption acts as a stress concentrator and reduces torsional stiffness, which is compensated for by the 25% code reduction per ASME B106.1M.

When should shaft deflection be checked instead of stress limits?

Stiffness and deflection should always be checked for long shafts, or shafts supporting precision gears. Excessive lateral or angular deflection at gear seats will cause gear teeth misalignment, noise, and premature wear.

Shaft Design Basics: Torsion and Bending

Power transmission shafts are the backbones of mechanical systems, subject to complex combinations of torsional shear stresses, reversing bending moments, and axial loads.

Torsional Shear and Bending Stress

When a shaft transmits power, it is subject to an applied torsional moment (torque) which causes torsional shear stress. For a solid round shaft, the maximum torsional shear stress is τ = 16T / (π·d³).

Stress Concentrations and ASME B106.1M

Shafts require shoulders to seat bearings, keyways to mount gears, and retaining ring grooves. These act as stress concentration zones. ASME B106.1M limits allowable shear stress to 30% of yield or 18% of ultimate, whichever is lower.

Formulas

Solid Shaft Pure Torsion

d = [(16 × T) / (π × τ_allow)]^(1/3)

ASME Combined Loading

d = [(16 × N_sf / (π × S_y)) × √(M² + T²)]^(1/3)

Worked Example

Solid steel shaft (S_y = 250 MPa), T = 350 N·m, M = 180 N·m, FOS = 2.0. Result: d = 25.2 mm.

Design Tips

Place pulleys and gears close to support bearings to minimize bending moments
Use generous shoulder radius to reduce stress concentration
Hollow shafts for weight-critical applications

Common Mistakes

Omitting bending moments — shafts rarely carry pure torsion
Neglecting fatigue — rotating shafts experience fully reversed cycles
Keyway correction — 25% allowable stress reduction per ASME B106.1M