May 2011

Engineering Case Histories

Case 62: Useful shaft stress equations to remember

Committing several equations to memory can be useful

Sofronas, A., Consulting Engineer

Remembering complex equations is not required for reasonable on-the-spot answers. Neither are sophisticated computer studies when a quick screening study will be satisfactory. This case history shows simple equations1 that have been useful for making quick decisions. Failed solid circular shafts. Too much shaft torque can cause failures; so it’s good to remember this torque equation: Torque due to horsepower (HP): T = 63,000 3 HP / rpm in-lb Torsional shear stress failure solid circular shaft is: Ss = 16 3 K 3 Tin-lb/ (π 3 Din3) lb/in2 K is the stress magnifier caused by sharp corners such as keyways and steps (shoulders). K = 2 can be used for both torsion and bending. Assum

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