Shaft Torsional Failure

To understand shafts and why they fail, you need to understand the relationship between stress and strain for steel. Stress is the force carried by a material per unit area, measured in psi (pounds per square inch) or Mpa (Megapascals or Mega Newtons per square meter). 

If a material is under tension, the stress is acting to pull apart the molecules that make it up, making it longer; if the material is under compression, the stress is pushing the molecules together, causing the material to get shorter (and fatter as the compressed material “bulges” outward) if enough stress is applied. Strain is the change in the length, or elongation per unit length, of a material under a tensile stress. Most shafts are made of hot-rolled carbon steel, but for more specialized loads or environments, you may see shafts that are made of alloyed or stainless steel. When a tensile stress is added to a material, the material begins to deform at a certain level of stress. This applies to steel, wood, concrete or any other “engineering” material. In the case of a motor shaft, the material is steel. The deformation due to the tensile stress is elastic until the stress reaches its yield strength point for the steel (typical carbon steel = 73000-psi or 503-Mpa). The yield strength will vary with the material. For example, a 416 stainless steel shaft, while offering corrosion resistance, will actually have slightly lower yield strength than a typical 1045 hot-rolled carbon steel.