Hi, There are tons of equations out there to determine the stresses in a material due to forces/torques etc. My problem is, I never know how much stress is allowed! I understand that different applications require different factors of safety, but I never really know what factor to use. Maybe an example will illustrate my point: I need to design a shaft to transmit torque. Lets say I want to use steel (Yield 220MPa, UTS 341MPa). I can vary the diameter to set the amount of stress the component will see. If I want the shaft to last 20 years, and it isn't a safety critical component, what nominal stress should I design it for? 50Mpa, 100MPa...? Cheers!