Shaft dimensioning

[mech666]

Hey guys, I was wandering if anyone could help me out with the process of dimensioning a shaft.

I vaguely remember a process(G. Niemann) by iterations with this formula:

d =[(16 x M)/ pi x tau)]^3

where M is the torque, and tau is shear stress. One then picks a standard size equal or greater than this. The issue is, I know my torque but I don't know what shear stress to use or what the iterations consist of.
Any recommended links?

Thanks

 

[mech666]

Actually that was "[(16 x M)/ pi x tau)]^-3", cubic root.

 

[desertfox]

hi mech666

I don't know the iteration your talking about however your Ultimate shear stress is normally about 75% of the Ultimate Tensile stress of your material, what you then do is apply a safety factor to this value ie: divide it by 2.
So if you select a material and set your shear stress,you can then re-arrange the standard shear stress formula to give you the radius of the shaft which I have worked out for you below:-

r = ((2*T)/(pi*tau))^0.3

This should give you the radius of your shaft to meet a certain shear stress.

regards

desertfox