# Design of a shaft subject to torsion/bending/fatigue

1. Dec 14, 2011

### danz001

I have attached the design task I have been given and I am unsure as to what to do next! I will explain that which I have done so far and if anyone can point me in the appropriate direction that would be greatly appreciated!
1. I have created a Free body diagram (FBD), and drawn in the unknown two reaction forces at bearings and the torques where known.
2. For the unknown torque force 'F' I have assumed there is no net torque for the system, and stating my positive Z direction and using T=(f2-f1)xr, have deduced that F=1.1KN
3.I have then gone back to my FBD and drawn in the newly calculated force, and by considering moments throught the first bearing support A and deduced the reaction force at B is 5.877KN. Using the sum of Y forces = 0, I could then calculate for at A=6.023KN.
4. I then drew my shear force diagram, followed by my bending moment and torque diagrams!

Now I am stuck!
I am lead to believe that I must use the relevant equivalent bending moment and torque diagrams, and tau, sigma and twist rate values calulate a suitable diameter for the shaft.
Where:
Me= 0.5[ Mmax +[Mmax^2+T^2]^0.5 ]
Te=[Mmax^2+T^2]^0.5
Is my Mmax value simply deduced from my highest value on my BM diagram, and what T force do use?
I'm not sure if these questions are rather elementary, apologies if that is the case!
Many thanks
Dan

2. Dec 14, 2011

### danz001

didn't seem to attach