Shaft fatigue analysis predicting infinite life for Al-6061 under 700N load

[guy_alon]

hello,

im trying to do a fatigue analysis on a shaft but keep getting that it will never brake,
which is impossible , because of various cases that it did after something like 3.5 years
the shaft is made of Al-6061 and a divided force of 700N is working on him.
the linear speed at the edge is 1m/s or 382 rpm and it has 1 bearing on each side.
i attached a file with an illustration with more detailes and a picture
i want to find out why it fails and how much weight i need to substruct in order for it to work.

thank you
Guy Alon

 

[Q_Goest]

What stress concentration factor are you using for the corner where it's breaking?

 

[guy_alon]

do you mean Kf from the soderberg equation?
i calculated it and i got
Kf = 1.72

 

[Q_Goest]

What I've generally noticed is that when something fails in fatigue (high cycle fatigue) when it was otherwise not expected to fail, is that the calculation doesn't take certain factors such as stress concentration, surface factor, gradient factor, etc... into account properly. Check these factors; here's one for bending of a stepped shaft for example.
http://www.tribology-abc.com/calculators/e1_1b.htm

 

[john.phillip]

Based on Peterson's Stress Concentration Factors 2nd Ed. I'm getting Kt between 2.51 and 2.58 under bending only, for D=25mm, d=17mm and r=0.5mm:

https://efatigue.com/ (click on stress concentration)

According to the drawing, it seems that the bearings are positioned where d=17mm, is this correct ? If so and considering a distributed load, I'm expecting the nominal bending stress to be at least 35MPa at the fillet between 17mm and 25mm diameters, without applying the estimated Kt. How much are you obtaining ?