Shaft Step Stress Concentration

[DAVIDSTECKER]

I am working on the design of an output shaft for a gearbox and need some input about a type of fillet for the step between two different diameters.
I know the general rule is to use as large of fillet as practical, but some time ago I ran across a design that actually plunged into the smaller diameter then went straight up the face to the larger diameter.
This was supposed to be better than just a fillet in the 90 deg. step.
Does anyone have experience with this practice and is it the better way to go?
Best regards, David

 

[rb1957]

you can do this under-cut ... i think it's main advantage is simplicity in manufacture, but i doubt it has a better Kt than a conventional shoulder radius ... try this link to calculate Kt ...

http://www.fatiguecalculator.com/finders/findkt.htm

 

[DAVIDSTECKER]

Cool web site, thanks.
The under cut I saw came in at a slight angle as it approached the step, and then left the radius. Not quite a "full" groove.
Actually this design was shown to me by someone at a major electric motor company and he said that this practice all but eliminated failures.

 

[DAVIDSTECKER]

I did a search and came up this thread that sounds some what like the relief I'm talking about.
thread2-106261.
What do you think?

 

[TVP]

David,

The stress concentration will be at a minimum when using a large radius. The groove feature that you mentioned is better than a sharp corner, but not as good as a large radius. It is used when a radius cannot be accommodated, due to a bearing sitting on the face of the larger diameter portion, etc.

 

[Fabrico]

Gearboxes and PTO's often use these to define a break point if there is gong to be one.

Step downs are also used at the end of splines. The jury is still out as to whether a step down is better or worse than spline groove ends.