stepped shaft stress concentration

[senthil83]

we are using a stepped shaft and i found in all machine design books, to avoid the
stress concentration in the change in diameter we have to provide a fillet radius
to avoid stress concentration.

why can't we taper the change in diamater of shaft for a slope 1:4 so that,
there will not be a abrupt change in cross section and
stress concentration is also reduced?

please clarify

 

[desertfox]

Hi

You can taper the shaft if you wish but if the difference in diameters is large then you will need a long taper, also depending on what's mounted on the shaft,a taper might not be practical due to space available.

 

[rb1957]

as above you can taper if you've got room for it, and provide transition fillets at the ends of the taper. i think there'd still be a stress concentration effect, though it would be much smaller than a traditional fillet. i assume the change is small and you'll have something like an S transition (ie horizontal tangents at both ends) ?

Quando Omni Flunkus Moritati

 

[dvd]

How the tapered surfaces meet the cylindrical surface is important. If there is a small radius blend, then this can be the site for crack growth to initiate. Generous blend radii and polishing of the blend will help greatly.

 

[Cockroach]

Try using a fillet as one quarter of an ellipse in the ratio of 3:1. I have found a marked performance improvement in torsion. So the semi major axis is three times that of the minor axis for all those geometrically impaired by mental visualization.

I suggest you FEA a shaft with and without such a feature as an educational experience to the suggestion.

Good luck with it.

Regards,
Cockroach

 

[Tmoose]

As others said, if you have room to develop a taper, AND provide proper details at the ends of the taper it could work well.

Some regions of some shafts are so lightly stressed that steps, sharp corners and torn surface finish won't reduce fatigue life a bit. So nice finishes and transitions can only be justified under workmanship.

 

[bigTomHanks]

Taper is fine but it adds weight to the shaft that may not be needed. Depending on the application this may have some very negative consequences.