pic: Broken Output Shaft

[AdamHeard]

I'm certain the large slot for the dog shifter was more of a stress riser than the snap ring grooves.

And Chris, not that most teams use snap rings on the extremes of the shaft, where there is no load being applied; it's less common to see them between loads like this.

Originally Posted by EricVanWyk

Could someone with some MechSkills post an FMEA of the stress in a design like this vs a key-way?

Thanks!!

It wouldn't be a universally accurate demonstration.

The location of all such features and loads would have a large effect on the result.

[Ian Curtis]

The chart on this webpage should sum it pretty nicely. For some reason I can't directly link to it, and google images doesn't turn up any other good ones, so go to this website and search for "Graph illustrating the effect of proper fillet radii on stress concentration" (minus the quotes) and there it will be.



The factor on the left (Kf) is a multiplier to the max torsional stress (and when you exceed the max torsional stress of the material, it will fail!) you find if there was no change in diameter. So, you can see it isn't hard at all to multiply your stress significantly for relatively small changes in shaft diameter.

[JamesCH95]

Originally Posted by AdamHeard

I'm certain the large slot for the dog shifter was more of a stress riser than the snap ring grooves.

And Chris, not that most teams use snap rings on the extremes of the shaft, where there is no load being applied; it's less common to see them between loads like this.




It wouldn't be a universally accurate demonstration.

The location of all such features and loads would have a large effect on the result.

This. They're fine if they're no appreciable torque is being transmitted through it, which is the proper way to utilize snap rings. Heck, two of the FSAE cars I worked on had drive shaft components retained with snap rings in no-load areas.

About an FEA model of the stress concentrations... in my experience many FEA programs don't pick up on stress risers much of the time. I just tried to do it with Solidworks Simulation and the program didn't pick up on the groove as a stress riser. This is a problem when a designer doesn't use common sense and simply trusts FEA results without question.

Here is an interesting FSAE thread on a similar topic with a very good explanation about why the drive shaft failed at the groove.

Again, non-torsionally-stressed grooves are okay. Torsionally stressed grooves will very likely fail.

[mplanchard]

You have a large aspect ratio in this problem - long shaft and short, small keyway.
Make certain you Add-in SolidWorks Simulation under the Tools menu. SimulationXpress will only give you a simple approximation. You need full SolidWorks Simulation.

You will need to adjust the mesh to be finer around the keyway.

Marie

[Alan Anderson]

Originally Posted by mplanchard

You have a large aspect ratio in this problem - long shaft and short, small keyway.

What keyway?

[mplanchard]

sorry - tried to answer this on my iphone. maybe it is a grove?

[JamesCH95]

Learning how to use COMSOL 4.0 at work (an excellent FE program for those who are interested, much better than COMSOL 3.5) I decided to see if it could show the stress risers in this particular situation, and it did so quite well. I've attached two plots. One plot is of the surface stresses, which does indeed show the stress concentrations in the groove. However, I find the slice plot much more illuminating as to why this groove is a stress riser. If you look at how the shear stress flows through the part you can see that it likes to stay near the outside surface, but the snap ring groove abruptly forces stress towards the middle, compressing the stress (if you will) around the groove.

Pay no attention to the magnitude of the stresses, I just picked a load number out of the air that was close-ish.

PS- I hope no one is too upset about reviving an older thread.

[JamesCH95]

Stress plots posted above.

[kibbs425]

No FEA is necessary. Contrary to popular belief. A lot of good analysis can be done with a book, pencil and a napkin! All good ME design books will have a reference to Stress concentrations. They are generally relatively simple to use as long as you can read a chart.

Engineers edge (www.engineersedge.com) is the 1st pass that I usually take when I'm looking and I don't have my book handy and the internet is available. Here is the section on stress concentrations: http://www.engineersedge.com/calcula...tion-shaft.htm

This won't give you the stress unless you know the rest of the numbers, but you can at least compare Kts for the known geometry to determine what has less of an impact.

Shigley's Mechanical Engineering Design is my text book of choice. It was required in my college ME Design class. It is still something that I use regularly and has a lot of great references.

Good luck!