Steel Shaft Failure 4

[MetalGearMan]

Hi. I am a materials engineer and currently have a project that I require some help with. It is a fairly straightforward setup that I am dealing with. I have posted the setup as an attachment. The drawing was done after a modification to the system was made. The modification was moving the gear motor box further away from the main shaft with the use of a drive chain to reduce the stress on the shaft. This was done because in the original setup where the gear motor was connected directly to the main shaft, the shaft failed (almost like snapped in two). I am trying to figure out why it failed in the original setup. The shaft material used was carbon steel AISI 1080. The yield strength of this material is about 375.8 Mpa. I have done some torsional analysis on it and came up with a relatively small shear stress on the bar which is well below the yield strength. Maybe I am negelcting some stresses or loads in my calculations? It must be some other loads or stresses that caused it to fail like it did. I would really appreciate any help or ideas with this. Please do not hesistate to throw suggestions at me.

PS. The componenets resting on the main shaft and the other shafts are made of natural rubber and rotate with the shafts to move materials upwards.

Thanks

 

[rb1957]

guys,

i question the value in trying to teach someone fatigue analysis and the practical ins and outs of fatigue when they clearly lack the stress analysis basis to understand the work, no slight intended MGM.

but it sounds as though changing from a direct drive (having the motor directly connected to the shaft) to an indirect drive (using a chain) has fixed the problem. doesn't that suggest that the problem was the "directness" of the drive ? wouldn't the fatigue loading on the shaft be same in both drive configuations ??

 

[gwolf2]

MetalGearMan

Well OK, I'll be a little more blunt.

Having carefully read through your comments I conclude that you have such a poor understanding of stress, fatigue and dynamics fundamentals that I doubt that there will be any benefit in you continuing with this thread.

Unless someone on this thread walks you through every single detail of the concepts and equations and guides you through the measurements and data which you need, you are never going to get to the bottom of this.

I think that you are way out of your depth on this one and need to seek local help.

This is a tips forum not free detailed consultancy service.

gwolf

 

[MetalGearMan]

Hey Guys

Thanks for all your help and suggestions. I figured out that the loads on the shaft from the rubber components along with the vibrational stress are what caused the shaft to fail. Once the modification was made, the vibrational stress was reduced greatly on the shaft. Also, the new shaft material 4140 gives greater durability and is less brittle and provides for a less likely scenario for crack propagation. I now have a variety of options to make a more permanent solution in the future thanks to you guys.

Cheers

gwolf2: No commment.

 

[gwolf2]

I rest my case