Steel Shaft Failure
Steel Shaft Failure
(OP)
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
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





RE: Steel Shaft Failure
RE: Steel Shaft Failure
I looked up the yield strength from a few websites that had that figure listed. I just needed an approximate value don't know why I used four significant figures.
RE: Steel Shaft Failure
RE: Steel Shaft Failure
Thanks
RE: Steel Shaft Failure
The shaft looks thin and there looks like plenty of non-structural mass - do you have a pipe whip problem causing bending fatigue.
Thats all I can think of atm.
gwolf.
RE: Steel Shaft Failure
Thanks for the suggestions. I do believe that it is a cyclic bending problem. We are currently using a higher carbon steel 1440. It seems to be working along with the modification to the setup. However if fatigue from cyclic loading is the problem then I would need a better solution. Any ideas?
RE: Steel Shaft Failure
Next check out the natural frequencies of that shaft and compare with driving frequencies.
I also wondered whether the stuff you are moving is occasionally jclogging and giving the shaft a kick every now and then.
RE: Steel Shaft Failure
htt
RE: Steel Shaft Failure
The material that I am moving is recyclable materials such as newspapers, pop cans etc so I doubt it is having that effect of the shaft.
RE: Steel Shaft Failure
Doesn't matter much; if the chain drive helped its survival, that suggests the shaft is _way_ too skinny, and is bending under the weight of all the crap lying on top of it, or beating on it.
Mike Halloran
Pembroke Pines, FL, USA
RE: Steel Shaft Failure
RE: Steel Shaft Failure
RE: Steel Shaft Failure
Johnny Pellin
RE: Steel Shaft Failure
As materials guys, we know parts can fail at less than yield by 'fast fracture' or by fatigue mechanisms such as this. For the theory behind your photo of the broken shaft, look up the 'Paris Law'.
Besides the brute force solution of a bigger shaft, other fixes can include a round bottom keyway, tougher material, shot peening the surface, higher chemical purity in the steel, finer grain size, repositioning the bearing(s), or combinations.
Also was that a typo in describing the steel that you switched to? I don't recognize 1440; maybe you meant 4140, which can be heat treated to have better fracture toughness than a hypereutectic steel like the original 1080. (All those carbides are great for strength but can nucleate cracks.) I believe lower, not higher, carbon is better in this case. Remembering the tradeoff between YS and K_1c, I would suggest selecting the lowest carbon content consistent with the yield strength needed, and with a tempered martensitic structure.
RE: Steel Shaft Failure
I recommend that you find a stress specialist in your company or outsource it.
RE: Steel Shaft Failure
You can ask in the vibration forum for additional information.
RE: Steel Shaft Failure
Yes that was a typo I meant 4140. You are right the higer the carbon content the more brittle your material gets and more chance to nucleate cracks. Your suggestion about a tempered martensitic structure is great since it blalances out the compromised strength from choosing a lower carbon steel.
It is clear to me now that the mechanism of failure was due to fatigue. I am having some difficulty in calculating the distributed load on the shaft from the rubber components since they have an eccentric shape and the shaft is driven through the middle of them. I need this load to get the amount of deflection and see if that is what caused it to fail. In regards to 'Paris Law' how do I determine the crack size?
gwolf2
I would not be on this forum if that was an option
RE: Steel Shaft Failure
Yes your shaft picture is a classic fatigue failure.
Have a look at this site:-
http://w
desertfox
RE: Steel Shaft Failure
You asked about crack sizes to use for the Paris Law. The final crack size, of course, occurs just before the break, and is determined by the 'plain strain fracture toughness' of the material, K_Ic (worst case), and the max applied tensile stress.
a_f = 1/pi*(K_Ic/sig_max)^2
Initial crack size, if unknown, can be taken as the smallest detectable flaw with whatever NDT method you are using. None? Eyeball? Then how about
a_o = 0.03 inch.
Previously you had asked what cyclic loading is. You probably know by now - as the beam rotates, the deflection/stress at any fixed point (off axis) is a sine function of the rotation angle. This is the cyclic loading; this is the range for finding delta-K in the Paris eqn. (The o'clock position of the longitudinal groove, however deep it is, will also throw in a bias.)
On shaft deflection: Reasonably conservative assumptions and best engineering judgments can simplifiy long hand calculations on fatigue life. From the drawing, it looks like you can assume a beam with a uniform distributed load for a first approximation. (or x-number of point loads superimposed).
Some other thoughts on a material fix – specify "aircraft quality" steel. Consider nitriding or carburizing both of which are supposed to improve fatigue life, I think, because a thin surface layer is placed in compression. Lastly avoid corrosive environments, plating or phosphating.
RE: Steel Shaft Failure
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 ??
RE: Steel Shaft Failure
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
RE: Steel Shaft Failure
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.
RE: Steel Shaft Failure