question about spline design
question about spline design
(OP)
Hello everyone, I designed a spline in this thread: http://www.eng-tips.com/viewthread.cfm?qid=373420
Now I'm doing an FEA torsional analysis of my shaft to check for fatigue issues. I modeled the spline along with the shaft. Peak stress is showing to be about 80MPa (12ksi) localized around the teeth roots.
Now, when I did my spline calculations, everything seemed to be within the design stress limits as specified by the machinery guide, but now I'm worrying because the fatigue limit for my steel is suggested to be about 35MPa (5200psi) with a safety factor of 2. Is this stress FEA is giving me a fatiguing stress? Do I need to be worried about this?
This has to be a fatigue stress~~~ a start/stop fatigue. This stress would not exist at zero speed. Therefore the spline will fail, yes?
I also have a very nasty torque profile. Peak torque is about 2X higher than the average torque at constant speed conditions. If this torque pulsation is damped away, then my stress would be reduced by 50%, putting me in acceptable stress ranges. The question is though......how much damping will occur. Unfortunately I can't answer that.
Have a good evening.
/PR
Now I'm doing an FEA torsional analysis of my shaft to check for fatigue issues. I modeled the spline along with the shaft. Peak stress is showing to be about 80MPa (12ksi) localized around the teeth roots.
Now, when I did my spline calculations, everything seemed to be within the design stress limits as specified by the machinery guide, but now I'm worrying because the fatigue limit for my steel is suggested to be about 35MPa (5200psi) with a safety factor of 2. Is this stress FEA is giving me a fatiguing stress? Do I need to be worried about this?
This has to be a fatigue stress~~~ a start/stop fatigue. This stress would not exist at zero speed. Therefore the spline will fail, yes?
I also have a very nasty torque profile. Peak torque is about 2X higher than the average torque at constant speed conditions. If this torque pulsation is damped away, then my stress would be reduced by 50%, putting me in acceptable stress ranges. The question is though......how much damping will occur. Unfortunately I can't answer that.
Have a good evening.
/PR





RE: question about spline design
Fatigue is an issue. The 2x is a design margin and may not be enough depending on your loading and material. You can get SN curves for most materials. The curves are not application specific so you may need to test to verify it meets the requirements.
Reverse bending is the worst condition so preloading in compression usually produces the best results. Sorry but your start stop sounds like reverse loading unless you can preload the spline some how.
Also on the SN curve look at your design cycle life. You may find a higher stress is acceptable if you have a short design life. Some steels are considered to have infinite life at 10^6 cycles so if your stress is below the corresponding value you are good. But if you only need 10^4 then your stress could be as much as 50% higher and still work.
Good Luck
RE: question about spline design
RE: question about spline design
Regarding your FEM results, you need to be careful in how you interpret them. Accurately modeling something like a spline joint is very tricky since even extremely small variations in the tooth flank surfaces, alignment between the matings parts, or stiffness of the associated structures can have a huge effect on load distribution and stress levels in the spline teeth. If you are analyzing for HC fatigue with your FEA, then you should be using a composite load case that is a cubic mean of all the torques/cycles the spline is subject to over its anticipated service life. Each start/stop only amounts to one load cycle, which may likely be fully reversing. But if there are any torque variations during a single rotation of the spline in constant speed operation, then these must be taken into account in the composite load case.
RE: question about spline design
@tbuelna, good points. Yeah, I wish I had more experience with FEA modeling this type of system. I've already noticed that small changes in the constraints has a fairly significant impact on the results. The nuances of this analysis has taken me a bit by surprise, because the analysis is ultimately a simple torsion study.
RE: question about spline design
The fatigue analysis should be giving you the stress level needed to survive. Based on your design, materials, load profile, and cycles your max stress should be 35MPa. You are at 80MPa.
Good luck with the modeling.
RE: question about spline design
The more I look at this, the more I think it will be fine. I redid the simulation to improve the accuracy and reduce the stress concentrations, and the stress seems to have gone down to be more inline with machinery guide spline calculations. If these results are right, then I have meet all my design guide recommendations. These results seem to make sense, because the rest of the shaft isn't likely to see stress much greater than what exists in the spline itself, and I believe the spline itself will be robust.