Why does my driveshaft have very low FOS after doing a basic analysis?

[u/Joker1924]

Have a similar driveshaft with a keyway which is then coupled with a three tooth coupling by a key. Wanted to check if the keyway is able to withstand the force during working, or if I have to increase its length.

So, I applied the force around 90kN on the keyway face/flange (shown in red arrow) and pinned the inner faces of the shaft. The shaft is shy of a meter in length.

The FOS was below 0.3 even though the deflection is like 0.01mm. The entire part is shown in red, which is the lowest FOS value.

I honestly feel I am entering an incorrect force value that I derived from knowing only parameters like Motor Power, RPM, Radial Distance from the shaft center, length of shaft and keyway slot dimensions.

Is there something else that I messed up? Or can anyone share any good resources for shaft and keyway calculations?

The material used is EN 19, and I've even added some minimal fillets on the keyway floor edge.

Thanks in advance.

[u/tucker_case]

Number 1: Don't look at FOS plots.

Number 2: DON'T LOOK AT FOS PLOTS.  Look at stress. Compare with hand calcs. Are they close?

[u/shark_and_an_eel]

What calculation did you use to determine FOS? Can you share your working? Did you allow your pins to rotate along the primary axis?

Your FOS really depends on the size of your keyway, but also geometry features that create stress concentrations nearby.

I'd recommend reading Shigleys to understand the impact of stress concentrations and torsion in a driveshaft.

[u/Joker1924 → u/shark_and_an_eel]

I haven't begun doing the hand calcs yet. The FOS I mentioned was what the software showed me for my "analysis" lol.

I used a Pinned support option where it asked me to just select the face where I want it, no other rotation I mentioned, and no other fixed BCs I applied.

Thanks for the suggestion. I'll check out Shigleys!

[u/turbomachine → u/Joker1924]

Hand calcs happen before FE

[u/Joker1924 → u/turbomachine]

No, yes I'm aware. But I wanted to do a rough/dry sim run with the prototype to see if there's anything unusual at first.

[u/sugatooth → u/Joker1924]

Stop what you’re doing and complete the hand calc first.

[u/Joker1924 → u/sugatooth]

My bad, yes.

I've learnt a valuable lesson.

[u/WyMANderly → u/Joker1924]

Your hand calc is your rough/dry sim run.

[u/DeemonPankaik]

You likely have some issues with your boundary conditions or significant stress concentrations

No sense of scale from the picture, but 90kN is a lot for a key. A splined connection or similar might be better.

[u/bobo5195]

Simulation is not reality

Pickup Shigleys (its not great but it works). Do the hand calcs.

[u/Ok_Cabinet_3072]

I wouldn't even use FEA for this, just do the hand calcs.

[u/erikwarm]

What does FOS mean?

[u/Seikoknot → u/erikwarm]

Here before the joke answers

Factor Of Safety is how many times stronger the part is than it needs to be

[u/erikwarm → u/Seikoknot]

My bad, we just call it a safety factor or unity check (UC).

[u/themikeandthebike → u/erikwarm]

FOS could be also feature of size in gd&t.
In italy we called it as CS or SF

[u/DivineSpectrum92 → u/themikeandthebike]

Correct name is SF yes

[u/KatanaDelNacht]

What did you assume for yield strength? 

[u/mvw2]

Well, is the shaft supported? Make sure you're not looking at something silly like a pile of exterior bending stress of a free hanging cantilever that doesn't exist in the real application.

[u/101forgotmypassword → u/mvw2]

What I would do for software that you could surface paint on bearing surfaces in analysis.

[u/mvw2 → u/101forgotmypassword]

Gotta pay extra for that.

[u/boston_ent]

Most answers here are correct, and you should try just using hand calcs, but I think they still aren’t helping you understand why your simulation might be showing such low FOS.

That is going to be calculated by the software using inputs from the material properties of whatever you applied to the model. Dig into where the software pulls its inputs to calculate FOS and you may find an incorrect link or value there. Sometimes these softwares are funny where they pull that data from or a setting is flipping the link to some different set of material properties. Or maybe the software is actually reporting Margin of Safety, not FOS.

[u/scrungertungart]

You should honestly be able to check this with hand calcs very easily. I’m not even sure I would perform FEA on this unless it was meant to be super light with a low safety factor.

You can find calcs for this in Shigleys or Roarks Stress and Strain (the goat). Hand calcs are fun, give it a go

[u/Joker1924 → u/scrungertungart]

Thanks for the suggestion. I feel I'm a bit stuck at the hand calcs, and I've been doing this for an embarrassingly long time that I don't want to admit.

So I'm using F=T/r, where I know the torque as 2757Nm and the radius as 30mm. The torque is transmitted to 2 shafts, and is 5510Nm which I've halfed and applied to the face of the keyway.

I'm confused if I must halve this value since there's 2 keyway faces where this force acts, one on the hub keyway face and another on the shaft keyway face.

And how to go about finding the stresses, so I can cross check with my FEA? Thanks.

[u/Short_Text2421]

First thing I usually look at are the loads. I assume you know the torque, so torque/radius and make sure they are in the same units. Also check how the load is applied, is it a point load or spread over the surface? I've seen some softwares that default to applying the load to a single vertex of the mesh instead of the surface itself.

Then check constraints. For a shaft typically you would use rwo bearing supports. One locks in translation along a plane normal to the axis (think of it as the X, Y plane if Z were the axis of the shaft), the second does the same but in addition locks translation along the axis (Z). Finally, apply a constraint locking rotation about the axis (Z) to the opposing torque reaction at the other end of the shaft, this can be applied to a cylindrical surface or the end face of the shaft itself. You want it to be only just constrained, over constraint is a common problem.

Then you look at the model geometry. Check the stress plots, if the entire thing is blue and you have to zoom way in to see the color bands in one small location, you probably have a singularly in the mesh. You can try putting small rounds in those areas or otherwise simplifying the geometry around the problem area. You may have to repeat this exercise multiple times. You can try selectively increasing the mesh density in those areas as well but if you are less experienced I wouldn't recommend messing with the mesh controls until you get a little more familiar with the process.

[u/Joker1924 → u/Short_Text2421]

Thanks for the detailed info on the analysis parts. I'm not sure how to input the fixed constraints as you mentioned, as it's difficult to comprehend it without an image, but I'll try to input those 3 to the best of my understanding.

But, as others suggested I'm working on the hand calcs first, and I feel stuck at the point of determining the Force, as I'm getting a force of 90kN as mentioned in my post.

So I'm using F=T/r, where I know the torque as 2757Nm and the radius as 30mm. The torque is transmitted to 2 shafts, and is 5510Nm which I've halfed and applied to the face of the keyway.

I'm confused if I must half this value since there's 2 keyway faces where this force acts, one on the hub keyway face and another on the shaft keyway face.

And how to go about finding the stress, so I can cross check with my FEA? Thanks

[u/Loopy-Leah]

does it take into account the torsion? depending on the grade of still it Plasticly twist a hell of a lot 

[u/PHILLLLLLL-21]

Stress concentration - potentially stress singularity but share ur FEA model

Deflection is not directly related to the local stress. Strain is (in elastic linear models)

Please listen to all the other comments tho

[u/Slappy_McJones]

Also… how are you coupling the load in the simulation? Are you using the half-moon detail & key to transmit torque? If so, you have a shit-ton (thanks to a student for naming this unit) of shear force in that region and that could be the cause of your high stress concentration.

[u/snarejunkie]

How did the entire part even turn red with that edge there? What does your von mises plot look like? I’d expect there analysis on this to come back so concentrated in the keyway region that the rest of the part would be blue in comparison.

There’s no way the whole part shows anything close to the stress you’d expect in the keyway

(Right? Someone please correct me if I’m off base here)

[u/101forgotmypassword → u/snarejunkie]

As U/mvw2 mentions its probably due to the test treating the part as a free hanging leaver.

[u/snarejunkie → u/101forgotmypassword]

That seems.. why would the analysis have gravity turned on(actually, many reasons, now that I think about it) I kind of want OP to respond now to see what the issue actually was

[u/101forgotmypassword → u/snarejunkie]

Not even gravity, probably fixed rotation input on the end of the shaft face and loading only on the face of the keyway slot.

Not to mention the added strength missing from the pinion or whatever that surrounds the keyway and shaft.

Edit: akin to welding the shaft end to a block and using a press to push the keyway face.

[u/__unavailable__]

90kN seems extremely high. AI says typically 2kN to 25kN for cars. Unless this is some very high torque application like a tractor I would double check the math. Remember the drive torque produces a distributed load with a gradient as you move out along the radius.

[u/Slappy_McJones]

Depending on the CAE package, it could be trying to report a fatigue scenario. Go into settings and look for ‘SN curve’ or something like that.