Can anyone do a calculation for me - my engineering maths is a little past it's sell by date!

What would be the maximum torque you could safely transmit with a 32mm hollow mild steel tube (ERW) 1.5mm wall thickness & how much is it affected by wall thickness decreasing to 1.2mm or increasing to 2mm

Thanks in advance if you can help!

havnt got a scooby but i am sure someone on here https://www.mig-welding.co.uk/ would be able to help you out

Can't help with the calculation either.

Do you need to consider how each end of the tube is/would be fastened to the next part? Weld, spline etc?

Suspect that there are variables, length, method of securing each end etc but again I don't know the answer .

According to this page https://www.schweizer-fn.de/festigkeit/wellen_achsen/wellen/wellen_torsion_rech.php it would be 38Nm for 1.5mm. τ zul = 18, di/da = 0,906.

[Edited on 30/10/18 by Fant]

Ok, did a litle more Googling & I think I've found what I need here

Link

Substituting my measurements, using the thinner wall to get the lowest likely figure. so 32mm OD, 29.6 ID & taking Tmax to be 100 MPa (yield strength of mild steel is 250MPa - so I reckon that gives a 1.5 times factor of safety?) I make it that max torque would be 172 Nm

Could someone please check my maths for me? I thought the 38Nm that Fant had found so crazily low

Thanks for help so far!

The problems you have with designing something like this are:

Using Mild steel, the quoted mechanical properties are likely to be very unreliable.

Depending on the application, designing for a fatigue life is a much bigger concern than UTS.

If it's for the drive shaft you talk about elsewhere I would want to use more like a factor of safety of 10 to give you a chance, and use a proper steel you can get a geniune material certificate for.

Cheers

Fred W B

Fred, yes, a very good point, & I appreciate it's not just max torque of engine you need to allow for in case someone wants to be a plank & wind the revs to silly levels & drop the clutch (tho' torque would then be limited by traction) - initial shock torque must be limited to a certain extent by propshaft rubberised joints at both ends as well

I doubt these are likely to be highly stressed or very high mileage vehicles, but nevertheless I'd rather have something a little heavier that's going to last than something which is always a reliability concern - bomb proof is good! I can fairly easily go to 38mm with 2mm wall

RE "proper steel with a certificate" - CDS adequate? Any ideas? - I believe CDS has a yield strength around 1.5 times that of mild steel, but apparently still has quite variable properties - I really don't want this to get into a ridiculously expensive exercise as defeats the original point of the project, nor do I want a prop which weighs 100kg!

Russ,

Something you also need to consider is the torque load on your propshaft will be different in every gear due to the the gearbox torque multiplication.
Your ~95Nm max torque in first gear will give an effective torque of nearly 400Nm, in an impulse situation like dropping the clutch at high rpm this will multiply the torque significantly for the time it takes to either launch the vehicle, get wheelspin or encounter clutch slip.
With a tubular shaft, if the wall thickness is low there can also be a failure mode where the tube collapses and twists on a 45° spiral. This can happen before the maximum torque is reached.
Also to consider is the resonant frequency of the complete shaft, which combined with any imbalance can cause complete fatigue failure of the shaft.

While having a quick look, I have found a gkn industrial shaft catalogue, some of the shafts listed have tubular sections are rated for torque.
You're looking at 580Nm for a 40x2mm tube,1300Nm for a 50x3mm tube, 2500Nm for a 60x3mm tube. In comparison, the same joints with a solid shaft are rated at a significantly higher torque, which indicates the tube is the weakest point.
I would think that those figures would have an allowance for a degree of shock loading, there is also a section of joint/shaft sizing charts at the back of the pdf that may be useful to you.

http://www.rowlandcompany.com/pdf/gkn-driveline-constant-velocity-driveshafts.pdf

If you're looking to get custom spline sections and shafts made, this company is worth contacting.
http://www.broadoakgears.com/

Dave