Could someone help with steel round tube buckling load calculations?

[tigdlo]

I have used a few online calculators to make a chart to compare the weight, yield and buckling load of different size steel tubes to use for car suspension links but I have no idea if it's correct or even correct enough to be of any use.

I used this moment of inertia calculator, https://calculator-online.net/moment-of-inertia-calculator/ and this for the buckling load, https://www.omnicalculator.com/physics/buckling

For a tube 31.75mm OD, 27.69mm ID, 2.03mm wall thickness, 600mm long, Young modulus 186Gpa, yield 370N/mm2, pinned-pinned k factor 1.0, I got a moment of inertia of 21013.75mm4 and a buckling load of 58664N.

Would someone please check if the calculators are correct? Tubes from 1"x16g to 1.5"x12g appear to have a buckling load from 75% to 88% of the yield in tension, does that sound about right?

 

[Misterg]

For a tube 31.75mm OD, 27.69mm ID, 2.03mm wall thickness, 600mm long, Young modulus 186Gpa, yield 370N/mm2, pinned-pinned k factor 1.0, I got a moment of inertia of 21013.75mm4 and a buckling load of 58664N.

I basically concur - 53.6 kN critical buckling load using the SkyCiv moment of inertia calculator and the amesweb buckling calculator:

Free Moment of Inertia & Centroid Calculator | SkyCiv

Calculate Moment of Inertia · Centroid · Torsion Constant · Statical Moment of Area of a Beam Section usign SkyCiv's Section Buiilder

skyciv.com

Column Buckling Calculator

Tubes from 1"x16g to 1.5"x12g appear to have a buckling load from 75% to 88% of the yield in tension, does that sound about right?

Can't say without doing the calculations, but doesn't sound unreasonable - Instinct says this may be on the high side. Real world results will be heavily influenced by imperfect geometry and loading inherent in real world fabrications, so allow plenty of safety factor.

 

[tigdlo]

I found about 4 that gave similar results around 53kN, 2 that gave 107kN which I find unlikely and one that had mm4 for the CSA that a result of less than 1N which is obviously nonsense.

Can't say without doing the calculations, but doesn't sound unreasonable - Instinct says this may be on the high side. Real world results will be heavily influenced by imperfect geometry and loading inherent in real world fabrications, so allow plenty of safety factor.

The standard car has 30mm x 2mm round tube, I will probably stick an original and a replacement in a hydraulic press if I ever start the project, for now I'm just planning/over thinking.

 

[Misterg]

I found about 4 that gave similar results around 53kN, 2 that gave 107kN which I find unlikely and one that had mm4 for the CSA that a result of less than 1N which is obviously nonsense.


The standard car has 30mm x 2mm round tube, I will probably stick an original and a replacement in a hydraulic press if I ever start the project, for now I'm just planning/over thinking.

Out of interest I stuck 5mm of eccentric loading into the calculator and it reduced the buckling load to 30kN.

In all probability, the original was either what 'looked right' or one size bigger than something that broke.

Diameter helps a lot with buckling - 30mmdia x 2mm wall has ~20% lower moment of inertia than the 31.75mm dia x 2.03mm wall (17 vs 21x10^3 mm^4) and the buckling strength is reduced to ~48kN, or 26kN with 5mm of eccentricity.

Always worth trying to apply the 'real world' test to the answer: Will a 2' long piece of 1.25" tube hold up 5 tonnes? I'd be twitchy. 2 tonnes? Probably OK.

 

[Erie Fred]

Maybe run FEA on the complete assembly ?

 

[tigdlo]

Maybe run FEA on the complete assembly ?

I do use the free version of simscale but you only get 10 free simulations, I noticed I got another 10 last time I logged in but I don't know how often it resets. The problem with FEA is it's only accurate if you know what you are doing, fine for comparing 2 designs loaded in a very simple way but something like a complete suspension assembly is well beyond me.

For now I'm happy to rule out certain tube sizes like 1"x12ga, 1 1/4"x14ga is very close in CSA, weight and strenght in tension but has nearly 20% greater buckling strength.

Out of interest I stuck 5mm of eccentric loading into the calculator and it reduced the buckling load to 30kN.

Thank you, I'll have a play with different eccentric loadings of different tubes next, I could just plan to use what is common, like the rear links sold for RWD rally escorts, 1 1/8" x 14ga is considered heavy duty but they could be using something like T45.