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  #1 (permalink)  
Old 5th January 2012, 09:29 PM
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How do you calculate load bearing of metal tubes ??

This may be a question for someone with an idea of engineering calculations.
I've tried to find a chart or similar on the net with no luck.

I'm designing a bike carrier to go on the back of a Landy wheel carrier and I'm trying to work out what tube to use for the main carrying arms.
The two arms would be similar to the picture below, they would be approx 400mm-500mm long and will need to carry up to 60kg (4 bikes) shared between the two arms.
The OD of the tube must be 25.4mm (1") as that is the size of the fittings I want to use.
So I guess what I need to know is the wall thickness of the tube to suit the length and load bearing.

Is there some standard formula to work out what's needed in either mild steel, stainless steel, or aluminium ?

Thanks for any help.

Cheers, Murray

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Old 5th January 2012, 09:43 PM
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formula yes, but then you need to know a heap of other information as well.

manufactures give at least 5 mechanical property on there "Form" ( profile). and of course the grade will give you the balance.

easier is to just use there charts.

the first problem you have is to calculate your load, 60Kg going BOING BOING etc
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Old 5th January 2012, 10:05 PM
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Well, as Mr Squiggle would say, "That's a tricky one Miss Pat!".

You could start by weighing the bikes to get a static load, apply a generous factor to take into account dynamic load from bouncing about, and then halve it for two arms and add a small factor to take into account potential asymetric loading of bikes.

Then prepare a shear force and bending moment diagramme for a simple cantilevered beam with a number of (bikes) point loads.

Then establish tensile strength of available material.

Then establish second moment of area of 25mm solid bar.

Then using second moment of area of hole in centre of bar as unknown, transpose the trusty old m y on I formula to establish delta d, half of which should be wall thickness of tube required.

But, before you do all this, consider carefully what your time is worth, what the cost of material to suit your available fittings is, how current your maths skills are, how good your welding is and all sorts of other things versus buying off the shelf.......

Or if you have your heart set on making your own, from a pragmatic and practical point of view, and to save a fair few trees and a lot of valuable time - check out something similar that may be available commercially.

And, to double check, go down to the local recycling place, and look for similar things that are broken, and work out why and how to improve things..........

Cheers,

Andy
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Old 5th January 2012, 10:42 PM
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The easiest is to get some test material, cut to length, and apply force until it bends.

Then you have a baseline.

If a 25mmx25mmx1.6mm square tube of length 1m bends when loaded with 100kg, then decide:
a) of you are likely to ever apply 100kgs to the end of the tube or
b) whether to buy a thicker wall tube.

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Old 5th January 2012, 10:49 PM
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Quote:
Originally Posted by 85 county View Post
...............the first problem you have is to calculate your load, 60Kg going BOING BOING etc
Thanks, yeah, I have thought about the 'BOING BOING' factor on the back of a 4WD, that's why I have to make sure it's strong enough.
The reason I'm thinking 60kg is the Thule carrier in the picture uses 1" tubes and is rated at 60kgs so it's at least a starting point.

Quote:
Originally Posted by Gumnut View Post
.............But, before you do all this, consider carefully what your time is worth, what the cost of material to suit your available fittings is, how current your maths skills are, how good your welding is and all sorts of other things versus buying off the shelf.......

Or if you have your heart set on making your own, from a pragmatic and practical point of view, and to save a fair few trees and a lot of valuable time - check out something similar that may be available commercially.

Cheers,

Andy
Thanks Andy,
Have you seen the price of (quality) 'off the shelf' units. But apart from that, I'm making a custom unit to fit on a spare wheel carrier, so off the shelf doesn't work in this case. Depending on availability of the components needed I am confident of making a quality unit for a lot less than you would expect to pay for something commercially available.

I knew the calculations would probably not be straight forward, but no harm asking in hope

Looking at some size charts it appears that stainless tube is only available up to 1.6mm wall thickness, same with aluminium, but mild steel goes up to 3.2mm.

Maybe I just forget the engineering and try a couple of different tubes and see how they stand up in the real world, say, 25.4 x 1.6 stainless and 25.4 x 2.0 mild steel.

Cheers, Murray
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Old 5th January 2012, 10:57 PM
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Quote:
Originally Posted by jakeslouw View Post
The easiest is to get some test material, cut to length, and apply force until it bends.

Then you have a baseline.

If a 25mmx25mmx1.6mm square tube of length 1m bends when loaded with 100kg, then decide:
a) of you are likely to ever apply 100kgs to the end of the tube or
b) whether to buy a thicker wall tube.

Good idea, thanks. I'm probably over thinking things. It wouldn't be too difficult to clamp some pipe to a bench and hang some weights off it.
Obviously all the weight on a bike carrier is not hanging right at the end, so if I hang 80-100kg on the end, it should be a good indication.

Cheers, Murray
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Old 5th January 2012, 11:00 PM
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Murray, the steels will have a strength in MPa specified. From that, the load, etc, you can calculate the size you need.

Assume 60 kg as a point load at the end of the length of beam(s) you have. Probably double that to 120 kg to account for F=ma over bumps.

However, the main issue won't be bending under load, but rather fatigue. Using a more ductile metal will help with that.
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Old 5th January 2012, 11:08 PM
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Quote:
Originally Posted by isuzurover View Post
Murray, the steels will have a strength in MPa specified. From that, the load, etc, you can calculate the size you need.

Assume 60 kg as a point load at the end of the length of beam(s) you have. Probably double that to 120 kg to account for F=ma over bumps.

However, the main issue won't be bending under load, but rather fatigue. Using a more ductile metal will help with that.
So from the fatigue perspective mild steel would probably be more suitable in this case?

Edit: Another idea that might be useful considering most of the load is vertical - how about some internal support in the tube, like a piece of flat on edge running the length of the tube. Would only need to be tacked in a couple of places if it was a reasonable fit.

Steve
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Old 6th January 2012, 02:32 AM
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rijidij,

If this diagram is a fair representation of one of your tube carriers then I think that the material that you will join the tube to will be a good guide as to the wall thickness required of your tube. Given that the wheel carrier is made of mild steel and if you are to weld to it or even if you clamped somehow I'd want about the same wall thickness on both pieces. To be sure go the 3.2mm wall thickness but 2.0mm might do it, I suspect that it is not worth the effort to save on weight or cost and just go with the 3.2mm.

Gumnut could probably help Mr Squiggle out as he has laid out a method.

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Old 6th January 2012, 07:11 AM
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Where the 25mm tube is welded to the upright is where trouble will start. The stress concentration at the weld point is the main weakness. For the minor weight penalty I would use at least 3mm tubing, or add a collar of thin tubing over the 25mm at the anchor point, then weld that too. Like a bicycle frame lug.

I broke a lot of steel tubing as a lad, I built a few dune buggies.
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