Thick walled vs solid

[mattsdragons]

Trying to figure out new axels for BB but this is also a general point

I will use my calculations as an example

I have 2 options:
20mm SOLID Stainless Steel round
OR
20mm x 5mm WALL Steel round

Obviously 20mm solid stainless will be strong but when we are getting into 5mm wall at quite a small radius which is also slightly lighter, which would you pick?

[maddox10]

The hollow one will be stiffer than the solid one.

[geeza]

If you are using tube straight of the shelf it will not be as round as bar will be so you may run into trouble with your bearings. But I agree with the above

[mattsdragons]

Only be shortish sections bought from a local engineering firm so hopefully I wouldn't run into too many issues

[Eventorizon]

The hollow one will be stiffer than the solid one.

How can that be true?

[Liftoff]

Diameter for diameter the solid shaft is stronger than the hollow tube....marginally.
But weight for weight the hollow tube is way much stronger, so if say you increased the diameter of the tube slightly to 25mm you could get the same torsional rigidity of the 20mm shaft for half the weight.
The central part of the shaft does not carry the same load as the outer surface, much less infact, thats why you're better off with a tube

[Eventorizon]

I understand and totally agree with you. The original statement is incorrect: a hollow tube is not stiffer than a solid one. But since he needs the weight saving it is better to go hollow.

[adz600]

Personally would not use stainless as it bends easily, good seamless tube would work well

[maddox10]

Have some reading guys.

http://en.wikipedia.org/wiki/Neutral_axis

Tube is a lot stiffer/stronger weight for weight than solid.

[Eventorizon]

I know you are 100% right, we did it in my first year at uni (Had to learn to do all the hand calculations: melted my brain) But it is useful to know all that, thanks.

[Liftoff]

Have some reading guys.

http://en.wikipedia.org/wiki/Neutral_axis

Tube is a lot stiffer/stronger weight for weight than solid.

Exactly my point...

[Garfie489]

The actual material in a hollow tube is stronger than a solid one, because its likely to have had some form of additional cold work performed upon it, which increases strength, but reduces ductility.

[Liftoff]

Size for size, ie if you want a 20mm dia by 100mm long shaft, a solid shaft will be the strongest option you can get. A hollow shaft of the same external dimensions will be weaker, depending on how much material is removed but weighs much less than the solid one.

Now lets say the 20x100mm solid shaft weighs 500g, for that same weight you could get, say a 30mm dia by 100mm long hollow shaft. Now the hollow shaft will be much stronger than the solid, even though they weigh the same.
I'm talking same materials here...obviously.

So in conclusion I'd say, if you're limited to a 20mm dia, but weight is not a problem, then go with a solid shaft as it is the strongest option you'll get.
On the other hand if weight is a problem, but diameter dimension can be increased, then use the largest diameter tube you can use.

[mattsdragons]

Ordering parts today, after some opinions

Is it worth saying the extra for silver steel shafts or am I fine to just go with normal stainless steel ones?

[psycho_fling]

I'd go for Silver steel, stainless is a 'soft' metal when compared to steel or silver steel