Hi

I need to replace a steel tube 1500mm long (25mm OD - not sure yet what ID is) with one that is made from your roll wrapped tube range. The Tube is supported at either end and is loaded along its length. No Torsional forces are acting on the tube.

 My question is: can I simply replace with a CF tube that is approximately 25mm OD? Will this new CF tube be over or under engineered.

Any help or advice would be greatly appreciated.

Russell

Thanks for replying

I am trying to make a child's hammock/cot that is attached to the door pillars of a camper van. Basically 2 steel poles that hook on to some brackets that are fixed to the door posts - this attached picture hopefully shows what I mean. I do not know what type of steel was originally used as I have never actually seen one with my own eyes.

I was planning to have some inserts made up incorporating some sort of hook (maybe in aluminium or steel) that i will bond into the ends of the tube. I'm not sure how long the inserts need to be so that they will effectively spread the load from the end into the tube.


What do think?

Ok, now I see where you are going. I would imagine that the steel tube would be straight forward lightweight ERW. Carbon Box Section would be the strongest but Carbon Tube would give you more "flex" which is probably more suited to your needs. I would use end supports that allow the Tube to bend under load ie not fixed. So a simple "hook" as you suggest would work. I would make the "hooks" as wide as possible like a coat hook rather than a cup hook to help spread the load.

I agree with Warren.

The important thing here is spreading the load.  In theory you could have quite thin tubes and it would stay intact, even perhaps pultruded tubes.

However, in practice this wouldnt work.  As you know the load (or kids in this case!) will move around, bounce up and down etc etc.  Not to mention sitting on the edge of the hammock and causing a point loading force. 

Although pultrusions are generally stiffer down thier length due to all the fibres in one direction, this leaves them weak when it comes to point loading.  So what would happen is it would be fine while they are on the hammock laying down, then if one sat on the edge, the tube would snap due to high point loading.

Hence this is where roll wrapped tubes come in as they have fibres both down the length of the tube and across it meaning they can resist point loadings far better.

In terms of specific tube sizes, our RollWrapped Carbon Fibre Tubecome in several sizes but are all sized by thier internal diameter as they are mandrel made. We do specify the outside diameter but this can vary slightly due to manufacturing process and tolerances.

The closest we have to your initial ideas is a 22.2mm ID Carbon Fibre Tube which has an O/D of 25mm.  That should be reasonably stiff for you.

Even if you dropped down a couple of sizes, it would unlikely break although the tubes would bend considerably.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Many thanks for the replies. 

Without knowing exact weights and dimensions and measuring it, it would be impossible to work out the precise deflection. 

As a rough guide, you can compare carbon roll wrapped tubes to steel equivilents of the same diameter and wall thickness.  You will get roughly the same kind of deflection and performance with the bonus of it being less than half the weight.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Russell,

to be fair, with the basic mechanical data and simple maths you can calculate the strength of a known cross section tube or rod for simple bending forces.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant