Rivet nuts

[Ian_Edwards]

I'd like to fit an additional mast step to my Seldon Carbon Fiber mast. The existing mast steps are secured using what looks like 6mm SS rivet nuts.

I'm new to the world of rivet nuts, I've never had to use them before, so what type of SS rivet nut should I use?

Having looked online, I see that most are designed for fixing to thin plate, the CF mast is at least 15mm thick where I want to fit the step, so I don't see how the standard rivet nut will work.

I seems to me that a ribbed outer shell and some epoxy glue, so that when the barrel is expanded it grips the hole in the CF mast and the epoxy to give some "belt and braces" might make a more secure fixing.

Any advice on the best type of rivet nut to use and where to buy them from, from those who know about these things?

 

[splashman237]

Ian
I've only experience of putting alloy riv nuts into glassfibre on a car restoration and they seem to be fine even if they dont go all the way through thickness . Some are ribbed so they resist turning when you put in the srew/bolt your using to fix with.
For thicker materials like your carbon fibre mast you might want to look at MEMserts they are swaging inserts that are designed for blind holes and thicker materials , dont know what material you can get them in. On alloy dinghy masts I prefer to use Inconnel rivets as they dont seem to have problems with salty water etc. Presume youve checked with Selden , sure they could tell you what the originals were.
Regards

 

[Ian_Edwards]

Thanks for the reply and the suggestion of MEMserts, I'll research that online. I haven't tried Seldon on this one, but my previous experience of Seldon in the UK is that they are about as much use as a Chocolate Tea Pot!

I get much more useful answers to question like this from this forum.

 

[Rigger Mortice]

Forgive the slight thread drift but I'm astonished that the thickness is 15mm on Carbon Fibre. Whereabouts is it? Just curious, but I would have expected a thickness of just a few millimeters.

 

[Norman_E]

Any links for MEMserts? Google is coming up with men's shirts and all sorts of rubbish.

 

[Seajet]

I presume by ' rivet nuts ' you mean anchor nuts, the things to screw into held by two rivets ?

I'd be wary of working on a carbon fibre mast without specialist advice; I've built several alloy masts & booms but carbon fibre is a nasty material, even a slight impact can make it delaminate inside like fractured plywood, while appearing fine externally.

I photographed a lot of problems with C/F wings and structures, and the repair schemes a colleague came up with.

 

[Jabs]

Thanks for the reply and the suggestion of MEMserts, I'll research that online. I haven't tried Seldon on this one, but my previous experience of Seldon in the UK is that they are about as much use as a Chocolate Tea Pot!

I get much more useful answers to question like this from this forum.

Can I suggest you talk to Allspars at QAB, Plymouth, Seldon distributors.

Tony.

 

[Ian_Edwards]

Thanks for the responses.

I phoned Allspars this afternoon and I'm waiting for them to phone me back. I've always found them very helpful in the past.

By rivet nuts I mean nuts which are inserted into a hole and then expanded, in much the same way as a pop rivet. It allows a bolted fixing when you can't access the inside. http://www.rivetwise.co.uk/rivets/rivet-nuts.asp

Carbon Fiber mast in question is over 20m long, weighs about 300kg and carries over 1,000sqft of upwind sail and a 2,000sqft asymmetric down wind, so guess it has to be reasonably thick to be strong enough.

I found a link to MEMserts, but they only seem to sell them in steel, no mention of SS on the website. http://www.memfast.co.uk/shop/Vprod2.asp?cat=2250005114

 

[Bobobolinsky]

You can get them in monel, but you need long handled tongs to set them

 

[Ian_Edwards]

You can get them in monel, but you need long handled tongs to set them

Do you a link? I'm looking for 6mm MEMserts (or similar) in Monel, in theory I only need 3, but would happily by a few more if there is minimum order.

 

[West Coast]

Otherwise known as rivnuts. We use them in the GRP fabrications we manufacture for offshore oil platforms. You can get them in stainless steel but only A2 grade. Gesipa and others make them. We generally use M8 size.

Fixture thickness is an issue however, the biggest we can get will work for 6mm GRP and holds well. We carried out shear and tensile tests and were surprised with how good the results were. You do need the correct tool for setting them properly.

However, in your case, I would be very wary as they set up a lot of stress in the GRP material as you set them. We find they work best in a laminate of a high density of woven matt. A laminate with higher longitudinal roving content (likely in a mast construction) could be vulnerable to cracking round the hole.

 

[paul-essex]

http://www.bollhoff.co.uk/en/uk.php

I use these and have a tool for them but it is possible to set them with a bolt at least as deep as the rivet plus a nut and a pair of greased washers

 

[penfold]

I helped a X-yachts agent dress a CF mast for a new X43 a few years ago; to attach stuff to the mast he had me drilling and tapping holes and screwing fittings on with SS machine screws.

 

[Martin_J]

I would be worried about any oncontrolled point stresses in the carbon fibre as the rivnut is pulled in..

Also, as West Coast says, I think you will find it hard getting rivnuts that will set in any panel over about 6mm thick.

I know they are in a different league but being based in Portsmouth, I would probably make a quick call to Magma Structures in Portchester.

They are currently constructing three 100m tall masts in cabon fibre so they probably know the best answer..

http://www.yachtingworld.com/supersail/blog/534573/monster-rigs-revealed-by-magma

and their website can be found here...

http://www.magmastructures.com/

 

[Rigger Mortice]

Carbon Fiber mast in question is over 20m long, weighs about 300kg and carries over 1,000sqft of upwind sail and a 2,000sqft asymmetric down wind, so guess it has to be reasonably thick to be strong enough.

I'ld suggest you check that thickness. The thickest wall that Selden have on their current site is 6mm.
15mm does seem to be very thick for carbon.

 

[Rigger Mortice]

Carbon Fiber mast in question is over 20m long, weighs about 300kg and carries over 1,000sqft of upwind sail and a 2,000sqft asymmetric down wind, so guess it has to be reasonably thick to be strong enough.

I'ld suggest you check that thickness. The thickest wall that Selden have on their current site is 6mm. That weighs about 7kgs per metre so, if your 300kg is all up fitted out weight that would sound about right.
15mm does seem to be very thick for carbon but, if the 300kgs is just the mast without any fittings then 15mm will be about right. I'm surprised though.

 

[Ian_Edwards]

I've just spoken to Allspars, the answer is screwed inserts, apparently for a 6mm fixing, you tap an 8mm thread and screw the insert in. Allspars will supply the few I need, so it looks like I have a practical solution. it wasn't one I'd thought of but it makes sense.

Thanks for all the input, a great help in sorting thing out.

 

[West Coast]

I've just spoken to Allspars, the answer is screwed inserts, apparently for a 6mm fixing, you tap an 8mm thread and screw the insert in.

That sounds a good solution; again we have good experience of securing M8 fasteners into tapped holes in thick walled, good density laminates. Minimal stress set up round the hole. Would suggest you put some epoxy glue into the tapped threads and the fastener before final fitting to seal the FRP and secure the fitting in the tapped hole. Clean the fitting with degreaser solution (eg acetone) before hand!