Ideally this is aimed at those who have actually used 'Rivet Nuts' in a professional / commercial environment please ...
I am considering using said to help restore a professional 'take down' motorcycle recovery trailer where many of the fastenings would be inside a rectangular steel box section which would make fitting nuts on the back of machine screws complicated (I'm aware it's possible with nuts welded to strap or spacer tubes and bolting though etc).
In most cases these fastenings would either be in shear or simply holding a joint already under compression in place. And example would be a tie-down 'loop' that would be fastened to either side (front and back) of the axle tube.
So, I was wondering if these Rivet Nuts could provide a practical solution (assuming they could replace conventional nuts etc) and if so does anyone have any practical / personal recommendations re makes or suppliers please?
I'm probably looking at M8/10 machine screw sizes going into ~3mm rolled steel box and angle iron, I'd be using steel Rivet Nuts (I'm not sure stainless Rivet Nuts would be ideal into mild steel?) and only need 10's, not 100's etc.
Should we use the special tool (again, production speeds aren't an issue here) or can a simple bolt and back nut and a thrust bearing or some washers provide the same effect?
I've used them a lot. Although generally smaller sizes.
A decent tool is IMHO essential for setting them correctly. And make sure spares are available for it.
I use M3 quite a bit and it is tricky to get exactly the right setting without snapping off the arbor on such a small size. Needs to be a very high quality steel used to pull it tight - a normal bolt will likely not be strong enough. And turning a bolt rather than pulling on it more likely to damage the thread.
So saying, larger sizes seem easier.
I'd guess a tool which automatically produces the correct 'pull' for any given size ain't going to be cheap. Most of the lower priced ones rely on you providing the correct force. Although can be set to a limit. If you are doing lots into the same material, you could experiment for the best settings on some offcuts. And keep those settings.
Not really, but I used at home to fix a tubular stair handrail
I certainly got rock-solid fixing from them
I used A2 stainless grooved rivnuts from About Town Bolts
I got the el-cheapo Silverline tool, which was OK for fitting a dozen rivnuts, but probably not designed for stainless as it came off slightly worse for wear.
Would you think that in a worst case situation (tension) they would offer as much strength / pull-though resistance as a conventional hex nut Dave? I was thinking a nut would offer a greater surface area but maybe the bolt or it's thread would fail first in any case?
Understood. That may be on my wish list though, depending on the price.
Understood.
Ok.
I think I would use the 'backnut' process where the bolt thread stays still in the rivet nut and the rivet nut just drawn in without the thread turning. I might also turn up a 'tool' to ensure they are held in firmly and square etc.
My fear also ... and I'm not sure would be justified in this scenario and for these quantities.
Understood. I have several torque wrenches. ;-)
Given you don't need access to the back (to hold a nut) then I was thinking I could use more of these than I might with a conventional nut and bolt, think riveting up a ships hull plate versus fixing an air vent into place etc. ;-)
I could turn up a tool to allow the nut to be drawn 'square' and as long as you don't turn the thread in the insert whilst drawing, I can't see how doing it that way would be much different to using the tool etc. At the end of the day, even if it wasn't fully seated during the fitting process, it would be when the fastening was finally made?
I should be able to 'see' (even if I can't actually get to) the finished nuts and as Dave suggested, I could do some tests on some matching scrap in any case.
You don't turn the bolt in the riv nut you screw a nut down on a rod threaded into the riv nut if you do not have professional tools and that is perfectly satisfactory although time consuming and you can put a thrust race under the nut if you have one.
Have you actually done this? As I said a size I often use is M3. Just to provide an easier fixing than fiddling with nuts an bolts - so access to both sides. And haven't found 3mm screws strong enough to set it. Might work if you can source high tensile types, though.
Thing is that M3 arbors are pretty expensive. So I'd guess made out of expensive material.
You would think the rivet nut companies would state how close they get to matching a conventional solution in compression (of the rivet nut head versus the bolt head though the material), tension (possibly the failure of the bolt itself) and shear (that might be a close run) but maybe not if it's not that close? ;-(
If the figures don't come close then I still can weld some self locking weld nuts to some plate and then screw that inside the section (so the plate doesn't fall off when removing all the main screws, or, drill right though the box, open up the back hole large enough to take a thin-walled socket and use a nylock nut and washer though the hole.
I don't think it would weaken the section sufficiently (it's much bigger than the original) and I could plug the holes with blanking grommets to resist the weather getting in. It just won't be so easy to disassemble as with captive nuts. Or I could turn up some through spacers (to stop it squashing the box section) and use bolts with nuts and washers on the outside (I could even bond the spacers and washers in place with some metal epoxy).
Well I have used quite a lot of Rivnuts. Only in a DIY environment, but I have done a fair bit of professional work involving design and assessment of various structures and mechanisms, including threaded components, and especially the lubrication of mechanisms.
They are ideal for fixings to box sections. The advantage you will have is that you know the thickness of the metal, so can pick the right rivnut. I've used them to fix things like tie rings and toggle fasteners to welded box section structures in my horsebox, where you are often not sure what the metal thickness is (and sometimes, because of accessibility, it's not that easy to confirm the thickness after drilling).
Fastenings in pure shear are basically as strong as the bolt from the design viewpoint. If you can get the design right, friction provides some extra shear strength. In practice, many fastenings are going to see a bit of tension as well as shear.
When you use a nut and bolt in tension, the weak link is usually the bolt, unless you are connecting such thin (or weak) material that the bolt head or nut and washer can "pull through". With a fully clenched rivnut, there is not usually as much "meat" as there would be in a conventional nut (unless it is a thin nut, as sometimes used for sheet metal shelving, for example). My experience is that rivnuts are usually not as strong in tension as the bolt they are used with. They have to be made from relatively weak steel so that they can be deformed into the final shape when cold. As with a pop rivet, you may be able to get a stronger fixing by adding a washer at the back side.
I've been happy with the ones that I have bought from eBay. IME it is important to have drills fairly close to the recommended size, and worth having really good quality drills. One trouble with box section steel is that it is not particularly "free cutting" (to use the machinists' term) and is sometimes a bit difficult to drill near the weld.
You may get away with this in your case especially if you make up proper jigs and spacers. I'm assuming you would use a rolling element thrust bearing, also high tensile bolts, and an ordinary mild steel back nut? Worth using a high moly disulphide lubricant on them. The nuts won't last long.
If you are buying a tool for general purpose use, then forget the "cheap" pliers type. Something like the Sealey AK3984 (see eBay) doesn't look too bad (there are more substantial versions). I uspect M8 and M10 will need something more meaty.
I have one a bit like this
formatting link
which is great for getting into tight spaces.
For most jobs, I use one of these
formatting link
and I love it, even though from the theoretical viewpoint it should not be as good as a "straight pull" type. It actually works by turning a hex headed bolt inside the threaded portion (so it puts the rivnut in torsion as well as tension). You do need to use a good moly disulphide lubricant on the screw to get good results. Mostly I have used 4 to 6 mm.
Used these a long time ago to secure amongst other things about 5kg of fire extingusiher and 10kg of SLA battery to a bracket that was then bolted to existing fixings in the floorpan and bodywork crossmember of a competition vehicle.
formatting link
I used two fastenings for the extingusiher and four for the battery
It was sufficient to satisfy me and also pass scrutineering for racing. I used M8 nuts fixed with the proper tool into ERW steel box section about 60mm x 20mm with 2mm wall thickness.
I also used them for oil cooler and radiator bracketry both into steel and aluminium
Unlike copies they provide real data on load capabilities
No doubt they are fine for lightly stressed applications. The real question is are they as strong as a decent nut and bolt for more critical apps - like say where a torque setting is given?
M8 insert, 5kN per fastener pull out load = approx 500kg - that was nearly the weight of the entire vehicle
But it all depends on your definition of lightly stressed, the direction of loads and your factor of safety. The extinguisher fastenings might survive a
200 G event. But there would be f*ck all vehicle or driver left to extinguish.
I recently helped someone doing a lot of rivnut fixings on a frame assembly . The tool used was from Memfast
formatting link
Tool was about £150 with a complete set of mandrels but worked very well including for 10mm steel rivnuts (although 10mm were moderately hard work).
We started by trying to use a fairly cheap pop rivet type tool but the results were poor with some of the set nuts rotating. It was difficult to hold the tool perpendicular to the workpiece while applying the large force bigger nutserts required to set them. The Memfast tool produced no failures and was easy to use.
Can I confirm that they are like pop rivets (that is, they fit through a hole then are pulled back to deform a tube) except that you can bolt and unbolt things once they are set?
Pulling back done either by the bolt you plan to use, or a stronger bolt/ stud/whatever?
Again, a bit like wall plugs for fixing through plasterboard?
I assume that the major issue is keeping everything flush with the external surface until the distortion of the tube is enough to hold everything in place. Haven't worked the tools for that out, yet.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.