Rivet related question, how many and what size?

[TucsonAZ]

Hopefully somebody with some sort of an engineering or technical background can chime in here with some actual information on shear and load and such but it all basically boils down to this.

I'm replacing the floor of an all aluminum trailer (7'x13') as part of a camper conversion. I will be using 11 gauge (0.12") 6000 series aluminum and the trailer floor beams are currently 24" centres (mix of tubing and i-beam) but I'm planning on welding in some 3/8" x 1.5" aluminum angle between the existing beams to shorten that span to under 12 inches.

Loads will be human weight at most in any given sqft with the worst case being say a piggy back with feet closely together so let's say max load ever under 300 pounds in any given square foot.

My question is, what size rivets and what type of either pattern or spacing? My plan was/is to use 120 degree countersink 3/16th aluminum blind rivets with a stainless mandrel but I will adjust as needed.

Some areas (back fold down door for example) will also have a second layer of thin 20 gauge-ish) diamond plate on top of the 11 gauge so I'm not sure if that impacts things.

THANK YOU so much in advance for any help you're able to offer.

 

[TucsonAZ]

As a side note, it's REALLY good to be back here. Life has gotten busy and I really haven't been able to focus on what I love or this build which I've been putting off for a year now. I've been on IRV2 and some other forums, this is my home, it's amazing that people in so many of those groups think that $20,000 isn't a lot of money and look at you like you're insane when you tell them you want to travel in a $20k motorhome.

I've changed my mind a few times this year on how and what I want to travel in but I'm back to my roots now and it feels good.

 

[Trebor English]

What was there before the floor replacement? That might be a helpful guide.

 

[hugemoth]

Google rivet formula. There is a straight forward formula to give you the answer.

 

[TucsonAZ]

Mostly this applies to different applications than mine like structural fuselage and so on. For example, it calls for a spacing no less than 4 and no more than 8 times the rivet diameter. So in my case that would mean a minimum of one rivet every 1.5" which I think would be far too much for my application. I could be wrong but if 3" would overkill and 1.5" would be absurd I would like to just shoot for overkill as it will half my work and time installing. Were I placing a truck on it I could see that, if 1.5" spacing meant not having to add the additional angle iron I could also justify that but again, somebody smarter than me knows the answers to this.

 

[maki2]

Grip length on the pull rivets must be at least equal to the thickness of the material you are fastening.

When you use countersink heads you need to have sufficient thickness of material so that the hole diameter is not enlarged during the process of countersinking. There are charts for that which state the degree of countersink, the diameter of the hole and the minimum thickness of material that can be used for that diameter of countersunk rivet. So you don't need to be an engineer for that part of the job, it is just chart reading.

If you are concerned about the strength of the rivets use stainless steel rivets. You won't want to be hand pulling that many rivets anyway.

Also order 3 dozen or so clecos that match your rivet size, You can get them for a reasonable price from Ebay, don't forget the cleco pliers and also a hole deburring tool. You will need both of those as well. If you are don't countersunk rivets then a piloted stop countersink is an absolute essential or you will never get the counter sink perfectly centered and flush each and every time. You get the countersink and stop countersink body from aircraft supply sources.

I am not an engineer but I worked as a riveter at Boeing. I do know how to do these things.

 

[Doubleone]

If you're using diamond plate, I'd suggest to use regular button head rivets.  They'll fade visually into the pattern. That countersink just adds a whole 'nother degree of complexity.  A countersink needs to be an exact depth or the rivet head will either rest above or pull through the sheet.  Unless you have access to actual countersink tooling with depth stops... I'd stick with standard button head, plus cheaper too.  Big button heads are more for fixing softer composites and plastic.  Stainless rivets are bullet proof and anti-corrosion but hard to set.  You'll need a good air powered gun.   I'm not an engineer, I only know enough to be dangerous...  I learned material strengths and fabrication working on race cars and off-road race trucks.  I'd guesstimate rivets spaced every 6"-8" for a top mounted floor supported underneath by the frame.  Maybe go even much as 12" depending on the frame construction (I can't see it from here...).  We're not talking about an air frame where a failure means everything falls out of the sky...  Worst case, if you go light on rivets and later inspect to find deformed rivets, you can always go back and halve the distance, adding in more rivets.  Flat floors might flex and twist but really aren't under much tension unless the frame diamonds, and that would likely involve a collision.  On the other hand, exterior walls of an aluminum skinned trailer are definitely a stressed-skin design and can oil can, tweak, and pull around the rivets with varying loads.

An afterthought... Aluminum is kinda fickle. You might want to reconsider welding on the cross members if they're aluminum. It's a can-o-worms kinda deal. Some aluminum is designed with a heat treat for strength. You could inadvertently weaken the structure by welding, by causing localized soft spots, and/or differential heating without annealing could cause cracks on the weld margins. If you're absolutely intent on welding, at least check with manufacture and find out the exact frame material and compatible filler metal, to let your welder know.

 

[TucsonAZ]

Thank you both...a lot!! And you're right, I was just looking into countersinking and it's likely more trouble than any upside to doing it. I was looking into the tooling and while I'm willing to buy it just finding the proper #12 120 degree 3/16th is going to be a challenge as I'm only finding 100 degree.

So in short I see very little upside to them and a good enough potential downside to not make it worth the effort.

I did just buy the Milwaukee M12 2550 cordless riveting tool. I'm thinking about going every 4" and can add between if I feel the need and in the end I'll have 600 rivets into just the flooring so there was NO WAY I wanted to do those all by hand!

Thank you all for the thoughts on this, super helpful and gives me some additional information to help me plot my course.

 

[maki2]

or you can bond the panels to the beams with polyurethane adhesive caulking and space the rivets a little further apart. Just a thin coating applied and still wet when you get ready to actually install the predrilled and deburred panels to the structure.

 

[highdesertranger]

or use 3M Very High Bond(VHB) tape. no drilling, no riveting, no mess, 100% bond. highdesertranger

 

[maki2]

or use 3M Very High Bond(VHB) tape.  no drilling, no riveting,  no mess, 100% bond.  highdesertranger

That is a good idea and with the cost of supplies, tools and the amount of labor involved in installing rivets taken into consideration the overall cost of VHB tape would be a decent bargain. But carefully follow the directions and choose the correct VHB tape for the application. There are many different variations of VHB tape around.

 

[Doubleone]

You know... exterior VHB or urethane bonding will tend to let the floor float around a bit (not a bad thing).  On the race cars/trucks we typically oversized holes on the top panels and just let them find their own home.  It's more tolerant for flexing and collision damage.  

If I were mounting an aluminum trailer floor I'd probably go 3/16" aluminum rivets at 6" spacing all around.  I think 4" spacing is way over the top over kill.  Using 3/16" stainless I'd likely set 'em at 6" on the perimeter and 12" on cross members.  Cross member rivets pretty much just keep the floor from rattling.  Rivets are much much stronger than you may realize, especially steel or stainless.  It's very likely that plain old aluminum rivets would do the job just fine at 6", but you're going with stainless which are super strong.  Consider a single stainless riveted plate mounted in single sheer would likely support weight of an adult male all by itself, and you're about to install 600...  Just know there is point of diminishing returns.  Please do whatever you think is right, but to me 4" seems way excessive.  I guess I could maybe see 4" if you're hauling something super heavy like a forklift with small contact points (tires) that deflect the floor in between cross members, but then I'd wouldn't be going 11 gauge...  Perhaps on an airframe or stressed skin would be a different deal, but on a trailer floor over cross members all you really need to do is hold it from the perimeter, attachments at the cross members are a bonus.

 

[TucsonAZ]

Thank you all for your insights, I've posted in a few groups mostly engineering related but these are the most helpful responses Iv'e gotten, maybe because we all have some common sense. 

I'm going to use the AFM Almighty adhesive versus the VHB, it's no voc which is a plus and it will create a more airtight bond.  Hmmmmm, I wonder if I could even install the panels with that first, add a ton of weight to the top until it bonds and then do the rivets.  I guess it wouldn't work as well that way but it would make things easier. 

Soooo, maybe I'll do 4" around all the edges and 6" down all the cross sections.  I like overkill so why not is the way I see it.  I have been thinking the rigidity would actually be a huge positive.  I could be wrong there but it makes sense in my head. 

Anyway, I still need to learn to tig, I have the machine but haven't used it so hopefully that process will go smoothly as I've done wirefeed for a couple decades but never tig.  Ideally I'll be getting started in about a week and I will keep you all posted as I go.

 

[Doubleone]

It's a funny thing... ask an engineer a technical or theoretical question, and you'll get a technical or theoretical answer... In the real world, at some point, we just need to acknowledge that some deflection is bound to occur. In fact, with the off-road trucks we actually plan for deflection and build in bushings to allow it to flex, because overly rigid parts crack and fail... Post up some pics for us as you go...

 

[VanTalk00]

Grip length on the pull rivets must be at least equal to the thickness of the material you are fastening.

So I just watched a video taught by an instructor at a trade school (it looks like) about rivet sizing.  According to the method he uses, to rivet together two 1/8" pieces of metal, you would need a rivet diameter of 3/8" and a length of 13/16".  This seems like way too big to me, so I tried to confirm the info via Google (of course).  I did see some reinforcement of this idea, but generally, very little advice in sizing.

What diameter and grip would you recommend for two 1/8" pieces?  Do you have a rule of thumb?  What about two 1/4" pieces?

Jim

 

[maki2]

So I just watched a video taught by an instructor at a trade school (it looks like) about rivet sizing.  According to the method he uses, to rivet together two 1/8" pieces of metal, you would need a rivet diameter of 3/8" and a length of 13/16".  This seems like way too big to me, so I tried to confirm the info via Google (of course).  I did see some reinforcement of this idea, but generally, very little advice in sizing.

What diameter and grip would you recommend for two 1/8" pieces?  Do you have a rule of thumb?  What about two 1/4" pieces?

Jim

the match is easy, .125 x 2 equals .250.   I  have been putting in lots of 3/16 rivets with a 1/4 grip length to join together two layers of 1/8 aluminum for my cabinet build. It is the perfect and correct grip length.  But I was not adding any adhesive between the pieces and that would effect the grip length. There are some multi grip length rivets I used when I was putting 3 layers of 1/8" thick aluminum together.

You will need some fluid tight rivets for the floor panels. Just go shopping on Ebay or Amazon and buy them in quantity if you are doing a large area or go to a store that is specialized in fasteners. If it is less than 100 you can go shopping at your local hardware store.

 

[abnorm]

JIM ^^^^^^^^

The instructor is sizing SOLID RIVETS...……...You need enough material to peen over to make the join...…..

 

[Doubleone]

Blind rivets usually specify the 'grip' range right on the box. Just buy good quality rivets...