Fastener Torque resource for thin material 2

[ColonelMonk]

Howdy

I've got a 1/4-20 fastener into a .188 thick wall in square tubing.

I normally like to run with thread depth = diameter x 1 but in this case, there's nothing much I can do. I have 6 fasteners total holding on a stout trailer fender on an equipment trailer I'm designing.

Were I tightening the bolts, I would not worry about it much, but I'm afraid without a torque spec the production guys will be stripping these out occasionally.

Can someone refer me to a torque chart that takes material thickness into consideration? OR help me figure this out? OR give me your best guess based on experience?

Was thinking about having the machinist make me a sample I could use to find "yield torque" where the fastener strips out the wall, then back it up 10% or something.

Thanks for the help!

CM

 

[dhengr]

ColonelMonk:
And, while the fender only weighs a few pounds, what makes you think these fasteners won’t just vibrate loose in a few hundred miles, whatever the torque. Furthermore, torque is really a poor way of determining the tension in threaded fasteners, even under controlled conditions. There are some blind side (rivet like) fastening systems, something like a pop-rivet, which might be a better choice. You might also weld some doubler tabs on the side of the tubing so you are tapping through 3/8" of mat’l. instead of just the 3/16" tube wall thickness, but I’m not sure I’d trust this from the vib. standpoint either. Dig out your Strength of Materials or Machine Design textbooks and do a little reading. The question is..., how much bolt tension does it take to strip (shear) the male or female threads, whichever controls. On thin material this becomes an iffy calc. effort because you usually are inclined to discount the first and last thread because you don’t know their condition. Your sampling torque test would be a good idea to get a feel for the problem and torque magnitude. Then a few thousand samples later you might have a value you can hang your hat on. Until you get the next lot of bolts, or the tap wears a bit.

 

[ColonelMonk]

Thanks.

Well, You might be right. We do use some rivets on existing equipment in other places....

What makes me think they won't just vibrate loose, is the fact that we already do this today and don't seem to have trouble. If torque is such a poor way, then why does it seem to be the primary method? I do appreciate what you've written, but if you have a common and better method for determining the "tightness" of fasteners do tell!

Now, I failed to mention that this is a redesign of an existing unit. Same 1/4-20 fasteners are used on 4 of them. The other two are "self-tapping" #14.

The 1/4-20 are threaded into a projection weld nut, with no more thread depth than the tapped hole in question. They were able to use the weld nuts because the fenders were attached to these welded "spacers" that were open on one side.

These tapped holes are in the middle of a 9 foot piece of tubing so there's really not an easy way to thicken the material, at least not from the inside. I thought about thickening it in the area that is tapped, but we are trying to get away from such time consuming fabrication.

We have used NORD-LOCK washers on the current arrangement and to my knowledge have had no problems at all with it loosening.

Without formal testing my assumption was that the weld nuts were probably a harder material and not as susceptible to stripping, thus I was hoping to find a rule of thumb for torque in a thinner material.

Anyone else?

Thanks again for the reply

CM

 

[desertfox]

Hi

Off the top of my head could you possibly use 1/8" screws and put more screws in, that way you get at least 1d engagement

 

[ColonelMonk]

Yeah, I thought about changing the screws to #10 and then add a few extra, but as I said we are already using 1/4-20 successfully. The weld nuts are the same thickness but likely harder. Assembly guys use cordless impact wrenches where they shouldn't, and this is what I'm worried about. It's unfortunate that we don't really have skilled assy techs.

Getting them to use a torque wrench is probably impossible, but if I put the spec that works in the work instruction and they strip them out, it's their problem.

We are building the first prototypes next week, so 1/4-20 it is. If it doesn't work thru our testing, that's fine, we'll make a change.

I hadn't given the riveting much of a thought but in the event of an accident, it is still "replaceable" enough. We do our own fabrication, so it's not that we can't weld them on, but I don't want to do that, if a fender gets munched and needs replacement that will make painting necessary.

I know someone here knows the answer, but if not I will get a test sample made and find the sweet spot. I'm not worried about applying maximum tension for a 1/4-20 screw, this is, after all, just a fender. We are going to test the trailer plenty, it'll end up with thousands of miles on it before we sell one, so if the bolts won't remain tight we will make adjustments, perhaps go to riveting.

CM

 

[CoryPad]

ColonelMonk,

For thread engagement (or nut thickness), the equations are available:

http://en.wikipedia.org/wiki/Bolted_joint#Thread_engagement

http://www.engineersedge.com/thread_strength/thread_minimum_length_engagement.htm

http://www.engineersedge.com/thread_strength/thread_minimum_length_engagement_fed-std-h28.htm

http://www.roymech.co.uk/Useful_Tables/Screws/Thread_Calcs.html

You then will need to match torque and preload with another equation:

http://www.efunda.com/designstandards/screws/fasteners_intro.cfm

http://www.roymech.co.uk/Useful_Tables/Screws/Preloading.html

 

[drawoh]

ColonelMonk,

I have designed tube structures to go into aircraft. To get mounting holes, we drilled holes in the tube, welded in a plug, then we drilled through the plug. Now, you are clamping solid material!

How about thread inserts, like rev-nuts?

--
JHG

 

[jayrod12]

I didn't consider 1/8" thick to be thin material. I guess it would really depend on how hefty of a fender it really is I guess.

 

[Tmoose]

As suggested by others, RIVNUTS are a nice upgrade.

http://www.rivet-nut.com

sure is an awkward danged website.
Here at least is a list of available sizes.

http://www.rivet-nut.com/Rivnut.pdf

 

[ColonelMonk]

The Riv-Nuts are just about perfect! Plenty of thread depth. I will have to check that out.

The welded plugs are a good idea, but we are trying to take some of the manual manpower out of our manufacturing. We are getting busy and don't quite have the space to start another line so are automating as much as possible and reducing stuff that requires manpower. I have just done this redesign not only to add features, but to simplify. About a 40% reduction in the number of frame members. It was never an engineered structure, was probably designed by a welder. It was as if the guy wanted to make sure that welding was the bottleneck so that he'd never be laid off!!

 

[badservo]

http://www.flowdrill.com/

 

[ColonelMonk]

Oh wow. Thanks bad servo! I have seen assemblies with tapped holes created by such a feature and I didn't know how they were formed. And my memory being what it was it sure didn't come to me for this thread.

I like the rivnuts, but the more work that can get done in our Haas without having to handle by man-hours the better.

I'll give those a closer look.

CM

 

[Tmoose]

The general Flowdrill specs says for material up to 10 mm thick.
Sounds like they have you covered.

 

[ColonelMonk]

Wow. I guess so!

Looking forward to learning a bit more. I'm guessing that all of the flow drill sizes are tap drill sizes. The trick will be getting our beginner CNC programmer to figure it out. I'm sure someone out there has done this.

We will see about the tapped thin holes for this proto, might have to buy one of the manual rivetnut installation tools to retrofit it IF they won't hold. Or just tack weld it. The PROTO is not going to be sold, we will have it in our fleet for some years. It'll be torture tested just as soon as the build is complete.

CM