Material around internal threads - again, new scenario 1

[coyoteboyuk]

I'm attempting to identify the miniumum material required around an internally threaded hole to maintain thread/material integrity. The advice from the other threads I've read has been to use the AF size of the fastener, however the scenario I'm considering is slightly different and the standard part I'll be modifying already doesn't meet this rule. The part is a wheel hub/flange with threaded holes for the the wheel bolts. The bolts are M14x1.5 and there's about 3mm clearance to the OD of the hub as

https://www.carpartsdiscount.com/au...001_wheel_hub_oem_8d0_407_615e_8d0407615e.jpg

. Rather than using offset bolts (wobble bolts) I am planning to re-drill the hubs to the new PCD (114.3, old was 112) but naturally this will take the thread outer diam just over 1mm closer to the outer edge.

The hub is 12mm thick steel (not sure of the grade yet) and naturally the joint interface will be with the large wheel back-side face but in either the original or new configurations the outermost section of the threaded hole is likely to be unsupported due to the large clearance used for wheel bolts in alloys.

I've not found any guidance in the machinery's handbook or a few other books I have access to, nor any online resources. Can anyone point me in the direction of the information?

 

[Goahead]

Why should the material left between the bolt and the outer edge bother you? The stressed material upon tightening the bolt is at the thread root. If you care to remove possible stress raisers and shot peen the threads you would not have to fear of developing cracks.

http://www.welding-advisers.com/

 

[coyoteboyuk]

Because while the thread root might be the area that takes the load, but it has to be supported and reacted by surrounding material. Take it to an extreme - if I put the thread so close to the side that the root material was the outer edge, then took it to failure you'd rip out a strip of material X wide (X being some portion of the diameter of the bolt) and the full thickness of the plate. At this point I do not believe you'd have no, or the same, deformation of the remaining "hooks" of material near the failure site - I think the two hooks would be significantly more deformed than the material on the opposite side of the fastener. Do you believe this is incorrect and that material ouside the root of the thread is unaffected by load on the thread? A random bit of FEA I've scavenged (yes, I know but it's demonstrating what I mean)

To the left it's clear to see that the loads spread out into the supporting plate, not just taken as a discrete shear anulus around the thread.

 

[racookpe1978]

OK.

But doesn't that same FEA analysis show almost all of the stress is concentrated in the base material at a distance of less than 1 thread root depth? That is, it looks like you'd need 2x - 3x thread depths around the tapped hole to prevent pullout axially.

 

[coyoteboyuk]

Indeed, I'd agree (though that image is a bit limiting in some ways). Hence my wondering if there were established rules for this. I could make a best-guess stab at it but that's not really an engineering solution and on a safety critical part I wanted some theoretical or established backup before choosing a cut-off point to say no at. As it turns out I have a change of plans and will be going the other way, but the question still remains for future work and for if I want to lighten the part.

 

[hydtools]

NASA recommends no less than 1.5D edge distance:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900009424_1990009424.pdf

page 21.

Ted

 

[coyoteboyuk]

Thanks Ted, that's a really good find. I do wonder if it's considering that for bolt-through use? I know NASA work to stringent standards but if it's the nominal standard that means a certain car manufacturer throws all that to the wind and does what it likes!

 

[hydtools]

The existance of standards does not mean everyone uses them.

In my past life I used 1.0D edge distance to tapped hole. But with testing and proof of application.

Ted

 

[KENAT]

The typical 1.5d or 1.0d edge distance are rules of thumbs, often most applicable to rivet patterns in sheet metal etc. Many standard fasteners probably don't meet 1.5d. (I couldn't get the link to open, apologies if it was indeed talking threaded holes)

In other lightly loaded situations I've just worried about the remaining wall thickness being something sensible - less than about .050" and I'd start to be concerned for my typical applications.

If you have the time and skill set to actually analyze it, and no industry standards directly apply, then why rely on rules of thumb?

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[dinjin]

Can you drill the part between the existing holes for the
original number and size of the tapped holes?

 

[coyoteboyuk]

I do have time to analyse it but I was hoping to draw on the knowledge of my engineering forefathers rather than repeat a task already done. As it seems there's relatively little information on it it's sparked some curiosity in me.

Can you drill the part between the existing holes for the
original number and size of the tapped holes?

I'm a bit confused as to why, but yes - are you considering destructive testing?

I suspect that in comparison with many applications an M14x1.5 torqued to ~100Nm to hold a wheel on is relatively low load, especially considering the application of spigot rings to take the primary shear load.