1/4-28 Threaded Ground Stud.

[joepme]

I need to ground an aluminum box used in an aircraft. The grounding strap has a 1/4-28 nut attached to it. I was going to us a PEM self clinching stud pressed into the housing, but they no longer produce this size. What other method could be used to attach a stud to the outer wall?

 

[HANDCALC]

You might consider using a nut and washer, per Advisory Circular AC 43.13-1B, Section 15, Tables 11-14 and 11-15, (pages 11-78, 11-79). Attached is a weblink to make your life easier.

http://www2.tech.purdue.edu/at/courses/at308/Technical_Links/Ac43-13-1B/CH11_15.pdf

 

[joepme]

Thanks for the link. It will be helpful in arguing my dissimilar metals issues.

However, in this case, the head of the bolt would need to be inside of the enclosure, and I am having trouble convincing (and believing) the bolt won't turn when a grounding strap and nut are attached to the bottom of the bolt.

 

[HANDCALC]

Have a look at the EM Stud PDF document from ALCOA at the following link:

http://www.alcoa.com/global/en/products/product.asp?prod_id=458

Look at page 36 of 39. Could you use something equivalent to MIL-T-83454/5, Blind Stud 0.250-28 UNJF-3A?

(I'm Structures, not Electrical)

 

[joepme]

Thanks, That EM stud would work great, however the quote was a bit high and the lead time was 8 months out...

I may just come up with a better mechanical lock for the bolt head inside the enclosure.

 

[groovitude]

One option that you might consider is an ST series stud. I know for a fact that they are readily available, and not too expensive. I can't think of the part number right off hand, but I know I have the number at work and could get it to you easily if you need it. the studs are made of sst and come in a variety of sizes. They are also available with a shoulder if desired. the shoulder could help with your worries about the stud turning when the nut is being installed or removed. Another option is to use one of the loctite brands of threadlocker to prevent loosening. One thing to look for when installing them though? They break very easily if you over tourque them! you may need to experiment with different sized taps to acheive your desired results. I've had a lot of problems with these very same studs. The dissimilar metal issue can be overcome by treating the threaded hole with irridite/alodine or some similar process that will prevent corrosion between the two parts. Gaulling is a whole other issue, which is why I recommend using an O/S tap to make the threads.