Any aircraft mechanics in here? Bolt question....

[Bloo]

I have some close tolerance aircraft bolts here to bolt my rear axle ring gear on, for the 1936 Pontiac. The bolts are NAS6605-14 (5/16-24 threads, Thread class 3a, 160,000psi, cad plated). The nuts I have are MS21042-5 stop nuts (5/16-24 threads, thread class 3b, 160000psi, cad plated, style B flange, jet nuts).

 

These will bolt a hardened ring gear to a cast iron or maybe nodular iron differential case, with a washer under the nut. Loading is primarily in shear. I cannot find anything online that addresses this combination. Apparently thinner nuts are generally used with these bolts, due to the short thread length.

 

If I have measured correctly, I will have one thread down the hole (mostly in the washer), and two threads sticking out the top of the nut. Isn't that about perfect? I have 2 different thicknesses of washers here (AN960-516 and AN960-516L) in case there is a little variation in the thickness of the parts.

 

How tight should this combination be?

Edited March 23, 2019 by Bloo (see edit history)

[1950panhead]

Grade 8 5/16-24 torque is 20 foot lbs

[Bloo]

Yeah, and the aircraft stuff I could find (not my exact stuff) was less than half that for tension, and even less for shear bolts. I would have expected it to be similar to grade 8.

[Gary_Ash]

Specs for those bolts/nuts says 330 in-lb/20 ft-lb torque.  See www.coastfab.com for more info about them.

 

 

[Bloo]

Hi Gary, I was on Coastfab the other day. Is that 330 in/lb figure going to be OK with the thin-headed shear bolts? Or will it have to be less because of the thin heads? I couldn't seem to sort that part out when i was poking around over there.

 

I was also wondering if that "torque test"  value was normal torque, or absolute maximum before failure?

 

Thanks!

Edited March 23, 2019 by Bloo (see edit history)

[Spinneyhill]

Ensure you use torque tables for nuts of the height you are using.

 

Two threads in the nut is not much. If you are torquing them, the bolts must go right through the nuts. Note the torque figures you are looking at above for 5/16-24 is for a nut height of 0.251-0.266" high (H). Two threads at 24 tpi is 0.083". You will strip them if you use 330 lb.in on them.

[Bloo]

Two threads THROUGH the nut, sticking out the top, one thread below so the threads don't bottom. I just went back and edited that because it was ambiguous.

Edited March 23, 2019 by Bloo (see edit history)

[Gary_Ash]

Let's start with the bolts:  The diameter d is 5/16" = 0.312" and the tensile strength of NAS 6600 bolts is 160,000 psi.  In the ideal world, you would want to load them to 85% of the tensile strength or 136,000 psi.  The cross-section area of the bolt is .0764 in² , so the clamp force F would be 136,000 lb/in² * .0764 in² = 10,397 lb.  To compute the torque T required to achieve this clamp force, T = K*F*d.  K is controlled by whether the bolt is dry or lubricated and whether zinc plating is used, etc.  The NAS locknuts are cadmium plated, so we'll use K=0.18.  Hence, T=.18*10397 lb*.312 in = 584 in-lb.  That is how much torque would be needed to load the bolt to 85% of its tensile strength.  At 330 in-lb of torque, the bolt is only at 56% of that clamp force, so there is little/no chance of breaking the bolt due to torquing the nut to that level.  I'm surprised it is this low as it's normal to stretch the bolts to maintain the clamping load and not have sideways shift of the parts because that tends to loosen the nut.  Maybe it's because the nuts are intended for shear applications, not tension.  The bolt will still be well below the yield strength, as well, and the bolt head won't pop off.  Just don't oil the bolts or nuts for assembly.

 

Note that the shear rating of NAS 6600 bolts is 95,000 psi, so your bolts have a shear strength of about 7260 lb.  I'm sure that is way higher than the rivets or bolts originally used.

 

See https://www.engineeringtoolbox.com/bolt-torque-load-calculator-d_2065.html

 

[Bloo]

Gary, Thanks!

 

These nuts are rated for tension, it is just the bolts that aren't. If I am safe at 330 Inch Pounds then thats what I will use. The load is nearly all shear, although i'll take any clamping force can get. Loosening would be bad, but something breaking and falling off in there would be really bad. They will be lubricated with Loctite.

[Real Steel]

I was trained as an aircraft mechanic, and I still have my FAA A&P licence.  My directive was usually simple: replace hardware with the specified hardware, and fasten it per the specified method.  Your question is too complex to pull the answer from a typical mechanic's resources.  Instead, you need Engineering help...and it looks like you're getting it.  Best of luck!

[chistech]

On 3/23/2019 at 11:05 AM, Gary_Ash said:

Let's start with the bolts:  The diameter d is 5/16" = 0.312" and the tensile strength of NAS 6600 bolts is 160,000 psi.  In the ideal world, you would want to load them to 85% of the tensile strength or 136,000 psi.  The cross-section area of the bolt is .0764 in² , so the clamp force F would be 136,000 lb/in² * .0764 in² = 10,397 lb.  To compute the torque T required to achieve this clamp force, T = K*F*d.  K is controlled by whether the bolt is dry or lubricated and whether zinc plating is used, etc.  The NAS locknuts are cadmium plated, so we'll use K=0.18.  Hence, T=.18*10397 lb*.312 in = 584 in-lb.  That is how much torque would be needed to load the bolt to 85% of its tensile strength.  At 330 in-lb of torque, the bolt is only at 56% of that clamp force, so there is little/no chance of breaking the bolt due to torquing the nut to that level.  I'm surprised it is this low as it's normal to stretch the bolts to maintain the clamping load and not have sideways shift of the parts because that tends to loosen the nut.  Maybe it's because the nuts are intended for shear applications, not tension.  The bolt will still be well below the yield strength, as well, and the bolt head won't pop off.  Just don't oil the bolts or nuts for assembly.

 

Note that the shear rating of NAS 6600 bolts is 95,000 psi, so your bolts have a shear strength of about 7260 lb.  I'm sure that is way higher than the rivets or bolts originally used.

 

See https://www.engineeringtoolbox.com/bolt-torque-load-calculator-d_2065.html

 

Damn, I always said I’d never need that algebra they made me learn in school and along comes Gary!😃 Once my brain stops spinning, I’ll read that again.

[Guest]

Hot rivets are sure easier.  I wish you all the best with bolts.  I know one person who used bolts and nuts and one u

who threaded the carrier and used capscrews.  Both have worked well for years as far as I know.

[Bloo]

I cant find anyone who will rivet it. Are you aware of anyone?

[Spinneyhill]

On 3/24/2019 at 4:05 AM, Gary_Ash said:

you would want to load them to 85% of the tensile strength

I think 70% of minimum tensile strength is more usual. There is not much margin for safety at 85%. Indeed, the Engineering Toolbox says 70%.

[Gary_Ash]

Yes, I’ll agree with the 70% torque limit.  I’ve seen postings that say torque wrenches and ratings are good to about +/- 25%.  So, if you torque a bolt to the nominal rating and it breaks, back off half a turn...