Self locking bolt question

[cbvet]

Loctite is cheap, readily available, & it works.
Eric
CBVET

 

[Jones]

Yer right Eric. I can't make it through a project without a bottle of red Loctite at my elbow.
It may not be whiz-bang, high tech but it sure does the job. The only thing it insists on are clean surfaces.

 

[cranetruck]

Thanks for all you replies, much appreciated.
I don't want to substitute the specified bolt, MS35764-669, but would like to know by what action it locks.
The safety wire would only be to keep them from turning more than a fraction and not fall out and yes the bolts would have to be torqued first.
There are no traces of sealant or patches of nylon or loctite.
Locking tabs are not good since the bolt may loosen up unless the washer/spacer is a "ground hardened" one from what I read.
I'm still pretty sure that the locking feature is in the head design of this bolt. Googling the MS and NSN numbers yield nothing, so far....

Like I stated, this series of trucks had a lot of one-time (designed for it only) parts, including big ones like the engine, tires and transmission so why not one or two little things, like the self locking bolts. Since the trucks were not generally used by the military, these design solutions were lost along the way.

 

[Jones]

And how old are the bolts in question? The first time they were dunked in solvent or wire brushed, any trace of locking agent would have disappeared.

As far as incorporating "head-flex" to achieve locking action goes; you could be looking for a high tech design explanation for a low tech part.
Sometimes just knowing that a part or principle works reliably is far more critical than how.

Granted, they may have called for XM757-specific parts on some of the bigger assemblies but I seriously think fasteners would have been off the shelf items. Otherwise you'd have trucks in the field dead-lined for the lack of a washer or a bolt.

And while I applaud accuracy in restoration and can be as finicky as the next guy; we're not talking about pulling out the original engine and slapping in a small-block Chevy.
Why not just shrug, grab a bottle of red Loctite and move on. No point stalling out just because you can't find an exact replacement.

 

[cranetruck]

I like the "detective" work, besides we learn a lot from asking these kind of questions. A lot of my posts and articles come from digging deeper, some of it has become "common knowledge" on this site as a result.

Anyhow, I went to the source, at least I was hoping I did and this was the response:

"The old RPS stopped making MS hardware in the 1960’s. Rockford Products Corporation went out of business last year after filing for bankruptcy. The new Rockford Products LLC only manufacturers large diameter special product made to customer print specifications. I am sorry that we can not provide answers to your questions."

The bolts are all, apparently, original and some may have been installed by the manufacturer. The ones I have removed were very tight and required the 3/4 air wrench to remove, so whatever action that keeps them self-locking, they are still good.

I found one patented self-locking screw by Rockford, #3763909 and it's not it.

In a 1983 fastener handbook, I found one reference to self-locking bolts that relies on "a flexing head design produces axial locking spring tension when driven flush against a rigid seat", but no examples...

Don't know if the NSN would show anything other than the description. Perhaps the MS# would, but I have no access to that standard.

 

[JasonS]

My REO and GMC engines use the self locking bolts for the main caps, flywheel bolts, etc. The only obvious thing that I can see in common is the head construction. A cursory search on google yielded a patent for locking fasteners using differing coefficient of thermal expansion to prevent loosening. That patent was awarded in 1952. You may want to see what patents were referenced in that patent and which patents referenced the one in 1952.

 

[cranetruck]

Jason, I think you are getting warm, my best conclusion, so far is that the bolts are "close tolerance" types and the self locking action is tension, the same as found in cylinder head bolts, machine tools and examples you listed.

 

[cranetruck]

Turns out that the locking action is in fact in the head design of the bolt. One patent that is the basis for the bolt used on the xm757 is #2,543,705 (see images below).
Ford had hoped that this series of vehicles would be adopted by the army and wasn't afraid to innovate, parts that no longer exists in the inventory...

 

[DanMartin]

I see how that would work...the cuts in the top will allow the bolt head to deform outward under torque, and the undercutting of the bottom will lessen contact area, providing a tight fit. I would imagine that dirt/oil/grease would be an issue though...could undermine the locking ability of the bolt.

If it was a really good idea, I'm sure it would be used more often today. Clearly there are better solutions in use today.

An interesting research project Bjorn, but IMO I would swap these bolts out with something that will actually hold together for that 757.

 

[cranetruck]

Found a supplier, Aetna Screw Products.

"The unique head design accounts for the superior performance of The Aetna 6 Slot GM* Type AA Hex Head Place Bolt GR.8. There are six slots on the top, a recessed center, and an undercut at the base of the head that meets the shoulder. This design creates an elasticity, so that when torqued down, the head distorts and maintains a pre-set clamp load under very adverse conditions."

FWIW, also used by Ford in flywheel to crankshaft applications part # B8AZ-6379A...known as a "Place Bolt".