Self locking bolt question

After reading the thread by Russ and his drive shaft "incident", I started to look at the xm757 and it uses self locking bolts as shown in the images below.
My question is, what part or feature of the bolt makes it "self locking"? It screws easily all the way in and uses apparently ordinary 1/2-20 threads. There is a slight under score (?) on the bottom side of the head so that the head rests flat.
The part number is MS35764-669. The tightening torque from the TM is 90 to 100 ft-lb, which is lower than the recommended value from tables (130 ft-lb).

Any insight here would be appreciated.

Any thoughts on using safety wire type bolts?

There are several types of self-locking bolts. I have some that have “normal” threads for the first few threads then they become “oversize for the remainder of the bolt. They are usually used in aircraft. After they have been used they loose there locking properties and are not to be reused. Perhaps the bolts on your 575 have been off and on enough that they no longer lock.

Scary thought, I will replace all of them for sure, what do you think of safety wired bolts?

BTW, I checked the TM specifically for the "throw away" instruction, but there was none for this part....(NSN 5306-00-071-4473)

Safety wiring bolts can prevent problems down the road. Just look how it is used on the bolts for the CV boots on the front axles of the 5 tons. As far as safety wiring the bolts for the U-joints I don’t see any problems with balance. Just wonder if the wire would be too exposed for cross country use, may have to keep an eye on it.

Safety wiring will at least keep the bolts from turning more than a fraction of a turn, it won't actually "lock" the bolt in place. The fan blade bolts on the xm757 engine are safety wired, no doubt, to keep the bolts from go flying if they come loose...
Another observation, with the wire in place, it would be difficult to re-torque without replacing the wire each time..
Oh well, it's always something, all things considered, I'll drill the heads of the bolts for the safety wire and go from there.

I have found threadlocker Loctite to work very well.
Regards Marti

Ferrari does it.

Why not just use self-locking bolts like the TM says to do?

Jones said:

Why not just use self-locking bolts like the TM says to do?

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That is really what my intent is, but I have no source for new ones and as stated in the first post, I don't know what feature of the bolt provides the locking action and they thread easily in and out of the tapped hole....unless there is something about the head design (flexing?).
The safety wiring would add a measure of safety to be sure.

There are several types of self locking bolts/nuts currently on the market. Many of the ones that ship with replacement u-joint clips are of the type that has a threadlocking compound (Locktite) aready applied to some of the threads. Others are of a type that has slightly deformed thread areas (as Scounger stated) or other machining.
Reading of the MilStandards is a bit dry... I have not looked at the one referenced in Bjorn's illustration yet, but a description of a self locking bolt that meets the current MIL-F-8961 is:
"The Dyna-Thred II lock is made by forming a close tolerance hole axially into the threaded end of a fastener. A controlled area of the cavity is dilated to expand the periphery of a controlled threaded area. The pitch diameter of the first thread is left unchanged for starting ease. The result: an easily installed, highly reliable, self-locking bolt or screw."
Ref: http://longlok.com/products_dynathred2.htm ,headquartered in Cincinnati, OH

These things are commercially available from a variety of source that can be found by searching a directory such as Thomas.net

papercu said:

http://www.boltscience.com/pages/vibloose.htm

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Hmm, very interesting. I noted the comment that good ol' split lockwashers were found to aid loosening instead of preventing it. I wonder how the Nord lockwashers that the Hummer guys like to install on their halfshafts would fare? I bought a set of them a while back, and intend to install them on my HMMWV halfshafts once my batch of round to-its arrives in the mail.

nf6x said:

..... I bought a set of them a while back, and intend to install them on my HMMWV halfshafts once my batch of round to-its arrives in the mail.

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What's currently used, any problems with them?

what does the bottom of the head look like?

plain bolts with a circumferential row of teeth under the washer head. These are ramped, allowing the bolt to rotate in the clamping direction, but lock into the bearing surface when rotated in the loosening direction.

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cranetruck said:

nf6x said:

..... I bought a set of them a while back, and intend to install them on my HMMWV halfshafts once my batch of round to-its arrives in the mail.

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What's currently used, any problems with them?

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Split lockwashers are standard equipment. "Common knowledge" (for whatever that's worth) in the Hummer community appears to be that halfshaft bolt loosening is a common problem on the Hummer/HMMWV, and the Hummer guys seem to swear by the Nord lockwashers as I recall from back when I spent time on the Hummer forums.

Oh, I assumed that you were asking about the lockwashers. If you were asking about my round to-its, I can assure you that they're always in short supply!

I'm tellin' ya, these are the ticket:

http://www.stage8.com/home_indus.html

No way the bolt can back out...

Gottcha Dan, but I would like to know what I have on hand, this truck (xm757) has shown me more than one unusual solution to old problems.

rizzo said:

what does the bottom of the head look like?...

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A little hard to scan, but here is a side view. The underside is flat except for an annular grove, which makes me think that there is some flex involved to hold it.

Self locking bolts have several mechanisms as mentioned above that cause a bit of interference (pressure) against some portion of the thread. When metal sits against metal with enough PSI, it kind of "sticks" together. That's what gives you a good crisp snap when you release a fastener that is not frozen or rusted, but has been together for some time. Self locking bolts rely on "exaggerating" this process.

The fact that the "self locking" relies on an interference fit, means that in time the female threads will wear to accomodate, and render the self locking bolts to be about as effective as a regular bolt. Standard practice anywhere I have seen when this is suspected (or possible, depending how critical the fastener) is to use a mechanical lock on the head. That means safety wire, or a foldable locking tab, usually in the form of a washer with two tabs, one to fold against the bolt, and one to fold against the retained component, in this case the u-joint strap. I'm gonna assume that If you ask the counter guy at Auto zone for new flanges and caps and locking bolts, that he won't even get off his chair to go look, and I assume that you'd still like emergency servicability to be reasonably practical, so mechanical external locks are my recommendation.

As for the undercut, I don't know. I don't think a bolt that is stretching at that point will necessarily cause a locking effect the same way as stretching the threaded area will do. I'ts fairly standard to have the opposite of that for the purpose of consistant equal torqing, as a flat hexagon will "dig" into the fastened component a little bit and really mess with the actual torque applied to threaded part of the fastener. I wonder if the bolt you show is intended to use this effect to your advantage as well?

Almost all self-locking bolts and nuts are one time use items.
Using a bolt with distorted threads as a locking concept isn't a good first choice. The female threads in the parent part, if made of softer material, may eventually become worn out requiring part/housing/whatever replacement.
Safety wire doesn't allow you to check tightness. It's only a fail-safe against loosening; the bolts still have to be tightened to their proper torque first before wire is applied and twisted.
With the groove under the head you probably have some old self-sealing patch type locking bolts (see pictures of various styles).
Still currently available.

I work for a mining machinery manufacturer. We use Nord-Lock washers with good results. I recently specified them on a new design of conveyor chain that hauls the coal from the front of the machine to the back. They hold the side straps of the links together. The application is subjected to severe vibration and impact. So far, so good, but the product has only been on the market since April. We buy the washers by the tens of thousands, so price is reasonable. I think they are pretty pricey in small quantities.

http://www.nord-lock.com/

I also tried a Nord-Lock washer on my finish sander with triangular head that fits into corners. The head would come loose and require re-tightening of the bolt every few minutes. The Nord-Lock did the trick.

They did very well in the Junker Vibration Test as described in the Boltscience website.
http://www.nord-lock.com/default.asp?url=7.16.37

Hmm, very interesting. I noted the comment that good ol' split lockwashers were found to aid loosening instead of preventing it. I wonder how the Nord lockwashers that the Hummer guys like to install on their halfshafts would fare? I bought a set of them a while back, and intend to install them on my HMMWV halfshafts once my batch of round to-its arrives in the mail.

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