Self-locking nuts and bolts

gregs210 said:

Each has their respective place and purpose, and personally I would not use loctite where a locking fastener is called for. I'm not an engineer and as you note MB knows what they are doing, so it's a bit illogical to think they wouldn't specify one over the other for a functional reason.
Locking fasteners can be pricey...but unintended disassembly is decidedly more costly.

Click to expand...

Fasteners are complex beasts that comprehend a family of technologies to apply, design, evaluate, and fabricate. It is a particular area where I'd not say I'll never do or use X.

Perhaps you are not aware but thread sealant products, among which are the Loctite's, come in quite an assortment of varieties which run from the ones for fasteners to be disassembled without effort, to those that may require the heat of a torch to loosen up the mating threads (e.g. Loctite blue). The problem with liquid fastener locking products is that they may need to be applied by hand and/or that they may contaminate an incompatible joint or assembly.

As for the other varieties of self locking fasteners, the teethed washer head screws or nuts are the most dependable, from a reusable stand point. Deformed thread fasteners, or those with locking inserts, is worth to replace them whenever we take them apart. So, it can become expensive if instead of purchasing them at a shop like Universal Fasteners, or McMaster Carr, you purchase them from your local stealership. Whichever way you go, make sure the markings on the fastener match or exceed the ones you removed.

Cheers, JV :thumbsup:

nvl said:

I've been working on my Benz, following the shop manual instructions. It seems that every bolt and nut is the self-locking variety, and available mainly through the dealer at exorbitant prices. To be honest, I had never heard of such beasts, so I did a little research. Actually, they do exist, and the science behind them is quite sound. Those Benz engineers do know what they are doing. Here is a very useful link, and video clip:

Vibration Loosening of Bolts and Threaded Fasteners

The article states that "...the chemical locking category provides the greatest resistance to vibration loosening...". Perhaps, hardware nuts and bolts with Loctite will do the trick?

Does anyone have any information on this?

Click to expand...

loctite, when used properly can provide a very good locking against vibration, BUT, proper application with the proper selection of loctite is the key. However, there are many things that can go wrong and weaken the bond. The joint must be absolutely clean of dirt or oil, the proper prep agent is highly recomended, and the locktite needs to be fresh, not old stuff sitting around. You also need to know what materials and platings are on the part to select the proper adhesive, they are not compatible with all platings or all materials if you want the rated strength.

You also need to make sure you use the proper locktite for the application, e.g. low strength, regular strength, or the high strength heat removal type specific for the bolt size (diameter). The max temp rating needs to be taken into account and if the assembly will be exposed to oil or other fluids, which affects which locktite product to use. So, it isn't as easy as "grab a tube of loctite and I'm done." In fact, unless you are replacing a lot of fasteners, you could easily spend more on the proper adhesives, prep agents, cleaning agents and bonding agents than you will on hardware.

Instead of paying dealer prices for hardware, any good old fashioned hardware store will have self locking hardware, either of the "nylock" variety or self locking flange or regular nuts that have been deformed of the proper grade rating.

One other thing to remember, if the original application used self locking hardware, the torque spec's took that into account. Switching to locktite will change the required torque downward since you don't have the torque of the interference to overcome.

Again, each has it's advantages/disadvantages, personally I don't interchange locktite and mechanical previailing torque fastener for any application that is safety related or internal to an assembly

Blue locktite (or however it's spelt) = breakable with a wrench

Red locktite = may need heat or a lot more grief.

Both are, IMHO more blind faith or luck rather than judgement as things like grease or corrosion on the threads can stop a proper bond.

Shake-proof washers, hmmm....

Locknuts (one nut tightened down on top another) okay, but obviously can't be used on blind bolt holes (i.e. where the bolts don't go through and out the other side).

Nylocs are fine for general use. Again can't be used on blind holes.

Flange bolts (like a regular bolt with a larger base to the head, kind of like a washer, and serrated on the underside) are okay, but not great. Flange nuts likewise.

Aerotight nuts (see picture) better, but can only be used once as the splits in the top of the nut are supposed to deform when tightened. Again can't be used on blind holes.

Castellated nuts with a split pin: superb. Most motorcycles used to (still do?) use them on their wheels.

Best of all: lockwire.

If you look at racers they lockwire just about everything, as do vintage planes and most helicopters. But it's probably overkill for most car applications.

True story about shake-proof washers:

Back in the 80s I used to pull pints in a pub in England. The landlady, Sue, once worked at the old Triumph motorcycle factory. Her job was to put the tail-light clusters onto the back mudguards of the old Bonnevilles. She had a line of Bonnevilles, a rack of light clusters, a bin full of bolts, a bin full of nuts, and a bin full of shake-proof washers.

Now Sue had never seen a shake-proof washer before. She knew what a regular washer looked like, but not a shake-proof one.

So she figured they were all broken. And didn't use them.

So, for about six weeks every Bonneville came out of the factory without shake-proof washers holding on the tail-light clusters, and, of course, if the problem wasn't caught, they all fell off.

Luckily for motorcyclists everywhere, Sue soon married 'Enry and began her career in the licensed trade.