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Reusing locknuts revisited

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steveg

steveg

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There was some discussion of this before and I ran across this statement by Pegasus on their site. See bottom paragraph in screenshot.

I found the FAA citation listing the minimum Prevailing Torques for reusing locknuts. Prevailing Torque is the amount required to move the nut on the threads once the locking mechanism is fully engaged. Not the same as Final Torque, which is tightening it against the object.

FWIW - did a little test: 2+ foot-pounds were required to tighten a dry, new 1/4-28 nylok or stover locknut on a new bolt.


PegasusCitation.jpg FAAchart.jpg
 
In a similar vein, the lock or spring washers we dutifully reinstall don't really lock at all. Once the fastener is fully tightened the washer flattens out and has no more locking ability than a plain flat washer. I learned this in the racer's bible, Carroll Smith's "Prepare to Win" Probably a good practice to use a bit of Loctite if you're trying to maintain an original appearance.
 
In my business (aviation), the rule of thumb is that locking nuts will keep their minimum required locking ability for 5 uses. It is rarely worth keeping track of those uses, so we almost always require that they be replaced every use.

At home, I pay attention to the torque when removing, and if they are close to free turning, I throw them away.
 
HealeyRick said:
In a similar vein, the lock or spring washers we dutifully reinstall don't really lock at all. Once the fastener is fully tightened the washer flattens out and has no more locking ability than a plain flat washer. I learned this in the racer's bible, Carroll Smith's "Prepare to Win" Probably a good practice to use a bit of Loctite if you're trying to maintain an original appearance.
Click to expand...

I've read this many times, usually in connection with aviation or racing, yet in practice remain unconvinced that lockwashers are worthless. Have noticed the original split lockwashers have a curled-up barb on the end and do truly seem to grip. When you undo one of them it takes a significant amount of torque to break it loose. These washers do not sag or flatten out. IMHO the British steel from back in the day is better than the modern stuff I've encountered.

Aviation and racing are not the same as our street cars - the level of risk and the investment of time and materials are higher. On my car the only bolts that loosen from time to time are the front shocks - everything else stays put with their lockwashers and some locknuts.
 
steveg said:
... On my car the only bolts that loosen from time to time are the front shocks - everything else stays put with their lockwashers and some locknuts.
Click to expand...

Use Grade 8 flatwashers under Grade 8 bolts with blue threadlocker torqued to 35ft-lbs (obviously, all threads must be clean to start). They will not come loose again. OTOH, I tried same on the rear shocks and one came loose; then when back to Grade 8 bolts, flatwashers and lockwashers and they've held for many miles.
 
I agree that the lock washer being useless thing is not universally accepted, but it has credence in NASA, the military and racers. Even though our cars (except the racers) aren't subject to similar stresses and vibration, a dab of blue Loctite couldn't hurt on critical components. Bob, if the idea of a lock washer is to "dig in" to the mating surface, doesn't the use of a Grade 8 flat washer underneath the lock washer prevent that? I'm not an engineer, so not arguing, just trying to think it through.
 
I don't use a lockwasher under the flatwasher; for the rear shocks I put a Grade 8 flatwasher under the bolt head on the shock and a Grade 8 lockwasher under the nut on the mounting plate. The shocks--front and rear--have a flat area that appears to be milled or at least cast; the flatwashers just fit.

IMO, a lockwasher against the soft shock metal is worse than useless; they dig into the metal and release the pressure.
 
Lock washer is also called a split lock washer or spring washer. Even when fully compressed it is still exerting force against the surface below and the surface above (the nut), which works to act against rotation (of the nut), in effect locking the nut, hence the name lock washer. Still not convinced? See attached screen grab from McMaster Carr.
 

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nevets said:
Lock washer is also called a split lock washer or spring washer. Even when fully compressed it is still exerting force against the surface below and the surface above (the nut), which works to act against rotation (of the nut), in effect locking the nut, hence the name lock washer. Still not convinced? See attached screen grab from McMaster Carr.
Click to expand...
The text on that screen grab is way too small to read.
 
I like finding fasteners that have lost torque, especially when fixing them makes that new thunk or clunk go away. Makes me feel needed and even a little smart on occasion. I guess I need to qualify that because I should say I like finding that stuff before the wheels come off and that can be a fine line to land on. Good reading and I will only add that in the world of helicopters most critical fasteners are drilled shank bolts with castellated nuts and cotter keys. Most are required to also have plastic thread inserts in addition so you might have a chance if a cotter pin were to be missed. I have always questioned the use of split washers or star washers unless the material was hard enough to bite both the substrate and the bolt (unlikely).
 
I had great success with Nord-Lock washers. The rear shocks use to need tightening every 3 months. Now they have not loosened in almost 2 years. I'm not sure they will work on all applications. Based on how they lock, I would guess they can be used over and over.
 
tahoe healey said:
I had great success with Nord-Lock washers. The rear shocks use to need tightening every 3 months. Now they have not loosened in almost 2 years. I'm not sure they will work on all applications. Based on how they lock, I would guess they can be used over and over.
Click to expand...

I've had problems with my front shock bolts loosening lately. Had been using belleville washers for years with no or very little loosening, then went to the split rings and had one of my long bolts nearly fall out (tube shock setup).

This video shows how the Nord-Locks compare to various locking methods in Junker tests:
https://youtu.be/mgwmuZuJ02I

McMaster-Carr seems to have good prices: 20 pairs of 3/8" Nord-Locks for $12.38.
 
Got my Nord-Lock washers from McMaster. Installed them on one side of my front shocks (other side to come) and had to back one out. The amount of leverage required for loosening is quite amazing compared to that for typical locknuts or lockwashers.
NordWashers.JPG
 
I don't agree with the expert opinion about split washer effectiveness...not saying it's the very best option, but it does add tension to the joint, which helps to mitigate nut loosening. Think about it...it's a spring, albeit a very flat one...ever notice that some carburetor adjustment screws have a tensioning spring under the screw head? It's there to prevent the screw from turning due to vibration.
 
IMHO the carb spring is not exactly the same because there's no sliding motion between the base of the screw and the screw head. The Junker tests in the videos resemble our front and rear shocks where two layers are moving relative to each other, and this has the effect of rocking the bolt and unscrewing the threads. The places all over our cars where the split-rings do not loosen may not in fact need the lockwashers at all. Also in the forum discussion they seemed to say modern cars no longer use lockwashers. Also on our cars the safety-related connections are all handled by locktabs, nylocks and castellated nuts.
 
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