Nut keeps coming loose - any long term solutions?

[Rosteri]

I’ve noticed quite a few Grade5 (3 stripes on the head) bolts in Caterhams nut and bolt packages, which I try to replace with Grade8 (6 stripes) or AN aircraft quality when I can. My point is, that I don’t trust Caterhams bolt quality - do check and replace them.

 

And if nothing rotates and the nuts don’t come off, bolts are ok, then I’d stop retorquing and just drive. If the chassis/strut parts yield instead of the bolt or seat themselves from the movement, then you are not doing yourself a favor by re-tightening.

 

As a general rule in aircraft mechanics no nylocs in rotating assemblies or the engine bay (or anywhere with heat). In these cases a castle nut and a cotter pin is always used instead. This is a real pain with metric HW, as the metric castle nuts are huge for tiny cotter pins and all bolts need to be custom drilled. Also all commercial drill jigs are for imperial bolts.

 

I didn’t follow the rule either when building mine… but it felt wrong pretty much at every nyloc nut.

 

 

Edited February 23, 2022 by Rosteri

[mrmustang]

Ok, I'll admit, of breifly skimmed over the posts in this thread and I am surprised nobody has mentioned, if the bolt itself is not stretched, the use of a nylock nut to prevent loosening and the need to retorque as quickly as the OP has been doing. Either that, or the cross drilling and safety wiring of the same to prevent vibration of the chassis and associated parts from causing the same. Treat the car like the race car it was designed as, and use the appropriate safety measures.

 

Sometimes the solution is the looking right at you, all you need to do is see it as the simple solution it really is.

 

Bill S.

[ashyers]

"I can only assume no movement, but how can I know for sure?"

 

If there is movement it will likely show up as galling/wear on the clamping surfaces. That would indicate a loss of clampload do to some part in the stack yielding.

[KnifeySpoony]

10 hours ago, Rosteri said:

I’ve noticed quite a few Grade5 (3 stripes on the head) bolts in Caterhams nut and bolt packages, which I try to replace with Grade8 (6 stripes) or AN aircraft quality when I can. My point is, that I don’t trust Caterhams bolt quality - do check and replace them.

 

And if nothing rotates and the nuts don’t come off, bolts are ok, then I’d stop retorquing and just drive. If the chassis/strut parts yield instead of the bolt or seat themselves from the movement, then you are not doing yourself a favor by re-tightening.

 

 

 

 

 

 

I think I will try a stronger bolt just to eliminate bolt stretch as a possibility. Although I don't see why re-tightening is a bad thing if it's actually wear on the chassis boss that's causing things to loosen.

5 hours ago, mrmustang said:

Ok, I'll admit, of breifly skimmed over the posts in this thread and I am surprised nobody has mentioned, if the bolt itself is not stretched, the use of a nylock nut to prevent loosening and the need to retorque as quickly as the OP has been doing. Either that, or the cross drilling and safety wiring of the same to prevent vibration of the chassis and associated parts from causing the same. Treat the car like the race car it was designed as, and use the appropriate safety measures.

 

Sometimes the solution is the looking right at you, all you need to do is see it as the simple solution it really is.

 

Bill S.

LOL well, it's in my first post that I already have a nyloc on there. And also, the nut is not loosening, so safety wire will not accomplish anything...

 

4 hours ago, ashyers said:

"I can only assume no movement, but how can I know for sure?"

 

If there is movement it will likely show up as galling/wear on the clamping surfaces. That would indicate a loss of clampload do to some part in the stack yielding.

I do think this is a distinct possibility, though I don't fully understand the mechanism. At some point I will pull it apart and look.

 

 

My only goal with this is to prevent catastrophic failure on track which would undoubtedly lead to loss of control. There is a thread on blatchat about the radius arm bolts shearing off or falling out causing crashes. It was from back in 2001. Interestingly, torque spec on those bolts back then the spec was 35FTlbs. Now it's 34Nm.

[Christopher smith]

Everyone should consider a good read of Carroll Smith's book Nuts,Bolts,Fasteners and Plumbing. It covers how Indy Cars were designed/assembled in 1990. Part that I felt was particularly important is the proper choice of bolt "grip" length ( that is the non-threaded section) in shear applications. So if anyone is buying replacement bolts that can be critical in highly stressed applications (as in 7s) be careful.

[KnifeySpoony]

Interestingly, I found this tidbit in the UK 7 race rule book:

 

"" It is permissible to replace the rear-most mounting bolt of the Watt’s link for a longer bolt of similar specification, enabling a nyloc nut to be fitted to the end of the bolt protruding from the chassis bush, preventing the bolt from loosening in use."

 

https://cms.caterhamcars.com/wp-content/uploads/2021/05/2021-Caterham-Seven-Championship-UK-Regulations-Published-Copy.pdf

 

So, this is clearly a known issue. However, I already have a nyloc as supplied in the original kit, and as above, the nut isn't actually loosening. 

[CarlB]

When you were checking the torque did you use your torque wrench to loosen the bolt. Torque wrenches are not as accurate removing. The torque can change because of the friction between the bolt and the clamping surface. If it was lubricated you generally get more consistent clamping. Lubricated bolts have a lower torque setting. I would use a Allen head fine tread machine bolt. They are better than grade 8. Use a nyloc and if that doesn't work double nut the bolt. 

[KnifeySpoony]

I did not slacken off the bolt. I just put the torque wrench on and tightened it to spec. There is no clearance to put the torque wrench on the nut, so I have to tighten the bolt while holding the nut with a wrench. The bolt is lubed and secured with a nyloc.

[MV8]

Hello,

 

I have no first hand experience with your model but I have a lot of experience and I also stayed at a Holiday Inn Express recently.

 

There should be a steel sleeve within the bushing to act as a positive stop for the bolt and nut to clamp against. If the sleeve is there but too short, the added washers can help loosen the nut.

 

Sometimes instead of a sleeve, a positive stop is provided by a shoulder on the bolt near where the threads end and the unthreaded shank begins.

 

Either way, the sleeve or shank (step/shoulder to head) should be as long as the width of the bushes.

With a step shank versus a sleeve, washers reduce the effective length of the shank.

After that, jamb nuts, a new nyloc or other prevailing torque fastener such as a nord loc should work great!

 

A pic of the hardware or exploded view of the assembly may shed more light.

 

[KnifeySpoony]

Agree with everything you say - however, as later clarified, the nut is not actually coming loose. The issue seems to be something else. I will update my first post.

[MV8]

I looked at a 420r watts kit and see they use long shanks with minimal threading which is good. However, it could be the bolt shank is not threaded far enough to clamp the inner sleeve. It may be from tight build tolerances and/or a hardware mismatch. Washers generally are not made to a precise thickness. If you had a dial caliper you could measure the grip length and the sleeve length and compare. The bolt is just a clamp for the inner sleeve. If you don't have that then just add another washer with an id smaller than the step in the shank the washer causes the nut to stack tight further out and see how it goes. You should still have a few threads protruding above the nut.

Edited March 25, 2022 by MV8