Fastener help

[Mtsoxfan]

On our new UTV, the plow set up keeps coming loose. I've already replace the two mounting bolts already, and need to replace one again. I will end up doing both based on info gained here.
The bolts in question are bolted through the mounting bracket into a nut welded to framework. I've already used flat washers and lock washers, but get same result. My next idea is to get bolts that are longer than required, and add a second unwelded nut behind the welded one. The originals had a shoulder like a washer attached to them. Not available around here, so I went with cap bolts with washer and lock washer. Both have failed.
Is my double nut idea a good solution? Note, I can see the welded nuts, but there is very limited room to do anything other than add a nut.
We get snow often here, News just said how it's been years since we've seen so much snow in Dec. so I'm looking to get repaired before next snowfall.
Thanks all.

 

[2manyrocks]

I think the quality of some bolts isn't what they used to be in the sense that I suspect the thread dimensions are too large and/or the metal itself isn't as good so the fastener won't hold torque. You might buy some good quality bolts from McMaster-Carr and see if they make any difference and/or try some locktite on the threads.

 

[Alan W.]

On our new UTV, the plow set up keeps coming loose. I've already replace the two mounting bolts already, and need to replace one again. I will end up doing both based on info gained here.
The bolts in question are bolted through the mounting bracket into a nut welded to framework. I've already used flat washers and lock washers, but get same result. My next idea is to get bolts that are longer than required, and add a second unwelded nut behind the welded one. The originals had a shoulder like a washer attached to them. Not available around here, so I went with cap bolts with washer and lock washer. Both have failed.
Is my double nut idea a good solution? Note, I can see the welded nuts, but there is very limited room to do anything other than add a nut.
We get snow often here, News just said how it's been years since we've seen so much snow in Dec. so I'm looking to get repaired before next snowfall.
Thanks all.

Have you tried using loctite. If I was going to add a second nut it would be a nyloc nut.

 

[mrmikey]

Gr8 bolt, flatwasher and locknut on top of the fixed nut, nothing gr5 as they're softer and also Loctite as was suggested. If the bolt has come loose a few times the welded nut is probably wallowed out by now. Lockwashers are good when the nut comes loose and that means nothing.
Failing that, is the bolt large enough diameter that you can use a bolt that's been drilled to be able to use a castellated nut/cotter pin on top of the original?

 

[CalG]

It sounds as if the bolts are loaded in SHEAR with frequent load reversals.
First rule would be to pinch up all parts a full "nominal" tightening torque. There are tables readily available on line that would provide values for YOUR CAP SCREW SIZE.

After tightening to the correct torque, a locking nut is a GREAT idea.

The bolt Grade, whether 3 5 or 8 (or metric equals) have little to do with loosening. All steels have the same elasticity modulus.

 

[Mtsoxfan]

H

It sounds as if the bolts are loaded in SHEAR with frequent load reversals.
First rule would be to pinch up all parts a full "nominal" tightening torque. There are tables readily available on line that would provide values for YOUR CAP SCREW SIZE.

After tightening to the correct torque, a locking nut is a GREAT idea.

The bolt Grade, whether 3 5 or 8 (or metric equals) have little to do with loosening. All steels have the same elasticity modulus.

How do I determine the metric strength class?

 

[CalG]

H

How do I determine the metric strength class?

Google has tables galore!

 

[CalG]

Google has tables galore!

Oh sorry, The fastener class is imprinted on the cap screw head. i.e. 8.8 , is a pretty "high grade" metric equal.

ps
Stay away from any fastener that has no grade markings. They are all made of "chinesium" and fail in the most trivial of applications.

 

[Mtsoxfan]

So, an M10 8.8 is 36 ftlbs. Doesn't sound like much, but it is what it is.

Thanks, fyi, Google has tables galore really doesn't mean much to those not in the know of fasteners. Lots of techy jargon. I had to make some assumptions.

 

[ultrarunner]

As others have said there are many options better than lock washer.

Antique cars subject to rough roads back in the day use castellated nut with cotter pin.

This was before loctite and nyloc nuts.

Still plenty of drilled bolts with wire in aircraft.