Threadlock and torque specs 1

[SEBASTIANOFA]

Does the use of threadlocker change the tightening torque of the bolts? I would like to use it to seal the thread from moisture and corrosion but if the tightening torques change, it is a problem. I have to use torque-to-yield bolts, for a car subframe and engine mount, this bolts are designed to stretch during the torquing and I need to set them on the correct torque.
The workshop manual says 100nm + 1/4 of a turn for the subframe and 40nm + 1/4 of a turn for the engine mounts

 

[Lou Scannon]

Good luck with that. I don't suppose you have a way to measure bolt stretch and install to a target stretch value?
If you know the target preload value, and can get guidance from the threadlocker manufacturer about the coefficient of friction, you could source your own fasteners that can handle the preload and torque them accordingly.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[SEBASTIANOFA]

No, i can’t misure nothing and the Audi manual say nothing about it.
The loctite technician say that I can use the dry torque value but…I’m not really sure and since are quite important bolts I want to make a good and definitive work.
We should know whether the difference in friction between dry and wet, only for the thread, is relevant or not in practical terms.

 

[TugboatEng]

It's not relevant. Why are you so concerned about this?

 

[SEBASTIANOFA]

My intent is to avoid air (so corrosion) in the thread using some sort of isolation , threadlocker, grease, oil….but I’m worried about break the thread or the bolt itself

 

[TugboatEng]

I use silicone grease for our exposed marine applications. Loctite works as well. Anti-seize does not resist corrosion well and should be avoided. Modern automotive fasteners have a lubricant pre-applied. If you're buying aftermarket that may not be the case. Torque is a very nebulous number outside of a very controlled environment. In your case use whatever lubes or fluids you want. You'll be fine.

 

[pmbrunelle]

You have a torque+angle tightening procedure to follow.

You torque the bolt to get the joint seated. Then, the 1/4 turn provides the bulk of the bolt stretch.

The entire point of rotating the bolt a specified angle is that the presence of friction/Loctite doesn't matter.

 

[drawoh]

SEBASTIANOFA,

My machine design textbook, Design of Machine Elements, by Virgil M. Faires, provides the torque equation

T = CDF[sub]i[/sub] in inch-pounds,​

where F[sub]i[/sub] is your initial torque, D is your major diameter, and C is the torque factor. Faires provides the torque factors 0.2 for as-received, and 0.15 for lubricated. The Loctite threadlocker 242 is a lubricant, before it hardens anyway. I would expect any liquid to provide some lubrication.

If you are doing 1/4 turn of a wrench, lubricant should have little or no effect on the tension.

--
JHG

 

[enginesrus]

There are some gasket sealers that work good to inhibit moisture or other fluids from entering the threads of threaded fasteners.

With the torque turn method you mention, the lube on the threads does affect the initial pre torque. What size are the threads and what is the initial torque?

 

[SEBASTIANOFA]

The threads are around 1-1,5cm. Torque is 100nm and then 1/4 of turn. I then should try to check how much difference there is dry/wet for the 100nm marking the bolt and the chassis

 

[drawoh]

SEBASTIANOFA,

You need a precise knowledge of your thread's major diameter and its pitch to do any meaningful calculations. You need to be way more precise than 1-1.5cm! The metric unit of length on engineering drawings happens to be millimetres.

By the sound of it, you are being asked to torque your bolts to 100N.m, then, turn the bolt another 1/4[ ]turn. This makes sense to me. You preload the bolt to some known value, and then apply a fixed rotation to it. I usually think in terms of finger-tight plus nominal rotation, but just what is finger tight? Any liquid will give you better control over friction.

I wonder about your bolt sizes. I am just quickly evaluating some metric grade[ ]10.9 bolts here at 75% of yield stress and a friction factor of 0.2. The 0.2 friction factor is generally used on the assumption of no-lubricant, which means you have poor control over friction.

M10x1.5, [—] 72N.m
M12x1.75 [—] 126N.m
M14x2 [—] 201N.m
M16x2 [—] 312N.m

Your initial torques have to be comfortably below your initial yield stress ones. In other words, you can apply 40N.m[ ]torque plus 1/4[ ]turn to an M10x1.5[ ]bolt. You cannot apply 100N.m.

My best guess is that your original manufacture made assumptions and they did calculations. Their instructions are based on these, so you had better follow them. Your next choice would be to do your own analysis and figure out what assumptions they made. Friction factor? Percentage of yield?

--
JHG

 

[MintJulep]

Loctite will tell you that 242 doesn't change the friction factor very much. They will also tell you that if it is very important to you, that you should test your exact application.

Or, apply the green, wicking Loctite 290 after you tighten the thing.

 

[SEBASTIANOFA]

I think that by tightening the bolt to 100nm, marking where it arrives, removing it and putting it back by applying the lubricant until it reaches the same point is an adequate solution, after which I can give the last 1/4 turn. I will use the original bolts and stick to the workshop manual data, simply mark the finish point with dry thread and bring the bolt to the same point but lubricating it. Should work fine?

 

[gruntguru]

Should work fine.

je suis charlie

 

[drawoh]

SEBASTIANOFA,

Are you allowed to re-use your bolts?

I suggest that you work out the strain from the 1/4[ ]turn of your bolt, and the resulting stress. Your existing bolts have been plastically deformed.

--
JHG

 

[TugboatEng]

Exactly. In plastic deformation (yield), the amount of tension created by the bolt is constant for a very wide range of angles. The angle provided in the torque spec is only meant to land you somewhere on that flat part of the stress/strain curve. It is the properties of the bolt that provide clamp load and not the torque applied or friction in the threads. Manufacturers switched to TTY because it's supposed to simplify tensioning of fasteners.

 

[SEBASTIANOFA]

I know , I will use the old bolts to mark the sign and then switch to new ones

 

[TugboatEng]

That's a terrible idea. If the old bolts have progressed past ultimate tensile strength and have started necking they will rotate substantially further before reaching torque. TTY is supposed to be a simplified fastening system. If want a true tension you must use a hydraulic tensioning system such as SKF Hydrocam.

 

[SEBASTIANOFA]

Mmm ok , do you think I can use new ones for dry marking till 100nm , remove, lubricate and then retorque till the mark and 1/4 turn? I don’t think they will stretch for 100nm, since they are made for 100 and 1/4 turn

 

[TugboatEng]

Do it how they do st the factory. Install a new fastener and tighten it according to the procedure.