Torque values with and without lubricant?

[mark5pt56]

So, not to add complexity to the torque wrench thread, curious on this one

I understand that the metals involved, tolerance of the parts(threads, etc) and lubricants utilized-is there a formula or general percentage that one can apply?

Note: I know the values for the barrel nut are with. specific lube and orientation of the tools. Just curious on mainly screws as typically utilized on a rifle of various types.
My only reference and example is on a pistol and the Venom optic, IIRC when I called Vortex, they said 15 dry, 10 wet.

Mark

[MistWolf]

It's important to verify if the torque spec is a dry torque or with some kind of lube applied. Allowance for dry vs wet installation varies according to alloy, diameter and other factors and isn't universal.

Torque value is a way to indirectly measure bolt stretch. Bolt stretch is important because it lets us achieve maximum clamping pressure without exceeding stretch limits and damaging the bolt. Measuring bolt stretch is a more accurate method but using torque values is much more convenient.

What torque is needed to achieve correct bolt stretch can be calculated by knowing bolt diameter, alloy and probably thread pitch. Charts are available detailing standard torques. Engineers also have info available in case they need to make calculations for torque values for their specific applications.

Another factor for calculating torque is friction. If it take more friction to tighten a bolt or nut, it takes more torque to achieve the desired bolt stretch. Most torque value charts are calculated with the threads dry. If lube is applied, friction is reduced and the bolt will be stretched more at the same torque value. The difference could be enough to damage the bolt.

Some applications require dry installations. For example, it's come to be widely accepted that fasteners used for mounting optics must be clean and dry and installed with a drop of thread locker to ensure they don't loosen over time. If they have any grease on them at all, they won't stay tight.

[Ned Christiansen]

If you research this too deeply you'll drive yourself nuts with all the variables!

The Machinery's Handbook tells us that the variance can be 30%. That is, dry vs/ lubed, dry it could take 30% more tq to realize the same clamping pressure. But if you were to bump up the tq on carrier key screws by 30% because they were dry you would be getting well into the region where you might pop the head off.

I have twice attempted to ask LocTite if thread lockers constitute a lube during installation, and they either "don't no nuthin'" or don't thing customers should get into it at that depth.

[m4luvr]

i’m just a shlub but if i was using a threadlocker while torquing a fastener i would consider it wet

[Joe Mamma]

If you research this too deeply you'll drive yourself nuts with all the variables!

I agree with this 100%. Even the type of lubricant is an issue if you look into this deeply enough. That makes sense but, I've seen charts that even distinguish between the viscosities of the oils used as a thread lubricant.

Also, just like Ned said, thread lockers could be considered a lubricant. But no one really talks about this. After they have hardened up, they could affect torque in the opposite way--you might actually have to apply more torque to get the desired affect. Of course that depends on what thread locker you use or reuse, etc.

Joe Mamma

[JiminAZ]

The short answer to the OP's question is "no". But if you held a gun to my head and said "name a percentage or else" I'd say about 20-25% between dry and lightly lubed.

I am an engineer who deals with bolt stretch/torquing all the time in industrial applications. Our bottom line is that if it is critical, we measure the stretch. Keep in mind most of what I deal with are 1/2" to 6" diameter bolts. But the principles still apply.

The formulas for calculating torques required to achieve a particular stretch are simple, and the ASSumptions required to do the calculation are pretty much unquantifiable.

As alluded to above, the friction coefficient between the surfaces of the threads and the underside of the bolt head are assumptions. Torque required is linear to the friction factor. What that means is for a given change in friction factor, you will get the same percentage change in torque requirement. It also turns out that the friction factors are small numbers, so small changes result in big torque changes.

I know blah blah blah engineer crap. Here's the bottom line.

T=f * (other stuff). Or torque equals friction factor times some other stuff. The other stuff has to do with pitch angles etc and doesn't change for a given bolt.

Values for "f" (the friction factor) tend to be either side of .11 or so.

• Dry bolt maybe .15
• light lube maybe .1 to .12
• Slicker lube or thread compound maybe .08. I have seen as low as .06 published.

If you do the math the percentage changes associated with these "assumptions" gets big. Going from .1 to .14 is a 40% change! Or from .1 to .08 is a 20% change

I have tested these assumptions many times in our shop. Some of my more interesting findings:

• Results are not repeatable. Minor imperfections in thread surfaces (which raise friction) get burnished off on the first tightening and things are slicker the second time around. Also the opposite can happen with a dry film lube or oxide finish which wears off in the first tightening and the second tightening things are less slick. Even with lube.
• There are differences between cut and rolled threads. Big enough to matter in critical applications.
• Many lubes have friction factors that are a function of pressure applied. In other words the friction factor changes as more pressure is applied. WD40 is one of these, actually gets slicker with pressure. So the formula above is over simplified.
• There are noticeable friction factor differences in metallurgical makeup of the components that have the same thread callout (say 300 series stainless vs hardened alloy steel).

For our gun stuff, I'd say just stick with the manufacturers recommended torques. One of the bigger benefits of using a torque wrench is that even if your torques are not "perfect", they are consistent (when pulling down scope rings or mounts, that sort of thing).

On a well engineered system the bolt is specified so that even if the install is not optimum, you are still in the working range of the bolt. In general this means the bolt is a bit oversized for the job. Things get more tricky where weight and size are considerations (aircraft, lightweight or miniaturized devices, that sort of thing).

One other thing that I find is lost on people is that generally speaking you don't want to tighten your bolts to the point of impending yield (just below where it begins to permanently deform or stretch). The reason is this - the bolt is preloaded (stretched) when installed. Then in service it may see additional loading that would put it into the yield range. Think of how the fastener is loaded under recoil or gas pressure and you'll see what I mean.

[mark5pt56]

Wow! knew there would be difference but not that wide of a range. As mentioned, the variables are there and not predictable. Follow the guidelines and hope the manufacturers did the homework.

We all appreciate the time and effort to type and the great information.

[hk_shootr]

This is huge in the aviation world. Here is a basic chart just to show a bit of the wet/dry difference
Dependent upon the lube and fastener, it’s generally 7.5-12% torque reduction for a wet fastener



7576EBDB-E012-4DBF-9191-66FF9E3F0E32.jpeg

[lysander]

So, not to add complexity to the torque wrench thread, curious on this one

I understand that the metals involved, tolerance of the parts(threads, etc) and lubricants utilized-is there a formula or general percentage that one can apply?

Note: I know the values for the barrel nut are with. specific lube and orientation of the tools. Just curious on mainly screws as typically utilized on a rifle of various types.
My only reference and example is on a pistol and the Venom optic, IIRC when I called Vortex, they said 15 dry, 10 wet.

Mark

Generally, what you are trying to achieve with specifying a torque is a clamping force range. This clamping force will establish, not only the load the assembly will hold, it will establish the friction loads between the threads and whether the assembly will generally stay screwed together.

For example, the barrel installation of the AR calls for a lubricated torque of 30 to 80 ft-lbs. This equates to a clamping load of 1800 to 4800 pounds and a friction force of 288 to 768 pounds.

If you don't put any grease on the threads, your clamping force drops to 472 to 1259 pounds.

I recommend that unless the assembly specifically calls out the assembly should be dry, use a little lubrication. 1) less change of galling, 2) higher clamping load for the same stress on the bolt/screw, and 3) easier removal.

[lysander]

One other thing that I find is lost on people is that generally speaking you don't want to tighten your bolts to the point of impending yield (just below where it begins to permanently deform or stretch). The reason is this - the bolt is preloaded (stretched) when installed. Then in service it may see additional loading that would put it into the yield range. Think of how the fastener is loaded under recoil or gas pressure and you'll see what I mean.

Unless you designed the joint to use TTY bolts.

As almost all factory heads are these days.