Torque values with and without lubricant?

AR/M4 AR Technical Discussion
1

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

2

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.

3

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.

4

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

5

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

6

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.

[ul]
[li]Dry bolt maybe .15[/li][li]light lube maybe .1 to .12[/li][li]Slicker lube or thread compound maybe .08. I have seen as low as .06 published.[/li][/ul]

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:

[ul]
[li]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.[/li][li]There are differences between cut and rolled threads. Big enough to matter in critical applications.[/li][li]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.[/li][li]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).[/li][/ul]

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.

7

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.

8

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

9

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.

10

Unless you designed the joint to use TTY bolts.

As almost all factory heads are these days.

11

That’s about 1/4 the clamping force theoretically. I had no idea the difference could be that large. Thanks for posting lysander.

Joe Mamma

12

So one bottom-line takeaway here, I think, is that not over-torquing is always a good thing.

Let’s talk carrier key screws. If you are installing them with crunchy powdered glass all over the threads and under the head, much of your 50-58 inch pounds is used up in the friction caused by it. The clamping force is greatly reduced, let’s guess here for discussion, it was reduced by 70%. But, if your contact surfaces were clean and flat you would still have a seal. Assuming the bearing surface of the screw (underside of the head) is now clear of grit and is making metal to metal contact, I think in this scenario your installation is good “enough”, given that the screw gets staked in somehow so it can’t come loose. You didn’t overstress the screw with too much torque. All we’re doing here is making a seal; it’s not a bridge or a skyscraper or a rocket ship so contact is contact; the screw is not going to get any stress on it really; it’s not going to stretch due to a load up its axis as the “lift” on firing would be less than twenty pounds.

Next setup, all clean and lubed. You get all the clamp that can be, limited by the condition of the threads (rough, smooth) and the materials in play, and whether or not the screws are plated (as I recall zinc plating puts you in the “lubed” category). Again you did not overstress the screw by going tighter “just to be sure”. This is a better install, no doubt.

I know this is not the carrier key screw discussion but my ultimate install of these besides good staking is clean, dry threads, and a little red locker on the threads of the screws (not in the holes) and four little toothpicked-on dots of it on the underside of the head. I figure it “can’t hurt none” and IF Loctite constitutes a lubed installation, than that’s an ever better one (but is it just me or does it seem like LocTite ought to friggin’ KNOW that answer and be happy to share it…?!).

The big thing here is don’t overtorque stuff because it it does more harm than good, if it ever does any good at all. Those specs should be taken as the limit of your fastener, beyond which, as mentioned by JiminAZ and Lysander, you are just stretching the screw without the benefit of an actual increase in “tightness”.

13

It’s in their best interest to not share. There are too many applications for Loctite to keep up with. Let the company calling for thread locker bear the responsibility.

The big thing here is don’t overtorque stuff because it it does more harm than good, if it ever does any good at all. Those specs should be taken as the limit of your fastener, beyond which, as mentioned by JiminAZ and Lysander, you are just stretching the screw without the benefit of an actual increase in “tightness”.

Not only does it not increase clamping pressure, but over-stretching the bolt will reduce clamping pressure

14

This reminds me when a young kid in the shop picked up his rifle and scope and I offered to mount it for him. I did tell him to use a torque tool and follow the directions. A week later he was sending his scope back because he didn’t use a tool and “cranked” the screws to use his words when installing.

15

Loctite will act as a lubricant.

As to carrier key screws, they are 8-32 UNC threads torqued to a maximum of 58 in-lbs. These are intended to be installed dry.

Why?

Torquing dry gives a clamp load of 1,824 lbs, and a total stress of 130,285 psi. If installed wet the clamp load goes to 2,146 lbs with a stress of 153,285 psi.

The proof stress load for an ASTM A574 alloy steel socket-head, cap screw with a nominal diameter less than 1/2 inch is 140,000 psi. (Corrosion resistant screws are less by a good bit) And one must remember the clamp load is just the load from installation, shooting will introduce service pressure loads on top of the loads needed to just hold everything together.

I think the big take-a-way should be: follow ALL of the instructions, don’t add your own ideas, unless you know what results you are going to get.

Besides, if you stake, why do you need glue?

Sometimes stretching beyond yield is good in that it gives a more uniform clamp load across multiple fasteners.

16

If you are using a screw on the carrier key that was required to be coated in a lubricating oil after the finish was applied would that be considered a dry connection when you used it?

17

Interesting… I actually installed my last gas key using moly grease as lube for the gas key screws, but I only torque to 50 inch pounds. Should I be worried?

I always thought the previous torque spec of 35-40 inch pounds was adequate and I always used moly grease for lube and torqued to 40 inch pounds. I only recently decided to give the low end of the new 50-58 inch pound torque spec a try.

18

The recommended maximum torque for A574, 8-32 screws is 48 in-lbs with lubrication. So, you might have stretched them a bit and are not getting full clamp, but I wouldn’t get all excited about it, the rifle isn’t going to blow up or anything.

If they ever come off, install new screws as directed by the appropriate technical manual.

19

Thanks lysander, I’ll definitely keep this all in mind during my next gas key installation.

20

Speaking of loctite, was reading it’s good for two years unopened, one year once opened-never knew.