Torque values with and without lubricant?

[Joe Mamma]

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.

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

[Ned Christiansen]

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".

[MistWolf]

....does it seem like LocTite ought to friggin' KNOW that answer and be happy to share it...?!

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

[mark5pt56]

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.

[lysander]

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".

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?

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

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

[curious1]

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?

[556Cliff]

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?

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.

[lysander]

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.

[556Cliff]

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.

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

[mark5pt56]

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