Pressure and metal walls

I have seen some references to really stiff hunting charges and can't figure out why drums and nipples don't get blown out from such charges.

We look at barrel wall thickness etc, but rarely consider the lack of material that holds the drum in place when screwed up to the barrel flat. In some drums the material thickness between the flash channel and the threads of the drum next to the barrel flat is only a couple millimeters. It would seem that a stiff charge would cause the drum to separate from it's threaded shank and go flying. That does seem to be the weakest point of some percussion styles. Any explanations?

That's a very good question. The only guess I could have is because the area of the threaded portion of the drum is relatively small, there is less area for the pressure to act upon as opposed to the breech or barrel walls. This is similar to when you fire a flintlock how almost all the gases go out the end of the barrel instead of jetting out of the touch hole. The touch hole is so small that there is very little opportunity for pressure to escape from there. Hope this helps.

Well put.

Many folks state that they wouldn't want less than .010 of barrel left under a lug cut out, yet we wouldn't dream of having a whole barrel that thin (even if it would be stiff enough to function).

The thin areas are able to "borrow" reinforcement from the surrounding areas in supporting the thin area on all sides. The larger the thin area, the thicker it will have to be to have the same resistance to catastrophic failure.

I think you would find that blown out drums, nipples or vent liners were "primarily" the result of damaged threads.

Secondary to that a thin walled barrel would only intensify the issue.

How I mean this is - you have slightly damaged threads on your (drum), but because it's screwed into the side of a 45 cal 15/16" barrel the wall thickness is perhaps .233" (accounting for the rifling as well).

If the drum has 24 threads per inch (as an example) then fully 24 x .233" = 5 1/2 threads (5.59 actual) are "engaged/holding/gripping" the barrel wall.

If that same 45 cal is cut into a 13/16" barrel the wall thickness is now only .171".

That same drum is only held in place by 24 x .171 = 4 threads (4.10 actual).

So all else being equal the thicker wall provides for 40% greater "thread surface" gripping (I have no idea whether that results in 40% more strength or 200% more strength - perhaps someone who understands the math/physics can chime in with the % holding strength increase for each "extra" thread).

Now add in damaged threads (slightly stripped/cross threaded etc) or just plain "significant breech pressure" and the thin walled barrel would result in "failure" at a significantly lower pressure, than the thick wall.

I had never thought about thread count before. I know that scuba tanks have to have 8 undamaged threads to pass visual inspection every year, and they are made with 10. Maybe that's why vent liners are 1/4" x 32 thread count? I'm sure the deeper the thread the better the holding power too. I mean, breach plugs are pretty coarse, and we have 7-8 of them holding the plugs in place.

A few thoughts about threads:

Threads reach their maximum strength when the engagement with the mating part is about 1/2 of the thread diameter.
Ever think about the thickness of a nut? Most people don't recognize that they are only about 1/2 as thick as the threads diameter but they are.

That said, anything over 1/8" engagement for a 1/4 inch bolt is a waste of material

Put in a muzzleloading perspective, the nipple doesn't have to extend over 1/8 inch into the barrel to reach its maximum strength. Most have more engagement but that is primarily for safety in case the threads become worn.

Most wall thicknesses on a muzzleloading barrel are at least .200 thick in the breech area.
That gives a thread up to 3/8" in diameter at least 1/2 of the threads diameter of engagement, making it capable of withstanding its maximum working force.

The pressure inside the barrel during firing can reach some impressive pressures even though old fashioned black powder is being used.
For example, Lymans Black Powder Handbook says a .45 caliber 285 grain Buffalo HP Conical over a 80 grain powder load of Goex 2Fg can reach a pressure of 21,000 psi.

The important thing to notice is this is measured in Pounds per Square Inch, a measurement of force times area.

Now, the area of a 3/8 diameter thread is calculated with the old Pi R square rule so doing that we find the area of the thread is 0.1104 square inches.

0.1104 times 21,000 psi results in a force of 2,319 pounds of pressure trying to blow the thread out of the barrel.

That's pretty impressive until you find that even with a weak steel like most drums use, a 3/8-16 thread can take a load of 3,200 psi before its material fails. That's over 880 pounds of reserve strength.
(If the thread is a 3/8-24 UNF it can take a load of over 3600 pounds.)

If we are speaking of a nipple, the larger ones commonly used on a rifle has a 1/4-28 thread.
The area for a 1/4 inch thread is 0.0491 square inches.
The 21,000 pressure times 0.0491 area = 1031 pounds of force.

A 1/4-28 thread in mild steel can handle 1500 pounds.

Most of us shooting patched roundballs will have breech pressures around 9,000 psi or less.

9000 psi X .0491 sq in = 442 pounds force for the nipple and 9000 psi X .1104 sq in = 994 pounds force for the 3/8" drum threads.

I know there have been a lot of numbers in this post and some of you don't like numbers but they were needed to really explain what is happening to the threads in a muzzleloader. :grin:

:wink: Lets hope every one understands .

My brains hurt..... .

I have read somewhere that once there are at least 3 FULL threads engaged, any additional are not needed. I have no references though.

As a follow up, which is better, finer or coarser threads. It would seem to me that coarser would be stronger, but I really don't know.

I need to make a drum for a project and there are no commercial drums large enough. It will probably be at least a 5/8 inch diameter at a point where wall thickness is 1/2 inch. I was thinking of perhaps using a threaded stainless steel bolt as my stock to make the drum. I think I can probably find a 5/8 x 18 threaded stainless bolt.

Zonie.
Fantastic info. Only an engineer can explain it so well.

Zonie, since you obviously understand this stuff, how about a "layman's" (not Lyman) answer.

So from your numbers you state a 1/4-28 thread can withstand 1500 pounds, but even a 21,000 pound breech pressure might only put 1031 pounds of force on the nipple - quite a bit of "safety factor" there.

Now what if those threads were stripped or cross threaded - how much would that diminish their hold strength if say the (area) of the thread was cut in half?

Is it a linear kinda thing - pristine threads = 1500 pounds, threads diminished by 50% = 750 pounds or is it more complex than that??? :idunno:

No. It's not a linear thing and there is no way I can answer the question, "What about damaged threads?"

It depends on the amount and type of damage and I can safely say, don't trust any damaged thread if it is used in a pressurized area on a firearm.

Trying to "put it in a laymans language :hmm:

Pressure measurements need to define the pressure. Duh. Pressure is good but it doesn't really mean anything unless it has something to push against.
In fact, if it doesn't have anything to push against it will quickly become no pressure at all.

OK. So we know that pressure will try to push stuff that's holding it in but how big is the stuff? (If the size of the stuff is large the pressure will be able to push against it with a lot of force. If it is small, the pressure will be able to push against it rather weakly. After all, if it is small, there isn't much area to push against.)

To define a usable value for size, in America we decided to use a 1 inch X 1 inch square. That would be a good area size to base the pressure value on and that is the reason we measure pressure in PSI or, Pounds (of pressure) per Square Inch (the area).

Using this value, if we know the pressure and the size of the thing the pressure is pushing against it is fairly easy to figure just how hard any object is going to be pushed by any pressure that wants to do the pushing.

You mentioned the 21000 PSI value?
OK, if the area it is pushing has a area of 1 square inch, that pressure will create a force of 21000 pounds trying to get the obstruction out of its way.

But, what if the object is less than one square inch? What if it is only 1/2 square inch?

If it is 1/2 square inch, there is only half as much area to push against so the result is it can only push with half of its power or 11500 pounds.

If the area is only 1/10 of a square inch in size, the 21000 psi pressure will only have 1/10 of the area to push against so the maximum force it can create on the object will be 2100 pounds.

Speaking of 1/10 square inch, a diameter that is .357 inches in size has that amount of area.

What if the object is only the size of a #10 screw? A number 10 screw is usually taken to have a diameter of .190 inches. A .190 diameter circles area is 0.0284 square inches.

That 21000 pounds of pressure applied to a .190 diameter object will be able to push the object with a force of .0284 square inches times 21000 psi for a net force of 595.4 pounds.

I've been talking about the effect of pressure on a round object like a pin or thread but it also works just the same on plane old surfaces too.

If we want to talk about the inside of a guns barrel for instance we need to set a limit for the length of the bore so we can calculate the surface area.

Lets say we have a breech pressure of 7000 psi inside a .50 caliber bore.

To calculate the surface area of a 1 inch length of that bore we need to find the circumfrunce of the .50 diameter circle. That would be equal to Pi (3.1416...) times 0.50 which is equal to 1.5708 inches. A 1 inch length of that size results in a total surface area of 1.5708 square inches.

Figuring out how much actual pressure is trying to enlarge the bore with a 7000 pound breech pressure we will multiply the area times the pressure to come up with an answer of 10995.57 pounds. :shocked2:
This is not how the barrel wall thickness is calculated but it does come into that. It also gives a person a hint of why the breech area of a barrel needs to be fairly thick.

Before saying good night I need to mention the strength of materials.

The strength of a material is measured several ways. One of these ways, and the one that is usually quoted (because it is the biggest number) is the tensile strength. That is the strength of the material when it fails.

One of the leaded steels people around here talk about has a tensile strength of around 68000 psi.

Does that mean it can withstand a internal pressure of 68000 psi? No.
If there is less sectional area in the barrel than 1 square inch of material it will fail at pressures far lower than 68000 psi of pressure.

This makes sense if you think about it.
A paper thin barrel made out of ultra strong material with a tensile strength of 240,000 psi might not be able to hold in a pressure of just a few hundred psi without bursting.

Hope my rambling helps.

I pulled a lot of tensil tests on threaded fastners when I worked in the lab for a jet engine manufacturer. Three full threads, for pratical purposes 4 full turns of a bolt as the first thread on the bolt and in the nut aren't full threads, are all that were required for a test. The bolt will break before the 3 threads will pull out.

Well in laymans terms , any stripped threads will greatly reduce the holding strength of the threaded part, because it reduces the contact surface area of the 2 threaded parts eg :- stripped threads on wheel studs are going to not hold your cars wheel on for long . :thumbsup:

Great post, but it created more questions. Tensile strength and elongation values for instance.

Ok, tensile strength of a certain material is 100,000. It's a number. And in a vacuum. For our purposes, how does that actually relate to what we do?

For instance let's use a big square fully supported sheet of aluminum foil as an example. If you put a fully distributed 10 pound weight (like a gallon and a quarter of water) on it it would support it no sweat. If you concentrate the same force (like with a pin) it will fail,(because the psi force is far greater in the small pressure area.

Other posts were looking for the practical application of failure points.

When I posted this, I was actually questioning the rather thin drum walls at the point where the threads stop and the drum is next to the barrel flat. (I realize the threaded portion would be supported by the barrel wall. For instance, if the drum shank is 5/16 and the interior flash channel is 1/8. That only leaves a drum wall thickness of 3/32 (less than a tenth of an inch) where the shank transitions at the barrel wall to the wide point of the drum. It would seem this would be the weakest point and the metal would stretch and eventually fail.

However, I find the discussion of threads very interesting and I am glad the thread took that turn. It answers other questions I had.

Great explanation, Zonie. Something that needs to be understood if you're building smokepoles!

The presures that are generated are in pounds per square inch. That means that the pressure distributed according to the exposed surface area of the part in question. In the case of the breach plug of a .50 caliber rifle, the face of the breach plug is a bit less than a quarter of a square inch. About .2 square inches. A heavy load in a .50 cal rifle is likely to develop a breach pressure of around 28,000 pounds per square inch. That means that the face of the breach plug will have about 5,600 pounds of pressure being exerted on it. Since it is the threads that hold the breach plug in place, this pressure will be distirbuted against the total surface of the threads. A breach plug is screwed into about 1/2 to 3/4 of an inch of threads. You would have to figure the surface of the threads and then realize that this 5,600 pounds of pressure is distributed against the total surface of the threads. The face of a 14 X 28 nipple is only about 0.05 sq. in. Therefore, the total pressure exerted against the face of the nipple is only about 1,400 pounds. Again, that is distributed across the total surface area of the threads. Given the strength of modern steels, these pressures, while impressive, are not enough to cause catastrophic metal failure.

These pressures are based on 120 grain charges of Pyrodex. Black powder will present lower pressures.

Oops, manure!!! I just went back and read all of the previous answers and Zonie had already covered it. I just repeated what he said. S'cuse me Zonie! :redface: :surrender:

It depends on the amount and type of damage and I can safely say, don't trust any damaged thread if it is used in a pressurized area on a firearm.

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The cardinal rule.