Author Topic: Breech design question. (Read 4311 times)

cannonmn · « **Reply #30 on:** April 20, 2009, 03:27:41 PM »

Quote

the Whitworth a rotateing hammer

I think there may be some confusion on the purpose of the "tappet" arrangement that allowed maybe 45 degrees of freewheeling before the heavy arrangement whacked the breech screw. The Whitworth breechloader used a metallic cartridge which was what sealed the gas when it was used as a breechloader, so you really didn't need to close the breech super-tight. The tappet, as far as I know, was only installed to make it easier to open the breech after firing. I don't think it was used to close the breech.

When the Whitworth b/l was to be used as a m/l, you would install the copper cup seal-thingy that came with the gun as an accessory, and the first shot would seat it to prevent gas leakage. Then before you used it as a b/l again, you had to get that sealing cup out of the gun, and I suspect that would take some time-definitely something you would not do in action.

This stuff is described in the Whitworth B/L Operator's Manual I found in the Library of Congress and put on the web a couple of months ago.

GGaskill · « **Reply #31 on:** April 20, 2009, 04:21:42 PM »

If you look at the various full scale artillery of the post 1898-era, you will find either cartridge cases or the De Bange obturator. Closely machined fits wear, and the goal of mass produced anything is to not require close tolerances.

gary michie · « **Reply #32 on:** April 20, 2009, 04:25:59 PM »

It's me again

I thought that is where I got the notion that when not b/l-ing you installed the washer and made it tight; my bad!

GGaskill · « **Reply #33 on:** April 20, 2009, 04:47:06 PM »

For example, let's say a 1" bore with a 3" diameter 4130 steel breech. A 1" thick steel disk at the rear, attached with 4 x 3/8-16 cap screws (each having >100K psi yield strength), in an equally spaced bolt circle. Length of engagement into the breech, 1".

I'm not sure anyone actually addressed this idea, so I will. The problem I see is leakage. If chamber pressure is allowed to attack the whole area of the external disk, you will have a lot of force to deal with. With a 1" breech plug and 20,000 psi, you would have pi r² area (3.14159 x .5x .5 = .785) times 20,000 = 15700 lbs. Should the whole 3" diameter be exposed, you would have 141,000 lbs to be held with the bolts. So clearly, you need to have a guaranteed seal against chamber pressure if you use a closure like this.If chamber pressure is allowed to act on the flat plate closure, the plate will bend slightly and the bolts will stretch slightly and admit pressure to an ever widening area, drastically increasing the force the closure is required to resist. Once the seal starts to leak, whether from its own erosion or corrosion of the seating surface, the risk of failure grows quickly.

Also, remember that a bolt with 100 ksi yield strength has the actual strength of the cross section of the minor diameter of the threads (.297" for a 3/8-16), so a 3/8-16 bolt would have an actual tension strength of only about 6900 lbs. Doing a little more lower division engineering, the bolts would stretch about a half a thousandth at 20,000 psi chamber pressure. The aluminum foil powder bag would probably maintain its seal although I think I would go with 6 bolts instead of only four.

Double D · « **Reply #34 on:** April 20, 2009, 04:58:48 PM »

So maybe I was comparing apples to oranges. The breech plugs I built were plugs. Serving the same purpose as a breech plug in a muzzleloader. They are intended to be remove only infrequently.

Breech blocks for breech loaders are something entirely different

Victor3 · « **Reply #35 on:** April 21, 2009, 02:10:06 AM »

Quote from: GGaskill on April 20, 2009, 04:47:06 PM

For example, let's say a 1" bore with a 3" diameter 4130 steel breech. A 1" thick steel disk at the rear, attached with 4 x 3/8-16 cap screws (each having >100K psi yield strength), in an equally spaced bolt circle. Length of engagement into the breech, 1".

I'm not sure anyone actually addressed this idea, so I will. The problem I see is leakage. If chamber pressure is allowed to attack the whole area of the external disk, you will have a lot of force to deal with. With a 1" breech plug and 20,000 psi, you would have pi r² area (3.14159 x .5x .5 = .785) times 20,000 = 15700 lbs. Should the whole 3" diameter be exposed, you would have 141,000 lbs to be held with the bolts. So clearly, you need to have a guaranteed seal against chamber pressure if you use a closure like this.If chamber pressure is allowed to act on the flat plate closure, the plate will bend slightly and the bolts will stretch slightly and admit pressure to an ever widening area, drastically increasing the force the closure is required to resist. Once the seal starts to leak, whether from its own erosion or corrosion of the seating surface, the risk of failure grows quickly.

Also, remember that a bolt with 100 ksi yield strength has the actual strength of the cross section of the minor diameter of the threads (.297" for a 3/8-16), so a 3/8-16 bolt would have an actual tension strength of only about 6900 lbs. Doing a little more lower division engineering, the bolts would stretch about a half a thousandth at 20,000 psi chamber pressure. The aluminum foil powder bag would probably maintain its seal although I think I would go with 6 bolts instead of only four.

Thanks GG. Lots of things to consider there.

I'm thinking that a breech seal for this design might be better accomplished with a longer (~1") close-fitting plug made from harder elastic (Shore D-70+) material with a steel washer ahead of it. That should isolate much of the pressure impulse ahead of the rear plate upon firing.

With the above-described sealing element, I don't believe that four Grade 8 3/8-16 SHCS will ever see anywhere near their elastic limit. Neither would a 3" dia x 1" thick steel plate captured by them.

On a side note, even of the seal completely failed in this design, I don't believe that nearly the full 141K psi would be acting upon the 3" diameter of the plate to push it rearward, considering the small gap that would be present between the plate/breech face (please correct me if I'm wrong here).

GG, could you expand on how you arrived at 6900 lbs for the tensile strenght?

GGaskill · « **Reply #36 on:** April 21, 2009, 12:06:30 PM »

GG, could you expand on how you arrived at 6900 lbs for the tensile strength?

It should have been like this:

minor diameter of 3/8-16 bolt is .297". Cross-sectional area of .297" diameter ( 3.14159 times .297 time .297 divided by 4 = .069279 in²) times 100000 lbs/in² = 6927.1 lbs. I rounded to 6900 for convenience. Now that is not the failure load as the bolts will go to a little higher force while plastically deforming before actually breaking, but once we leave the elastic region, trouble is approaching. But we don't get there as long as the breech pressure is limited to the bore area.

If you assume the seal will limit the area subject to breech pressure to the bore area only, then the four bolts would have to resist 15700/4 = 3925 lbs each which is about 57% of their elastic strength.

Cat Whisperer · « **Reply #37 on:** April 21, 2009, 12:34:21 PM »

Interesting. From having observed failures of bolts used to secure a lid (14" diameter) on a hydraulic tank (bunches of PSI) secured by about a dozen 1" diameter 36" long bolts with fine thread; the bolts broke near the end, but never did they strip.

Stainless hardened bolts. Broke from the flexing - from pressure vs no-pressure - over and over. The effect on materials of the hydraulics is not unlike that of firing cannons.

The nuts were about an inch long (engagement of threads).

GGaskill · « **Reply #38 on:** April 21, 2009, 01:06:50 PM »

...the bolts broke near the end, ...

Meaning just under the heads or at the beginning of the threads?

Cat Whisperer · « **Reply #39 on:** April 21, 2009, 02:05:30 PM »

Sometimes an inch below the nut (bolt passed through the plate with the nut on top of the plate), sometimes two or three inches below. Sometimes several would break at the same event - putting a bit of hydraulic oil all over.

GGaskill · « **Reply #40 on:** April 21, 2009, 03:50:52 PM »

Let me rephrase my question, was the break in the threads or in the unthreaded part?

Victor3 · « **Reply #41 on:** April 21, 2009, 11:22:45 PM »

Quote from: GGaskill on April 21, 2009, 12:06:30 PM

GG, could you expand on how you arrived at 6900 lbs for the tensile strength?

It should have been like this:

minor diameter of 3/8-16 bolt is .297". Cross-sectional area of .297" diameter ( 3.14159 times .297 time .297 divided by 4 = .069279 in²) times 100000 lbs/in² = 6927.1 lbs. I rounded to 6900 for convenience. Now that is not the failure load as the bolts will go to a little higher force while plastically deforming before actually breaking, but once we leave the elastic region, trouble is approaching. But we don't get there as long as the breech pressure is limited to the bore area.

If you assume the seal will limit the area subject to breech pressure to the bore area only, then the four bolts would have to resist 15700/4 = 3925 lbs each which is about 57% of their elastic strength.

Thanks GG.

Your 6900 number threw me because I'd calculated ~10,000, but I forgot that this was for 3/8 - 24 Grade 8 cap screws (130 ksi & larger minor dia). I'm confident that using four of these would provide a more than adequate safety factor.

As far as using six rather than four cap screws, I was thinking of slotting the holes in the breech plate and adding through holes to clear the head (.562"). This would allow for fast removal of the plate without having to remove the screws. Using six screws in a 3" diameter would remove too much material from the plate.

Cat Whisperer · « **Reply #42 on:** April 22, 2009, 12:46:17 AM »

Quote from: GGaskill on April 21, 2009, 03:50:52 PM

Let me rephrase my question, was the break in the threads or in the unthreaded part?

yes and no; both occurred.

Victor3 · « **Reply #43 on:** April 22, 2009, 01:18:01 AM »

Quote from: Cat Whisperer on April 22, 2009, 12:46:17 AM

Quote from: GGaskill on April 21, 2009, 03:50:52 PM
Let me rephrase my question, was the break in the threads or in the unthreaded part?

yes and no; both occurred.

That's unusual. Failures usually occur in the threaded section.

I'm curious CW. What kind of a tank was this? My only experience with hydraulic 'tanks' have been with reservoirs not subject to pressure. Sounds like you're talking about a cylinder of some sort(?)

Cat Whisperer · « **Reply #44 on:** April 22, 2009, 07:05:57 AM »

It was on a rotary press, which loaded - unloaded the cavities which were filled with brake 'friction' and underlayer which were in sequence bonded to the back-plate (of a disc brake pad) under high pressure and temperature.

The pressurized tank was there to provide the volume at the needed pressure. IIRC it was about a 75 ton press. Some of the hydraulic lines were 2" ID. There was of course the resevour tank, the pumps and the cooling stuff. We had a number of manually fed 50 ton and some 600 ton presses as well.

GGaskill · « **Reply #45 on:** April 22, 2009, 08:00:41 AM »

... 3/8 - 24 Grade 8 cap screws ...

I wondered why you had said 3/8-16.

Bornsouthern34 · « **Reply #46 on:** April 22, 2009, 04:18:39 PM »

The solution I came up with was to put a standard threaded breach plug and then pin it. Similar to a shear pin. The shear strength of a small bolt or stainless pin should far exceed that of the threads on the plug. To access the breech, pull the pin and unscrew the plug.

Victor3 · « **Reply #47 on:** April 23, 2009, 02:01:42 AM »

Quote from: Bornsouthern34 on April 22, 2009, 04:18:39 PM

The solution I came up with was to put a standard threaded breach plug and then pin it. Similar to a shear pin. The shear strength of a small bolt or stainless pin should far exceed that of the threads on the plug. To access the breech, pull the pin and unscrew the plug.

Bornsouthern34,

Some breech designs I've seen use cross pins to add strenght.

However, it's important to remember that a carbon steel pin's shear strength is only ~60% of the tensile strength of a plug the of the same minor diameter/material. Also, many common (304, 17-4) stainless pins/screws have less tensile strength than graded (5, 8, etc.) carbon steel ones do.

Considering that a cross pin might be only ~1/2 the diameter of the breech plug, there isn't as much added strength in this design as might be assumed.

Note - The above isn't completely applicable to a plug screwed into a solid breech since the strength of the joint isn't dependant upon tensile strength of the plug. However, it's something to consider in any cross-pinned design.

Bornsouthern34 · « **Reply #48 on:** April 23, 2009, 02:17:40 AM »

If the pin is offset, would that increase strength by increasing the total surface area of the shear area on the pin/bolt?

GGaskill · « **Reply #49 on:** April 23, 2009, 11:28:47 AM »

I really think that the cross pin is only keeping the threaded plug from unscrewing. Unless there are only a couple of threads engaged, the threads are more than sufficient to resist the chamber pressure.

Bornsouthern34 · « **Reply #50 on:** April 23, 2009, 05:12:03 PM »

That makes sense. Another suggestion brought up to me was cutting an inverted trapazoid slot, with the widest side facing the bore. Any of you heard of this? I guess it would take a very patient machinist to accomplish.

GGaskill · « **Reply #51 on:** April 23, 2009, 06:11:31 PM »

... an inverted trapezoid slot, with the widest side facing the bore.

This is for both the breech and the plug? How would you insert the plug? I must not be understanding something here. Is this for a sliding breech?

I can see how it could work the other way with the threads engaging on the last turn (small end forward.)

Bornsouthern34 · « **Reply #52 on:** April 26, 2009, 07:53:41 AM »

You are correct both with the direction and having the breech slide up and down or side to side. Let me see if I can come up with a drawing.

Bornsouthern34 · « **Reply #53 on:** April 26, 2009, 08:14:21 AM »

Here you go.

GGaskill · « **Reply #54 on:** April 26, 2009, 09:56:00 AM »

I don't understand why you have the screw plug if you are going to have the sliding breech block. Put a seal in the rear of the powder bag and the sliding block should be adequate.

Bornsouthern34 · « **Reply #55 on:** April 26, 2009, 02:08:05 PM »

I guess it would be for looks. Not sure. This is a design that someone suggested to a friend for his cannon.

Victor3 · « **Reply #56 on:** April 26, 2009, 08:51:51 PM »

Quote from: Bornsouthern34 on April 26, 2009, 08:14:21 AM

Here you go.

B34,

Thanks for posting that. It's another option. While not exactly what I've seen on historical designs, it's similar in the basic idea of a sliding or removable block.

A trapezoid would work, but getting the shape of the hole would be difficult. A square block would do the job.

Even more simple would be round block. Then all you would need is a flat-bottom round hole. Or, you could bore it clean through and use a longer block using your screw to hold it in place. This would also make it easier to remove the block if it got stuck

I used to have a book on how to build a falling block rifle using a round breech block. The reason the author did it this way was because of the ease of machining the hole in the receiver for the average guy in a home shop.

In your design, the screw you show might be helpful to hold the block in place if some gas leaked under it and forced it upward. A detent drilled into the block to accept the screw tip would be good. You could even install the screw from the side and leave the hole in the rear clear. Then you wouldn't have to remove the screw to load/clean. In this configuration, a smaller than bore diameter retaining screw could be used, making it even more simple to machine.

ratpatrol · « **Reply #57 on:** April 27, 2009, 11:03:47 AM »

On your original post and design,BAD idea 3 inch Od and 1 inch Id give a 1 inch wall .minus 3/8 hole for bolts ,5/8 metal total 0r 5/16 0n a side to hold 25,000 psi. I don't think so. On your grade 8 bolts they have a high tensil straingh yes, but no strech they brake all at once. Sounds to me like a lot of pieces flying around.
On your other thoughts, It's very hard to mill deep designs,squairs,rectangles in metal,Cutters arn't that long and they flex, I know I'm building a sliding wedge type breech right now, 5 inch OD. 3inch ID to be threaded on a 3 inch 42 long tube 1 1/2 bore, Well be useing home made cases or the De-Banger system for sealing.

KABAR2 · « **Reply #58 on:** April 27, 2009, 07:17:28 PM »

Quote from: Bornsouthern34 on April 26, 2009, 08:14:21 AM

Here you go.

Why not build an Armstrong? With your breech and a bolt locking it in you are half way there.

Victor3 · « **Reply #59 on:** April 28, 2009, 02:00:57 AM »

Quote from: ratpatrol on April 27, 2009, 11:03:47 AM

On your original post and design,BAD idea 3 inch Od and 1 inch Id give a 1 inch wall .minus 3/8 hole for bolts ,5/8 metal total 0r 5/16 0n a side to hold 25,000 psi. I don't think so. On your grade 8 bolts they have a high tensil straingh yes, but no strech they brake all at once. Sounds to me like a lot of pieces flying around.

Actually ratpatrol, even if we assume (incorrectly, as it is not applicable here) a simple 5/16 wall tube with a 1" bore in 4130 material, it would still have a bursting pressure of >34,000 psi. We're talking about a 1" wall tube and four 3/8 holes (filled with screws) in the center of a 1" wall. Apples & oranges.

As far as the fasteners go, I think GGaskill and I have hashed out the engineering considerations fairly accurately above. Did you find an error in our calculations?