Thoughts on trunnion attachment

[CU_Cannon]

I am planning on building a 1/2 to 1/3 scale mountain howitzer.   I would like to cast the barrel but since that is not a possible right yet it will be machined from solid.  I was wondering what works the best for attaching the trunnions.  The options I have found are press fit, high strength epoxy, screw-in, and welded.  I'm not confident enough with my machining to try a press fit on something critical.  The epoxy sounds like a good option but I'm not sure how much I trust it.  Screw-in would work well.  Welding would also work well but I would be worried about cracking and hardening of the steel.

Thoughts, comments, concerns?

[Evil Dog]

Just a thought, but how about screw-in with epoxy?

[Double D]

I use William Greens method of attaching trunions.



Machine the trunnion recess in the tube before turning the shaft.  Make a plug to fit each trunnion recess for turning.  I find it easier to get the trunnions square to each other and the bore doing them first.  The plugs off course are to close the  hole swhile make the tapers.  The plugs are removed after the cannon is turned and the trunnions are installed.

I was just installing the trunnions by screwing them in and setting them interference tight with a barrel wrench just like when you screw a rifle barrel in a rifle action.   I was shooting with some folks in Northern California and they required trunnions to be integral or welded in.  When I pulled my barrel to have the trunnions welded I found one of them loose--finger loose.  

I weld all cannon trunnions now.

[CU_Cannon]

Screw-in looks like the right option.  If I decide to later I can epoxy or weld the trunnions in place I can.  

Thanks!

Now to find some steel.

[GGaskill]

Unless I have a cast barrel with integral trunnions, I weld them or at least braze them.  One of the advantages of using known materials (rather than scrap yard finds) is you know whether they can be welded.  I have thought about using a bolt through the trunnion center into the barrel, and may find an application of that technique, but so far have welded everything.

I know there are strong epoxies but I wouldn't trust epoxy by itself for this application.

Welding would also work well but I would be worried about cracking and hardening of the steel.

This depends on the steel used.  Mild steel such as 1018 or 1020 will not have a problem because it cannot be hardened with heat treatment.  4130 is weldable but 4140 requires substantial preheating and post-heating.  Generally, the more carbon in the alloy, the harder it will be to weld easily.

[Double D]

This is a good discussion topic for builders.  It is also a good topic for buyers to read.  You need to ask how trunnions are attached before buying.  I agree with George about attaching trunnions with epoxy, I would  think a stronger method of attachement is advised.

NSSA doesn't say anything about trunnion attachment but they do say installing a liner with high strength adhesives is ok.  


I know that one of the sellers on the internet list machines some beautiful cannons then attache trunion with JB weld.

When asked about it he used the same line another cannon maker who insists on using seamless liners uses " I sold a bunch of these things and never had a complaint"  May be true but do you need complaints  to understand it may not be safe.

[Cat Whisperer]

A couple of comments on trunion attachements.

First, if screwed in, after about three threads in depth you add little to the strength by adding more threads.

Second, there is a quite legitimate use for epoxy or jb weld and  trunions - take for example a trunion at the very base of the mortar - with a drilled hole and the trunion rod going completely through - to fill the spaces with epoxy to fix it in place works well.  As does a DEEP pocket with a close fitting trunion rod on one side pressed into it - the epoxy simply takes up the spaces locking the trunion rod in place - the brunt of the recoil is taken up trying to compress the epoxy not rip it out.

[Cat Whisperer]

Unless I have a cast barrel with integral trunnions, I weld them or at least braze them.  One of the advantages of using known materials (rather than scrap yard finds) is you know whether they can be welded....

Overbuilding things that go bang is OK!

Knowing what you're working with is also a very sane approach to using explosives.

[CU_Cannon]

I like the idea of screw-in and welded.  I'm sure that epoxy would work great, in some ways almost better than welding but I would like to be a bit of a purest.  I'm pretty sure that they didn't have epoxy in the 1800s.  Of course welding has advanced since then also.  Six of one half a dozen of the other.  I just feel welding is a more permanent solution.

Over building is always a good idea when in comes to safety.  It is really easy to scale down a cannon but you have to remember that physics doesn't scale down.  If you make something half the size it has one quarter the volume.  Say you take a 2 in square rod that can hold 1000 lbs.  Stress is equal to the load / area.  In this case the stress would be 250 psi.  If you take a rod made of the same material that is half the size the stress would be 1000 psi!

[Slam-Fire]

I learned this from the structural engineers while building the Freemont Bridge in Portland Oregon, circa 1974.

Strength in cap screw (bolt) is maximized at the inside (bottom depth of the thread) diameter of the screw shaft. Shear strength in the connector is only as strong as the shaft diameter. Sinking the threads deeper than the shaft diameter does not add to the shear strength of the connection.

When you thread a trunnion into a barrel, you reduce the wall strength of the barrel. I don't have any calculation formulae for this, I just know the wall strength is reduced.

I'd be leary of epoxy connections for trunnion to barrel -- epoxies have good adhesive strength, but they seem brittle and lack shear strength. Trunnion connections are all about shear.

Welding is the way to go. Take the parts to a metal fabricator and have it professionally welded. It's worth the expense to know it's not going to come apart. Welding trunions might run about $50.

[paulj84003]

My two cents worth. I  have only built 6 cannons and mortors so not an expert. But to affix the trunnions to the  cannon barrel I use  Bill Greens idea with a modification. Instead of turning threads on the trunnions I tap them and tap the barrel using an Unbreko set screw of the appropriate size  in between. As well as using loctite retaining compound.  Jim

[Cannoneer]

The options I have found are press fit, high strength epoxy, screw-in, and welded.

   Would someone kindly clue me in on what " press fit " actually entails. I know there was a discussion of this subject not that long ago but I don't remember if the topic was ever resolved, I think some people were referring to what they called a " sweat fit ".

[dan610324]

now is the crazy swede here again with an idea, hope you will excuse my lack of English language knowledge when it comes to difficult technical language .
sorry if I'm only talking in the metric system here but I'm not familiar enough with the inch measuring system when it comes to extremely small tolerances .
I have been looking here at the "William Green" method , but for an projectile shooting barrel I didnt like it .
you will always get empty space in the bottom of the drilled and tapped hole , and at those drawings attached here it seem to be very little material between the bore and the tapped hole .
maybe the barrel wouldn't crack open , but it could at least become an cavity from the inside of barrel where the trunnions are attached if its done that way when shooting full service loads .
and it takes a lot of machining skill to do that in an good way .

wouldn't it be much easier and much safer to shrinkfit the trunnions in the barrel and tig weld them to the barrel ??

as an example , lets say we make an barrel with an 30 mm bore, if you follow old dimensions you should have approximately the same diameter of the trunnions.
where the trunnions is placed on the barrel you have approximately 24 mm wall thickness , if you are milling an diameter 30 hole there 12 mm deep and makes an trunnion 6 - 7 hundreds of an millimeter larger and shrink fit it to the hole .
you have an absolutely level surface in the bottom of the hole , that mean you have absolutely no air between the barrel material and the trunnion . then when the trunnion is shrink fit to place and tig welded I would guess that it is very close to as strong as if it had been produced in one piece .

ideas and opinions please ??

[dan610324]

press fit is when you have an smaller difference between hole and axle and use hydraulic to press it together .
when you shrink fit you have an much larger difference in size and cool the axle to have it to shrink and heat the hole to have it to expand and then put the parts together , when both parts are back to normal temperature the axle that are larger than the hole is stuck there

[Double D]

I liked Greens method also until one day I found both trunnions loose.  I now weld, solder or braze trunnions.

[GGaskill]

A press fit is a fit where the hole is a slight bit smaller than the piece going in the hole.  By slight, I mean about .001" plus .001" per inch of diameter.  Depending on the geometry, sometimes you can heat up the piece with the hole such that it actually expands to a diameter greater than the inside piece, which makes assembly easier but requires quick work to get things in place before the parts have lost their temperature differential.  For small parts, it is sometimes easier to simply chamfer the edge of the inside part and use a press of some sort to force the pieces together.

The "sweat fit" mentioned was in reference to the N-SSA liner construction rules.  The term seems to be common in the Northeast but equally seems to be unknown elsewhere.  It means the same as heat shrink or shrink fitted.

[Cannoneer]

   Thanks George, I phrased my question very poorly but I think you still managed to answer it. So, if I'm talking with a machinist and the topic is attaching trunnions to a barrel and he tells me he press fit them what he would actually be telling me was that he milled or drilled a pocket or hole of x diam. then used significant force to "press" the trunnion which was made to have a slightly larger diam. than x into the pocket. If on the other hand he told me that he shrunk fit the trunnion on the barrel he would actually be describing using heat to expand the pocket in the barrel and perhaps cold to the slightly larger diam. trunnion to shrink its circumfrence and then also use pressure to join the trunnion to the barrel and when the metal returned to an ambient temp., the trunnion would be solidly gripped by the pocket. "Sweat fit" being only a regional or certain profession's term for describing a "shrink fit".

Added 8/10/ 08 8:05P   Dan, I'm sorry I missed your answer the first time I checked this thread, I don't know how I did but it happened. You answered first and I thank you for your response.

[accuratemike]

Ok, I have a question too. Is there a minimum wall thickness that should be maintained between the bottom of the trunion hole an the bore? You walk a fine line between having the pockets deep enough to sufficiently support the trunion and too little material left around the bore. The 1 caliber rule at the breech goes right out the window for the profile of most barrels, when it comes to the trunion area. Looking at many designs, I see that the walls are pretty thin at the base of the trunion pockets. I have not seen any rule of thumb for "safe" minimums though. MIKE

[GGaskill]

Here is one of the advantages of welding--the strength of the weld backs up the bore wall.  Since it is almost impossible to cut a curved bottom hole that would parallel the bore wall, one is stuck with flat bottom holes that have thinner walls in the middle and essentially full thickness walls at the edge of the rimbases.  Sometimes, I will not bore the pocket to the full depth such that it is not circular but leaves openings at the edges.  Then I will fill the gap with weld material.  This gives me a little thicker bore wall than if I had bored the pocket to completely contain the rimbase.

This issue becomes more of a problem when you are making a tube that is exactly scaled based on bore diameter.  When underboring, it is less of a concern since the bore walls are already thicker than proportional.  So maybe something to take away for subscale builds is to make the bore a little less than scale (or conversely, make the exterior a little oversize for the bore.)

As far as rules of thumb, I don't have any beyond trying to make the wall as thick as possible within the limitation of the geometry.  Since the pressure in black powder artillery peaks very soon, it has gone down significantly by the time the shot has reached the trunnion area due to both heat loss and expansion of the chamber volume to include about half of the length of the bore.  If the piece has a sub-bore diameter chamber, the expansion is even greater and the pressure reduction is similarly greater.

[Double D]

The one caliber rule applies over the breech and no where else.  Making pockets for trunnions is one thing. But drilling deep hole for threaded  studs halfway down the barrel is another thing.  I guess I would be a bit concerned if the holes were going to be left empty.   

Also consider that you may indeed mill pockets but you are filling this pocket back up with metal hopefully welded or brazed in place. Just the same as weldoing a breech plug in place.  It is something to think about, but as George says pressures are dropping and lower in area of the trunnions.  I have never heard of a barrel failure in the trunnion area, but then I also am not aware of every cannon failure.  The greater fear is the trunnion will shear off. 

[Articifer Tom]

 Yeah  , I live in the northeast and nothing will make you SWEAT and have a Fit  like your two pieces not going all the way together while  heat shrinking parts.

[Cannoneer]

Yeah  , I live in the northeast and nothing will make you SWEAT and have a Fit  like your two pieces not going all the way together while  heat shrinking parts.

Very good Tom, I almost worked up a "sweat" with the convulsions of the laughing "fit" that your humorous puns caused me to have.

[Frank46]

This is one topic that should be made a "sticky" as I've often wondered myself as to properly fix the trunions on a cannon bbl and not have to worry that have the bbl itself become a projectile when fired. Frank

[seacoastartillery]

     Frank,  These moderators are pretty sharp; they already made it one.  Take a look at the list of Stickies.  It's the one called,   "STICKY Books---------------How to Build"

It's not just you, Frank; they are way ahead of us too!

Regards,

Tracy and Mike

[Frank46]

Mike and Tracey, thanks guys. Thats what I like about this forum. If you don't know about something, you'll either get a bunch of answers or its already here. You learn something every day. Frank

[Cat Whisperer]

Mike and Tracey, thanks guys. Thats what I like about this forum. If you don't know about something, you'll either get a bunch of answers or its already here. You learn something every day. Frank

Thanks, Frank (and Mike and Tracy too) -  This is exactly what Douglas and I set this up for - so WE could learn and enjoy others' experiences.  Graybeard did his part too in humoring us.

[dan610324]

I can only agree , this is the best and probably the one and only forum where its possible to share thoughts and ideas about cannons .

[Double D]

Mike and Tracey, thanks guys. Thats what I like about this forum. If you don't know about something, you'll either get a bunch of answers or its already here. You learn something every day. Frank

Thanks, Frank (and Mike and Tracy too) -  This is exactly what Douglas and I set this up for - so WE could learn and enjoy others' experiences.  Graybeard did his part too in humoring us.

Tim's exactly right we started this board, so WE could learn and we still are!!!!!  I don't think  Graybeard had any idea how popular this forum would become.  I know even tough I had been on the old Gun Knife BBS Cannon forum and it was slow, but consistant, I knew there was an interest. 


We do have a depth of interest that is not the same as other cannon forum I have seen in over the years...I mean where else have you seen a discussion on how to attach trunnions with such  an eagar sharing of knowledge on the subject.

[de_lok]

I put to mind to resolve the trunnion attachment issues over 20 years ago when I first started making cannons as one of the first proto-types sheared the bolts attaching the trunnions. I was not aware of William Green's designs at the time, but my solution could be considered an upgraded version of his method.It has worked so well I kept it a well guarded "secret" as long as I was selling cannons. This method has proven to be well engineered, incorporates most of the more reliable methods mentioned in this thread, and can be done by anyone capable of doing any of the above mentioned methods. Double D is correct by stating the structural integrity of the tube should be intact because the voids are filled with metal, but then again you always need to know what you are working with. I use a tre-pan flat bottom slot and matching counterbored trunnion. A set screw is screwed into the barrel then trunnion screwed on. You can either put some JB weld or epoxy in the slot before attaching the trunnion to fill the gaps, or a better way is to silver solder the seam. It is important to note I have used this on scale models no larger than 1/3 scale. I have not proven it on 1/2 scale up. I had to dig abarrel out of the scrap pile to take pics, sorry the resokution isn't the best, I will post better pics on request...................

[de_lok]

More pics............