Firearms Design and Engineering

[Johnm1]

In another post I mentioned that the S&W New Departure (Safety Hammerless) had a rebounding hammer that was controlled by the mainspring tension. I spent some time looking at how it works and it just amazes me how that was engineered to work.

This started with the observation that my S&W Model One and One Half Single Action did in fact have a rebounding hammer. I studied the parts diagram looking for a piece that would make the hammer rebound and couldn't find one. Being a single action that action is very simple. It wasn't until I started to diagnose a sticking firing pin on a New Departure (Safety Hammerless) that also has a rebounding hammer that I read that the rebounding hammer is accomplished by the mainspring and its tension!

FYI - for this discussion the 1 1/2 Single Action has an exposed hammer and the New Departure has an enclosed hammer with a frame mounted, rebounding firing pin.

Basically, a properly tensioned mainspring with the correct amount of 'curve' in it drive the hammer to a point where the hammer is not only at rest, but actually imparts a decent amount of backwards force on the hammer when at rest to not only place the hammer off of the firing pin/primer but also require a significant amount of force to push it forward to contact the back primer, in the case of the Single Action, and off of the frame mounted firing pin in the New Departure.

Here are some pictures to illustrate the arrangement. These will be for the New Departure but the single action is very similar:

This is a picture of the hammer at rest


Notice the top of the hammer has backed off of the back of the firing pin. In this position the mainspring is now imparting some backwards force on the hammer. If this were a hammer mounted firing pin it would take significant force to push it back into contact with the back of the frame mounted firing pin. I can confirm this by trying it on my single action that has a hammer mounted firing pin and an exposed hammer.

It is only the weight/momentum of the hammer that 'pushes past' the at rest position and contact either the back of the firing pin (frame mounted firing pin) or the back of the primer (hammer mounted firing pin) when the trigger is pulled.

Another thing to note. Notice the curve of the mainspring in the above picture. This is showing the curve of the mainspring with the strain screw tightened fully. As the strain screw is loosened the curve of the mainspring 'flattens'. I find that the clearances between the stirrup and the relief in the hammer is so close that with the strain screw loosened the stirrup contacts the relief in the hammer and binds the action. I wish I had taken a picture of the curve of the mainspring in the cocked position but I didn't. Here is a picture of the stirrup/hammer in both the cocked and at rest position.

At Rest Position


Cocked Position



So the point of this post is the engineering that went into the design of the hammer, mainspring, stirrup to accomplish both the reliable firing of the firearm and the rebounding hammer. Today we would just model the arrangement and find out where to put the stirrup pin and how much to relieve the hammer to avoid interference between the two. But how did they engineer this back in the 1870's? The math existed, but it was complicated and not straight line. As the hammer is pulled back the curve of the mainspring changed and thus so did the place the interference would occur. And just how did they determine where the 'at rest' point was for the hammer in just the right spot so that it not only provide the energy to fire the weapon but to also provide backwards force to keep the hammer off of the firing pin/primer in the at rest position?

Inquiring minds want to know.

 

[Johnm1]

So I may need to figure out the actual geometry of the rebounding hammer. Below is a comparison of the mainspring geometry of the above Safety Hammerless/New Departure in the rebounded position compared to a 38 Double action advertised as having a rebounding hammer in what should be the rebounded position. The 38 Double Action’s hammer does not rebound and I don’t know why. We can see the angle of the stirrup is
different but I’m not sure that is the reason.

Both

38 Double Action that doesn’t rebound

Safety Hammerless/New Departure that does rebound


The actions are different between the double action only and the double action. But the angle of the stirrup may be a clue.

Plus the 38 Double Action needs a new rear sear. I replaced the rear sear because the original was “chewed up” and the action worked almost perfectly for a short time except for the rebounding hammer part. I have to figure out why the hammer fly isn’t protecting the rear sear surface.

here is a link to an animation of the Double Action in motion. It helps a little.

 

[PRD1]

The geometry required for rebounding locks was pretty well understood before it was used in the construction of the S&W revolvers. British and European sporting sidelocks were constructed with rebounding hammers considerably earlier, and I'm sure that the design requirements were understood by firearms designers on this side of the pond, as well.

PRD1 - mhb - MIke

 

[Johnm1]

The geometry required for rebounding locks was pretty well understood before it was used in the construction of the S&W revolvers. British and European sporting sidelocks were constructed with rebounding hammers considerably earlier, and I'm sure that the design requirements were understood by firearms designers on this side of the pond, as well.

PRD1 - mhb - MIke

Interesting. I’d like to read about that history/development.

So the big question is if that development, whenever it happened, was achieved through math or trial and error. I suspect math.

 

[PRD1]

A brief 'net search showed that the first British patent for a rebounding lock was recorded in 1866, and others followed. The patents were illustrated, so could be read. It's a good question whether math played much part in the development process, but considering that much of the historical evolution of firearms was empirical, I'd be surprised if much sliderule work was needed in this instance. Still, I'd be as interested as you to know the facts.

PRD1 - mhb - MIke

 

[4v50 Gary]

Very good thread.

 

[Johnm1]

considering that much of the historical evolution of firearms was empirical, I'd be surprised if much sliderule work was needed in this instance. Still, I'd be as interested as you to know the facts.

I wonder if it is a question that can be answered? The math or trial and error question. It's not like people were declarng the answer and showing how they did it.

This from another Knight of the Woeful Countenance.

 

[Johnm1]

Very good thread.

I may not know much, but I hopefully can initiate intriguing conversations.

 

[Monac]

Driftwood Johnson explained that Smith & Wesson Model 1878 Double Actions designs did not have a rebounding hammer in post #16 of this thread: https://www.thehighroad.org/index.p...ican-revolver-conundrum.852669/#post-11157764

I was very surprised by that. I have never owned an 1878 type S&W DA, but I do have a Marlin 1887, which is a copy of the 1878 with some small changes to avoid S&W's patents. The single action sear on my Marlin is broken. I thought the fact that it did not have a rebounding hammer was either one of the evasions or another broken part, but Driftwood showed that it is not.

A DA revolver without a rebounding hammer seemed very odd to me, but then, a lot of things seem odd to me.

(I hope this is relevant and that I have not misunderstood what is being discussed in this thread.)

 

[Johnm1]

I hope this is relevant and that I have not misunderstood what is being discussed in this thread.

It is relavent. I have a book on disassembly/reassembly of vintage firearms that specifically states that this model does have a rebounding hammer. The book is not a S&W publications so there is the possibility that it is incorrect. I'll read through Mr. Johnson's post on the referenced thread when I get home tonight. It's just an interesting topic. I'm really interested in how they did the design. Trial and error takes forever and the math was there to do figure it out at the time. I just don't if they did the math or not.

Related, I find that the hammer fly is a real important part of the double action function. Basically the hook on the rear sear has to 'ride up' the face of the hammer fly for the action to cycle correctly (3 clicks). As I read the disassembly book, the hammer fly was an addition to this model (or maybe just an improvement incorporated into this model) to protect the rear sear from the 'half cock notch'. And I find that after the trigger is released and the rear sear is seated on the hammer fly, it takes only a small amount of trigger movement to the rear for the firing pin/hammer to retract in what looks like a rebounded position. I don't know what would provide the force to do this. Could the geometry of the mainspring be such that when the hammer has released the energy from the mainspring and struck the firing pin that the hammer 'bounces' off the frame with enough energy to 'rebound' the hammer back to this position? I don't know. I do know that when the hammer is in this position the cylinder stop has dropped a bit and doesn't reliably engage the cylinder. Though that could be a function of a worn cylinder stop (a known issue with my revolver) or another worn part). FYI - the cylinder stop in its entirety is a spring and that spring force is all that engages the 'nub' on the cylinder stop to engage the cylinder.

I have stared at the parts and the animation until I'm bleary eyed. The one thing my observations cannot detect is if there is a worn part that isn't performing all of its designed functions. And the last person who was a part of the design and could answer that question died probably 75 years ago.

I'm not sure that any of this means anything in the big picture of knowledge or not. But I did stare that the rear sear long enough to determine that hook on mine was out of shape and that out of shape prevented the hook from riding up the hammer fly. I could see the original marks on that surface that was made, probably during manufacture, and cleaned that surface up to make it work with the hammer fly. Just like an assembler would have done in the 1880's. That was kinda cool.

 

[Johnm1]

I should have read the referenced post before my last post. In the referenced post Mr. Johnson stated

"This photo is of a S&W 38 Double Action 4th Model ........... With both of these, the hammer stays down all the way after a shot is fired. "

The odd part is that I was a part of that thread 2 plus years ago and have no recollection of being a part of it. Another sign of OGD (Old Guy Disease) or just a case of cerebral flatulence (Brain Fart)

And we all know Mr. Johnson knows his stuff regarding S&W.

 

[jmorris]

Today we would just model the arrangement and find out where to put the stirrup pin and how much to relieve the hammer to avoid interference between the two. But how did they engineer this back in the 1870's?

Build things until they work, often called “research and development”.

We have all heard the stories about Thomas Edison successfully finding 10,000 ways a light bulb won’t work…for example.

 

[Col. Harrumph]

Iver Johnson hammerless (meaning, shrouded hammer) revolvers were DA-only and their hammers had to rebound. But the hammer models did not rebound, and had a sear notch cut similar to that of the Colt SA "safety" notch. You had to retract the fired hammer manually to get the firing pin to clear the spent primer. The 2nd pic in the 2nd post is looking similar....

 

[Johnm1]

But the hammer models did not rebound, and had a sear notch cut similar to that of the Colt SA "safety" notch

That is true for my H&R hammer model as well. And I’m pretty sure mine was made in the 60’s or 70’s. As in 1960’s or 1970’s.

So a little more research on my part could have answered my own question. Well that and remembering the post from 2019 where Driftwood Johnson answered the question directly.

I have a book on disassembly/reassembly of vintage firearms that specifically states that this model does have a rebounding hammer. The book is not a S&W publications so there is the possibility that it is incorrect.

I also need to improve my reading comprehension. I reread that book and it does not say that the DA has a rebounding hammer. It does say that for the 32 Single Action. I just inserted what I thought to be correct into that paragraph.

 

[Monac]

You know, after re-reading Driftwood's 2019 post, I think I ought to understand why the "New Model" single action S&W's had rebounding hammers while the 1878 double actions did not. But I don't. Can anyone dumb it down for me?

 

[Johnm1]

Accidental double post. Not sure how I did that.

 

[Johnm1]

I cannot explain it either. As I understand it, my 32 Single Action uses the geometry of the main spring to control the rebound. Keep in mind I'm trying to get my head around that concept, so what I think I know may not be true.

but if it is in fact the geometry of the mainspring that controls if you have a rebounding hammer, I'm not sure why it isn't incorporated into teh double action revolvers. I have to consider that the double action mechanism is significantly more complicated than the single action and the geometry of the double action main spring may need to be different. Though I'm struggling to understand why that would be the case.

I spent some time evaluating the parts of the single action last night and there isn't a separate piece that I can find that would make the hammer retract like it does other than the main spring. Antique Firearms - Assembly - Disassembly states that the rebounding of the hammer in the single action is performed by the main spring and in the section on removing/replacing the hammer stirrup warns to put the stirrup back as it came out as this is key to allowing the rebounding of the hammer. Here yhou can see the slight curve in the hammer stirrup:

 

[Johnm1]

Here is a picture of the hammer stirrup curve without the main spring in the picture.

 

[Johnm1]

Build things until they work, often called “research and development”.

We have all heard the stories about Thomas Edison successfully finding 10,000 ways a light bulb won’t work…for example.

I can't argue with that. I'm not sure it would fly today what with computer modeling and such, but that isn't the question today.

Can you imagine trying/failing and trying to make revisions by the 1000ths of an inch? And what direction to move the last failed hole? Do you base the location of the hole for the stirrup on the location of the hammer pivot point, the main spring curvature, or the curve in the hammer stirrup. Which do you change and how much?

I don't doubt that there was 'Research and Development' in the time these were designed, I'd just hate to be the person responsible for coming up with the absolute answer by trial and error. I'd at least like to apply some math to the task of finding the correct place for the hammer stirrup. With a little (well, maybe a lot) work I could pull the math from my distant memory to define and plot some of the locations of things that might minimize the amount of trial and error.

Maybe it was done with a large 'mockup' of the assembly. That would be easier and when the answer was found the measurements could be scaled back to the original size. It had to be written down somewhere. Didn't it? Anybody have access to 150 year old shop manuals?

 

[jmorris]

Can you imagine trying/failing and trying to make revisions by the 1000ths of an inch? And what direction to move the last failed hole? Do you base the location of the hole for the stirrup on the location of the hammer pivot point, the main spring curvature, or the curve in the hammer stirrup. Which do you change and how much?

Yes, I have been there and done that. I don’t know how to really explain it, as far as “seeing” a working design in your head but I don’t guess someone that could draw a life like picture of another person could explain that to me. If that’s not there, modeling software is your friend.

Experience is the best teacher, as long as your not one of those people that will repeat the same mistake twice.

 

[Johnm1]

I don’t know how to really explain it, as far as “seeing” a working design in your head but I don’t guess someone that could draw a life like picture of another person could explain that to me.

They say that Beethoven saw entire sheets of music when he closed hie eyes. Some people just have it.

I just took a video of the single action that demonstrates the main spring reounding. I'm going to add some pictures to go along with this. But I want to post it before I lose the link.

EDIT: Added a second video that has more light. I still struggle with pictures.

https://photos.app.goo.gl/FSRYQhj8SzNT5SRC8

https://photos.app.goo.gl/FSRYQhj8SzNT5SRC8

Hope that worked.

 

[jmorris]

I don’t think you need one of a kind or once in a generation kinds of skills but those that all may not possess.

Kind of like some people would need this drawing to install a new belt and some would not.

 

[Johnm1]

I would need the diagram!

I can play a horn (commonly called a French Horn) at a pretty high level. But I couldn't make it at even the college level because of my poor ability to sight read music. I can read the notes on the page and I don't have to convert the written notes into a thought and translate it into fingerings, but I can't hear the music in my brain as it is written in time on the page. I have to hear it before I can play it in time. From there I can become quite good. Just a limitation of my brain.

 

[Johnm1]

I added a second video that has more light on what I was trying to show. You can see my thumb push the hammer forward from the rebounded position anf the hammer stirrup lift off of the main spring. This shows that the main spring has reached its resting point. When fired the hammers momentum carries it further to strike the primer. Here is a picture of the parts involved and the only part that can provide the rearward motion of the hammer from the striking position back to the rebounded position is the trigger spring. It takes quit a bit of force to manually push the hammer forward into the fired position. For this discussion we'll call the position when the hammer/firing pin touches the primer the fired position. And the trigger must be pulled to allow the hammer to travel from the rebounded position to the fired position.

Chiccone's book says the following

The .32 S.A. also sported a new rebounding hammer, an important engineering feature shared with the much larger .44 New Model 3 single action, also introduced in 1878.

The hammer stirrup (#28) may be removed by drifting out its pin at the rear of the hammer. Be sure you make careful note of the position of the stirrup (#28) for reassembly later. Its position is critical to rebound operation.

As for why the rebounding hammer isn't a part of the double action design of this same time is still a mystery. Though the above rebounding design must rely on the interface between the trigger and hammer along with the trigger return spring to accomplish the rebounding. And there may be a reason that this system won't work in a double action design.

Do you suppose this same conversation was had between designers/engineers in the 1850's or '60's.

 

[Johnm1]

I struggle with photographs and videos. Hopefully this one is a bit lighter and more to the point.

https://photos.app.goo.gl/HFe5TDF2cjitqpTL8