OK. Go ahead. The only additional question I can think of is do trains carry spare knuckles or couplers?
I notice that one can buy a brand new knuckle for about $150 and a complete coupler for about $1000, depending on type.
I'll send the message out to him, so he will have it tomorrow morning, and I will post the message and his reply.
They do carry spare knuckles on trains. The caboose I looked at yesterday had a pair of spare knuckles hanging on a rack under the sidewall. I am not sure about the whole coupler, but I doubt they would carry spares, since it would require power handling equipment to install them.
This has certainly turned into a fascinating discussion! Still I'd appreciate it if you would ask if there is anyway to date the coupler that I found.
cvt
Chuck,
At some point, maybe we can discover a resource that details the exact evolution of couplers from the original Janney patent forward. So if there are no dates on your coupler, maybe its timeframe can be established by style. Do you know the date of abandonment of the railroad on the grade where you found the coupler? That would establish the latest possible date of the coupler. Was the coupler completely buried or partially buried? That can hint at the timeframe since iron pieces naturally bury themselves with the annual frost heave. However, it is also possible that iron has gotten buried by ditching operations. And sometimes ditching digs up really old iron as well. Old railroad grades are littered with an endless variety buried iron fragments. Some are over 100 years old.
Bucyrus wrote: I'll send the message out to him, so he will have it tomorrow morning, and I will post the message and his reply.They do carry spare knuckles on trains. The caboose I looked at yesterday had a pair of spare knuckles hanging on a rack under the sidewall. I am not sure about the whole coupler, but I doubt they would carry spares, since it would require power handling equipment to install them.
Locomotives carry spare knuckles also. Usually an E and an F style knuckle. Some even have an E and F stenciled over the holder, but when getting a knuckle it's best to visually check the knuckle itself. Going by the stencil can lead to embarrasment.
Jeff
jeffhergert wrote: Going by the stencil can lead to embarrasment.
Like, "I carried this heavy thing 50 cars back and it's the wrong one!?!?!?"
Larry Resident Microferroequinologist (at least at my house) Everyone goes home; Safety begins with you My Opinion. Standard Disclaimers Apply. No Expiration Date Come ride the rails with me! There's one thing about humility - the moment you think you've got it, you've lost it...
tree68 wrote: jeffhergert wrote: Going by the stencil can lead to embarrasment.Like, "I carried this heavy thing 50 cars back and it's the wrong one!?!?!?"
Something along those lines. Usually the condr radios the engr to throw off one and pulls the condr up. Then, or after shoving back if the broken one was on the standing portion, the condr radios for a different knuckle. Using the radio makes it so much better because everyone near by (railfans included) can hear what's going on.
My incident was a bit different. A train on the other track stopped to help. The old head (20+ years) condr got off the help. I told him I needed an E knuckle. He went and pulled one out of the holder. I held up the cut lever while he placed the knuckle into the coupler housing and I placed the pivot pin into it. We could close the knuckle but couldn't get it to lock up. Although he actually got the knuckle, I felt no less dumb because I never looked at the knuckle myself, even after it was in the coupler . About this time a car man drives up. Takes one look and says we need an E knuckle. We had a F instead of an E because of the stencil on the engine.
Two lessons. 1. Don't go by the stencil. 2. Old heads are human too, they aren't always perfect.
I usualy find ore cars with broken couplers, like this here. The trains here are very bifg and sometimes the couplers brakes when after a sudden stoppage. these gondolas carry 120 tons of iron ore and run in group of 160 cars.
Pedro
This is a great question - and susbequent discussion - about one of the quintessential elements of modern railroading that we all take for granted, and have apparently overlooked.
In my collection of Trains from the mid-1960's (and some from before then), I don't recall seeing any kind of explanantion as comprehensive as that put forth here.
Unfortunately, the late, great John Armstrong (the O scale model railroad and prototype railroad author, also a mechanical engineer) would have been an excellent choice to write and illustrate an article on this. I remember the article he did on the 3-piece freight car truck, from the 1980's I believe. (There are at least 3 trucks on display as parts of sculpture here in the Lehigh Valley - 2 in Allentown, and 1 in Bethlehem.)
Which leads to a suggestion, for someone who's feeling ambitious, inquisitive, and maybe has a little spare time and some skills or tools (or both):
1. Obtain and transport a coupler - such as one that's headed for the scrap heap anyway;
2. Cut it safely (like with diamond blade or abrasive saw) to expose its inner workings. A horizontal cut for a longitudinal section, at about the level of the top of the moving part of the knuckle, seems the most likely spot;
3. Color and light it best for photography;
4. Take a sequence of photos - or even a video - that shows how these parts work together in opening, closing, and under load;
5. Post those photos here, or let's have an article appear in Trains, or the video on YouTube or similar, so that we can all see and learn from it;
6. When done, donate it to a local railroad museum for use as an exhibit.
(This is all kind of like a medical school's use of a cadaver . . .)
For example, I'm wondering if a suitable coupler could be obtained from either the NS Car Dept. people at the nearby Allentown Yard/ Terminal, and/ or the McConway & Torley plant in nearby Kutztown (see previous posts about contacting their Director of Engineering). When done, it could then be donated to either the Smithsonian Institution in Washington, D.C., the Pennsylvania Railroad Museum in Strasburg, Steamtown in Scranton, or the Wanamaker, Kempton & Southern tourist railroad (also nearby, in Kempton, PA), etc. I'm sure people in other locations could think of similar sources and displays.
What would be needed- after asking and obtaining the coupler - is a vehicle capable of hauling it (query: about how much would one weigh, if it was cut off at the shank ?), the saw, the photographic skills, and the ability to post the photos. Anybody interested ?
In any event, I look forward to seeing what happens next with this thread.
- Paul North.
Paul,
I think an article on the details of couplers would be a great idea. We are still trying to sort out the internal functions which I suspect may prove to be quite intricate in purpose. I see the same force that tries to rotate the knuckle also applying friction brakes to impede that rotation. It may be that there are some very well illustrated diagrams that fully detail the coupler parts and their function. Disecting a coupler to photograph the functions would be a promising way to demonstrate the operation. I would think that all the parts exist as 3D cad models, so an illustrated demonstation could be built on the computer. I have Solidworks cad, and would attempt to model the parts if I had a sample coupler. Those are not simple parts to measure and model, however.
I remember that article on the 3-piece freight car truck. I thought that was one of the most interesting ever. There has been a lot of emphasis on the human element of railroading lately. I like the mechanical engineering element. I would like to see a comprehensive article on diesel locomotive trucks that would discuss all the pros and cons of the various styles and features.
I sent a message to Joe Gagliardino yesterday morning inquiring about coupler function, but I have not heard back from him yet. Here is the message I sent:
Dear Mr. Gagliardino:
I am participating in an Internet forum discussion of the functional details of automatic railroad couplers. On behalf of the participants of this discussion, I am trying to resolve the question of how train-pulling force is transmitted through the coupler components. It seems apparent that the pulling force tries to open the closed knuckle, but the lock prevents the knuckle from pivoting open. When pulling on the locked knuckle, there is a reaction that forces the knuckle sideways toward the knuckle pivot pin.
Does this reaction force cause the knuckle to contact the pivot pin and impose a shear load on the pivot pin? --Or-- Does the side-shifting knuckle first come into contact with the round bosses on the inside of the knuckle pivot yoke features of the coupler body? And if the latter is the case, does that prevent the pivot pin from receiving any shear load?
By "round bosses" I am referring to the bosses that are a part of the coupler body, and surround the pivot pin above and below the knuckle. These bosses correspond to the circular arc segment, counterbore features of the knuckle. I would like to know if there is specific nomenclature for these bosses and counterbore features. It is apparent that if the pivot pin is missing, the loaded knuckle imposes a sideways force against these bosses, and the knuckle will continue to function for pulling a train. However, with the examples that I have inspected, it is apparent that the pin takes the load if it is in place. I assume that either the pin or the set of two bosses can bear the load, but if the pin bears the load, the two bosses cannot, and visa versa.
Closer to the locking end of the knuckle, there is another set of circular arc segment bosses that are also concentric to the knuckle pivot pin. Judging by the fact that they are shiny, they apparently take a load as they bear against corresponding arced surfaces that are part of the coupler housing. There is one set above and one below the knuckle. I would like to know what these features are properly named, and how they are intended to share the load with the bosses near the pivot pin. It appears as though they prevent the direct pullout of the knuckle, in line with the train.
If that is the case, the loading of these surfaces would seem to add friction that would resist the pivot of the knuckle, and therefore, lessen the knuckle's rotational force against the lock.
So the basic question is this: How do the features of these four sets of mating, circular walls, plus the lock, and plus the pivot pin with its hinge bores, all interact to transmit the pulling force through the coupler? Can you provide an explanation or tell me how to access a drawing, force diagram, and/or explanatory text that details how the coupler transmits the pulling load, and how the load is shared between this group of features?
I very much appreciate your taking time to clarify these matters or to offer advice on available references that will do so. Thank you.
Sincerely,
[Bucyrus]
....Sure looks like this thread and the very knowledgeable participants are really going to get the info on here yet to show us {that don't know}, how a coupler really takes all that force. Thanks.
Quentin
Here's a really crude drawing of that cross section, at least as I'm seeing things:
This would be the coupler in the closed position as seen from the side. The green shaded area would be stretch forces, the red would be buff forces. The blue line is the pivot pin.
Compare this with the previously posted photos and I think you'll see how I'm thinking this whole thing works. The ridges on the knuckle mate with corresponding grooves in the coupler body. That's why the pin carries no actual load, as has been discussed.
tree68 wrote: Here's a really crude drawing of that cross section, at least as I'm seeing things:This would be the coupler in the closed position as seen from the side. The green shaded area would be stretch forces, the red would be buff forces. The blue line is the pivot pin.Compare this with the previously posted photos and I think you'll see how I'm thinking this whole thing works. The ridges on the knuckle mate with corresponding grooves in the coupler body. That's why the pin carries no actual load, as has been discussed.
I always just assumed that the knuckle pin carried the load - I guess I never really thought about it actually.
However, based on the photos, the discussion in this thread, and the results of my knuckle pin stress calculations I posted a while back, I agree with tree68's explanation.
Anthony V.
This issue of whether the knuckle pin carries a load has not been clearly resolved. It is apparent that the pin can be missing, and the closed, locked knuckle will still perform in a train-pulling application because the load will be carried by the bosses inside of the coupler hinge yoke engaging pockets on the knuckle. So a missing pin obviously carries no load. However, from what I observed in two samples, if the pin is in place, it carries the load, and the bosses then do not carry the load.
If this is the intent of the design, then the bosses are merely redundant backup features to pick up the load if the pin breaks, thus keeping the train together.
There is also the issue of the plactic pins that can be used as an alternative to a steel pin. It appears to me that the theory of the plastic pin is that it will bend slightly under load, thus allowing the load to be taken up by the bosses. But the steel pin will not bend enough to allow the load to transfer to the bosses, so the steel pin takes the load. In other words, the plastic pin functions like a missing pin. It yields to a point where it gives up its load to the bosses.
However, this pin load is not the direct tractive effort load in line with the train. It is a reaction load that is perpendicular to the in-line, train-pulling load. As such, this pin load will be far less than the full tractive effort load. The same can be said for the load that the knuckle lock sees.
It appears to me that the direct, train-pulling load is transfered from the knuckle to the coupler body through the other set of force bearing ridges that are located closer to the lock than they are to the pivot pin.
Bucyrus wrote: This issue of whether the knuckle pin carries a load has not been clearly resolved. It is apparent that the pin can be missing, and the closed, locked knuckle will still perform in a train-pulling application because the load will be carried by the bosses inside of the coupler hinge yoke engaging pockets on the knuckle. So a missing pin obviously carries no load. However, from what I observed in two samples, if the pin is in place, it carries the load, and the bosses then do not carry the load.If this is the intent of the design, then the bosses are merely redundant backup features to pick up the load if the pin breaks, thus keeping the train together.There is also the issue of the plactic pins that can be used as an alternative to a steel pin. It appears to me that the theory of the plastic pin is that it will bend slightly under load, thus allowing the load to be taken up by the bosses. But the steel pin will not bend enough to allow the load to transfer to the bosses, so the steel pin takes the load. In other words, the plastic pin functions like a missing pin. It yields to a point where it gives up its load to the bosses.However, this pin load is not the direct tractive effort load in line with the train. It is a reaction load that is perpendicular to the in-line, train-pulling load. As such, this pin load will be far less than the full tractive effort load. The same can be said for the load that the knuckle lock sees.It appears to me that the direct, train-pulling load is transfered from the knuckle to the coupler body through the other set of force bearing ridges that are located closer to the lock than they are to the pivot pin.
Bucyrus:
I would like to be more precise about my position. I think that the ridges are intended to carry drawbar load, not the knuckle pin. The pin probably experiences some load due to "slop" in the coupler mechanism due to tolerances and wear.
If you go back my earlier post discussing the stress calculations, the results indicate that the pin would carry only about 40,000 lbs (within the limits of my assumptions). This is only about 1/10th of the rated coupler load of 390,000 lb. These results are consistent with the notion that the pin carries only a very small fraction of the load at best.
Anyway this is a great discussion.
I looked at two more couplers today and found them to be different from the two I looked at a few days ago on a C&NW caboose. After seeing these latest examples, I can clearly understand how the forces are transferred.
To clarify my terms, I am referring to left/right directions when viewing the coupler from the coupling end. I am referring to knuckle rotation of CW/CCW, when viewed from the top. To clarify which sets of force bearing ridges I am talking about, I call the ones near the lock, "primary force ridges" and the ones near the pivot pin, "secondary force ridges."
When the knuckle is closed and locked, there is still a lot of slop that allows it to move around. When the knuckle closes and locks, it usually is pushed all the way inward. Then when a pulling load is applied, there are three separate, sequential stages of movement of the knuckle. Each stage allows travel up to a mechanical stop, which then causes the next stage to occur.
1) First the knuckle rotates clockwise until its tang or narrow end (moving rightward) makes contact with the lock, which stops that rotation. The axis of this rotation is the knuckle pivot pin.
2) Next, the knuckle pulls straight out until the primary force ridges contact, which stops that pullout.
3) Finally, the knuckle again rotates clockwise until the secondary force ridges (moving leftward) make contact, which stops this rotation. The axis of this rotation is the contact point between the lock and the tang end of the knuckle.
So once the loading is fully applied, all three of these stages of contact points bear some of the pulling load. However the contact of stage #2 bears the vast majority of the tractive effort, pulling-load even though the contacts of stages #1 and #2 are also bearing some load, because they cause a bit of a zigzag in the main load vector, which would otherwise be a straight line.
There is a curious point about stage #1. Although some load is transferred to it during a pull, stage #2 impedes that load transfer to the lock. This is because the loading against the lock requires the knuckle to rotate, and this rotation would require rotational slippage between the mating surfaces of the primary force ridges. Slippage here would be like a wheel trying to turn with a brake shoe pressed against it. This strikes me as an ingenious way to help minimize the rightward knuckle force against the lock.
In the first rotation of stage #1, it appears as though the knuckle pivot action is being carried by the pivot pin. This rotation is very easy with little friction, and as it rotates, you can see that the secondary force ridges are not contacting, so the pin must be controlling the center of this rotation like an axle.
The final stop described in stage #3 is definitely by the contact of the secondary force ridges. However, it also appears to be shared with a contact of the pivot pin, which tightens up at the same time the secondary force ridges make contact. It would seem to be practically impossible to try to engineer these features so the ridges and pin would bottom out simultaneously and share the load equally. These are un-machined, as-cast features that are bound to have fairly substantial tolerances. However, if you designed the pin fit and the fit of the secondary force ridges to make simultaneous contact, and they were off a little due to tolerances, they would probably wear in so that their contact eventually would become simultaneous.
Overall, I conclude that when you pull through the coupler, the knuckle pulls straight out and is stopped by the primary force ridges. The knuckle also is induced to rotate clockwise, which places a rotational load to the right against the lock, and to the left against the combination of pivot pin and secondary force ridges.
Let me try to help with the force load on the pivot pin. Not very much. Coal trains with rotary couplers will sometimes lose the pivot pin when the car is rotated to dump the load. They then proceed to the mine where the cars are reloaded. At no point in the process do the cars become uncoupled account the knuckle pin fell out during the reloading. Indeed, the pins can take some of the load but it is relatively insignificant. In train forces can indeed break the pin without breaking the knuckle or drawbar and that results in the broken pins you find along the tracks. Each of those did not result in a train separation but they may have.
The knuckle pins are a good grade of steel and are about 4/4 or 5/4 in diameter. The plastic pins were and attempt to make a lighter product for field repairs. They were intended to be replaced at the next service facility but the railroads have since found that is not happening so they are on the way out. If you note some of the spare knuckle racks on locomotives the knuckles are held in place by pivot pins. So not only do you have a spare knuckle at each holder you also have a spare knuckle pin.
Once you have an 85# piece of steel drop from about 3 feet towards your toes account a knuckle pin was missing when you opened the knuckle in a switching move, you quickly develop a work flow to look for the top of the pin in the drawhead before opening the knuckle up.
Thanks for the information arbfbe. That confirms my conclusion above. It seems that there are some couplers that transfer all of the leftward knuckle deflection to the pivot pin, and some that transfer that deflection to a combination of the pin and the coupler housing bosses that surround the pin. But in either case, the deflection force is only a fraction of the force that pulls the train. But apparently it is still enough force to break the pin occassionally.
I can sure understand the issue of having a knuckle drop on your foot.
arbfbe wrote:What keeps the knuckle from deflecting leftward or opening under load is the lock block.
I understand your description about the operation of the lock and the fact that it prevents the knuckle from opening. However, in my description, when the knuckle loads against the lock, it moves right, not left as you stated. Perhaps you are referring to the pulling face of the knuckle swinging to the left as the knuckle opens, and it is the lock prevents this. So we are both seeing the same thing, but applying the left/right terms differently.
This is what I mean: The loading basically rotates the knuckle clockwise when viewed from the top. When you view this circle of clockwise rotation, the top of the circle moves right, and the bottom moves left. So it takes two opposing stops to prevent the knuckle from rotating to open.
Therefore, when the knuckle opens, the small end of the knuckle moves right and contacts the lock, and the big end moves left and contacts the bosses on the inner sides of the pivot yoke; i.e. the bosses that surround the pivot pin. The elimination of either one of these opposing stops would allow the knuckle to rotate open under pull. So for opening and closing the knuckle, one of the two stops (the lock) is either inserted or removed. The other stop (the bosses) remains permanently in place. But both stops are required to keep the knuckle closed under pull.
The knuckle pin has no real force exerted on it when the knuckle is closed.
All it provides is a pivot for the knuckle into the coupler body.
It may take a small amount of binding due to wear on the knuckle, but it makes no difference to the integrity of the knuckle if the pin is missing, once the knuckle is closed.
I have seen the pin jump up several inches during slack action.
What often causes the pin to break is having the knuckle closed, and kicking or humping cars against the closed knuckle...a stress fracture can develop, and the pin will fail at some point in time.
We routinely swipe a pin from the next track over when we need one...as long as no one open the now pinless knuckle, no problems.
The photo is from the AAR Field Manual, 2003, Interchange rules.
Blow it up and you can see how the knuckle fits...the load bearing surface is what you thought, the "groves" in the knuckle tongue take the load, along with the inner face of the knuckle...but not the pin.
23 17 46 11
If "borrowing" a knuckle pin is common practice, do you have special insurance for your feet, as some piano players have for their fingers?
No, they lift right out in fact.
Some have a cotter pin through the bottom to prevent them coming out in rotary service, but most are just dropped in and gravity holds them there.
You don't have to open the knuckle to get the pin out, there is no pressure on it at all, just pull it out.
Old habit, when you go to open a knuckle, you either look to make sure there is a pin, or you run your thumb over the top to feel for the pin...and you learn right quickly to never put your foot under the knuckle...they can cost you a toe when they fall out.
The new composite rubberized pins are a pain, they don't "wear" but bend and get misshapen, which make them hard to pull out.
cordon wrote: If "borrowing" a knuckle pin is common practice, do you have special insurance for your feet, as some piano players have for their fingers?
So, how much does the knuckle weigh?
Here's a closer photo of a coupler without the knuckle on a historical diesel in Denison, TX. You can see the knuckle thrower on the left. Couldn't find the accompanying knuckle.
Yes, the "Knuckle thrower" or kicker pin is the hook shaped object in the left part.
The knuckle lock is the square thing in the middle, which in this photo is about to fall forward and out.
It is held in by the pin lift on the bottom, and in use, it will fit up inside the top of the coupler.
Lifting it up releases the tang or tongue on the knuckle, earlier referred to as the" grooved load bearing surface".
This tang in what bears the forces.
Close the knuckle, and gravity will drop the lock down, in front of the tang, and lock the tang inside the grooves.
The shape of the tang keeps it trapped inside the coupler, and when the knuckle is locked against another knuckle, the knuckles can not move sideways, the other knuckle prevents this.
(note: the knuckle can not move inside the coupler, but the entire coupler can move sideways, depending on the type of car anywhere from a few inches to several feet)
The purpose of the knuckle pin is providing a pivot point when opening/ closing the knuckle, and you can close one without the pin if you line up the grooved surfaces first.
When under tension, the tang is pulled directly forward against the grooves inside the couple head, and against the side of the lock pin, which will have dropped down and locked the end of the tang, preventing it from rotating.
You can not open the knuckle when under tension, you can not generate enough force, even with the leverage from the cut lever, to push the lock pin up...which is why, when you hear crews working on the radio, they ask the engineer to give them "Slack for the pin," or "Give me a pin" when un-coupling cars...this is the pin they are referring to, the lock pin.
Once the slack bunches in, and pulling tension is removed from the tang, you can open the coupler with ease.
Trust me, I open about 100 of these things every day...the knuckle pin has nothing to do with keeping the coupler locked, it provides no purpose other than a pivot point when opening or closing the knuckle, and once closed, you can remove the pin with ease and not affect the ability of the knuckle and coupler to function.
I change or replace about two knuckles a day...its part of my job as a switchman.
edblysard wrote: When under tension, the tang is pulled directly forward against the grooves inside the couple head, and against the side of the lock pin, which will have dropped down and locked the end of the tang, preventing it from rotating.You can not open the knuckle when under tension, you can not generate enough force,......the knuckle pin has nothing to do with keeping the coupler locked, it provides no purpose other than a pivot point when opening or closing the knuckle,
You can not open the knuckle when under tension, you can not generate enough force,...
...the knuckle pin has nothing to do with keeping the coupler locked, it provides no purpose other than a pivot point when opening or closing the knuckle,
I can see how the pull on the knuckle would transfer straight through to the ridges on the tang of the knuckle that in turn pull on the ridges inside of the coupler housing. However, there appears to be a slight offset in this line of pull force at it is passed through the knuckle. It appears as though that offset would create a tendency for the pull force to rotate the knuckle toward opening. I assume that the lock bears some load as it prevents this rotation of the knuckle. If so, there must be a counterforce to this knuckle rotation that bears near the knuckle pin. Since the knuckle pin does not bear this counterforce, I assume that this force is borne by the coupler bosses and knuckle pockets that surround the knuckle pin. Is this right?
Yes, you have it correct.
If you could look directly down at a knuckle, or better yet, look at the page from the AAR rules I posted, you will see the knuckle tang curves to the outside.
When under tension, or being pulled on, the forces tend to pull the tang forward, and the grooves direct it so the tang "hooks" into the coupler body, there is a small gap around all of this stuff, because these are fairly rough castings.
Most of the load is borne by the tangs and the coupler body, the lock pin just prevents rotation, and when coupled to another knuckle, the knuckles are trapped inside each other, with little sideways play, so the majority of the forces are linear or straight line.
When you unlock the knuckle, the knuckle pin provides the pivot point, and the pull from the other knuckle on the inner face will rotate it open till the tang hits the small teat on the lock pin face...look at the photos posted, and you can see the teat sticking out of the front of the lock pin, the "nose" on the face of it...this rides in the top groove of the tang, and limits how far the knuckle will open.
When you lift the cut leaver, it forces the lock pin up, and the kicker hook will nudge the knuckle over just enough so that when you let the leaver go, the lock pin will drop down and this nose or teat will drop into the upper groove of the tang, holding up the pin and at the same time limiting the rotation of the knuckle.
If you remove the pivot or knuckle pin, and open the knuckle, the mass and weight of these things is enough that the pivot point moves forward just enough that the knuckle will fall out.
Look at how thin the casting is around the pivot point...the knuckle rides on a very small surface there, let it move forward a small amount, less than a inch, and it will just fall out.
What breaks knuckle pins is the forces created when you kick or hump cars against each other with the knuckle closed.
Most of the broken pins you will find are sheared about 3 or 4 inches from the top...right about where it enters the knuckle.
The forces created when you kick against a closed knuckle are straight line, and upwards.
The car can actually jump a little if you kick too hard.
Because the pin fits loosely in the hole, and there is a small amount of slack in the tang and the grooves, the knuckle can move inward about a quarter inch, and it can snap or shear the pin at the point where the top of the knuckle meets the coupler body.
The broken pin then drops out the bottom, and the sheared top stays put because of the "pin head" shape.
Sometimes, the pin jumps up at impact, and the shear leaves enough pin so that, when the pin drops back down, there is enough of it left to still fit into the knuckle top.
This is often enough to allow the knuckle to still function, and you don't find out the pin is broken or missing.
Keep in mind that when the knuckle is opened by another knuckle, as when you are un-couplng two cars from each other, the knuckle does not open all the way, it will only rotate to the point that the other closed knuckle clears, so it rarely reaches the open limits or stop created by the lock pin...and because it does not rotate to its full extent, the broken pin provides enough pivot pressure to still work...you find the busted pin when the conductor/switchman reaches in and opens the knuckle to its limit.
The bottom of the knuckle will slip off the boss, and the knuckle drops out.
We open the knuckles to provide a larger "gathering" face, in case the other knuckle is not dead center or open all the way.
One of the first things you are taught when working in a yard is to open all the closed knuckles you walk by, and to look and make sure they are all open to the limit...this way, when I come against a track with a bunch of "gaps" or "joints" between the cars, I can bunch the track up, (gently shove the cars together) and couple the track up with out having to walk down to each joint or gap and open the knuckles...cuts quite a bit of time out of yard work and saves a lot of knuckle pins!
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