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DCC Uncoupling, 7mm Kadee Style

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My festive holidays although two and half weeks long have not allowed me the time to play as my wife calls it. But the one day I did mange to grab whilst my wife was busy allowed me to experiment with something that has bugged me for a while, the lack of DCC controlled uncoupling for 7mm scale models. I had set out the day to finish off my platforms !


My choice of coupling has always been Kadee as they worked for me when I modelled 4mm and they still work for 7mm but need the presence of magnets for them to work. So how could they be adapted to work with DCC Controlled Uncoupling


I had initially thought of using micro relays to actuate the mechanism but a relay is not very tweakable with regards the distance the mechanical portion moves need hands on to change anything to do with distance travelled, tension on the connecting linkage between the relay and the coupling.


So what was out there small enough out there that could be set up and tweaked via a computer and software. All my 7mm locos, not class 08 or class 108 DMU use the excellent ESU V4 Loksound XL decoder and have four physical outputs capable of handling servos. Looking into the configuration side of the decoder, a servo can be quite well controlled and set up via the very good Lok Programmer. No guessing CV's, its all done via a graphical interface and very little room for error.  Well worth the money if you pay a lot with your ESU decoders.


Speed, start position, end position are all configurable from the Lok Programmer so it seemed that less hands on was required once a servo and coupler had ben installed under the loco.


But what servos were available. Space in 7mm scale is far better than 4mm but still at a premium in some areas and the bogie are is once such place. So a search on the well know web search agent came up with Ultra Micro Mini Nano 1.7g Digital Servo.


This little monster measures just 13mm x 16mm x 6mm and weighs only 1.7 grams. The Kadee coupler is similar in size in some aspects




Next step, find a way to add the Kadee coupler and servo to a loco. Bring on a Heljan Brush Type 2.


There is room behind the buffer beam to take the servo and with some small changes to the buffer beam the Kadee coupler draft box and coupler is easily fitted.


As Kadee users know, the coupler is allowed to swing from side to side in the draft box to any linkage had to be flexible and extendable to allow this vital function to still be function. But as a servo movement can be tweaked so some slack could be allowed in the linkage between the servo and the couple head to allow the swing to continue. As the swing from side to side is from the centre line of the draft box, if the linkage is pivoted at the rear of the draft box and on the centre line the length should be equal when it swings left or right. This is almost true for Kadees depending where the linkage is attached to the coupler head. So some thought was put into where the linkage would attach to on the coupler head.


I have opted for an adapted trip pin. Its cut short and bent 90 degrees to what it normally is. Also would the return spring in the coupler head be capable of pulling the actuator wire with it so it could return to the closed position by itself when the servo relaxed.




Here are some more photos showing the installation.






Testing has shown that this works but there is room for improvement on the installation. The servo needs to be tuned to make the servo actuation arm more in line with the pivot point for the actuating wire. This is a minor point as it does all work anyway.


The actuating wire will be moved from the trip pin to the moving part of the coupler head. The trip pin will be totally removed then making the couplers look a bit more prototype.


The end stop on the coupler head that sets the distance the head opens will be filed back a little to allow the coupler to open a little bit more so it works better on curves. Also the lip on the front edge of the coupler arm will be filed flat so it is smooth and allows the arm to move more feely when the coupler is under tension with a wagon or coach attached.


More minor tweaks but wont go into them now.


Here is a video of what happens. Sorry about the refocusing of the camera but its only a little handheld.


The decoder is set up at the moment for play value, demonstration of the function. The coupler clunk is a tad wrong but the video shows the coupler cycle built into the Lok Sound V4 decoders. The loco pushes to take the tension off the coupler, it opens then the loco moves forward.


Of course the coupler can be set using the Lok Programmer to open can close with out the uncoupling cycle off a function button. In fact I have both set up on function buttons. One function button for the full uncouple cycle and one function button for just the coupler. Also you can have one for front and one for back or have coul4rs at both ends operate of a common function button. Up to the user.


Anyway, the video.




The wiring to the decoder is very basic, three wires and if you follow the common standard for servo wiring and read the manual for the decoder on page 23 its simple and not worth going into.


So, thoughts, comments, suggestions. I have a few more things in mind for this but it is all looking quite good for it to be added to all my 7mm loco fleet.


The set up in the photos is really a test set up with the servo just attached using blue tack and the actuating wire wrapped and twisted in place. The end result will be a lot neater, tidier and properly installed.   


As for price, the servos are just over a tenner for two and sourced in the UK. Possibly a lot cheaper if bought via E bay in China.  





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This is a very cool method of operation - I remember an article many years ago on converting HO Kadees to DCC utilising memory wire but this looks a relatively straight forward conversion.


Thanks for posting all the pics.

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Hi Karl,


I remember the memory wire solution but if my grey cells serve me right, it used a chain as the actuation connection which for me has too many variables about it.


The wire I have used needs painting black in the 'visible area' to camouflage it a bit.


Once I get a bit more time to play I will tweak things and post more info and findings on this subject. I have thought about fitting this set up to my guards vans as my wagons will be run in rakes so only the guards vans need to be uncoupled.

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An interesting solution.    There are smaller servos around, but they can be hard to find and sometimes quite expensive.  The "linear" ones might be useful in this application.


I'd suggest trying fine fishing line in place of the wire linkage.  It can be wrapped around the Kadee arm and secured with a dab of superglue or varnish. 




I have seen a Kadee solution for the HO/OO coupler which is broadly similar but doesn't need a servo, instead uses an electromagnet.   I saw it on an 08 shunter on the McKinley Railway:


  • A similar connecting wire is fitted to the one in the TTG design above.  The wire is attached to the end of a brass rod, and the brass rod has a small cylindrical magnet glued on the end. 
  • A hollow cylindrical electromagnetic coil is arranged around some brass tube, and fixed to the base of the loco.  (I think an electromagnetic coil alongside the tube would also work).
  • The brass rod (with link to Kadee) is threaded into the brass tube. 
  • Connect electromagnetic coil to a function output.
  • There is no precision adjustment required, just get the magnet/coil in roughly the correct position, and the spring tension in the Kadee takes care of the rest.


Memory wire is a lot of trouble in my experience, and rarely worth the effort over other solutions.  I have made it work in a 4mm scale wagon and a very small 7mm loco.  There are also two types of memory wire. The commonly available type requires a very strong return spring to bring it back to shape (strong enough to bend the underframe of a 4mm scale wagon, so a serious problem).  There is a less easily obtained type will return to its original length without a spring, so the spring is only needed to move the mechanism back to shape (MERG has this type until stocks are exhausted, then no more will be made by the originator.  I don't know of another source of this type of wire. ). 



An operating trick with couplings in guards vans or passenger stock is to put the coupling action on a "standard" function key, and then consist the vehicle with the locomotive operating the train.   Saves remembering to swap decoder to work the coupling.  



- Nigel   (making DCC controlled couplings in 4mm and 2mm scales since 2007 )

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Hi Nigel,


Thanks for sharing your thoughts on this subject.


I looked the linear servos and whilst very good the dimensions of the ones I looked at meant they could not be fitted in the space available as they had to sit inline with the coupler and this meant the length of the servo was too great for the space available. The ones I used were able to sit at 90 degrees to the coupler and at 6mm thick do not take up much room.


As for fishing line, yes agree it may work but I don't have any as thin or thinner than the wire I have. The wire is only 0.15mm thick so not too bad.


If you do know of smaller servos, do let me know as these so far are the smallest I have found. I would try smaller if I could get some.


As for using electromagnets, played with them a while ago and never really got a very good result from them., that's why I went for the servo solution. Perhaps I will revisit this idea but for now, the servo side of things seems the best way to go for now.

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Small servos,  two main sources of tiny ones that I know about; 

  • Toki Smart Servo RC1, but seems to be near unobtainable at present, and the few I've seen for sale recently are silly money.  When more readily available it was under £20.  Its quite long, but very thin.  I've used them with the servo outputs from Zimo, and would expect the similar output from ESU to work fine.
  • Devices from Plantraco Microflight in Canada.  Many of these are "Actuators" rather than servos, so lack the servo movement electronics.  As a consequence, treat them as if electromagnets and wire to a normal function output, fine tune the movement with the "brightness" on the function output CV's.   Also note that most are designed for 4.5v operation, so wire appropriate series resistors to limit current/voltage, or at least wire two units in series.  I use their smallest "Nano actuator kit" (which is just an electromagnet coil and a magnet in a plastic bag) in my designs. 



Strands of silk might also be a source of actuating wire; buy the wife a couple of silk scarves and keep one for yourself and unpick a couple of threads :-) 



If the cylinder sliding inside an electromagnet doesn't make sense from my description, I'll try to draw it.    It is very simple, and needs no precision adjustment/alignment.   I was impressed with how easy it was to make. 



I don't know how you're developing ideas, but many of mine are made up on a block of MDF(or similar) on the bench to prove parts work, and then rebuilt onto vehicles. 




- Nigel

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Hi Nigel,


Thanks for the info on the servos. But the dimensions are wrong for what I need. The length being a lot longer than the ones I use puts the servo actuating are well off to one side and 90 degrees out from what is needed as well. Nice height though, 3.9mm. Also the price quoted at £17.68 makes it a a lot more expensive. Yes would be interested at that price if they fitted all my requirements but they are a bit out of gauge as such.


As for the diagram for the electromagnet set up, no problem. I fully understand the principle and have seen it in operation at David's place in Bournemouth.


We do seem to be moving along the same lines with regards thinking. The silk thread being one of them and the test rig being another.

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Hi Karl,


I remember the memory wire solution but if my grey cells serve me right, it used a chain as the actuation connection which for me has too many variables about it.


The wire I have used needs painting black in the 'visible area' to camouflage it a bit.


Once I get a bit more time to play I will tweak things and post more info and findings on this subject. I have thought about fitting this set up to my guards vans as my wagons will be run in rakes so only the guards vans need to be uncoupled.

I think you are right about the chain - I vaguely remember seeing it in operation on a layout at the Bristol exhibition about 10 years ago but don't quote me on that.


I do like the sound of Nigel's solution for 4mm & would like to see a picture too.

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Diagram of the Kadee mechanism for Karl.   Its not my design, I've produced several others over the years, this one came from one of the McKinley Railway chaps.


Make the tube (green) as reasonably long as you can, this supports the rod with magnet on end. 

When the coil is energised, this pulls the magnet on the end of the rod into the tube.  The Kadee coupler will limit the movement.   

When the coil is turned off, the spring action in the Kadee will pull things back to the rest position.   


K&S brass tube and rod is suitable for this, adjacent sizes will slide inside each other as long as burrs are removed. 





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I did something very similar to this in 4mm that I wrote up for DEMU's UpDate mag a few years back, I used a small relay with a wire soldered to the armature as the operating mech, and a short length of black thread as the link to the bent trip pin. I also found I needed to clip off the limiting bump on the side of the knuckle so it could open slightly further, this allowed it to uncouple when only this coupler was opened instead of both.


Hope that helps



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Hi Andi.


I has thought about going down the relay route but to me it lacked flexibility in being able to tweak all the settings in software rather than hardware and the variations in builds for different locos.


I agree about the lip on the knuckle and removing it. It allows better uncoupling especially on curves.

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  • 3 years later...

Has anybody fitted Kadee couplings to the new Dapol 7mm Class 08?


I'm half way through a conversion which involves :-


1. Removing the screw-link couplings but retaining the coupling hooks both ends.


2. Obtaining a packet of Kadee NG #746 (long shank) couplings.


3. Pulling out the metal tab at the buffer beam to allow the knuckle entrance.


4. Assembling a coupling unit and sliding between the bottom of the buffer beam and the horizontal vacuum pipe.


5. Screwing 2 x M8 bolts 13mm long through the side holes of the draught box, thence into the interface holes in the chassis.


6. Testing with Kadee coupler height gauge.

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