As the HO Baldwin DT6-6-2000 project moves to the end of the testing phase, one of the key features I designed into the locomotive still needed testing: the powered Kadee couplings. I think these will be a great addition as this locomotive spent most of its life moving freight around yards, so this week I’ll share with you how it went.

For those of you who’ve been following the design and build of this locomotive from the start, you may remember I decided to use Preci Models DCC auto uncoupler system to automate the Kadee couplings.  You can read about that in my earlier post here.

Because I’m designing this locomotive shell from scratch I had the option to mount the Preci Models motor in a more convenient place rather than simply fixing it to the back of the coupler as shown below.

The tiny motor fitted perfectly into the shell, with the brass spindle protruding out just behind the Kadee coupler.  It’s very close to the shell mounting hole, but that’s the great thing about designing the shell as a 3D computer model; I knew it would fit.

The couplings I’m using are Kadee #148.  These are medium metal couplers with the Whisker®.  The Whisker® is the tiny wire you can see either side of the actual coupling arm which keeps the coupling centered in its box.

To make these work with the Preci Models motor, I needed to cut a slot in the rear of the box.  As the plastic is fairly soft this was easily done with a new blade in a craft knife.

The string, which is supplied with the Preci Models motor kit, can be laid through the coupling box.  The string needs to be parallel with the coupling arm otherwise it will put more effort into pulling the coupling to the side rather than opening the coupling knuckle.  The string also needs to run free from any obstruction inside the box; it will be above the Whisker® as shown below.

With the box lid fitted you can see how the string passes through.

There are different ways to connect the string to the knuckle but I like to use a drop of superglue. To do this I put a drop of superglue on a piece of card, use a small toothpick to place it on the right spot, then lay the string in the glue.  I also use a spray actuator to instantly set the superglue. It’s important to note the spring on the side of the coupling; you must not get any glue in this or the coupling won’t work.

The coupling can now be tested.  By simply holding the coupling box between my fingers I can pull the string down and the knuckle should open.

There should be no resistance, other than the small spring on the side, which should cause the knuckle to close with a little snap when the string is released.

The coupling can now be screwed to the shell.  Of course, I would recommend painting the shell before fixing the couplings as you don’t want to get paint on any of the moving coupling parts.

The string is pulled by the rotating motion of the motor and it’s important to keep the string as straight as possible, so in the picture below you can see I’ve wrapped the string around the motor spindle in an anti-clockwise direction.  This way the string will come off the spindle at the top and be in the right place.  To keep the string in place it’s actually tied onto the spindle, adjusted to the right tension, then fixed in place with another drop of superglue.  Again, I do this with a small toothpick to avoid getting any between the spindle and the motor.

Next, we need to power the motor.  It’s worth pointing out here that this system works on the principle that the motor spins as much as it can, which may only be half a turn, then stalls until the power is turned off and the spring in the coupling pulls it back.  If the motor is left powered on for more than 5 to 10 seconds it will overheat.  Also, if the motor is powered directly from the decoder or track, it will again overheat and probably burn out.  So the Preci Models motor kit has two 68 Ohm 0.5w resistors, one for each motor, which needs to be included in the wiring to limit the power running into the motor.

The motor needs a minimum of 150mA of current to work and not all decoders can provide this from their function cables.  It should say on the decoder documentation.  There are options for this in the Preci Models motor kit instructions but this locomotive will eventually be fitted with an ESU LokSound decoder that can handle this.  For this test, I have a basic ESU LokPilot which also has the required power output.  First I connected the red motor wire to the Blue DCC chip common and the black to the purple DCC chip function wire.

As the motor is a DC motor and the DCC decoder outputs +DC from the blue wire and -DC from the function wires, the motor spun clockwise when wired like this, and that’s not what I wanted.  So I swapped the red and black wires over and the motor now spins in an anti-clockwise direction.

With the chassis sat on my programming track and the DCC decoder plugged in I was able to test the coupler, and it worked well.  With the function wire activated, the motor span and the knuckle opened all the way as shown below.

And when the function was turned off it sprang back.

Here’s a very quick video of it working.

Once I was happy it was working properly I trimmed the excess string off the motor.

The last test was to fit the shell to the chassis and try it in situ.  The string on the knuckle is hardly noticeable, and I think once the shell is painted you won’t be able to see it at all.

I don’t have a long enough length of HO track on my modeling bench to do a full action test but I was able to simulate it using another loco in this video below.  The SP&S Geep looks high on the right because there was no track under the rear truck and I was holding it up!  But as you can see the powered coupling on the DT6-6-2000 works perfectly.

I won’t install the coupling at the other end yet, as I want to paint the shell first, but I’m very happy with how the automatic Kadee uncoupler works and I think it’ll be very valuable on shunting operations.

The very last test for this shell is fitting the etched brass parts and I’m very close to having them all drawn; it’s taken a little longer than expected, but I want to get it right and the better the design of the etch sheet the more parts I can get on to it, which in turn brings the cost down.  Hopefully next week I can share that with you as well.