Wednesday, 31 December 2008

Coronation class with vertical movement DCC controlled AJ's

The latest from the retrofitting workshop; a High Level Coronation class 0-4-0ST with vertical movement AJ's.

I fitted vertical movement rather than the rotary used in the Armstrong and the 2mm scale 04 because of various space constraints. Primarily, the relatively low footplate height, plus existing underframe components would have made a "through buffer beam" rotating coupling difficult to fit. In addition, I wanted to try out a vertical (normal) movement AJ.

The plan with the vertical movement had been to use the iron core of the coil to hold a permenant magnet in the "closed" position, and energising the coil would repell the magnet to the "open" position.

The main lessons :-
  • magnetic attraction back to rest is possible, but didn't quite work in this case due to lack of clearance around an existing brake shaft on the model. The iron core in the coil is too far from the magnets on the flap to pull the coupling back to rest, hence the need to add a counterweight (swings in space near gearbox).

  • the power required to pull an AJ down vertically is quite large, there is considerable drag as the tail of the coupling disengages due to the horizontal spring strength in the couplings. So, a more powerful mechanism is required than rotating. This required bigger magnets, and thus created the clearance problems in the first point above.

  • operationally the movement is better than rotation. The loco can be programmed to backup and uncouple, but does not need to have "move forward" in the programmed sequence. Therefore the movement is more prototypical, giving the crew time to disconnect the coupling before pulling away.

The DCC chip is a CT Elektronik DCX75, mounted in the bottom of the boiler underneath the motor. The coil is about 35ohms, and a further 33ohm series resistor is used to limit the current to the coil. The coil is wired to "half wave" (ie. to a track pickup rather than decoder blue), the DCX75 does not have a blue wire. Movement control is via various CV's which CT provide for couplings, the JMRI definitions file for this is available from me (and in time will be in a future JMRI release).


I am likely to rebuild this coupling in a few weeks with a further modification; a hollow electromagnet will allow the fixed magnet to move within the core of the coil, this should mean more power with a smaller fixed magnet. It will also make the magnetic "return to rest" more likely to work as the fixed magnet will be nearer the iron component.

Saturday, 6 December 2008

Smaller (!) DCC controlled couplers

I've had the 4mm scale loco with DCC controlled AJ couplings since the spring. But my aim has always been at "really small". 2mm scale "small". So, the last few months have seen various ideas tried, rejected, tried again.
The latest is for a 2mm scale Farish 04. Its a standard N item which has had the coupling pocket removed from the plastic keeper plate, plus 2mm finescale wheels (yes I know the spoke pattern is wrong for an 04!).

Picture shows coil on bench held on bit of bluetac. The coil wires are 45swg (0.07mm dia), or about the same as hair. The scale behind is a 1mm divisions.

The movie below shows it in operation. I will turn down the movement both forwards and backwards for real operations (various DCC CV settings) as they are currently a bit too large.


The chip driving the 04 is a CT DCX74. The DCX74 and DCX75 both support movement and coupling control from a single function press. It is intended for Krois uncouplers and I am exploiting the control here. The manuals from CT are fairly cryptic, but I think I have untangled most of the uncoupler issues. (The Zimo MX63 and MX620 are a bit larger, and also have functions for Krois couplers, though fractionally different in the way they operate. The Zimo documentation is a lot easier to follow). For those using JMRI (Decoder Pro), I have put my revised decoder file for the CT DCX chips in the JMRIUsers Yahoo group files area, when its had some more testing, I'll submit it for a future JMRI release.

I can see scope for a slightly larger version of this design (parts easier to handle!), with the frame and swing arm etched in nickel silver for 4mm use. I have a sketch design for the parts, and they should work with either a rotating AJ (like my earlier Armstrong Diesel) or the standard "pivoting" AJ with the pivot perpendicular to the coupling axis. I need to talk to someone who can put the etch bits on the side of a test-sheet, plus try out a few more electromagnet coil size options.