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.

No comments: