Thursday, January 08, 2009

Con rod fiddles

Con rodsIn his dated, but excellent, book on chassis construction for model railways, Iain Rice ruminates on the problems associated with using Romford wheels. While these are (IMHO) the best wheels on the market - strong, reliable, well engineered and easy to use - the crank throw is generic. Some locos needs something different and there isn't anything you can do about this.

The dock tank is a case in point. I'm not sure what the correct throw is but it's less than that provided by the wheels. For about 1/5th of a revolution the wheels are trying to push the cross heads through the face of the cylinder. To get around this I have had to (on both this model and my original) cheat and turn the holes at the fat end of the con rod into an oval slot. Thus when the crosshead reaches the end of its travel the rest of the movement is taken up at the other end.

Result - the wheels rotate smoothly. The fiddle is largely hidden behind the retaining washer and the rest of the valve gear, To see what I've done you'll need to peer pretty closely. This sort of bodge is why I always say I build "layout locos" rather than glass case models. As far as I am concerned a model should earn its keep so I'll prioritise working mechanicals over looks any day. But then, what looks worse, a stuttering but perfect model or a reasonable one that moves smoothly ?

Now you might say that a different set of wheels would be better and I wouldn't entirely disagree. Trouble is then you are in the realm of plastic centred wheels with round axle holes, home to nightmares of persuading things to stay quartered and the plastic not to deteriorate over time or contact with the wrong sort of fixative. Sorry, give me long term reliability and the possibility of repair without throwing the things away in the future !

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