Drawbar dreams

“I do all my best work in bed,” famously and salaciously declared Mae West.  And so it was, I awoke the other night with the solution to a problem I’d given up on long ago – the drawbar.

Confusingly, the drawbar of a locomotive is not the thing that pulls the train; I think that’s called a coupler, but the book that would confirm it is out in Mount Flood.  The drawbar is the frame member that connects the backs of the two sides of the frame, and to which the coupler is connected.  It takes up many pages in my locomotive engineering book because the weight of the train pulls on it transversely, rather than longitudinally.

On a 4-4-0, the drawbar extends past the frame sides and supports the house brackets, which in turn support the ends of the running boards and ultimately the cab, or I suppose “house.”  Invariably, the step hangs down from the end of the drawbar, and thus, the model must be robust.  At the same time, with a split frame design like this one, the drawbar can’t span the frames.

When I built #10, I soldered a solid drawbar across the ends of the frames and then cut a gap in it.  Later, one half or the other got too hot while I was soldering something else, and I had to solder it again. Several times.  I’m sure it’s just barely hanging on there now.

For #622, I had all but resigned myself to soldering on little bits of etched material to represent the ends of the drawbar.  Resigned until awaking in the middle of the night, shouting Eureka!

The idea leverages the fact that each frame is made of two layers of etch.  The outer layer incorporates the house brackets and wheel covers.  The inner layer includes a pair of flanges that overlap, but admit an insulating gap between them, which I will fill with epoxy.  By stealing a little bit of that flange, and bending it in instead of out, I can represent the drawbar ends without soldering them.  The title photo shows what I mean.

Of course, to make this change, I had to dig way back in the history of the OnShape model.  Along the way, I broke just about every feature and had to mend them again, and I learned that you shouldn’t add distinct tabs to two flanges made at the same time if you think you might have to go back and change the tabs later.  I spent a lot of time waiting for features to regenerate.

I’ve added a number of other features to the model as well.  There is now a generous slot down the middle of the pan to enable me to gap the firebox.  Each of the wheel pedestals has a flange to fold up and help clamp the bearing guides in place while soldering.  There is also a new tab to capture the back of the firebox.  Nearer the front, I’ve added some tabs to help align the two frame halves, and some tabs to support the end beam and pilot.

With all these tabs and flanges, the sheet metal model is starting to look very complex.  There are 19 folds in this single part.  I’m calling it framigami.



Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s