A few months ago I produced a 3D model for some replacement eccentric rods for a MRC/Rowa 2-8-4 Berkshire steam locomotive.  My Berkshire was missing one and although it ran well without it I wanted it to look right.  You can read the post here.  I have now had some of the eccentric rods printed and in this post I will share with you how they came out.

I printed the new eccentric rods in Shapeways’ Frosted Ultra Detail material; I did this just before they announced their new Frosted Extreme Detail material.  The new FXD would be ideal for these parts.  Anyway they arrived and once cleaned up looked very good as you can see in the image below.  The bottom one is an original from the other side.

In the image below you can see the two C shaped fixings which the eccentric rod clips into.  One is on the 3rd driving wheel and the other is under the bell crank.

The new rod clipped into the fixings fairly easily.  I found it worked best if you fit to the bell crank end first, then the 3rd driver.  As the pin on the 3D printed rod is very small there is the risk of breaking it off so I will be supplying them in a pack of four just in case.

The new rods moved freely and showed no signs of binding.

Next came a running test and the loco ran as good as ever but there was one tiny issue.  And when I say tiny I mean 0.34mm.  If you have a closer look at an original eccentric rod in the image below you can see the tiny pegs that clip into the C fixings.  They both appear to eccentric to the center of each end but in fact the left hand end is closer to the center, 0.34mm closer.

Now for such a small dimension you wouldn’t think it would make such a big difference.  Have a look at the video below that shows the locomotive running with the original eccentric rod fitted and see how much the bell crank rocks back and forth, or gives the impression that’s what it’s doing.

Then look at this video showing the locomotive with the 3D printed rod fitted.

Although the rod runs well the slight extra length corrects the offset the eccentric rod is designed to make and the bell crank is hardly moving.  Also it creates the visual effect that the end of the eccentric rod is closer to the center of the 3rd wheel, which would cancel out the movement.  Although this is only a cosmetic problem I would like to get it right and, happily’ this is an easy thing to fix.  I’ve moved the peg into the correct place in the 3D model and when I get a reprint done it will hopefully be all sorted.  I think for the next set I’ll try these in the new FXD as well.  So for now the Berkshire is back on the shelf.

For next week’s post I have something a bit different to share with you; on the weekend I was invited to a running session at the McKinley Railway here in the UK which I greatly enjoyed as it is more than just a model railway.  Next week I will explain what it’s all about and share some photos and videos with you.