Friday, September 29, 2017

An Extra 8.5g

As some of you figured out the new parts I showed in the previous post are a revised version of the keeper plate in the O14 Clayton battery electric locomotive I've been modelling on and off for the last two and a bit years, having started on it back in May of 2015. I did finish a complete model at the end of 2015 but that is now earning it's keep in Rhyd. Initial running trials at Rhyd showed that while the model worked okay it wasn't really heavy enough, especially if the driver was on the heavy side, and had a tendency to bounce. For the past year I've been slowly thinking about ways to add more weight and this is the result.

On the left we have the original keeper plate as fitted to the model now running on Rhyd. In the middle we have the revised keeper plate which is quite a bit thicker and has much taller ends. On the right we then have a new part which is purely to add weight; it doesn't have any specific function, unlike the keeper plate. This fits between the motor and the layshaft directly over the wheels.

The original keeper plate weighed 6.3g whereas the two new parts together weight 14.8g so an extra 8.5g, or viewed another way, an increase of 135%. Hopefully this should drastically improve the running quality of the model by helping to keep it securely on the rails. Hopefully I'll find out reasonably soon how much of an improvement as I need to get on and build the new model because, yet again, I've been commissioned to build it for someone else, so I still won't have finished one for myself!

As a bit of an aside, and because I think it's interesting, I thought it worth a few comments on weighting models and I why I've taken the route I have. Often when trying to add weight to models people use Liquid Gravity which is essentially a lot of tiny little heavy beads; would probably have been lead shot at some point in the past but health and safety rules means it's no longer lead based.

Liquid Gravity is nice and easy to use as you simply pour the beads into the available space within the model and keep it in place with a little superglue. I used this approach when building the Hudson-Hunslet model as it meant I could fill the tiniest of spaces to add extra weight. While the manufacturers don't provide any details on the weight of Liquid Gravity for a given volume I did find a review that had tried to estimate how heavy it really is. They found that it weighed roughly 4.15 g/cm3 which is actually quite light when compared to lead which weighs 11.3 g/cm3.

The stainless steel that I've had the parts 3D printed in is referred to by Shapeways as being 420 steel. Having had a hunt around I've found that 420 steel should have a weight of 7.74 g/cm3; so an 86% increase in weight for the same volume. Shapeways also give the material volume of each part and checking I found that the original part (with a volume of 0.8141cm3) should have a weight of 6.3g and the new parts (with a combined volume of 1.8215cm3) should weight 14.1g which matches nicely with the final weights of the printed parts.

When building such small models it seems silly not to take advantage of the extra weight of the stainless steel especially given that it can be printed to exactly fit within other printed parts and, in the case of the keeper plate, to serve a function at the same time. I'll certainly by continuing with this approach on future models, although Liquid Gravity still has it's uses.

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