Saturday, January 19, 2013

In Good Company

I've recently spent some time (figuratively) crawling around under coal wagons inspecting their braking systems, in order to try and design a simple, but accurate, representation that I can add to my 3D printed underframe. One of the things I've discovered (two excellent resources are here and here) is that while the brakes on my model coal wagon worked so well that the wheels wouldn't turn, in real life they wouldn't have worked at all, as I fitted them the wrong way around.

By the late 19th century most wagons were fitted with a simple lever brake on one side only. The diagram shows the break lever (in red) in the off position. To apply the brake the lever was pushed down. This turns the shaft (blue) and connecting arm (green) clockwise, which in turn moves the two push rods and attached brake shoes (black) outwards and onto the wheels. As you can see this is a really simple, but effective, braking system. The one downside being that if you had to apply a brake to a run away wagon you might have to cross the track to get to it, which may well be bad for your health and well being!

The easiest way to solve this problem is to simply fit a completely independent brake system to the other side of the wagon, but with the brake lever always at the right hand end of the wagon (so it appears to always be in the same place no matter which side you view the wagon from). This means that when you view the connecting arms from one side of the wagon, the nearest turns clockwise while the one on the other side turns anti-clockwise as the brake is applied. This means of course that you can't simply connect the two brake systems together by extending the shaft (blue) the full width of the wagon.

To overcome this problem, and to allow either brake lever to be used to apply both sets of brakes requires a clutch mechanism (shown in yellow) and for the push rods and connecting arms to now be mirror images of each other. This works, because if you push down the far lever (which is the same as previously) then it now turns both connecting arms such that the push rods are forced outwards applying the breaks, but because of the clutch it has no effect on the other break lever. If, however, you push down on the near side lever then, via the clutch, it turns the shaft anti-clockwise applying the brakes on both sides.

Not only did I manage to fit the brake system wrong on the Parkside Dundas kits I built, but it appears that I'm in good company as neither Dapol or Hornby seem to model fully correct brake gear either.
Firstly, on the left, we have what I accidentally ended up modelling. Two independent brake systems, but with the connecting arms and push rods on the near side fitted the wrong way around. This means that instead of applying the brake when the near side lever is lowered, the brake shoes will actually move further away from the wheels, which clearly is wrong. Mind you, on the right, you can see how both Hornby and Dapol model the braking system. They go to the trouble of modelling the clutch to allow either brake lever to be used to operate both brakes, but then don't bother to model the wagon wide connecting shaft, completely negating the point of fitting the clutch mechanism. I'm assuming they don't bother modelling the connecting shaft because a) it is out of sight under the wagon and b) would possibly be quite fragile, but either way they clearly weren't aiming for accuracy.

So which version am I going to add to my 3D model? Well given that there will be no difference in cost, if I can work out a sensible way of modelling the clutch, then I'll probably model all three correct versions. This way I can print which ever I want depending on what time period I want them to represent. This is one of the main advantages of 3D printing over traditional manufacturing; there are no upfront cost to create expensive moulds, which allow us to print variations at no extra cost.


  1. Modelling is a fascinating exercise. I used to enjoy building airworthy model aircraft. I was amazed how often I reinvented things.

  2. I'm beginning to feel a total inadequate here. I managed to follow the braking explanation - eventually but then realised that I've built made a model of anything so far as I can remember.

    1. Well if you're feeling inadequate already, then I'll apologise in advance for some upcoming posts! Now I've got the hang of 3D modelling and printing I'm on a bit of a roll. There is already a test print of a full wagon (minus brakes and some other important details) at the printers as well as what I hope will be a successful print of another item for Penistone Railway Works, and now I've figured out the brakes I think it won't be long before I'll be able to print an entire 7 plank coal wagon for less than the cost of buying the kits I used before. Now if only I could finish clearing away the stuff that accumulated in the dinning room over Christmas I might be able to actually get back to doing some hands on modelling rather than just sitting in front of a computer modelling wagons.

      As for never having built a model yourself, surely you must have built an Airfix kit at some point in your life (or helped Andrew or Gareth with one). I thought it was just something that everyone (or at least the male half of the population) did at least once.

    2. I was hoping you wouldn't remind me about my attempt with an Airfix model of a Spitfire (I think it was). Now ask me to dismantle and rebuild an M40A gearbox of a Volvo 122 and I'll happily oblige (or I would have done at one time). I just don't do small - I can't even tie a fly on to my line without lots of angst.