Saturday, September 6, 2014

Canopus: Building the Gearbox

As I mentioned in a previous post, I was a little concerned about building the gearbox for Canopus as the supplied parts differed from those originally used in the kit and it appeared that the new parts wouldn't build into a working gearbox. Fortunately I have now built a working gearbox, but before we get to that let me show you the problem with following the instructions given the change of parts.

As you can see from the first photo the gearbox fits between the rear driving wheels. The instructions say to solder the bearings to the outside of the gearbox and then file off anything that protrudes through to the inside. This is easy to do, but when you then fold the gearbox to shape you find that the clearance between the bearings on the outside of the gearbox and the wheels is very minimal. I couldn't actually see a gap although the wheels did turn freely without moving the gearbox. With such minimal clearance though any slight movement and chances are the wheels would touch the gearbox and this would likely lead to a short as the chassis is live. I'm assuming that this wasn't a problem with the kit originally but results from a change to the actual gears. Originally two separate gears were supplied rather than the new combined gear and I'm guessing that they fitted onto a thinner shaft and that the appropriate bearings had a smaller outer face.

The change in gears presents a second more important problem. The width of the two original gears appears to have been equal to the inner dimensions of the gearbox. With an interference fit between the gears and the shaft this would mean that they wouldn't move sideways within the gearbox. Unfortunately the new gear is much thinner. This means that the shaft is free to move sideways and any sideways movement causes it to drop out of one of the bearings. It would also appear that the shaft is a little undersized as the gear is a very loose fit; it measures 1.97mm and is loose in the bearings designed for a 2mm axle (they are the same as those used for the wheels). Fortunately it turns out that both problems are solvable without too much effort.

As you can see from the photo I've assembled the gearbox while completely ignoring the instructions. Firstly I've fitted the two bearings from the inside instead of the outside of the gearbox to improve the clearance with the backs of the wheels. Secondly I've replaced the shaft with some 2mm brass rod (yes I know I need to cut it to match the width of the gearbox but I've left it long so you can see it) as this is ever so slightly thicker (0.03mm to be precise) than the original and is a tight interference fit with the gear wheel. While the gear no longer slides on the shaft it would still move sideways in the bearings but I've solved this using a third bearing. The kit contained a spare 2mm bearing which just happens to be the perfect width to fill the gap. So now I have a gear box where there is no sideways movement, the gear wheel is a tight fit to the shaft, and the shaft rotates freely. A quick test with a 9V battery and the motor shows that it works perfectly; I haven't fully pushed the worm gear onto the motor shaft as once it is on it will be difficult to get off and it can only be fitted once the motor and gearbox are fitted to the chassis.

So I know have a working gearbox, I just need to trim down the brass rod before fitting it to the chassis. I'm thinking that the next step might be to paint the chassis because once the motor is fitted it can't easily be removed and I don't want to risk getting paint in the wrong place. Also it will be easier to paint the cranks etc. before permanently fixing the coupling rods and adding the rest of the motion.


  1. You seem to have got on top of this job.

  2. I'm catching up today and hadn't intended to comment on all your posts but I have to say that I read this intently. I am never likely to make a model but I can't help enjoying it vicariously.