London bus destination blind sign. Part 1.

August 8th, 2012

This project came about after I bought a London bus destination blind on TradeMe relatively cheaply. I say relatively since these things seem to vary widely in price. Seems they are an in thing at the moment for home interiors. Not that I know anything about that. I have a TARDIS in my living room if you need proof of that. But when I saw the blind I had to have it and make something for it. I want to make a display unit for it inspired by a London bus – the classic red Routemaster.

IMG_2647_1 Bus destination blind.

I am not sure exactly which route it is for but it is from South London and is dated July 93. It might have been trundling about on a bus when I was living in the UK in 95-96! And the locations on it are not that far away from where I was born.

The housing I am making will go on the wall and will be motorised. Mainly as an excuse to use a couple of neat old switches I have. A toggle will let you rotate the sign back and forth to show a different destination in a window on the front of the housing. To do this I will use two rollers and a windscreen wiper motor to power a chain to drive them. The sign is about 5m long so a little thought will make you realise two simple rollers driven by a chain won’t actually work.

With a chain drive over two equal sized sprockets the two rollers will rotate at the same speed. As the blind moves from one roller to another the diameter of the roller changes as the blind wraps or unwraps around it. This would cause the blind to become stretched or loose as the diameters change.

I had a plan to get around this. Ratcheting hubs! These are used on BMX bike rear wheels. The chain only drives them in one direction which is why on a bicycle you can pedal to drive the bike but the rolling bike won’t drive the pedals. I went to a couple fo local bike shops and asked to look through their bits bins (they ALL have bits bins) and I found two used  identical sprockets  for cheap.

IMG_2715_1 Ratcheting sprockets.

To make the rollers I am using PVC downpipe, 80mm in diameter. End caps are made from plastic chopping board machined to fit inside the pipe. I cut them out roughly on my scroll saw then machined them to a tight fit on the lathe. As I want everything to rotate I bought some cheap skateboard bearings which take an 8mm shaft. I also bought 2 lengths of 8mm silver steel to use as shafts.

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End caps.

To attach the sprockets to the roller end I machined up a small steel disc with a hole for the shaft through the middle and a taper on the edge. Three holes were drilled and tapped into the face of it. This disc is soldered to the shaft and clamps the sprocket to the end cap on the roller. The taper on the disc ensures the sprocket is held centrally. Three screws go through the end cap to hold everything in place.

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IMG_2739_1 IMG_2741_1 Sprocket attachment.

The sprockets of course ratchet in one direction only. I need the rollers to ratchet in opposite directions so I can arrange it so only the pulling roller is powered and the trailing roller is free-wheeling. It will rotate at the speed it needs to. To do this one sprocket is attached upside down compared to the other.

To finish off the rollers I screwed the end caps to the PVC with small, counter sunk screws. The three screws holding the sprockets in place are Loctited into position so they won’t come loose.

IMG_2780_1 Finished rollers.

That was the rollers completed. Next I made up small steel brackets to hold the bearings the rollers will run in. These brackets were simply sections cut from some angle iron I had around. To hold the bearing to the bracket I machined up some housings from more chopping board. These hold the bearings in place and are held with three screws.

IMG_2747_1 IMG_2752_1 Bearing holders.

IMG_2750_1 IMG_2777_1 Hole drilling template.

To get the three mounting holes even I made a little wooden template and used it as a drilling guide on both the plastic and steel. The hole in the steel and plastic is bigger than the centre of the bearing so that the bearing can rotate freely.

To check everything would work I temporarily attached the brackets to some wood and tested the rollers. They rotate very easily.

IMG_2782_1 Roller testing.

Next I looked at the drive. I am using a windscreen motor I have had lying about for years. I have no idea what sort of car it came from. Windscreen wiper motors are great for things like this. They have plenty of torque and often have two speed connections. These aren’t actually needed as the motor itself will run from 12 volts right down to 3 volts or less with a proportional reduction in speed. There are two things to watch with them though. Often they rotate faster in one direction then the other if you reverse the voltage. For me this won’t matter. The second is because they use a worm drive the motor can turn the shaft but the shaft cannot turn the motor. This can be a blessing if you are using the motors to drive a robot say since when not powered they are effectively locked and act as a brake. It can also be a pain since if you want your wheels to free wheel you must disconnect the motors from the drive train.

The motor shaft has a round shaft with flats on the side and a thread on the end. I machined up a steel plate to fit inside the drive sprocket I had (given to me free by the bike shop). I drilled a 6mm hole first and machined it to be circular. Then, when it would fit inside the sprocket (from which I machined away the inside thread), I filed the hole to fit the shaft.

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IMG_2787_1 IMG_2789_1 Drive sprocket.

I did want the metal disc to be an interference fit into the sprocket. You machine it slightly oversize then heat the sprocket so it expands, drop in the central disc and the sprocket then cools and contracts around it. It is how they used to fit the tyres on locomotive wheels back in the day. Unfortunately I muffed the machining and made it fractionally too small. So I simply soft soldered it in place. That will be more than enough to hold it.

The drive is transmitted via the shaft so the nut simply stops the sprocket falling off. There is no danger of the nut unscrewing when the motor reverses.

Next I need to make the base and the limit switch mechanism. I want the blind to automatically stop when it reaches the end of the sign at either end. To do this I will use a threaded rod driven off the wiper motor nut. The rod will spin inside a captive nut which will then move along the rod. With the rollers I have and the length of the sign it will take about 20 turns to go end to end on the blind. On an 8mm thread that is only about an inch of movement of the nut. To decrease the sensitivity of the system so it is easier to position the limit switches I shall use a bell crank arrangement to amplify the movement.

I have ordered a book about the old Routemaster buses that I am expecting to arrive any time now. With that I can design the base and the cover. That shall be made from aluminium as that’s how the Routemasters were made and also as an excuse for me to practice some aluminium metalwork before tacking the aluminium skin on my Austin 7 car I am making!

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