London bus destination blind sign. Part 4.

August 14th, 2012

Today I went for a walk at lunchtime back to the engineering shop (AES in Westhaven) and got some UNC bolts. When I got home I drilled out the roller brackets to fit those. I also added spacers made from clear vinyl tubing to the ends of the roller shafts to keep them laterally supported in the bearings.

IMG_2829_1 Vinyl tube spacers.

I also mounted the wiper motor using several more D nuts.

IMG_2831_1 Mounts for the motor bracket.

With the motor in place I looked at the chain. The one I have was too long so I removed some links using a chain breaking tool. This simply punches out the pins between the links allowing you to separate them.

IMG_2832_1 Chain breaking.

Unfortunately the chain was a little short. I could have added a link (actually two as there are two sorts of links in the chain) but that would make it too big and loose. Instead I was able to change the geometry so I could use the shorter chain. The drive system consists of three sprockets, one on each roller and the one on the motor. These are arranged in a triangle. By flattening the triangle I would reduce the length of chain needed. I did this by moving the motor up slightly (6mm in fact). I made the chain fit the two sprockets then I made two wooden plates to go under the motor bracket to raise it up.

IMG_2836_1 IMG_2837_1 Chain drive.

With the spacers under the bracket and the motor bolted down it puts enough tension on the chain to keep it nice and tight but still move freely. Chain drives work by pulling so it is better to have the driving sprocket on the bottom of the chain as I have it here so it is pulling the driven rollers around. The top of the chain doesn’t touch the top of the drive sprocket at all.

IMG_2839_1 Drive in place (sorry it’s a bit fuzzy).

With that done I was able to test it and it works! I made a little film showing it running and how only one roller is powered while the other can free-wheel. It’s taking an age to upload from my phone to YouTube but I will put the link here when it is done.

Tomorrow I can make the hold down clamps that will keep the sign on the rollers and maybe test it with a roll of wrapping paper I bought to simulate the blind for testing. Next I need to make the limit switch mechanism and then, finally, the aluminium housing.

Here is the film finally uploaded:


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