Wednesday 16th Chain drive to dome rotation crank.

 *

Wednesday 16th. A horribly wet day gives me a chance to work on the chain drive to the dome rotation. It shouldn't take very long once I find a suitable, bottom bracket cartridge. The axle has to be long enough to allow chain clearance from the timber work. If I use a separate supporting block that will give me greater freedom in placement. The only bottom bracket I had wasn't suitable.

Drove to town to find the bike shop closed for a week. Ordered online. Should have it tomorrow. Back to square one on the left hand thread, RH side, BB locking ring. I found one in my collection last time. Never found one online after days of searching. I used the ring to lock the BB to a thick alloy plate. I don't think it's the same size as the LH thread ring used on fixed gear hubs. No ring means no security for the bottom bracket when I crank the dome around.


Even Interpol couldn't find a bottom bracket locking ring with a left hand thread. I may have to make my own. The recycling centre is beginning to think all my swarf is coming from a business! I have to carry little bags of swarf in my pockets and tip them into the scrap metal container when the guards aren't looking.

Here's are reminder of the existing and very successful, counterbalanced, friction drive roller set-up. The drive ratio is 1:30. [4" roller : 120" dome diameter] The ratio was sheer coincidence but has worked very well without requiring much effort on the crank. I can use my pinky [smallest finger] to turn the crank.

The trick is to reliably maintain the upward pressure on the roller as the dome, base ring, rises and falls. The 5kg counterweight supplies the required pressure, with plenty to spare, thanks to the long, pivoted lever. A fixed roller would never work because the dome cannot be perfectly true.

Friday 18th With all the parts brought together I was able to mock up the dome, chain drive. I made a left hand thread locking ring with a tap. Simply by forcing the tap through a normal, right hand thread, locking ring. I clamped the ring in the 3-jaw chuck. Then backed up the tap with a centre in the tailstock. Cranking on the tap with a spanner forced the opposite hand thread into the metal. No power was applied to the lathe, of course. A crude but effective method.

Now I just need to make a support plate for the lower crank assembly. A chain tensioner may be necessary as the drive roller rides up and down under the base ring. Only experience will prove if a tensioner is necessary.

Click on any image for an enlargement. 

*