Dome rotation drive major rethink:

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When I fitted a hand cranked, friction wheel, dome drive I put it out of the way in the south east corner. It has worked faultlessly from first installation. See image right. The trick to getting it to work so well was the long lever providing mechanical advantage from the 5kg weight at the far end of a lever.

The friction wheel is pressed upward with probably 50kg thrust. Moreover, the pulley can effortlessly follow any undulations in the dome base ring. A sealed, cycle bottom bracket bearing supports the friction wheel and drive crank on opposite ends of the axle.

Yesterday, after lunch, I hit my head on the telescopes. Not just once but twice while ducking under them to reach the crank handle. What a silly Billy! In the afternoons the telescopes have flipped at the meridian and are now, largely filling the eastern half of the dome. All morning they have filled the western half of the dome.

I really need to move the manual drive system nearer my seat at the monitor. Just to avoid repeats of the head butting. Three telescopes have considerable moment and greatly resist being accelerated by human heads. So it's like banging your head against a brick wall. One with protruding objects which hurt! 

The northern wall is bad for the crank. I'd repeatedly bang my head on the crank. Absolutely 100% guaranteed! On the north east and north west walls would get in the way of the focusers when I am viewing objects low in the south. Which is the usual direction for solar and lunar viewing and imaging. Particularly in the winter in the case of the sun. I never look north because of the trees behind and above the dome. Much the same to the west except for higher up to south west. West and north west is all solid trees except very high up.

What are my realistic options without using electricity and motors? Well, I could easily fit a dropped chain drive to a lower crank. It is a bicycle crank and bottom bracket bearing set-up after all. The lowered drive crank could be low on the wall on either side of my chair.

I'd need another bottom bracket bearing for the drive crank. With just a sprocket up top beside the friction wheel on its axle. The minimum sized sprockets are limited to inner chainwheels. I could gear up or gear down. Or have a straight 1:1 drive. I find the present effort of turning the handle quite perfect. I used standard square ended cotterless axles for the system and have plenty of spares of those. I just need the sprockets [or rather chainwheels] and the cranks to go with them. The smaller the sprockets the better.

Having re-examined my chair relative to the monitor and computer desk I find it is asymmetrical to the dome walls. Being a few inches nearer the east wall. So using the north west corner post for the lowered crank bearing makes most sense. I need only turn slightly to my right to reach and effortlessly turn the crank.

A simple hole, bored straight into in the 4x4 post can hold the lower bottom bracket assembly. Half way up the post is fine. Then I can reach it comfortably while sitting or standing. A slit into the timber, centred on the hole, will allow a clamping bolt at right angles. Just to ensure the bearing doesn't move around in the hole. Or even pull right out.

Alternatively a split block can be bolted beside the NW octagon post to hold the sealed bottom bracket housing. That would save modifying the post itself and perhaps weakening it slightly. A block could easily be moved again if it was felt necessary. Whereas a big hole in a post is an eyesore if it is later abandoned.

I have just ordered two, rather smart pairs of square taper, 170mm alloy cranks with 33T fixed steel sprockets. Plus a set of plastic pedals. I can cut the right pedal down to the central bearing housing. Just as I did with the previous drive system.

Plastic is much nicer to handle than cold metal. There is no need for a pedal platform. Just a simple, cylindrical handle, of ample diameter has proved ideal. I shall have a disk behind the pedal spindle to avoid trapping my hand between the crank and the post. A white plastic disk will be ideal as it will be easily visible in the dark.

The upper chain wheel crank will be sawn off and then turned in the lathe to a neat boss. Only the sprocket and hub are really needed. I can't see the point in having a second drive crank higher up. It would only get in the way. Might even be a danger after dark. Total cost of the new components is about £45 including a new rust free chain. Should have all the bits together by Tuesday afternoon. And did, thanks to the usual excellent service from Cykelpartner.dk. The online bike spares store by which others should be judged. Order early enough and you will usually have it next day.

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