Dome drive crank?

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Wednesday: I have plans to add a crank to a supporting wheel to drive the dome manually and relatively effortlessly. Until now I have relied on dragging the dome around by sheer strength while pulling on the plywood ribs. The dome's turning friction is rather variable due to the uneven base ring. This is aggravating my "tennis elbow."

I have already discovered that I can turn one of the white supporting wheels/rollers with my bare hands. This achieves dome rotation but it needs great care to avoid my fingers becoming trapped.

It is hardly a sensible way to turn the dome through a large angle but the effort needed seems quite low. This is because the gearing is in favour of the much smaller wheel riding against a much larger one. In this case the larger "gear" is the dome base ring. Which is roughly 3m or 10' in diameter.

The white wheels are 160mm or just over 6" = ~0.5' : 10' [dome Ø] = roughly 1:20 with a similar increase in torque. Ideal for hand cranking but it would need a very low speed, gearbox motor to drive it with electricity.

The white dome support rollers typically run on needle roller bearings for low friction and high load capacity. Which wouldn't provide any drive [at all] via a driven axle.

So I think I will modify one of my smaller 4" industrial wheels and brackets. These have rubbery [polyurethane?] treads which should provide far more grip for driving and considerably lower gearing. 4" : 120" = 1:30.

The drive wheel [base] ought to be sprung to ensure continuous contact with the rather non-flat, dome base ring.

I have propped one of these 4" industrial wheels up on a block of 2"x 4" to show the relative diameters. If I remove the sealed needle roller bearings the bore is a good size for fitting a sturdy, fixed, drive axle. I'll need to think about adding bearings to the uprights of the sprung support structure. This will need to avoid flexure while I am cranking.

My real worry is having a crank projecting into the dome area precisely at head height. I suppose I could rely on the drive crank being left in an upwards position of its circular travel. Then it might miss my head as I move around inside the dome. Cranking at head height might become irritating after too much practice. Though I have no plans to drive the dome continuously.

Where to site the wheel and crank is quite an important matter. The telescope's eyepiece end sweeps low, mostly from east to west, via north. So the drive crank wants to be on the southern side of the building. Where the telescope dewshield is usually pointing well above horizontal.

The southerly half of the dome is the least cluttered area so most suitable. It would make sense to have the crank on an inside corner of the octagon where it would protrude the least. South east makes most sense since it is nearest the computer shelf. South west would be near the top of the stepladder. So would offers a risk of a collision as I stepped onto the observatory floor from the top of the ladder.

It might prove necessary to add some gearing to lower the drive effort even more. This would also allow a lower crank situation via [perhaps] bicycle sprockets and chain. Or V-belt and pulleys. Even if the direct gear ratio is perfect, a chain or belt would allow the crank to be placed where it is most comfortable to use. Though there is always the matter of the crank whizzing round if the dome is rotated directly by hand. I shall just have to resist the temptation to do so.

Click on any image for an enlargement.

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