Dome drive crank 3.

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Saturday:

I'm now looking at a simple lever to force the crank driven friction roller upwards against the underside of the dome base ring. On the far end of the lever will be a screwed rod [stud or all-thread.]

The leverage will amplify the upward pressure on the roller. Until the point where adhesion is guaranteed. The drawing shows the basic idea. The hand crank is not to scale.

It will need a bearing in the back of the friction roller and adequate support from both sides if heavy pressure is required. The BB axle adapter I made yesterday is just for ensuring the drive from the crank to the roller.

I shall make the lever as long as possible to maximize the leverage. Ideally it won't need a screwed rod for adjustment. A simple weight at the far end and the lopsided 'seesaw' should easily absorb variation in base ring level. Thereby maintaining enough friction between the roller and base ring for a nice, even drive without slippage.

The next step is to build a rough model from scrap timber to test whether the friction roller really will work reliably. I will also get a good idea of the force required on the crank handle to move the dome easily. Then I can decide if I need a further gear reduction. Or can safely gear up for faster slews.

The image [Left] shows the general idea as a mock-up. The batten shown is not large enough to enclose the BB cartridge. Which will require a hole with a horizontal slit and vertical clamping bolts to hold the BB firmly. Something like 2" x4" timber, set on edge should do for a first trial.

Note I have trimmed the plastic, pedal plates away to make a comfortably round, crank handle. I have "painted" over the supporting blocks in the image to make the mock-up look more like the finished design. As shown, a ratio of 12:1 or even 15:1 on the lever is easily achieved.

The friction roller and its supporting BB bearing are skewed at an angle to the batten to achieve a tangent to the roller's track on the base ring. This will not affect the lever system in any meaningful way. The pressed steel roller bracket, from which the friction roller was removed, will provide a firm pivot for the eventual lever and allow it to sit close to the friction roller. 

The image [Right] shows a plan view of the lever, roller and BB. The offset angle [12-15 degrees] is easily seen to ensure the roller lies on a tangent to the base ring.

It will be important to resist any twisting in the final mechanism. So that the crank turns in a true circle and does not hit the woodwork. There isn't much room to swing a crank due to the angles of the broad, timber, top ring of the octagon. The long lever can be sandwiched between vertical restraints to reduce twisting to a minimum.

Hopefully the closeness of the lever pivot to the roller and the restraint from the base ring on the roller, will minimize unevenness in crank resistance during manual dome "winding." The rise and fall of the far end of the lever will have to be checked during a full dome rotation to avoid collisions. The 2x4 lever can easily be trimmed to a taper, if necessary, to provide more clearance.

The last image shows a length of 2x4 propped up in roughly the correct position. It will have to move a couple of inches towards the white roller to surround the sealed, bottom bracket cartridge. The 2x4 will have to be drilled 31mm diameter at 12-15° to the face. Probably using a hole saw but I don't have one of that size. A long, vertical bolt will clamp the body of the BB cartridge into the hole through the 2x4.

I also need a left hand thread, fixed wheel sprocket, locking ring. A metal plate fixed onto the face of the 2x4 will be clamped to the near side of the BB cartridge, with the locking ring, to further secure it under load. I'm wondering if I can get a much longer axle in a sealed BB cartridge to give me more clearance for the crank. Sealed BBs are readily available in lengths up to 131mm and are quite inexpensive. Duly ordered a cheap 131mm to replace my present 118mm. This will give me another 12mm [1/2"] clearance. I am now looking at adding metal plates for both sides of the timber lever to make use of the threaded sections of the BB.

Click on any image for an enlargement.

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