2" shaft mounting. Pt 10: Adding disc bearings?

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The discussion on cloudy nights has raised the specter of flexure at the PA flange to shaft junction. The flange does not have enough depth to rule out flexure in the joint between them.

Rather than dump the original flange bearings idea I may add disk/plate bearings to this highly stressed area. The Fullerscopes MkIV cleverly used a combination of both 6" diameter disk as well as normal shaft bearings. This allowed it to carry far larger and heavier instruments than its [mere] 1.25" shafts might suggest. The disk bearings surfaces obviated the usual bottlenecks at the cantilevered junctions between axes. There being no bare-shafted overhangs. A disk of paper-thin PTFE/Teflon was used to reduce friction between mating surfaces. The wormwheels were sandwiched between the 6" diameter disks but deeply recessed to reduce overhang.

A disk [or plate] bearing uses reversed leverage to avoid flexure. Taken to extremes, it need only be two disk rubbing together with a central pivot pin for alignment. The disks are pressed firmly together by the loads placed upon them. In this case the weight of the Declination assembly, telescope and counterweights. Any tendency to tip or separate the disks is greatly resisted by the very poor mechanical advantage of the system. It would be akin to trying to lift something very heavy by pressing on the short end of a normal crowbar. 

The larger the bearing disks the more stable the arrangement. Downside is the increased friction with increasing disk radius/diameter. It follows that low friction combinations of materials become essential. Brass against steel or aluminium would work. As would a sandwich filler of Teflon/PTFE sheet between the disks. 

The Dobsonian altazimuth mounting is an example of a disk bearing being used between the rocker box and the ground board. To achieve low, but not zero, friction usually involves Formica and Teflon pads. There being no need for a full disk of the expensive Teflon material. Slight friction is highly desirable. A disk bearing can have its friction lowered by having only the rims in contact. This may also increase the stiffness of the arrangement.

I have some 180mm / 7" aluminium bar stock which could be used to make bearing disks to reinforce the flange/shaft bearing. The image alongside shows one way of doing this. Rather than reverse the PA/Dec neck flange a disk could be "wrapped around" it. This disk should be fixed to the neck flange by the same fixing bolts which hold the flange to the Declination housing. 

A second disk would be attached to the top PA flange bearing. I have shown this bearing reversed in the image but it be best to leave it flat side down to ensure proper seating on the PA housing. The lower bearing disk would then be bolted to the top flange bearings casting by the same fixing studs which reinforce the PA housing. It is not enough to have loose [bearing] disks. As they would not contribute as much stiffening as those which are firmly fixed to the bearing components themselves.   

A further complication is the addition of the large [and 16mm, 3/4" thick] wormwheel to this multi-layer sandwich. A ring or cup form of packing disk might be essential to increase the diameter of the wormwheel bosses to ensure the full diameter of the disk bearing is utilized. Where should the disk bearing, rubbing surfaces be situated relative to the wormwheel?

The wormwheel's clutch system must be carefully considered as it relates to the vital freedom to point or slew the telescope by hand while the drive motors and slow motions are simultaneously in use. Loading the wormwheel faces with the entire weight of the telescope, counterweights and mounting parts might make the wormwheel essentially integral with the Declination housing by friction alone. This would be highly undesirable as it would then be impossible to point the telescope except by using the wormwheel's drives. A very slow and impractical arrangement indeed! 

If the friction between the disks should prove too high in practice then an adjustable thrust, ball bearing could be employed. This could be placed at the base of the PA shaft or in a counter-bore between the disks themselves. The neck flange could be utilized to support a linear thrust bearing. Washer-like shims could be employed to adjust the friction levels. It might be worth investing in some PTFE/Teflon bearing material first to see if this overcomes the friction problem.

The disk arrangement illustrated above could be thinned by reducing the disks near their hubs to about 3mm or 1/8". Further aided by inverting the neck flange and sinking it into the Declination housing. Or, minimizing overhang at a stroke simply by moving the RA wormwheel to the bottom of the PA shaft as in the upper drawing.

The lower and final drawing shows the wormwheel used as a disk/plate bearing sandwiched by Teflon disks to reduce its friction independent of the shaft components. The neck flange is inverted and has a disk attached to its face to help to increase the socket depth [against rocking] and provides the top pressure disk for the wormwheel.

The wormwheel rests on top of a lower disk supported by thick, tubular spacers bolted to the bearing flange. This is a much stiffer arrangement since the wormwheel can no longer rock on its shaft. It effectively becomes a plate bearing. The wormwheel boss must rotate freely inside the lower disk and rests on a Teflon washer between itself and the extended, inner bearing race. No other arrangement offers the minimum of overhang beyond the top bearing. Moving the wormwheel to the bottom of the PA [Polar Axis] will reduce the overhang but only by the thickness of the wormwheel and its boss. Which will probably amount to 40mm or about 1.5".

This latest arrangement will help to considerably increase the cross sectional area between the top bearing and the wormwheel. Which greatly reduces the risk of the flange rocking on its shaft. The top disk accepts the heads of the through studs/bolts holding the Declination housing firmly to the 15cm, 6" diameter, inverted flange. While the lower disk accepts the nuts for the through studs compressing and stiffening the Polar Axis housing between the two flange bearings. These fasteners must obviously be sunk below the surfaces of the plate bearing disks.

Click on any image for an enlargement. 

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