A flange bearing and plywood equatorial mounting.

 *

 23rd May 2016. The Fullerscopes MkIV mounting had found its limits with the 7" f/12 refractor and 10" f//8 Newtonian. Having the Skytracker VFO pack up on me merely confirmed what I already knew. I needed a much bigger equatorial mounting to cope with the considerable moment arms of such long and rather heavy, OTAs. Friction and flexure were becoming a bore when trying to point the telescope. Or worse, when trying to center an object in the field of view. For imaging the MkIV's only advantage was having a variable drive. Which was now gone. Everywhere else it fell down on the job. 

I was quite tempted by the well proven pillar block bearing mountings. Beacon Hill has a successful mounting which uses pillar block bearings. What put me off was the need for considerable [and very very heavy] metal sections to keep the bearings aligned without twisting. Very large pillar block bearings are also very heavy due to their cast iron housings. 10kg [20lbs] each if I was to realise my ideal mounting with a considerable margin of stiffness without shaft flexure. Then I realised that going that large was unnecessary and shafts somewhere about 60mm were ample.

While searching for affordable pillar block bearings in the small ads website I discovered that flange [or pillow block] bearings were about the same price for the same bore when new. Moreover they [flange bearings] had some rather desirable features in comparison with the more popular pillar blocks. They could be easily attached to plywood without offset, cantilever or torque issues raising their ugly heads. The pillar block is offset from its support plate and the plate then tilted all over the place. The offset or cantilevered forces have to be countered with little more than a thick steel [or aluminium] support plate. The 60mm flange bearings weigh about 5kg or roughly 10lbs each. Though plastic flange housings are available too they are probably more expensive than cast iron.

While solidly made, birch plywood boxes with a flange bearing at either end has no great desire to twist. The boxes are as stiff, for the same weight, as solid aluminium if properly made.

The boxes could be laminated as thick as I desired to fill all the empty air spaces inside. Provided of course there was room for the axis shafts to turn. And all without adding much extra weight but adding considerable extra stiffness. Since the "packing" plywood would be within the visible outer shell it would consist of smaller pieces with lower weight.

Only waterproof wood glue would be necessary to hold it all together without any need for bolts, screws or nails. Or worse, welded, heavy steel plates and/or girders! I might actually be able to lift the major components of a plywood mounting without needing a crane or block and tackle! Nor a week's bed rest afterwards!

Furthermore, the flange bearings could be physically connected to each other and the surrounding box housing via lengths of sturdy studding. [Threaded rods] The bolt holes in the flanges are always generous in diameter and can easily be enlarged with a drill if needed. All this reinforcement would further increase the strength of the solid plywood boxes by compression and by virtue of the well spaced studs themselves while under tension. No loose sections or panels could ever detach from the axes boxes by accidental overloading.

Four, brand new, flange bearings can be had for about £60 in the 60mm bore size. [2 3/8" Ø] Easily enough to support either of my long OTAs with plenty of stiffness to spare. I seemed to be onto a winner!

Of course I still needed to source suitable shafts and  large wormwheels. I'd have an incredibly strong mounting for much the same cost as the lowliest, budget equatorial only suitable for a small refractor. I would prefer thick-walled tube for the axes to save a little weight without reducing stiffness. I shall have to ask around the local smithies to see what they have. 

The Polar Axis housing box would be supported and pivoted in a thick, reinforced, plywood fork to allow adjustment of polar altitude. A turnbuckle would provide fine adjustment to achieve polar altitude accuracy. Through bolts, or studs, using large, load-spreading washers, would then clamp the whole mass together as a solid unit to the thick base of the fork. All this requires is for the clamping bolts or studs to be raised or lowered to clear the axle.

The polar housing and fork will be bolted to the thick, welded steel, pier flange. Thereby safely avoiding all the usual bottlenecks seen in many commercial mountings. The center of gravity of the mounting [and counterbalanced telescope] lies directly over the pier pipe and thus avoid the usual cantilevered forces.  

The Fullerscope MkIV's flimsy, cast fork and tiny altitude locking screws are by far its weakest features. Making the polar tilt pivot in direct line with the polar axis made it impossible to have a through bolt. So it relies on short threads cut in very soft, cast alloy. The fork is thin and hollow section and readily crushed by over-tightening the existing and undersized, pivot screws. As I discovered when I re-cut the threads for much larger, stainless steel pivot bolts.

The plywood axis housings and fork could be painted for protection against the weather or clad in thin aluminium. Exposed, wooden windows can last for years with just a coat or two of good quality protective paint. So a waterproof, birch plywood mounting kept under a cover should last for as long as I will ever need it. 

Large and thick metal flanges would join the shafts directly to the plywood boxes to avoid flexure in these most highly loaded areas. I have some 180mm, 7" diameter aluminium round stock just looking for such a useful purpose. The shafts could be slightly beveled and hydraulically pressed into the pre-drilled and turned flanges. Though consideration for bearing removal will have to be considered.

The saddle can be made to any desired length from some heavy, aluminium U-form girder hanging around in my metal stock collection. Inverted over the preformed flange and with standard hinged rings fitted on the flat side it will be as strong as any cast saddle.It will also reduce overhang beyond the upper bearing which is a disaster in some mounting designs. Every millimeter of overhang requires extra counter-weighting and introduces unsupported shaft to potential flexure.

I could probably source an 8" diameter flanged and galvanized pier pipe, as long as desired, from an architectural ironwork manufacturer's heap of discarded projects.

Click on any image for an enlargement.

*