2" shaft mounting Pt4 : The vital PA-Declination socket.

I was unable to buy QD self-tightening bushes from the bearing stockists. So will have to look elsewhere for those. Ever open to a challenge, the absence of ready-made bushes might just lead to a better way to join the two axes. I'm trying to imagine a way to bring all four studs into play to spread the loads more evenly. While simultaneously enjoying a high degree of easy disassembly without flexure. Some sort of solid block encompassing the entire declination assembly would be best. A 6" cube, with four, through holes for the studs and one for the declination axis, should work. The block will be clamped by nuts on the existing studs which could leave the skeleton design open to view. Whether it actually wants to be open is quite another matter.

Once an ideal position has been found for the declination axis junction the individual rods could be covered in metal tube or pipe.

Am I throwing out the potential for stressed skin reinforcement of the axes assemblies by not covering them? I really don't think so. This would require the assemblies are actually able to bend under the likely loads applied by the telescope. Which are mostly static and minuscule compared with the capacity of such massive shafts, studs and bearings.

The weakest point is always going to be the "socket" where the polar axis fits the declination axis. Each set of four studs, bearing flanges and the shafts are arguably stiff enough as a unit provided the two axes can be joined in a way which denies any local flexure. Meanwhile, the 2" declination shaft blocks the PA shaft from passing right through the Declination assembly. So I need to use every millimeter of possible socket depth to good effect without direct contact occurring between the two shafts.

Where to find a [minimum] solid 6" metal cube? This is going deep into Unobtainium territory! It would also be far too large to swing eccentrically in my lathe for boring the very deep through holes square to the cube. Any collection of parts, to build the desired cube, would need to ensure the socket itself was made of hard and inflexible metal. This area must be a really solid part of the entire built-up unit. It must also be slotted to allow clamping around the PA shaft to permit easy, later disassembly and reassembly.

Birch plywood could easily be laminated into a suitable cube but [I think] it falls down on the ideal stiffness around the PA socket. Drilling the individual laminations for the rods and declination shaft would be easy enough using the pillar drill and a simple jig for accuracy. Solid, laminated plywood is easily stiff enough for the cube when further loaded by compression from clamping nuts [or load bearing plates] on the declination studs. The block would also further stiffen the entire declination assembly against bending or the threaded rods rotating around the shaft.

I keep coming back to the PA socket problem. There is no point [at all] in building a massive mounting like this if the socket flexes even slightly under local loading.

Perhaps a much large diameter, thick wall, metal tube could be fitted into the plywood block? A solid brass socket of greater outside dimensions would work. It would spread the loads into the plywood cube thanks to the increased surface area. Pressure per unit area could drop compared to a plain 2" hole bored straight into the plywood. The plywood cube could be turned on the lathe face plate to ensure squareness and a really accurate fit of the brass PA socket. The brass socket ought to be split on one side to allow a clamping effect via a bolt. The brass would not corrode onto the stainless steel shaft as it might well over time with a steel, stainless steel or an aluminium socket bush.

Extending the plywood block downwards towards the top PA bearing would allow a greater depth of socket. The measurement from the edge of the bearing flange to the shaft is only 45mm or  1.75". So the plywood cube wants to be bigger overall to extended the socket's depth. The block must be made at least made larger than the bearing flanges alone. This will help to move the compression loads from the thinner edges of the bearing flanges away from the more vulnerable and compressible edges of the plywood block. 

Area of the socket wall in the plywood assuming 2" axle depth. = 2" x Pi x 2 = 6.3 x 2 for the bare shaft = ~13"^2.

The maximum possible bush diameter is only about 90mm to clear the studs. 90:50 = A ratio of 1.8:1.

Not a dramatic increase but probably worthwhile and providing a far more robust socket for very little extra effort. Let us assume a socket 3" deep: 6.3 x 3" = ~19"^2.  90mm = 3.5" x Pi x 3 = 33"^2.

The studs must obviously not protrude into the volume required for the bush itself. [See images for actual dimensions.] If the socket was turned from a larger diameter stump of brass then a flange could be provided in situ to help spread the end loading into the plywood block. This would avoid the bush ever breaking through to rub on the declination shaft over time. If the original bush material were of suitable diameter then a much larger flange could be incorporated into the design.

I do have some 7" aluminium alloy bar which could be used for this very purpose. Though I'd prefer not to be buried under a huge pile of swarf by turning a "top hat" shape out of it. A 7" disk could be set on edge and bored to match the 2" PA shaft on a radius. This needs considerable further thought.  Perhaps two plywood blocks could sandwich a thick disk of aluminium set on edge to provide the hard socket material? Perhaps a simple brass socket bush is best with only a modestly wide flange. Again that removes the option to use a flange [plate] as a bearing surface to reduce flexure at the PA-Dec junction. Perhaps with PTFE/Teflon as a low friction interface material. Too many options for a hasty decision.  

I have been back to buy another length of polar axis shaft but chose 60cm, 24" long this time instead of the previous 40m/16". The new image shows 15" between flanges with plenty of overhanging shaft both ends. It occurred to me that I needed more space for the thickness of the large wormwheel flange as well as greater spacing between the bearings.

There is also the problem of the RA wheel's large diameter causing collisions with the telescope. Placing the wormwheel at the bottom end of the PA shaft makes good sense but it will be more vulnerable to attack when the telescope is moved about the sky.  Placing the wheel between the PA and the declination junction looks neater and provides more clearance for the telescope tube. I doubt that torsion effects in a short, 2" shaft really matter. So that does not enter the wormwheel position choice. I would like to place the worms on the side of the bearings so slow motion extension shafts can reach the eyepiece. The worm housing will add extra width which must not contact the telescope itself.

Both axes assembled for a quick photo between thundery showers. Shaft lengths are 24 and 32" with the bearing assemblies so far weighing 37 and 45lbs respectively. The finished distance between the bearing flanges will depend how much shaft overhang is required by the wormwheels and PA-Dec and Dec-saddle sockets.

Making the socket bush much larger in diameter might weaken the plywood block by reducing the containing wall thickness locally. The existing studs could be used to compress a slit in the block to coincide with a slit in the brass bush. This would avoid having to add a separate compression cross bolt and kills two birds with one stone. The orientation of the plywood laminations needs some careful thought. A vertical, multi-layer sandwich makes most sense to allow each lamination to be separately drilled for the studs and declination axle. No other orientation allows easy drilling over such a depth. The downside of this is the high visibility of the laminations. Though a thin, aluminium covering could be used if the laminations are not "pretty" enough.

Heavy, load-spreading roofing washers [or thick aluminium plates] are assumed to butt against the plywood block while the nuts on the studs are used for block compression. Otherwise the plywood block will probably compress over time under the considerable, local pressure applied by the nuts. Which can easily amount to several tons per square inch. A slit in the block should also compress the plywood tightly around the metal bush to achieve greatest overall stiffness. The brass socket would then become a solid part of the block and thence the PA shaft. While the socket flange would protect the plywood when placing the heavy declination axis assembly over the exposed polar axis during assembly. Though even this could be avoided by assembling the various declination parts together once the socket and block are safely in place on the polar shaft. The declination assembly must not be allowed to rotate on the metal socket or there will be no RA drive!

Having discussed the socket to death: The problem with a turned socket is that it is not a tool-free option for most telescope mounting builders. I am going to have to find a serious alternative which is readily available in different sizes from the everyday engineering world. The problem is the huge range of prices for clamping bushes. Something as simple as an exhaust pipe clamp won't provide a guaranteed square [perpendicular] mating surface. I think that is going to have to come from a flanged bush.

This is a Tollock locking bush from RS Components. I searched their UK website to find these. Then used the Tollock name and serial number to search the Danish website. £20 [equivalent] +VAT + P&P isn't the end of the world. Downside is the need for screws in the face which ideally which wants to attach firmly to the Declination housing. Presumably the screws  are not captive so could be replaced with longer ones to capture a larger adapter plate of some kind. The plate is then fixed to the Dec housing or to a very large dovetail strip. In the 50mm bore size the outside diameter is only 90mm.  I see that as a potential Achilles heel and would prefer a larger "flange." I'll keep looking for a larger version if one is available.

This is crackers! After searching for other bushes I returned to the original Tollock page on the RS website and the price has just doubled! The same has happened to the RS UK price when I go back to their English website. What is going on?

I thought of standard pipe flanges but that would require welding or thread turning and then facing on a lathe for accuracy. Shame that QD bushes aren't readily available as they are elsewhere. [US & Aus.] Does the American market have a monopoly of wholesalers who won't sell to the general public and hide their prices from non-registered customers? I can understand their reluctance to waste time on individual sales in small numbers requiring expert support. I just wish I knew which of the multiple names for clamping bushes [in Danish] would find what I am looking for. There seems to be no standardization between suppliers!

Click on any image for an enlargement.

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