2" shaft mounting Pt 22: Wooden pier.

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A wooden pier made from 4" square pressure treated timber can be bolted together. Wood naturally damps vibrations and needs no maintenance. Those with deep pockets can choose green or even seasoned oak and pretend they have a Great Dorpat Refractor. We have a local timber yard which offers large sections of oak to feed the half-timbered, thatched houses market. Curved braces to copy the Dorpat refractor may even be available. For the height needed [about 6'-7' or 2m] the cost is a fraction of wimpy, commercial, metal offerings which never attempt such pier altitudes anyway. And could never support a heavy, classical refractor in the first place. The height in the SketchUp drawing is truncated.

A four legged pier is far more stable than any tripod or other three-legged arrangement. Only on concrete will an infinitely stiff four legged structure rock. A wooden pier is not infinitely stiff and has easily enough mass to avoid any tendency to rock. On a lawn or gravel it will sink slowly under the combined weight until the pressure under the feet is more or less equalized. If it should list over time then it can be lifted back to vertical with a lever or crowbar and packed under the sunken foot.

A wooden pier offers advantages which a large steel pipe does not, except perhaps, cosmetic appearance. Stability from a large footprint is no hindrance except for viewing directly overhead. The large area of the top of the four posts avoids the necessity for mounting spindly mounting adapters. It provides an excellent match for the intended triangular pyramidal base I am planning to use. Adding heavy timber bracing to the heavy mounting is simply a matter of cutting suitable sections and bolting into place. Galvanized threaded rod [studding] is probably cheaper than long coach bolts in the likely lengths needed. If necessary the whole thing can be literally taken apart with a single spanner [wrench] and rebuilt elsewhere.  The image above shows the compliment of the 55° PA altitude angle does not suit the triangulating props at 35°. I need to add channel sections first. Then bolt the props to those to bring the props far more upright. First buy a 16mm drill!

I keep thinking that an adjustable height pier would be incredibly useful. Not least for mounting and dismounting a long and heavy OTA. I have looked at counter-weighted, parallel-paired, parallelograms but a counter-weighted see-saw arrangement might be easier to achieve and use in practice. Moving the heavy counter-weight between observing sessions might be quite a hurdle unless it could be easily spun on a threaded rod. Not impossible as large stud sizes are readily available in timber merchants. The counterweights could also be duplicated to halve the immediate workload.

The see-saw beam and pier would both have to be very substantial to avoid any flexure. A scissors jack could be used to raise and lower a pier telescopically but it has severe limitations on maximum vertical movement. Not to mention slop in the 'telescopic' arrangements. A lever arrangement could increase vertical range at the expense of heavier loads on the jack handle.

A long reflector and classical refractor have such extreme opposites in their needs for pier height that they almost preclude each other. It is difficult to see how the [very heavy] mounting can be easily and quickly moved between piers of only two feet high and well over six feet. The sheer weight of the mounting does not make the task appealing. Perhaps the mounting could be attached to a sled which rises and falls under the control of a block and tackle along a sloping pier?

Oddly, the two instruments are very similar in both length and breadth. Except hat one is viewed from up near the top and the other from the bottom. The arrangement shown is simple to achieve but has a massive overhang requiring equally massive counter-weighting. Having each instrument at the opposite ends of the declination axis means they would balance each other at not much extra weight.

Parallel mounting of two such instruments on the same pier makes little or no sense. Even though the reflector could be heavily top-balanced to allow a tall pier, the mass required could hold a lot of heat. Which is likely to be released into the light path for literally hours. The alternative is [perhaps] to arrange a mid-height pier but allow large differences in observer viewing height. A platform or stepladder for the reflector and an adjustable height seat for the refractor?

A more serious arrangement might be a car port to protect the instruments and refit the optics for observing sessions. Though I'd much prefer the instruments to be permanently set up ON TOP of the carport as a raised platform. Then all I need is an observatory to protect the very exposed instruments from storms and the elements. And, all on top of a very tall pier? Better, I think, to have a cross-axis mounting supported independently of the platform.

The observatory would have to be highly wind proof. It must also ensure against any chance of the observer falling from a great height when using a stepladder while observing with the 10" f/8 reflector. This is all getting very expensive and cumbersome and requires an awful lot of heavy woodwork before building the raised observatory is even considered.

I keep thinking a miniature "Nissen hut" could roll back on rails over the roof of my existing, pitched roof workshop. Such, tunnel shaped structures are readily available in all sizes for protecting farm animals. Or, a semi-cylindrical, roll back roof could protect the instruments with them both permanently mounted on a tall [long] cross axis. This would leave walls of a suitable height to protect me from the almost incessant wind. The degree of roof roll back could be adjusted to provide reasonable protection from the overhead projection of the 'barrel' roof. A semicircular, southern gable end could be folded down with a hinge at side wall height.

A much cheaper alternative would be a cross-axis mounting built onto a triangular timber 'trolley' with large, free running wheels. This could be wheeled out from under a protective cover and moved to clear sky. Not ideal from a number of standpoints. Not least rapid polar alignment and a lack of shelter. Then there's the matter of providing power and cables for computers for imaging and power for the drives. Never buy a garden with the house along the southern border!

Click on any image for an enlargement.

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2" shaft mounting Pt 21: PA disk bearing and support.

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I decided to use the Tollok bush in anger to turn the cylinder smooth all lover.  The jaws of the chuck were getting in the way. This worked a treat until I wanted to remove it all from the length of shaft. Whereupon I discovered the trick is to completely remove all of the "pull" screws before fitting the "push" screws to separate the coned flanges. I ended up with a serious tug of war between the two! I shan't make that mistake again.

The sum total of my work on the lathe produced a packed, pillow case sized bag of aluminium swarf. The image below shows the cylinder and boring bar arrangement. The cylinder now has another step to sink the Tollok bush almost flush. The cylinder is now almost the complete depth of the Tollok bush with only a thin wall remaining at the rear. I really must come up with a better name than "cylinder." PA/Dec joint? That's just as clumsy.

The final picture shows a finish I can live with.

This image shows the real [24"] Polar Axis propped up on a batten. The scale of the cylinder is now much more in proportion with the mounting.

As son as I had it set up for the photograph I immediately realised that I had no need for a full plate bearing beneath the turned cylinder. I just need to cap the tops of the studs with little plastic cups to have an immovable base for the cylinder to rub against as the PA linear thrust bearing. These pads would be the equivalent of a solid disk [bearing] with added low friction pads. Just as Dobsonian ground boards and rocker boxes have no real need for complete sheets to form their base bearing, nor does my cylinder and PA. All I need to do is arrange the tops of the PA studs to be level with each other before adding the little caps. Easily achieved with feeler gauges or even by eye alone. Note how close the studs are to the periphery of the cylinder. You couldn't hope for a better situation for the supporting pads.

Then there was the matter of applying a reinforcing plywood box to make the PA bearing housing. What if this box was more decoration than necessary? Could the studs themselves provide all the strength and stiffness required on their own? Note how the flange bearing orientation is still reversed to match the earlier, compact cross-axis design. This saves a small amount of unwanted overhang beyond the top PA bearing. Though the cup bearing pads will negate any worries on this score. Any flexure, however unlikely, will be strongly resisted by the pads. There is no bare shaft to flex between the top bearing and the cylinder anyway.

I now need a solid way to carry the loads from the mounting base into the PA studs and flange bearing 'unit.' The bottom flange bearing could be bolted into a wedge shape using longer studs. It would then become immovable. I now need a way to bring a solid support up from the base to the top of the PA to resist flexure. Any flexure will be from side to side [torque] and up and down. [Sag] Or any combination of the two. The four studs and flanges provide for a nicely rigid form to linear loads of compression or extension. If I can raise struts from a broad base I will have a perfectly inflexible triangular pyramid. [Or even a tetrahedron] The leaning battens are only a placed there as suggestion of the likely form. There is no reason why my earlier ideas for wooden pier would not suit a pyramidal base. With the exception of the PA cylinder the entire thing would still be a nuts and bolts project.

After waiting for 5 weeks for my wormwheels to arrive it seems they have been made with too small a bore size. I will now have to wait for another 5 weeks for new ones to be made. Even if I had a lathe remotely large enough, it is impossible to maintain the concentricity if the wormwheels are re-bored larger. The wormwheel "teeth" are cut with the wheel and its boss on a mandrel between centers. Any variation in concentricity will cause fluctuations in the telescope's drive speed as the radius of operation changes. I certainly cannot reduce the diameter of my shafts to match the much smaller bores. The whole point of building the new mounting was the increased size of the shafts.

Click on any image for an enlargement. 

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2" shaft mounting Pt 20: Round and round. PA/Dec connector.

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Having ditched the idea of a simple, bolt-together mounting I spent the day on the lathe turning a big lump of aluminium. I just happened to have a suitable 'slice' of the 7" diameter bar of about the correct depth just asking to be used for something.

This cylinder will sit on top of the Polar Axis shaft to act as connector and a reinforcing disk [or plate] bearing. With the Tollok locking bush hidden inside the cylinder holding everything together.

When the 10 screws are tightened, two steel cones are drawn into each other. The outer cone expands as the inner one contracts.  So the large lump of aluminium is gripped firmly from the inside. While the stainless steel polar axis shaft is simultaneously gripped equally firmly by the inside of the Tollok bush.

Turning the aluminium was a fairly slow process. It was at the limit of my 9"x18" lathe using the 4-jaw chuck. The surface finish is crap in places due to material build up on the tool tip. I need to grind a new tool to make a better job the surface finish. The ceramic insert bits I tried were unsuitable for aluminium. I ended up using a long boring bar with a ground tool tip for most of the day. Even using it to turn the outside of the cylinder. Nothing else would reach around the sides. The tool post collided with the face of big lump if I tried to get it to pass beside the big cylinder.

At the very end of the day I tried a spray of penetrating oil as a cutting lubricant but it didn't do much except for the polished tracks on the outer front face. Not pretty. I am highly allergic to WD40 which I have seen mentioned online as a cutting fluid for aluminium. WD40 brings on a severe asthma attack and I have never had asthma!

I was using my slowest direct drive speed to put it in the middle of the suggested cutting speed in Fpm. Experimentally increasing the speed just caused more buildup on the tip of the tool.

Long bolts or studs will hold the Declination assembly to the top of the cylinder. It looks large and rather clumsy at the moment but is only a little deeper than the Tollok bush. I didn't want to reduce the cylinder depth to an absolute minimum until I had finished the boring to match the Tollok bush.

At present the bush sits deep enough to bring the bush's second [expansion] flange slightly below flush with the cylinder face. I could continue deepening the two steps of the cylinder bore until the flange with the screw heads becomes flush. The cylinder would then be of the same overall depth as the Tollok bush. Which would reduce the cantilever of the Dec axis to the minimum.  There is still a little depth of material [12mm, ½"] left at the rear where I originally through-bored at 50mm to match the PA shaft.

This image shows the 50mm, 2" shaft passing right through the bush and aluminium cylinder for scale. Though the shaft won't actually exit on this side. There is probably no good reason for it to project beyond the top of the Tollok bush. Nor is there any real need for the bush to be sunk further into the PA/Dec connection cylinder except for appearance. Only the second flange expands to grip the cylinder.

Now I will have to find a longer length of tool bit to fit the boring bar to tidy up the appearance. I inherited a tiny length with the boring bar. I have just read of kerosene being used as a lubricant. I think this is known as paraffin in the UK. Domestic heating or lamp oil is the same stuff. I'll give that a try. With my slowest feed it is taking half an hour per finishing cut across the 7" face! Odour-free, lamp oil certainly helped the finish but the tool still needed to be sharp with no chatter. Not easy to achieve with such a long overhang of boring bar.

Click on any image for an enlargement. 

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2" shaft mounting Pt 19: Yet another rethink.

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While I have been waiting for the worms and wormwheels to arrive I have been drawing and thinking hard. Without knowing the thickness of the wormwheel on its boss I could make no fast decisions on where to place them nor how much room they needed. 

I was not very happy with the very deep Polar Axis framework in the previous design. [Pt.13]  The bearing support arms were so long that they provided a lot of leverage. With the likelihood of increased flexibility unless built massively. So I decided to reduce the depth of the 'U' shape considerably. Which didn't leave any room for a counterweight for the offset Declination assembly to pass through the PA frame.

The answer, of course, lay in having a weight extension arm clamped above the PA support frame. The RA wormwheel could also sit above the frame. Allowing an even shallower frame. The downside to this layout was nothing would be able to pass inside the PA frame. The limit, before a meridian flip was required, would be when the Dec housing hit the PA frame.  

I was tempted to saw up some slotted angle iron to build a trial PA frame. Then thought better of it and decided I'd use glued up 2x4's for the prototype. Or I could bolt together some 2x4 channel to make 4x4 'I' section.

The ideal arrangement for all sky access and compactness is a conventional German mounting. The Polar Axis 'skeleton' is shown alongside propped up at approximately the correct 55 degree angle. The distance between the bearing flanges is about 50cm.

Doubts were cast by one person on the CN discussion thread  on the stiffness of the flange which would join the Declination assembly to the Polar Axis. This criticism had seriously undermined my confidence in the German mounting due to all the weight being cantilevered off the top of the PA shaft. Subsequent purchase of the Tollok locking bushes has helped to rekindle my faith in the German mounting's potential. The German mounting is by far the most popular design for most classical refractors.

I have some 180mm, 7" diameter, alloy bar. A thick 7" diameter disk could provide a large surface area if the Tollok bush was expanded within it while simultaneously locking itself onto the PA shaft. The flange would rest on top of the RA wormwheel. With a further flat surface on top the PA housing. Though the wormwheel is not essential and could be placed elsewhere. The top disk would provide a larger surface on which to mount the Dec assembly. Teflon/PTFE would be useful between the surfaces of the disk to reduce friction. The Tollok bush has to be placed there anyway. So any worries about increased overhang are moot. The Bush can be sunk within the top disk to reduce any added overhang.

The images show a full sized mock-up of the conventional German mounting arrangement. The tube rings shown are for a 20cm [8"] diameter, main tube. The saddle is 70cm, 32" long. As is the Declination axis. The PA is 60cm or 2' long with 50cm, 18" between the polar bearings. The battens are just for temporary support for the picture. The PA Tollok bush [just visible between the Dec studs] would be buried in a thick 7" disk for reinforcement. By good fortune I obtained the massive lump of aluminium bar for small change at a flea market. Slices just fit in my 4-jaw chuck. 

After struggling with countless SketchUp drawings I have given up on trying to make a compact cross-axis design. They are too limiting of  the ability to see much of the southern sky without a meridian flip. No matter what I tried the declination assembly struck the Polar frame before the tube had managed much more than 90 degrees of RA movement.

A full cross-axis mounting needs towering support pillars and a huge polar axis for a large refractor. I did some mock-ups of those with lengths of timber and it would need to be absolutely colossal to support an 8' long refractor!  The telescope tube still has to be mounted as high as a German mounting to ensure comfortable overhead viewing. The main advantage I can see with the full cross-axis is isolation of the supports from a raised observing platform. For which it would be almost ideal. Long lengths of 10cm, 4" square pressure treated timber are available at modest cost. This would be far easier to build than a towering concrete pier which would need isolating from the platform.  

2" shaft mounting Pt 18: Plywood axis bearing housings.

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Let us assume that the plywood axis housings need be no larger than the outer dimensions of the flange bearings. Galvanized studs [all threaded rods] will join the flange bearings and compress the plywood structure between them. The heavy steel studs will add their own stiffness at some distance from the center of each axis shaft. Size matters where flexure is to be avoided. Depth is far more important than mere width.  Which is why rafters and joists are always set on edge to the applied loads.

Strips of birch plywood will be glued and stacked between the studs while leaving open channels for the studs and shafts. The diagram shows the basic idea without particular reference to scale other than the base image of the flange bearing as a guide. The square void in the center could be reduced further provided the shaft does not rub on the plywood. Quadrant sections could fill the corners but would not provide much extra strength being so close to the axis.  

If an aluminium plate was placed against the inside of each bearing flange it would spread the end loads on the plywood much more evenly. Thereby bypassing the voids in the flange casting recesses. The nuts on the studs can add considerable pressure so local destruction of the plywood must be avoided. Which is why I have made the plywood as solid and as complete in overall 'useful' area as possible.

Further layers of plywood could easily be added to the outside of the structure shown above but would not contribute to compression resistance unless the spreader plates were made oversized to match. Where does one draw the line? Plywood is quite cheap so adding layer after layer is easy enough. Does extra bulk add anything to the overall stiffness of the "box" structure? Being so far from the axis it should have considerable extra 'moment' over the inner layers for structural gains.

To maximize the extra support for the bearings perhaps a plate of plywood should be added to the inner sides of the flanges rather than aluminium? This would be glued to the laminated structure between the plywood plates for an even more homogeneous mass. This would help to resist the compression and bending loads over the entire and much larger unit by carrying the applied bearing loads more evenly into itself.

The image shows the basic dimensions of the inner face of the flange. If it were left bare [i.e. without a load spreading plate] the plywood edges would be crushed first where the flange is machined flat. But left completely untouched where the casting moulding is recessed. The recesses could be filled to spread the compression loads more evenly. A "rubbing" on paper could be used as a pattern for thin plywood to increase the active surface area for very little effort. 

It looks from the flange dimensions as if the required thickness can be made from three layers of 15mmm birch plywood. These would just reach the outer limits of the bearing flanges and just clear the shaft. If four layers of 12mm plywood were preferred then localized 'woodworking' could solve any shaft or bearing clearance issues.

Making the overall structure any larger demands oversized end plates. Though I am trying to resist having 'naked' edges to the plywood end plates. It would look much neater to have the sides overlapping the end plates for a more uniform appearance but completely ignoring compression resistance. A thin, aluminium "wrap" would easily enhance a roughly finished plywood block if an easy cosmetic skin is desired. Far easier than trying to sand the 'lump' perfectly smooth and square all over!

The entire structure could be fitted inside an 8" [PVC drainage] pipe for weather protection and an even smoother appearance. Drainage pipes have standard end caps which could protect the bearings with only a suitable center opening cut for the axis shaft to exit. The plywood structure could even be built up in extra layers. Perhaps until the plywood cylinder completely filled the 8" pipe if this was thought desirable for structural reasons.

The images show the simple process of making a paper pattern by 'rubbing.' The pattern was then cut out and glued to the 6mm thick, flange packing material. In this case a cheap plastic cutting board as I had no 6mm plywood handy. I then sawed around the pattern with a hand fretsaw with coarse omni-directional blade. The power jigsaw immediately welded its blade to the plastic regardless of using a very low speed or choice of metal or wood cutting or oil.

A little filing to "tidy up" the edges resulted in a solid surface instead of a large recessed area. The packing piece will never be seen so absolute accuracy was not important. [Interestingly[?] the pattern was handed and could not be rotated 180°] I think you will agree that the packing increases the flange's active area considerably and will help to spread the compression forces more evenly. Anyone who cares can overlay a grid to calculate the exact increase in area. At a glance, I'd say the area has nearly doubled. All of which helps to protect the end grain of the plywood laminations under compression.

The diagonal measurement of these particular 50mm flange bearings is 188mm. This will just fit inside an SN4 200mm PVC pipe.  But not the SN8 because of its greater wall thickness. Variations in flanges for different duty loads and by different manufacturers are available. A pipe would set the plywood size limit on a round form. Otherwise the corners would extend well beyond the near 8" diagonal of the bearing flanges unless rounded off. While a thin, aluminium covering can be any size and shape you choose.

Click on any image for an enlargement.

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2" shaft mounting Pt.17: Saddle and bush joined at the hip.

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The M8x25mm zink coated screws are only temporary.  I have some A2 stainless steel, socket head screws and large, thick washers on order. These will ensure a lack of flexure of the channel material at the bush face.

First I spotted through the holes in the larger bush flange with a felt tip pen held vertically. Then I used a compass and rule to confirm the hole's exact location and centration before carefully center punching. Note the witness mark to place the slot correctly each time.

Once the holes were bored out to nearly the correct size I bolted the flange into place through two of the holes. Then ran the finish drill size through the flange holes to true everything up.

The bush was less than 2mm too wide to go easily into the channel. So I used an angle grinder to put two, small opposing flats on the largest flange. A file was used to take off the sharp edges. The smaller flange fitted with plenty of room to spare. The next image is a posed picture with a length of 50mm stainless steel shaft in place in the locking bush socket. No attempt has been made to torque up the temporary screws.

I think you will agree the arrangement makes a very neat and secure job. The bush will be protected to a large degree by the channel section saddle. With the tube rings mounted on the flat face of the saddle's channel section there will be a minimum of overhang beyond the end of the Dec shaft.

Chocks away?

It seems I must try elsewhere for my stainless steel screws. 44 miles later and I have fetched the screws I needed on my touring trike. I found a large discount store in the city stocked the screws and load spreading washers. The even larger competitor next door had large, empty gaps in their stock display precisely where I might have had to rely on them.The first dealer only had M8 in 30mm which proved to be more useful than 25mm if I am going to plate over the saddle. The square washers I bought proved to be much too big to fit between adjacent screws. So my inability to obtain the original screws and larger, round washers proved to very useful.

The saddle looks smarter with a slight taper to the sides Here I have used galvanized washers but they aren't ideal and could be slightly larger with a smaller bore and preferably in stainless steel.

I am now leaning towards adding a top [and bottom?] plate to reinforce the channel section saddle material at the point where it is most likely to flex against the bush's flange. The ten screws and the face of the flange can do only so much with 5mm, 3/16" thick aluminium. Though any top projection on the saddle might limit clearance with the OTA. Steel has three times the stiffness of aluminium. So, where clearance is an issue, steel makes good sense.

However, unless I use stainless steel there will be a potential rust problem. Stainless steel is much harder to cut and drill with normal [amateur] equipment. Drills without a really sharp edge will tend to rub and work harden the steel. Tripling the thickness of aluminium plates will increase the stiffness at the bush flange. I could use an inverted length of channel section on top with the original with the webs cut down to allow OTA clearance. Or I could add a little packing under the tube rings if necessary. The channel section will not fit inside itself of course. So a simple plate would be necessary to go between the webs if I choose reinforcement internally.

Click on any image for an enlargement.

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2" shaft mounting Pt 16: Tollok locking bushes.

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The arrival of my Tollok TLK110 50/65 locking bushes has at last provided me with a firm mounting for the large saddle.

RS.dk accepted my order but the items were sent over [freight free!] from GB overnight via DHL Express. Tuesday delivery is excellent service for items ordered online on Sunday afternoon in Denmark! Full marks to both companies.

By tightening the multiple M8x21mm, socket head screws the two conical components are drawn tightly together. Since the outer, split component, with flange, cannot expand due to the slip ring [or fitted component] the inner cone must shrink. In doing so it grips very firmly onto the shaft without the need for key-ways, split pins, adhesive or grub screws. A fitted component would enjoy a matching degree of expansion inside its stepped bore. The machining quality and finish for what is an industrial component is really first class. Probably from tumbling in a gravel filled vibrator after CNC machining.

I could carefully mark out the 15 screw positions on the saddle by spotting through the larger flange. Carefully ensuring the correct orientation because the screw holes appear slightly different in spacing viewed from front to back. Though this may well be an illusion caused by the slit. Then I'd use the fixing screws to mount the saddle firmly to the shaft. The band should ensure sufficient retention but I ought to seriously consider turning a sleeve to share the load evenly along the shaft and the bush.

My original idea of trapping the saddle between the flanges via a large hole might be the best theoretical arrangement for stiffness. The flanges would provide even support from both sides. But might easily cause flange lock up before the cones were fully tightened together. Or, the flanges might not meet tightly enough to ensure a proper grip. I'd still need to drill all fifteen screw holes into the bargain. Preferably with slightly oversized holes to avoid circumferential tension as the two component parts shrink and expand differentially as they tighten together. After careful thought I think the best way to use one of these bushes to hold the saddle is to use slightly longer M8 25mm screws with large washers through holes drilled in the saddle. Though I could turn a ring to match the larger flange to 'sandwich' the saddle's channel material. This would avoid any risk of compromising the opposed taper action of the bush or a loose saddle. Tens crews[with washers] spread over such large area is probably as good as it needs to be.

 Here I have separated the components to show the apparently simple, dual tapered mechanism involved. No doubt a great deal of careful research went into optimizing the very large range of sizes in each model.They are available from the dinkiest of sizes right up to the absolutely huge!

The five empty, threaded screw holes are for separation of the cones for easy removal of the bush or its supported component after tightening. The fixing screws are loosened or removed and then five of these same screws are driven through the threaded holes to push the cones apart.

General view showing the split components and the deliberately loose, outer expansion/compression band.

The bore is 50mm in this case with a 65mm outside diameter. While the flange is a very handy 92mm in diameter to provide a firm base for a connected plate or component. The retaining ring is usually removed and the bush used to expand inside the stepped bore of a carefully dimensioned component, usually a pulley or sprocket but not exclusively. Bearings can also be mounted.

A perfect fit on my 50mm stainless steel shafts! No welding, grub screws or Loctite required. Just a hex key to obtain a firm and self centering grip on the shaft simply by tightening the screws alternately to a torque of 41nM or 30.2 lbs/ft. [NB: This torque figure is ONLY for the TLK110 50/65. Other sizes and models have their own torque requirements! Do not use any information here to employ a locking bush for any purpose. Do your own homework using the manufacturer's own literature or direct contact with them.]

The bush should be lightly oiled before use and the screws should be tightened evenly in a criss-cross pattern: 9-3-12-6-2-8-4-10-1-7-11-5, etc. If repeated removal of the bush is expected then the screws can also be oiled.

Tollok's English language catalogue: Catalogo TOLLOK inglese.pdf

The TLK110 details are on page 6. Note that the bore dimensions of the component to be fitted to the bush is vitally important to its designed grip.

The supplied expansion band would normally be replaced by a pulley or sprocket. Since the saddle channel is only 3mm [or 1/8"] thick I had better not try expanding inside that. Though a thicker collar could be turned to fit over the bush to supply the necessary resistance and even give a larger fixing plate for the saddle. A simple tube used in unison with the supplied band/ring would also work in my intended method of fixing.

Longer M8 screws are easily obtained if I decided to use these for holding the saddle with load-spreading washers. This would give the lowest profile fit for the saddle without visible protrusion on the upper face where the tube rings will sit. Since the bush does not move axially [along] the shaft during tightening there is really no need for the mounting shaft to protrude right through the saddle surface. Except as a visual check for monitoring the shaft's precise location. A smaller sight hole would do. Better than having the telescope tube [OTA] suddenly detach without warning having sneaked along the shaft unseen! However unlikely as this might occur in practice. This will depend on the exact details and care taken during fixing.

These locking collars might look relatively compact but a YouTube video shows some really hefty chain sprockets being attached with them! Which means they can cope with some seriously heavy torque and axial thrust.

Click on any image for an enlargement.

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2" shaft mounting: Pt 15. A 70cm saddle.

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While I wait for news of new flanges I have started work on the saddle. I intend to use 10cm, 4" x 2" channel section aluminium. Used inverted I shall line it with laminated birch plywood for extra stiffness. Some sort of flange will be vital to ensure a flexure free connection with the Dec shaft.

For scale, the rings are about 9" in diameter. The channel section is 70 cm or about 27.5" long. This length suits both the straight and the folded OTAs.

Another image showing how the saddle channel will be tapered towards each end.

Hopefully I shall be able to use an electric jigsaw to speed up the process. Provided the cut and the blade are kept well oiled a jigsaw, fitted with a metal cutting blade, works well on aluminium.


I am so sick of waiting for an intelligible response from the flange dealer that I am going to take the compression bush route. RS.dk sells a 50mm locking bush for about £60 equiv. This uses opposed cones and screws to tighten itself firmly onto the shaft.

I need a strong connection between the saddle and the end of the Dec shaft. Even if I waited for the flanges I'd probably need to have them welded to the shafts.  Even then I'd need to cut two 'ears' off the heavy stainless steel flange to fit inside the channel. Welding takes it beyond the unskilled, self-assembly brief I have set myself. The difference in price for the bushes compared with the larger flanges is peanuts.

The tension bolts can be used for saddle retention to the 92mm diameter head. By a happy coincidence, 92mm matches the internal breadth of the channel section saddle rather nicely.

By using all the tension bolts with load spreading washers I should obtain a seriously stiff connection. It may be that the ring flange is supposed to sandwich the intended load. Which is all to the better. The tapered channel section can be tightly packed with birch plywood to further stiffen it. Channel section is weaker in torsion than box sections or pipes. More to follow when the bushes arrive.

Click on any image for an enlargement.

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2" shaft mounting Pt 14. Ongoing.

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It occurred to me that the studs [screwed roads] could be interlaced to bring the axes much closer together. It is obvious that the shafts cannot physically pass through each other. So some degree of offset is essential to maintain long shafts and wide bearing spacings.

A 1/8" spacer would just fit between the shafts but this set-up was too unstable to photograph. Tying the interlaced studs tightly together confirmed a 3mm or 1/8" clearance between the two shafts and between shafts and studs. It would not be too difficult to overlap strip laminated plywood housings to cope with this interlocked stud arrangement.

It might make an attractive "skeleton" mounting if the studs could be left visible but covered in alloy tube. The axes shafts could also be covered with a larger tube. Though built-up plywood housings probably makes much more sense from a stiffness point of view.

The large journal bearings have a spherical outer race allowing them to effortlessly self-align. While any linear thrust loads are naturally resisted by the lock between the bearing race and its housing.

I continue to work on drawings in SketchUp to try and improve on the basic design. The ability to rotate and orbit the drawings is very useful for ensuring safe clearances as the mounting axes are moved around.  

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2" shaft equatorial mounting Pt 13: Back to basics:

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The need to unload the single flange supporting the entire weight of the OTA, counterweights and Declination assembly required a rethink. I have now placed the loads between two flange bearings. The plywood laminations under compression from studs [threaded rod] pressing the flange bearings together is back on the drawing board.

Here are couple of simple drawings using SketchUp for the first time:

The width of the Declination axis 'box' will be set to allow clearance of the OTA from the mounting and wormwheels. The Dec housing can be bolted right through the PA housing with studs to ensure maximum stiffness and strength.

Lead can be added to the rear of the PA 'T' form to balance the offset Declination axis and the weight of OTA. Though this will have no effect on the OTA's own balance point. The end to the 'T' must clear the mounting frame. I have set the drawing of the mounting to 55° to match my own latitude. A curved base to allow fine adjustment might follow.

In hindsight I should have built the detail onto the axes before adding the mounting and Polar Axis construction. Trying to add these details later was beyond my rudimentary skills with SketchUp.

I hope this new design lies well within the skill levels of any handy telescope maker. Hopefully providing a sturdy mounting capable of carrying long and heavy refractors without heavy expense.

More to follow when I had a chance to fine tune the design. I spent far more time undoing silly mistakes than actually drawing but it does start to go quite quickly with practice.
I can already see that the forward extended Declination 'T' can have the studs overlapped with the PA housing to reduce the degree of overhang. Though this might result in considerable complication in the 'woodwork' to achieve this.

A contact has kindly pointed out that during a meridian flip the counterweight must pass through the Polar Axis support frame. Here is a drawing with a clearance circle superimposed on the mounting. Shortening the declination shaft to match will require a much heavier counter balance weight.

Click on any image for an enlargement.

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2" shaft mounting Pt 12: Curvy alternatives?

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Yet another interesting idea has cropped up on CN. Instead of aiming for a traditional but boxy GEM I should consider curved forms. Building a mounting this way can avoid the serious cantilevering of the Declination axis, telescope and counterweights via a single flange and shaft socket. A large C-shape can provide polar altitude adjustment while neatly splitting the declination loads on the polar axis into two. 

Provided the curve is given an adequate cross sectional area it can be as stiff as aluminium. The image shows the basic idea. Thinner plywood strips are laminated over a former and clamped tight while the glue sets. Normally there is a degree of spring and the radius of the final result may increase. The flange bearings can be placed inside or outside the "arms". A bolt, fitted with load spreading washers or plates will clamp the curve securely to a suitably curved base.

The next step is to be equally adventurous with the design of the declination axis and saddle.

Here a similar, but tighter curve is nested within the polar axis curve to provide twin declination bearing supports. The downside of this arrangement is the severe obstruction of the telescope tube on either side of the mounting. To point at the Pole the telescope must lie alongside the mounting rather than above it. 

Since the two axes shafts cannot physically cross each other it is not possible to have the declination shaft perpendicular to the polar axis in the usual sense. [As shown by the red arrow] This would require a stub shaft and plate bearing impaled on the red arrow. A separate Declination counterweight shaft could be attached via a clamp but this adds bulk and complication. The telescope tube needs to clear the polar axis and large wormwheel. So the declination overhang/saddle must be extended considerably in the direction of the arrow.

The declination shaft could be offset slightly to the east or west of the PA shaft but this would introduce an offset force which must be counterbalanced. The offset force would also change with telescope weight so would need adjustable counter-weighting which is best voided. Just swapping a heavy eyepiece and star diagonal for a much lighter one might require re-balancing. Though a sliding counterweight could be fitted to the opposite side of declination shaft, from the offset, for rapid changes of lateral balance.

The lower image shows a circular declination assembly with a superimposed T-shaped box. All made form plywood laminations. The head of the 'T' provides the declination shaft housing and enough offset to avoid collisions between the OTA and mounting. The advantage here is that the declination assembly lies between the PA bearings and is evenly supported by flanges. So no stress raising cantilevering for the Polar Axis. The saddle is cantilevered but has a plate bearing formed by the wormwheel.

Click on any image for an enlargement.

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2" shaft mounting Pt11: When is a DN50 flange not a DN50 flange?

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The very pretty DN40-48.3 flanges turned up today but the bores were far too small at 42.4mm despite the flanges being marked 48.3mm. The 48.3 dimension refers to the exterior of the neck. Which I did not know when I placed my order.

Half the tapered flange neck would have vanished by the time I had bored the flanges out to 50mm. The next common size up is the DN50 which has a nominal dimension of 60.3mm. However, the bore is a couple of millimeters over 50mm at about 52.5.  I have emailed the supplier for further advice.

I have now discovered another flange which is much closer to a 50mm bore. Neck flange DN50-56.8 is sometimes listed as DN50-57. Sadly the supplier has been very slow to respond to my request to order new flanges and the return of the originals. It is just possible that this is due to the [unofficial] Danish national holidays so I shall just have to be patient. The problem is that I cannot make any real progress without them.

Many of the Danish suppliers seem to act as online agents for the main supplier, Indura. Unfortunately Indura will not deal with private customers via email. Presumably to avoid excessive need for technical support. The strange thing is that the main supplier has dimensioned drawings of their flanges on their website but never seem to quote the bore size. The precise bore size probably has little or no relevance to their usual customer. The standard sized connected pipe is welded over the extended neck and matches the bore size of the pipe itself.

But surely, denying the bore size to public view only increases demand for that information to be published? The mere fact that there are two different DN50 flanges suggests that there must be some demand for them both. Even if the DN50-60.3 is the "standard" variety the DN50-56.8 must have some purpose in order for it to be listed. A mismatch is obviously undesirable to avoid turbulence at a stepped bore.

Even if the final bore size proves to be 50.2mm then Loctite 638 may still not be stiff enough according to my responder on the CN ATM thread. It seems I must have the flanges welded on anyway. Albeit in reverse orientation to normal and with a solid shaft passing through the flange bore.

Click on any image for an enlargement.

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