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


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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