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The Tollok bush which holds the saddle onto the end of the declination shaft had little or no room for an expansion ring. The supplied steel ring would not fit inside the saddle channel section without surgery. This would involve thinning the ring considerably with the potential for disaster. The Tollok bush relies on opposed tapers to grip the shaft and simultaneously expand into something tubular which offers serious resistance. The grip on the shaft relies on the physical containment of the outer, flanged bush.
I decided to turn a full length cover for the Tollok designed to provide the vital resistance to expansion. Unfortunately I have no round aluminium stock in the necessary diameter. So I was forced to use brass. This is much heavier than I wanted for this situation and will require more counter-weighting. The advantage of the sleeve is that it protects the bare metal of the Tollok bush from unsightly rust. It also overlaps the flange bearing projection.
The images show the general idea. The piece of scrap, brass bar had been discarded by an educational establishment after a couple of knurling exercises. I left the knurling in place for decoration. The middle image shows the Tollok bush sunk flush with the brass sleeve. All much as I had done with the much large [7" diameter] aluminium cylinder on the polar axis.
Having the entire Tollok bush enclosed maximizes its ability to compress strongly onto the declination shaft without local stresses or flexure. I reasoned that since both halves of the bush are split, the full length enclosure would remove any chance of local flexure. Far better, surely, than a short ring?
The narrow front lip, which accepts the inner cone flange, set the maximum and minimum diameter which would just go inside the saddle's channel section. I deliberately allowed a little extra clearance here to avoid stressing the thinner lip by internal expansion. The much thicker, main body was made a closer fit where there was over a cm of solid brass to resist expansion.
I had bought some oversized, stainless steel washers to fit the Tollok, clamping bush screws. The idea was to spread the loads from the ten replacement, longer, stainless steel screws. Due to the screw spacing I had to turn the washers down in diameter to just clear each other. This ensures the maximum area and of the saddle is sharing the pull from all ten screws. Adding a load spreading plate under the screw heads would be pointless unless the plate was bonded on. It would also demand much longer screws with the potential for flexure.
The images show the general idea. The piece of scrap, brass bar had been discarded by an educational establishment after a couple of knurling exercises. I left the knurling in place for decoration. The middle image shows the Tollok bush sunk flush with the brass sleeve. All much as I had done with the much large [7" diameter] aluminium cylinder on the polar axis.
Having the entire Tollok bush enclosed maximizes its ability to compress strongly onto the declination shaft without local stresses or flexure. I reasoned that since both halves of the bush are split, the full length enclosure would remove any chance of local flexure. Far better, surely, than a short ring?The narrow front lip, which accepts the inner cone flange, set the maximum and minimum diameter which would just go inside the saddle's channel section. I deliberately allowed a little extra clearance here to avoid stressing the thinner lip by internal expansion. The much thicker, main body was made a closer fit where there was over a cm of solid brass to resist expansion.
I had bought some oversized, stainless steel washers to fit the Tollok, clamping bush screws. The idea was to spread the loads from the ten replacement, longer, stainless steel screws. Due to the screw spacing I had to turn the washers down in diameter to just clear each other. This ensures the maximum area and of the saddle is sharing the pull from all ten screws. Adding a load spreading plate under the screw heads would be pointless unless the plate was bonded on. It would also demand much longer screws with the potential for flexure.
Click on any image for an enlargement.
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