2" shaft mounting Pt.51: Saddle reinforcement.

*

Publishing one's ATM on a forum like Cloudy Nights is apt to throw up questions and suggestions.  The 100 x 50 x 5mm channel section saddle was questioned as to its stiffness. So I found some aluminium angle in 50 x 50 x 5mm and narrowed one web so that two lengths would fit side by side inside the saddle's channel section. The advantage of this arrangement is that the Tollok bush is bolted right through the now, 10mm thickness. As are the tube rings of course. The greater thickness should preclude twisting or other flexure modes without needing local reinforcement on the saddle.

If only life were that easy. There was no room for the Tollok bush in the narrower space without major surgery! I don't have any angle section large enough to meet on the outside of the saddle channel.

I considered placing an inverted channel on top of the original. If the two were bonded together the saddle would become H-section. The problem is that it looks rather ugly and contrived. Perhaps I should look for some flat plate or even box section which would fit between the webs of the original channel. Or I could contact an eBay aluminium stockist to see if they have an off-cut of something sturdier in channel section. They might have a short length which isn't worth listing.

Clamping the channel saddle at one end to the workbench showed it to be quite stiff except in rotation.  I then tried clamping one end of one of the 2m [6'6"] rectangular box, straight edges to the workbench. An 8" G-cramp [C-clamp] applied to the free end showed the beam  was very stiff in its deepest direction, remarkably stiff against rotation but rather floppy in the thinnest direction. All largely as expected.

Using this material in two or three layers for the saddle would both save weight and increase stiffness. The only problem is the ease of crushing with screwed fasteners. It really needs solid metal inserts to resist crushing forces. It would not be too difficult to push alloy packing strips down the channels to the center of the saddle. They could even have some epoxy added to keep them in place. This would give the Tollok bush screws and flange something hard to work against.

Beech inserts did not work when I used these beams for my 10" reflector. They did not have the necessary resistance to crushing forces. Which is a shame because I have lots of old beech bed slats saved for my construction projects. I could entirely fill the beams with those if it were worthwhile. I suppose I could have metal inserts at the center and ends with beech packing in between. Or, I could epoxy laminate a single beam on top of the existing channel section saddle to increase its stiffness. With the caveat that resistive packing would still be essential to avoid crushing.

Then I had the idea of adding a half pipe of aluminium to the back of the channel section saddle. My length of pipe managed to be 100mm in diameter. Exactly the same as the width of the saddle but not the 90mm I needed to fit nicely inside the channel. Grr? I wonder whether a half of the tube is capable of being gently squashed to a slightly smaller diameter? 

As can be seen from the images above I finally chose to add a slightly lower version of the existing [channel] saddle. The adjoining plates are now 10mm thick with ribs on both sides. The Tollok bush is bolted through both. As are the tube rings. Further fixings along the edges might be useful to reduce flexure until I can bond the two halves firmly together with epoxy. I have only run a 120 grit angle grinder wheel across both sections so far just to clean them up. Further smoothing work with finer abrasives will follow.

Click on any image for an enlargement.

*