Big GEM OTA balance weights.

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The need for more weight on the saddle required no weight be added to the OTA itself. Such weights may be convenient but add to the struggle of carrying the OTA up the steep stairs to the observing platform. My original sliding weight was transferred from the 7" f/12 refractor to the saddle of the big mounting.

On arrival the two, new, sliding weight bars, towel rails from IKEA, had shrunk from their original diameter but had grown in length. My home turned, brass, sliding weights had all been bored to 16mm for the old towel rail. Fortunately the difference between 13mm and 16mm was perfectly filled with tubular plastic, electrical conduit. Which needed drilling midway to allow the thumb-wheel screws to reach the rail. I added cut offs of Rawl plugs to be kinder to the rail. They also provide a nice slipping clutch effect. Allowing easy sliding, or stiffer movement, depending on the pressure from the thumbscrew. Metal against metal is more either or than how much.

I cut four new support brackets from some hefty aluminium angle on the miter saw.  Then bolted them to both sides of the mounting's saddle. This will avoid any asymmetric, unbalanced forces causing torque effects on the mounting.

The image shows the results of my handy-work. The stainless steel, tubular rails will not rust. Nor will the sliding brass weights. With such long travel most OTAs can easily be balanced without the weight colliding with the OTA. Multiple sliding weights can be added if desired or needed.

Note the ten, hex socket, [SS] screws holding the saddle rigidly to the 50mm [SS] declination axis shaft in the top image. This  is achieved via a Tollok, flanged, compression  bush. The spread of screws into a circle aids stiffness at the joint with the 90mm Ø steel flange. Thick [SS]washers have been shaped, by grinding, to nest, edge to edge, for maximum surface area beneath the screw heads.

The 50cm [30"] saddle is composed of two 10cm [4"] wide, 5mm thick, channel sections bolted and bonded, back to back, for a nicely stiff, I-beam cross section with a 10mm thick middle plate [or web.] Shaping of the upper channel section to a shallower taper avoids conflicts with tube rings without local weakening by cutting deep notches. The 6" Ø tube rings look far too small for such a large mounting.

Click on any image for an enlargement. 

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