Yep, single start worm is the only option.
The problem with 120 teeth gear is relatively small diameter (63mm - OK for smaller loads, up to 2kg methinks), which will mean higher torque required for bigger telescope (and possibly higher rate of wearing).
I would go for 360 teeth (as suggested earlier).
Don't worry about antibacklash worm gears, they are not really that useful (especially here, where moment/torque may be pretty high, and that must be counteracted with spring), as backlash can be adjusted if pair is made with sufficient precision.
Also, there will be additional friction present (because of that spring), which may be a problem for lower torque motor.
Quote:
Originally Posted by alistairsam
friction drive is an attractive option, but maintaining pressure on the rollers and concentricity of the rollers would be key factors.
if i can fabricate an assembly that can adjust pressure on the rollers, it might be practical.
|
Maintaining the constant and controlled pressure is not a problem at all (see picture below - my own "invention"
)
To manufacture the truly concentric friction drive is actually easier than truly concentric gear - because the whole operation can be done in one go, without taking the part off the lathe before the job is completed. BTW, 200" Mt Palomar telescope uses friction reduction wheels (I read that a long time ago in a book "the design of big telescopes" .. or something similar, I don't remember the exact title any more).