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Originally Posted by ballaratdragons
Mark,
The bearings will unscrew and can be moved anywhere but they are perfectly balanced where they are.
I like the look of the units (fork) in pics 1 & 2.
I also found this drawing for a simple GEM but I don't know how I would make it track!
http://home.fuse.net/astronomy/pipemount.jpg |
OK , the thing to remember with the smaller size mount is you will have to most probably use gear and worms and this is going to cost ya big $
This when i looked into it you would have to import them from the US or UK.
To import a 12" worm and wheel set was in the order of $1200-1500 each then you have to add controllers.
A smaller size say 9" gear for a 12"only scope will lessen the price.
This is why I went friction roller drive.
It does not require many $ to get a large steel disc machined.
You don't have to use steel , even ply with a SS band will do and this is what I am using for the DEC disc to cut down weight.
The thing for a friction roller drive to work with minimal slippage is you need the drive disc to be as large as possible .
This also helps in the final over all reductions required.
I have read at least 1.5-2x the size of the scope is normal.
So you would need 450mm drive disc for a 12" scope but will depend on variables.
There are ways of over coming this but then things get more complacated.
There was an article in S&S and S&T a year or so back about a guy who built a GEM using 18" ?? friction discs and a rubber belt traction drive .
This seamed a bit complicated for me.
This is why the split ring design is better suited to a friction drive due to the large ring for the drive to act on.
You can see this is why a friction drive is not really suited to GEM's --a bit hard to fit a large drive disc on a GEM with out it hitting the pier..
A largish fork is also better suited as well.
I was never able to find any one in Australia that makes large acurate low PE worm and wheel gears .
So the choices are narrowed to DIY or pay $12000 for a mount and why I didn't consider a GEM.
Here are some more smaller designs to study.
some made from ply and others from steel or aluminium , but all semi portable.
Also a Dob driver.
Mark