ATM Gear reduction for dobs

[Bob Segrest <bobslan@erols.com>]

Greetings,

I have been doing some home work on some of the reduction options available for people that want to motorize a dob using Mel Bartel's design.

Torque Transmissions
> I called and asked about a reduction gear, asking for .25" shafts in and out, a 60:1 ratio and 90 degree shaft offset.  I was pleased to hear that they carried a stock unit for $99 and some change.  It has a 5/16 input shaft and a 5/8 output shaft.  I asked about the backlash specifications and was passed to their engineer.  The engineer described the backlash tolerances as about 5 degrees and explained that this was largely the result of the product variations inherent in a plastic housing.

Allied Devices
> When I asked for the same basic reduction gear I was passed to Henry, the local gear box engineer.  Henry was able to find the right part numbers for what I wanted, promised to fax me a data sheet and passed me back to sales for pricing and a catalog.  The reduction gear with bronze bearings was quoted at $123.55 and with ball bearings for $143.10.  A few minutes later the data showed up describing all of the dimensional data.  It also specified  stainless steel worm, bronze main gear and a cast aluminum housing.  Backlash was specified as less than 30 seconds with the ball bearing variation.  I called Henry back and he told me that they only recommend ball bearings for start/stop applications and he indicated that could reduce the backlash number a bit by using a better main gear for another $25-30.

Edward R. Byers Co.
> As part of a recent trade with Dave Radosevich, I acquired a nice pair of 4.5" Byers worm gear sets.  The reportedly have an error specification in the 5 arc second range.  I designed a boss (hub) and gear box to use these lovelies and discovered it was going to cost me somewhere around $600 to get the parts made by my local machine shop at $50 per hour...  :0(

Well I don't have any great idea where to go next on this project.  It is too much effort to spend on a plastic gear box, I can't afford $600 for machine work or $2000 to buy my own machine equipment, and I am not sure I want to pay ~$300 for a pair of gear boxes that will end up with 8-10 arc second error after the final drive reduction.

I guess I'll have to ponder this one a bit more......

If anyone has a better source or bright idea, please let all of us know about it...

Bob Segrest