I posted this in Layout Building, apparently the wrong place. There were a lot of viewers but only one comment.
I am curious why the Geneva drive is not used for turntables. There is a slight hesitation every 15 degrees, but I am willing to accept it. The Geneva drive is so precise that it is used for film transport. It is also very inexpensive. Has any one had any trouble with this type of indexing?
I am told that the Atlas uses a Geneva Drive.
The Atlas is a small turntable. I will have at least a 16 inch bridge with 15 degree spacing. I think Atlas components are plastic injected. I machined polycarb, acrylic, and brass. This will give me precision location. Has anyone done this? What trouble could I expect?
The Geneva Drive will do away with senesors, electronics, and wipers for contacts. A geared 12 volt motor gives more torque than needed.
As you noted, a Geneva Mechanism is used by Atlas.
A Geneva Mechanism is expensive to machine; for example Stock Drive Products/Sterling Instrument (SDP/SI) charges $180- for a 90° unit.
Except for Atlas, most turntable models are very low volume, and can not justify the cost of tooling a Geneva Mechanism. Customers will also not be happy with the fix locations, and the step-step motion rather than continuous motion like the prototype.
You will need to set you track positions after you install the Geneva Mechanism, and you will not get exactly 180°, you may get close enough though.
Additional problems, see:
http://em-ntserver.unl.edu/Mechanics-Pages/em373honors-S2001/em373/geneva/geneva.htm
And then there is cost. You can make a stepper motor system for a LOT less.
Dont't know if this would be useful to you, but I've fiddled around with the Atlas turntable a bit, and think that it could be modified to serve as a drive for a larger turntable. I think a drive shaft could be made to replace the bolt/nut assembly in the middle of the original turntable., locating the Atlas mechanism under the table/pit. You'd need to figure a way to power the track on the larger turn table as it would be difficult to use the original atlas spring plunger system. The main down sides are that the movement is unrealistic, and the atlas motor drive tends to be on the noisy side. There also has been some play in some of the units I've experimented with, resulting in inaccurate indexing. Since they were second hand, it could be they were worn or damaged.
The plus side is that you can pick used units up at train shows and swapmeets dirt cheap. The last one I bought complete with motor set me back $10. And you're right, the Geneva mechanism is great fun to watch operate!
JBB
I designed a 24 station (15 degrees) Geneva Drive plate that my CNC cuts in 20 minutes. It is accurate within .0005 inches. I allowed .001 clearance between the drive cam and Geneva plate. this gives me repeatibility of + or - of less than .002 at each station. It will be driven by a 12 volt 12 RPM motor ($3.00). The material for the drive came from my scrap box.
The drive will be fastened to the 1/2 inch bridge shaft that will extend below the layout. The shaft (steel tube) allows the track wires to extend below the table, elminating wipers.
This unit is not installed yet, as it is still being developed.
Here is an image of the fifteen degree Geneva wheel. More details will follow.
This was posted as a test for pictures.
Hello. A few forum members have requested that I post the plans for my Geneva-drive turntable. I have posted a PDF that contains:
1) Composite photos
2) 15 Degree Wheel Plans
3) Cam Plans
4) Frame Plans
5) Assembly Drawing
The unit is powered by a 12VDC @ 12RPM geared motor by Mabuchi.
Plans for the 10 Degree drive will be posted in the near future. I hope that this will help some members with their layouts.
http://www.yesterdaysradio.com/rr/genevadriveturntable.pdf
sensors? electronics? geneva? What happened to the eyeball method?
The biggest disadvantage to a Geneva mechanism for a turntable is that it stops at every track, which real turntables do not do.
Disclaimer: This post may contain humor, sarcasm, and/or flatulence.
Michael Mornard
Bringing the North Woods to South Dakota!
Bayfield Transfer Railway The biggest disadvantage to a Geneva mechanism for a turntable is that it stops at every track, which real turntables do not do. And the real ones aren't, to my knowledge, indexed, either. Wayne
And the real ones aren't, to my knowledge, indexed, either.
Wayne
Agreed, and if your turntable is close enough to the operator to see, it doesn't need to be indexed either; I've operated on several layouts with 'forward' and 'backwards' buttons and you lined 'er by eye, just like the real thing.
Now, if your turntable is too far away, you can't do that... but why would you put serious "eye candy" like a turntable back where you can't see it? (Yeah, yeah, sometimes you may have to.)
Pretty sure the original poster has found a solution in the 7 years since he asked the question.
"The true sign of intelligence is not knowledge but imagination."-Albert Einstein
http://gearedsteam.blogspot.com/
Geared Steam Pretty sure the original poster has found a solution in the 7 years since he asked the question.
I plan to set up my TT with eyeball operation, worked for me before.
No point in wasting a perfecftly good thread. I don't see any expiration date on it...
If you have a double-deck layout or other sitituation that puts the TT at or above eye level, the Atlas or other geneva drive TT can be useful.
And if you have a snowshed over your TT, the Geneva drive is also simple solution for alignment...
Mike Lehman
Urbana, IL
And if you insist on using an Atlas turntable but still want a pit....
It takes an iron man to play with a toy iron horse.
Many years ago, before cameras were invented, I used an atlas TT under the table like that. I modified it so that it had twice as many stops, and ran abvout 90% of the time instead of about 45. It was an easy mod to perform, but I no longer have that setup.
Digital cameras that is. I Never got along with film.