The Geneva drive is very accurate. It was used to transport movie film. It repeats within .0001 of an inch. A home built drive will repeat within .001 of an inch. I have built both a 15 degree and a 10 degree drive. They are driven by a 12 RPM 12Volt reversible DC motor. It will rotoate with a 2 pound weight on the bridge.
The drive is mounted under the table. It can be used for a flush mount or a pit style turntable.
The forum informs me that modelers will not put up with the pauses that the Geneva Drive has as it rotates. Because of the mechanism's accuracy I can live with it.
I have posted photos of the prototype. Dimensions of the 10 and 15 degree drives will also be posted as soon as I finish drawing them on my computer. At the present time I am very busy but hope to post the drawings some time in November.
It would also be possible to build a 7.5 or 5 degree drive but it would require many mor pauses.
Jerry
To me, the most important advantage of the Atlas turntable and roundhouse is the small size, suitable for room-sized layouts. My layout is only 5x12 feet, so I don't have room for a complete engine servicing facility, much as I'd love to have one. I did squeeze a water tower in, but it's down the track aways. Here's an overhead shot obtained from top-secret high resolution through-the-roof government satellite imagery.
The overall left-to-right dimension from the edge of the turntable pit to the back of the roundhouse is 24 inches. (This is HO, remember.) The roundhouse itself is 16 inches wide, including the optional, moveable office building seen here at the top. The shed below the pit covers the motor assembly, but in the pit implementation the motor is below-grade and could be scenicked over. If you add the two spurs outside of the roundhouse (with the GP-9's on them here) the overall width is 24 inches.
It takes an iron man to play with a toy iron horse.
If you plan to "Index by Eye", you'll find it is difficult to stop the turntable motor accurately. It tends to have a flywheel effect and continue on past the alignment. It means you have to cut the power well before the track is aligned, and then hope it stops on the right spot.
Here is my tip:
Thinking back to my slot car days, I incorporated a sort of "dynamic" brake into the circuit that controlled the motor. I also made it operate at two speeds so that I could slow it down just before alignment.
Simply, the two wires of the motor are shorted out until you want to add power to turn the turntable. Adding power removes the short, and then as soon as you remove the power the "short" is again introduced to the circuit and this causes the motor to stop instantly.
Try it, it works!
Here is a link to a slot car site that descibes the process:
http://www.citizensoldier.org/howbrakeswork.html
Mike Sowsun
Oakville, Ontario
Now, that's what I'm talking about!
For example, I'll look to see if my old Atlas-conversion possibility only has 30-degree indexing, and how that affects placement on the layout. Perhaps a new N Scale Atlas Turntable will be more effective for the layout at 15-degree indexing.
How about the practical advice of: "To begin to be realistic, the distance between the roundhouse and turntable should be at least the diameter of the turntable. Add in some space for an ashpit (and possible water, fuel, and other servicing) on the arrival/departure track(s), and the minimum practical length comes up something like 5 times the turntable length."
There are just some things like those mentioned in this thread that are not covered in the articles.
Conemaugh Road & Traction circa 1956
Well I've been mucking about with the Atlas turntable lately. This started the way those things normally do. Saw one at a low, low bargin price on my way out of the swap meet, and so it came home with me. After fixing it's contacts and some other problems the old owner forgot to mention, I got to thinking that this would be an easy way to get a turntable up and running until I built the turntable I really wanted. Certainly would be more realistic for operation than nothing. Since I model in the pre WWI period, the short length would work ok with my motive power.
Next I discovered some old photos of covered pit turntable in use on the B&O in the late 1800's. Hmm, perhaps these things were more numerous than I realized. (If I remember right, the prototype for the Atlas table was located in New England...I think on the B&M) . The neat thing about the photos was that they showed all sorts of spare timbers and other assorted railroad junk piled on the planks covering the pit..just the kind of detailing opportunity dear to the hearts on model rails everywhere.
So now I'm thinking that I'm going to try to repaint , alter and detail the stock atlas table to see if I can get it looking a little more realistic. Maybe it will work out, and at worse I have a temporary working turntable.
As far as the mechanics go, the Atlas motor drive is a bit noisy. I'm going to try a new motor cover out of a more sound absorbing material. A better quaility motor might help too. My table seems to have a little play in it which means it lines up just a bit differently when operated clockwise from the stops when going counterclock wise. Don't know if they're all like this, since mine was used. I'm controling the table with an old train set transformer which allows slow speeds.
JBB
tgindy wrote: QUERY #1: What trial & error turntable construction experiences can be helpful?
QUERY #1: What trial & error turntable construction experiences can be helpful?
One I have learned the hard way, and read about: Don't cut a large circular hole to start your turntable project. You lose the center point, for one.
QUERY #2: How much layout real estate does the turntable & roundhouse require?
To begin to be realistic, the distance between the roundhouse and turntable should be at least the diameter of the turntable. Add in some space for an ashpit (and possible water, fuel, and other servicing) on the arrival/departure track(s), and the minimum practical length comes up something like 5 times the turntable length. Can you get by with less? Of course, but realism is going to suffer. But note the minimum possible length is on the order of 3 times the turntable length.
QUERY #3: What are your "Givens & Druthers" for a smooth-working turntable?
Givens:
- smooth, slow operation without any apparent movement in vertical plane (not strictly realistic, but the prototype looks better with vertical movement than models do)
- indexing that works and holds calibration over a long period of time
- reliable electrical pickup for the locomotives moving on/off the turntable
Druthers:
- prototype appearance
- can operate properly without continuous operator attention
- does not require a study of toggles/direction switches prior to moving locomotive on/off the turntable.
my thoughts, yours may vary
Fred W
Well, thanks for remembering the tutorial. I'm glad you found it useful. I've had my turntable in the ground and running for a while now, so I can add a few more things:
First, the major difference from my perspective is my false floor for the pit, which doesn't rotate with the bridge. This allows me to model a real, uneven dirt-and-turf pit floor. If you do that with a rotating floor, it will obviously move with the bridge. The key to getting away with the rotating floor is making it uniformly bland so the eye doesn't notice that it's not moving.
Again, the hardest part was aligning the bridge with the deck track. Mine is still just that little bit off, so it doesn't index perfectly in a couple of spots. If I were to do it again, I'd build in some "float" in the track-to-bridge attachment so that I could adjust that last detail after everything else is in place.
Mine has developed a hangup in the gearing somewhere. It may be just stickiness, and I found it worked better when I just let it run round and round for a while to loosen it up. Still, it does make me worry about maintenance in the future. I wish that I'd planned better for possible removal. The idea is that I can take it out from below, but I never really tried that during the installation, and now I'm afraid that I'd disrupt a lot of scenery if I did that.
There have been a couple of comments, including one of my own, about being able to stretch this into a larger turntable by simply using a longer bridge. There are 2 problems here. The first is the motor mounting on the gears. If the bridge were much bigger, the motor would get in the way of the pit rail. (You could cantilever the ends of the bridge a bit, and use a smaller diameter pit rail, so the ends of the bridge would actually extend out over the motor a bit.) The pit rail is necessary in my implentation with the false pit floor, because attaching the bridge only at the center doesn't provide enough support at the ends. The other problem with stretching the bridge is the 15-degree indexing. The stall tracks start getting quite far apart as you make the bridge longer.
I've got an old Atlas turntable sitting in my parts box that's probably 45 years old. It indexed at 30 degrees. Yes, it still works just fine, although the motor mechanism was shot.
This was a fun project, and I'm happy with the result. But, it was quite a project, and it certainly wasn't like putting together a kit. I probably put all my layout building time for a couple of months into this, so if time is an issue for you, it's something to think about.
But, if you can find a place on your layout for a turntable and roundhouse, it's certainly a focal point, and well worth the investment.
The October 2007 Model Railroader, featuring ABC's of engine shops, has a well-done turntable construction article, "An easy-to-build 65-foot turntable," in the issue's supplement: "Workshop Tips Servicing Steam."
The article is quite good, and reminiscent of Mister Beasley's Turntable Photo Tutorial...
http://www.trains.com/trccs/forums/1162840/ShowPost.aspx
...with the major difference being Mister Beasley embedded the Turntable Pit into foam as opposed to the recent MR article's use of a plywood base. In both cases, "Ye Olde-reliable Atlas Turntable" was used as the turntable base, and takes advantage of Atlas' 15-degree track-indexing.
While I model N Scale, this thread not to be restricted to that scale, but is intended to be an "All Scales" turntable thread. In fact, I still have 30 year-old Atlas HO turntable, and wondering if it might be adapted to my CR&T layout using these two tutorial's ideas and your insights.