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Turntable Setup
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 Posted: Tue Nov 28th, 2017 09:13 pm
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Tom Ward
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I need some help from someone knowledgeable about setting up turntables.
I'm just starting construction on my layout and have run into a snag in laying out the turntable in relation to the roundhouse track.  I'm using an HO scale Walthers Cornerstone 90' turntable.  This works out to about 49.5' for my On30 layout.  Right now it's not motorized but I was planning to set it up with a stepper motor.  The main lead coming in is perpendicular to the roundhouse.  Sort of.  When I laid it out full size on the bench top I found that there's a four degree offset for the tracks leading to the roundhouse.  Separation of the roundhouse tracks is 11.5 degrees.  My problem is that if I shift the roundhouse four degrees it places one corner very close to the edge of the layout.  Shifting the lead-in track is not an option.  This is a small room and space is a major limiting factor.  I'm also using dead rail so none of the tracks are powered.  So I have several questions:
1. Are the angles critical in setting up a turntable?  Will the 11.5 degree separation of track work or does it have to be in whole degrees?2. Will the four degree offset between the lead-in track and the roundhouse tracks matter?3. Is there a type of control other than stepper motor that would work better for this application?  Ideally I would want to be able to dial in a track number and have the turntable rotate there.  This is a possible Arduino application but I haven't really gotten very far with the planning for that.  I'd probably settle for manual operation if that was a better solution.
- Tom

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 Posted: Tue Nov 28th, 2017 10:04 pm
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Helmut
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The critical point in any application of stepper motors for turntables is the resolution per step which is crucial for safe operation. In order to stay on the track when passing from the table to the feeder, a lateral displacement of 0.2mm must not be exceeded. So your step width must be equal or less than 0.2mm. That rules out, even with microstepping, any direct driving of the turntable shaft. There must always be a gearing without slack to reduce the stepper's intrinsic resolution to the value that is needed. As you can see, the angle is not relevant at all. OK, an example:
Let's take a turntable dia. of 9 15/16 (Walthers) rendering 31.22" circumference or 793mm. Divide that by 0.2mm and you end up with 3965 steps. A top-end stepper offers 500steps/360°, so you need a 7.93:1 gear to achieve that resolution at least. The normal affordable steppers offer 200/360°, needing a 19.825:1 gear reduction. So a worm gear where you can eliminate any sideplay is in order. Take a 20:1 reduction and your resolution will be smaller ( the better ) to reach any track position you wish.

Last edited on Tue Nov 28th, 2017 10:17 pm by Helmut



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 Posted: Tue Nov 28th, 2017 10:13 pm
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Tom Ward
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OK Helmut.  Thank you.
- Tom

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 Posted: Tue Nov 28th, 2017 10:19 pm
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W C Greene
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Manual or "Armstrong" turntable operation is of course the simplest and easiest. But then, some may love the auto-matic-ness of motorized things. Since you are using r/c in the locos, there is no wiring BS to mess with, just getting the tracks to line up with the table. You may need to adjust the distance of the table to the enginehouse so the tracks can line up properly. And you may need to modify the kit to have one less stall if the structure is too close to the "concrete canyon" for you. I have 2 turntables and they have under the layout "controls" which are powered by 0-5-0 switchers. And on one TT, I have small "locking devices" to keep the table aligned with the tracks. Adapted from several real devices I have seen.
But then, with me, simple is the way I roll.

Woodie



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 Posted: Wed Nov 29th, 2017 04:57 am
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Michael M
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Tom,

As I recall the Virginia & Truckee had a rectangle engine house off a turntable at Carson City.  Seems that a rectangle building would be easier to construct, although you would have longer tracks leading to the engine house.

http://trn.trains.com/railroads/railroad-history/2010/07/virginia-and-truckee

Note how two of the smokestacks are missing some parts.




Woodie,

Found this old photo of V&T's three-stall engine house with a 3-way stub switch.

http://wnhpc.com/details/ssl0010



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 Posted: Wed Nov 29th, 2017 05:14 am
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jtrain
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Tom, you might want to think about mapping all this out in 1:1 scale on paper first, then use it as a template for the layout.

Other than that, I've got nothing to add.  I've never laid a walther's turntable before nor have I hooked one up that turned at the push of a button.  But thorough planning before doing anything else is essential.

--James



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 Posted: Wed Nov 29th, 2017 01:19 pm
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Tom Ward
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I appreciate the input from everyone.  This all started out because I was drawing out the track plan on the bench top in 1:1 to verify that my design was going to work in the space I had.  The angle offset of the roundhouse showed up then but not in the design program I was using.  Since I've never had a turntable before I had concerns about this all working out together.  After seeing your responses here I think maybe the thing to do is set the turntable up for manual operation, hand crank or something, and convert it over to automatic operation later on if I see the need.
So, if I can figure out how to set up a hand crank is there an easy way to lock the table in position when it gets to the right spot?  Would you use a solenoid beneath the table.  Is there a better way?
- Tom

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 Posted: Wed Nov 29th, 2017 01:23 pm
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Tom Ward
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Here's a photo of what I'm working on.
- Tom

Attachment: image.jpeg (Downloaded 33 times)

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 Posted: Wed Nov 29th, 2017 02:20 pm
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pipopak
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Decades ago I had a Fleischmann turntable. It had a solenoid inside the bridge with a pin that locked when aligned with holes under each track. Had 3 pushbuttons: two to start the turning motor ( one for turning in each direction) and a third one to activate (retract) the solenoid to skip tracks.
Jose.



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 Posted: Wed Nov 29th, 2017 05:34 pm
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Michael M
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The last turntable I had (HO scale) was manual.  I made a sliding bolt out of brass that I could throw into the hole for the corresponding track when I needed to lock it into position. 

On my current layout (35n2) I have a small turntable that is manually operated.  No locking device since there is enough friction to hold it into place.  Besides there are only two engine tracks so no need to get complicated.  Also the locos are battery powered so worries about polarity. 



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