Turntable with rim drive, rather than hub drive

[FowlmereRR]

Thank you, everyone, for your contributions.

Here in UK it is difficult to find the commercial turntables (except Peco, which is not very US-looking), hence my interest in scratch building one.

I was looking at a CMR TT, until my enquiry about shipping cost got the answer that they'd charge $96 to ship to UK! 

So, some design work and more research to do over the next few weeks till it is warm enough to work in the garage/workshop.

Stay safe, and thanks again,

Bob

[MisterBeasley]

I also "pit-bashed" an Atlas turntable to make it a pit structure rather than a deck.  I used an Atlas bridge as the turntable bridge.  My first run thru convinced me that it needed support on the ends, not just the middle, so my pit rail became functional, not just decorative.

[cx500]

FowlmereRR

Thinking about this did also raise a related question in my mind. I understand that when a loco drives on to a turntable, the aim is to centre up the mass as best as possible to balance the bridge, and almost "teeter" on the centre bearing. If that is true, how does that affect traction? I don't know if real world TTs were driven at both ends or just one. Just curious.

In my experience turntables had the motor only at one end.  Most that I saw were air operated single cylinders but I have seen and electric drive.  In the smaller places the air would usually be supplied by a hose from the locomotive itself.  Busier terminals would have a separate air supply.

I think the longer turntables generally were supported at both ends as well as the centre, although construction tolerances likely meant there could be some minor  teetering.  Shorter ones, especially the 70' ones that were very common on branchlines, were indeed mostly supported on the centre bearing and did teeter.  If turning by hand it was important to have the table balanced if it was to turn easily.  If a motor was available, just make the teeter such that the drive wheel was sitting on the ring rail.

[hon30critter]

Hi Bob,

The Atlas turntable is very shallow, and they can be kitbashed into a respectable looking turntable by adding walls, a floor and a bridge. Here is one under construction that I did a while ago:

Here is what the turntable looks like before modification. It is shown with the manual crank attached:

Here is the drive motor:

The Atlas turntable isn't very wide, but you can make the pit a little larger than the original flat platform. The diameter of the pit is limited by the location of the motor. I used an Atlas plate girder bridge and extended it a bit.

The drive motor sits beside the turntable. They provide a shed to hide it but anything would work.

There are a couple of issues with the turntable. One is that it is very noisy! The other is that the turntable stops at every track position as it rotates so it isn't particularly prototypical. Also, the stops are spaced at 15 degrees. Atlas makes a roundhouse with 15 degree stalls, but it isn't very large. Most roundhouse tracks are spaced at 10 degrees.

Dave

[FowlmereRR]

Thanks Randy and Kevin for your responses. I didn't realise that the Walthers TT worked with a toothed drive - not sure I would want that look.

The off-centre drive seems a better approach. I'll investigate that.

I did find a video online yesterday of a UP TT somewhere that was being demonstrated to visitors. The drive motor stuck out the side of the bridge where the operator cabin was, and traction was assisted by a humungous block of concrete attached to the top of it! Not sure that approach would scale very well if I tried that!

Cheers,

Bob

[SeeYou190]

My Bowser turntables have a large disk beneath the pit that is driven by an electric motor. This puts the turntable drive way off from the center of the pit.

Any sort of low profile motor could be used to drive the disk. I have built all my turntable drives myself from parts scrounged at Skycraft in Orlando.

-Kevin

[rrinker]

 That's how the Walthers built up turntables work, and to make it work, the compromise was the pit rail is toothed. So it's doable, but you probably won;t get sufficient traction to turn heavier locos using smooth wheels on smooth rail around the pit. And a dedicated drive wheel that rolls against the pit floor WILL leave wear marks and eventually be noticeable. 

 What you are proposing uis basically taking the Walthers turntable and using your own drive electronics instead of theirs - so that could be a starting point. It wouldn't be terribly difficult to rewire the Walthers bridge so that the drive motor wires go through the slip ring directly, to connect with your drive controls. 

                                    --Randy

 

[FowlmereRR]

I am looking at options for turning a (probably scratchbuilt) turntable (90ft, HO scale). Where the turntable will go has very limited clearance under the deck (about 2"), which rules out many of the commercially available drive mechanisms, which all seem to have the motor vertically mounted underneath and several inches deep.

I wonder if anybody has attempted to build a turntable that incorporates a motor in the bridge, and drives the wheels on the pit rail, just like the prototype? Providing motor and track power and other necessary connections, such as indexing sensors, won't be a problem, as I already have a bearing and slip ring assembly rescued from an old PTZ CCTV camera.

Assuming a small motor (stepper or otherwise) can be suitably concealed in one end of the bridge, I guess getting sufficient traction on the pit rail would possibly be a problem.

Thinking about this did also raise a related question in my mind. I understand that when a loco drives on to a turntable, the aim is to centre up the mass as best as possible to balance the bridge, and almost "teeter" on the centre bearing. If that is true, how does that affect traction? I don't know if real world TTs were driven at both ends or just one. Just curious.

So, if any of you have suggestions or comments, they'd be welcome as usual.

Bob