Concepts for the Roundhouse

It's really inconvenient to run all the way down to Golden Lake, the other end of staging (12 feet away) to turn the engine, and so, I'm starting to turn my attention to the Pembroke turntable.

One of the first things I need to figure out is the spacing of tracks for the roundhouse, and that required mocking up the structure. From the little information we have - a few photographs and no drawings - I conclude that the doors are probably 14' wide, and separated by about 5'. This makes for a more spacious roundhouse than I had anticipated. I mocked up a couple of versions in Sketchup.

If the engine house is an engine-length away from the edge of the turntable, it comes out over 14 real inches wide at the back end. By moving it away by another half-engine, it becomes a reasonable 11.5" wide. I'm not going to pull it out that far as I discovered I wind up with sharp points where the rails meet the pit wall. So, somewhere in between seems about right.

Either way, because of the way the lead approaches the turntable, there is no way we're centered: the straight-through track is the one closest to the main. Also, those 5' spaces between doors mean that there isn't enough space for the whole near wall. I think even with the wide version I get the full track length, but the building will be cut off at the edge of the layout. I hadn't planned for a detailed interior, but it seems it's going to be required!

roundhouse approach tracks configuration

Don't let the fact that approach tracks nearly touch at the turntable pit bother you. Many turntables of smaller diameter paired with roundhouses of only 7.5 degrees separation between stalls (yes, the approach to each stall was almost always longer than the turntable diameter or the length of the roundhouse stall) required the use of a frog near the turntable. The rails crossed near the edge of the turntable pit. This is rarely seen on a model, but was not rare at all in real life. Even 6 degrees is not an unrealistically small number of degrees between stalls. A 48 stall roundhouse would have a separation of 7.5 degrees to allow for a full circle. Usually a full circle roundhouse would not be built, too much traffic in and out over two tracks. In this circumstance, dual turntables and roundhouses would be built in a large terminal to handle the large number of locomotives requiring servicing.

Frogs on turntable tracks

Thanks for the note, Alan.  While I agree that overlapping tracks were not uncommon, I also think it would be unlikely in this case.  Knowing the builder, we can expect that they would have laid out the tracks in a convenient and inexpensive arrangement and then built the roundhouse to fit them.  There was plenty of room on the prototype, and so, there would have been no need to compress the roundhouse for width, or the house tracks for length.  I suspect, therefore, that the doors were somewhat less than 75 feet from the pit wall so that they didn't have to go through the expense of creating frogs.  So, as much as it would be fun to overlap the tracks, I don't think I will on this layout.

Thanks again,
Rene'

Roundhouse track spacing

Just to return to this question again (thanks again for raising it, Alan), a quick review of Engine Houses & Turntables on Canadian Railways, 1850-1950 by Edward Forbes Bush (thanks for the loan, Scott), shows that a very common format at least on Canadian railways was for adjacent rails to meet or nearly meet at the edge of the pit. There they were bolted together, probably with a block. The feet would have had to be planed, but we're at least not talking about a full crossing of the rails. That seems to have been very rare in Canada, and the book shows only one illustration where house tracks crossed over one another at the pit wall.

Making the assumption that Pembroke followed the common pattern, I came up with a roundhouse that looks pretty good, sitting 71' from the pit wall. What's more, it looks like all the tracks fully fit on the layout, and almost all the roundhouse fits too.

Thinking about the turntable

Dodging through traffic on my ride home is perhaps not the best time to be thinking about how to make the Pembroke turntable work. Indeed, usually when I'm riding my bike I can't think about much of anything. Yet tonight I had quite a good flow of ideas. Thanks to Mayor Gregor Robertson and all the bike lanes his council have pushed through, I managed to survive the experience.

It all started when I was tossing around ideas for indexing. I had been thinking of tiny wires sticking out through the pit wall, engaging in a hole in the end of the turntable. Thoughts of a disk beneath the benchwork were discarded because of the difficulty of aligning the turntable and the disk.

But tossing around that idea a little lead me to the idea of using another lazy susan base. This was very successful for the staging turntable, so why not do it again? The only difference is that now, instead of having a minimal height turntable, I am looking to build a representation of the bridge above the lazy susan.

Having a disk under the layout that rotates exactly with the bridge above simplifies all the mechanics substantially. For power collection, I envision two rails soldered to a PC board core with phosphor bronze wipers feeding electricity to the disk. The turntable can be soldered directly to the PC board, making it not only strong but electrically live.

The disk also makes indexing much simpler than the fine wire poking into the end of the turntable. As the sketch illustrates, I'm still undecided as to how many indexing rods to make. Should it be one for each track, or a single one. Despite the drawing, I'm now leaning toward a single rod, for which the semantics are "lock"

This will be a manual turntable. The real engineer and fireman had to jump down six times a day and heave the engine around in all sorts of weather. I don't see why you should get to stand in the comfort of my basement and simply push a button. You'll have to work, or at least turn a crank if you want to turn your engine around!

I'm thinking about using a Lego gear box for the mechanism. As long as someone doesn't try to turn the bridge when it is locked, I think Lego should be more than sufficiently robust.

Why not use a slip ring for

Why not use a slip ring for power connection? They are very robust and designed for this sort of thing.

http://www.slipring.com/miniature-slip-rings.html

Installed slip ring

Does anyone have a picture of a slip ring installed on a turntable? I see that most of the slip rings have a flange to keep the outside from turning but I can't envision how the inside, turning portion, connects to the rotating turntable shaft. blayne

Slip Rings

Thanks  for the suggestion, Graeme. Like Blayne, I would be interested to see how someone else has used  one for a turntable. 

Watching with interest - again

We don't have any bike lanes so my commute is fraught with excitement and little time to think of turntables and round houses. I finally got started on my "gallows" type turntable so am interested to see what you have in mind. 

Neil

The Pit Wall

One of the benefits of modelling a long-gone, poorly documented prototype is that occasionally you get to choose. Maybe you could call it a "benefit." I'm not so sure, it tends to send me into a whirlpool of research, trying to find typical practices to inform my decision. So it was today when I started to think about making the pit for the turntable. How was the pit wall likely to have been constructed?

The obvious choice would be concrete. The PS employed concrete in the abutments and piers for the crossing of the Bonnechere River at Golden Lake. So, we know that this was possible.

However, wouldn't it be fun if it looked like this one at Orangeville (from http://trainweb.org/oldtimetrains/whatsnew_2010.htm)?This one looks like a timber pit wall (note also the frogs on the near three tracks).

So, I abandoned the styrene for now, and went back to the virtual world. First I had to figure out the height of the pit wall. I figure on using some 28" N scale wheels for the ring rail, which scale out to about 15.25" in HO. So, the overall height of the wall comes out to about 2'4".

The timbers change from 12x12 to 8x8 about half-way along. I think it looks closer to the Orangeville photo with 8x8 timbers. But in either case, the Orangeville pit looks deeper.

Timber construction like this makes me wonder about the footing for the ring rail.  Unfortunately, all the turntables in Bush's book have concrete or masonry pit walls.  He doesn't indicate much about pit construction at all.

Getting the ends right

I want to solder the ends of the turntable rails so they line up nicely with the shore tracks and stay that way. At the same time, I don't especially want to use a PC board tie at the end of the turntable because I would like the first tie to look like wood. Not only that, but every joint I make will add to the vertical imprecision.

So, I'm considering a more complex shape for the structural flanges - a shape that connects the rails directly to the structural members of the bridge. This creates another cosmetic challenge around the first tie, but at least the rail should be fixed in place.

I feel I'm messing with convention by trying to fix the ends of the rails vertically. It seems the most successful turntables allow their ends to float, supported by the wheels on the ring rail. However, I strongly doubt my ability to make robust, well-aligned dolly wheels to support the ends. Also, the floating scheme requires some running clearance between the bridge and the central pivot block, which drives the turntable. Running clearance means in turn that the turntable ends will not line up precisely.

What I did

Very interesting design you've come up with. I like it.  When I designed my turntable, I decided to make the central pivot robust enough to take the weight of the model at the end of the bridge and still stay aligned.  I used a 3/8" brass tube as the axle, with a bronze bushing in the layout and another in the structure of the bridge.  Using a drill press, drilling accurate vertical holes for the bushings was not difficult. The resulting sructure was more than strong enough for even my heaviest HO locomotive, an Athearn super-power F7.  I used the ring rail for cosmetic and power transmisson purposes only.

The design worked well enough tat I am currently building another turntable using the same concept, except that this time the bridge is 3D printed and has the 3/8" bore integral.

Tube axle

Thanks for the suggestion, Ken.  Most people seem to go with something like you've described; I've been wondering how you maintain a positive registration between the tube, the bridge and the indexing mechanism.  Perhaps you knurl the end of the tube and the indexing mechanism?  

Manual indexing

Since all of the turntables on the Danville & Western were manually operated, I decided to go for the simplest and most prototypical solution - I push it with my finger!  Even if I were to drive the table remotely, I would still index by eye, I suspect.  The tube fits the bushings tight enough that it stays wherever I put it, but still turns easily.  If I wanted, I could solder the tube to the upper bearing (or glue, in the case of my 3D printed version), at which point the tube becomes a large axle which can be rotated, indexed, etc. from beneath the pit.

Manual Indexing

Ah, that makes sense, then. 

Thanks,

Rene

Wiring

Some things I can visualize in my head.  I can come up with a remarkably detailed construction plan on the way home on my bike.  I can conceive an image and sketch it when a pen and paper are handy.  Wiring, it turns out, is not like that.

So it was that I was carrying around all sorts of ideas about how to wire those V-shaped house tracks.  I had been thinking there would be wipers picking up from the underside of the turntable, switching their polarity so that they would work whenever the bridge was aligned for the track.  Because the bridge itself needs to switch polarity, there would be a reversing circuit mounted on the underside of the turntable too...

It seemed to work in my head, but it turns out it is much simpler than that.

When I started to sketch out the idea, to explore it and ensure it worked, or at least to design the gaps in the circuit board, I soon discovered that I was wasting my time. It turns out that the V's don't need to switch polarity. In fact, they don't even need to be gapped. The only trick is that the middle track will be wired backward - black on the near track, and white on the back.

The sketching process definitely paid dividends tonight!

A/R?

Auto reverser

Yes, that's exactly the plan, Mark. The question was whether I also had to reverse the house tracks because adjacent tracks join. 

Laying out the center lines

I had hoped to make a bunch of progress on the roundhouse tonight, but unfortunately, the Boy and I wound up talking about Bill C-51, and I had to promise to actually read the whole thing before he could relax and go to bed. Every Canadian should read it, along with the existing provisions in the Criminal Code and the Canadian Security Intelligence Service Act.

So, I didn't get started on the trains until 11:30, just enough time to tape down some rails so I could figure out where the center lines for the house tracks go. This is trickier than normal because adjacent house tracks need to meet at the pit edge.

It nicely matched the rough lines I drew based on the SketchUp drawing, even though the rails in the drawing were lines of zero width. So, I took the rails off and finalized the center lines. It doesn't look like I'm going to get the bridge in before the end of the month, though.

Roundhouse Excavations

Originally, I hadn't thought I would be building pits for the engine house as you weren't supposed to be able to see inside.  However, because the corner of the roundhouse is going to get cut off, I'm going to need pits.
Well, that's a great excuse to get the router out!  So, after a little bit of research, I figured out where the pits need to go, how wide (4') and deep (2-3') they should be and how long.  I cut them in several passes with my half-inch bit, freehand.  Probably I shouldn't have tried to be a hero, especially on the near pit, but there will have to be some sort of a veneer for the walls anyway, so I should be okay.

While I was at it, I also cut a relief under the bottom of the board for the door opening mechanisms.  There will be a big plate of PC board in this opening, where the plywood base goes down to about 1/16" thick.

I had hoped to get to the turntable pit too, but unfortunately, it needed a reinforcing plate beneath because it will also be very thin.  The glue holding the reinforcing plate on hadn't set yet, and so, I've deferred until another day.

MDF is the pits

Or the pit, as is the case tonight. I just finished roughing in the turntable pit, which I ultimately decided should be concrete, not wood.

The key dimension on the turntable pit is the elevation of the pit rail. Sure I'd like the pit itself to be perfectly round, but as long as it doesn't wander too far, and is pretty close to the precise dimension where the tracks cross onto the bridge, it actually doesn't matter too much if it's perfect. The height of the pit rail, relative to the tracks, however, does need to be accurate otherwise we'll have binding and flexing and all sorts of challenges.

So, I got out one of my favourite tools - my router - and began excavating the piece of MDF that I had cut to fill the original hole. I sneaked up on the proper depth over a number of passes, then slowly worked out to the line that was the wall. Then I excavated the inside of the pit, including the edge of the pit rail shelf in a number of concentric circles to make a basis for the sloping ground of the pit.

I left the middle intact so the router wouldn't fall into the pit. Notice how the last passes are a little ragged; I think the MDF was eating my router bit. Once I was kind of happy with the general shape, I sanded it roughly by hand, and took out the central pillar.

There is dust everywhere! I am happy to have the ventilator off, but now I need to have a shower before I turn in for the night!

Just read the last few posts here and..........

......I have found a single Frog Juicer from MRH Sponsor Tam Valley Depot to be the easiest and cheapest way to reverse the polarity.

Peter

Great Thread!

Hi,

wow, Vancouver must be a cyclist's paradise, you can even do detailed drawings while cycling

Your turntable is really nice! I like the idea.

Just a stupid question, as you have planned for a PC board as the lower layer of the turntable, you can cut the copper layer diagonally across the diameter of the turntable and just have the wipers opposite each other. That would get rid of the polarity problem I think and would be much easier than bending rails. Of course the original PC board copper would need some reinforcing. Well, I guess I'm always trying to simplify things

Have fun!

Slip Rings

I built my own slip rings. Works great.

http://melvineperry.blogspot.com/p/135-foot-turntable-project.html

Mel