Connecting Drive Shaft to Turntable Bridge

As I have been rebuilding my Diamond Scale turntable, I've discovered an important feature they possess that should have applicability to most all turntables in general. The connection between the drive shaft and the bridge structure should NOT be a rigid one,...not an absolute rigid affair.

This is particularly clear as we consider larger/longer TT bridges. As our bridge decks become ever longer it becomes a problem to have them contact their pit rails at BOTH ends simultanousely, while also being pushed up by their center drive shaft. We need to 'uncouple' this drive shaft's vertical motions, while still retaining its very positive rotation motions.

Diamond Scale accomplished this quite nicely by gluing a rectangular block of plastic onto the upper end of their drive shaft,.... then providing a slot in the underside of the the bridgedeck for this rectangular block to ride in. This uncouples the strictly rigid connection.

Here is the black piece of plastic stuck on the centering/rotation shaft,..and the slot in the bottom of the bridge deck into which it fits,..

(btw, there is a crack across that hole where the center shaft fits. Mine developed that crack somewhere along the way, and I had to glue it back together.)

Here is that bridge deck partially set down on that black plastic block. You can see by these 2 photos that the bridge itself is free to dip either of its ends to meet the pit rails at either end,..

Another nice feature of this arrangement is that a person can lift the entire turntable bridge deck up and off the layout without have to remove the turntable assembly itself !!

All of your photos are

All of your photos are missing just a big red X

Not necessarily

There are two ways to do this (well, maybe more). You can have the connection to the shaft be flexible, and let the bridge ride on the pit rails. Or you can make the connection be rigid, and construct it all very accurately, and then if you put bogies under the ends of the bridge at all, they don't apply any weight to the pit rail. But you have to get it right!

I think the larger turntables on prototype railroads were made in sections, hinged in the center. That way they could have support at three points, and not have to worry about the center and the two ends all being in a perfect straight line--there could be a hump or a dip in the center. A small one, obviously.

Big Red X ?

That's strange, a big red X at each photo location?....there are 5 photos.

lets see if this works,..

 

or this,....

That second one says it is working according to my screen??

Big Red X?

Never mind, I didn't read carefully enough.

Turntable

Hello, mine is still free and together and would love to send it to you, it's just setting there.

God Bless Ya   Dave

My First Home Built Turntable

Not too long after I was first getting back into the hobby, I acquired an unfinished Atlas plan 'Central Midland'. The stock plan did not have a turntable, but it did have a spot inside a big loop of track that was prime for such a structure. Since this layout was only going to be a short term experiment for me, I was willing to experiment with building a turntable for it,...rather than purchasing a commercial one.

My first thoughts turned to what item I might use for the turntable pit? Alum baking pans seem like a natural. Concurrently, what item might I use for the bridge deck,...some sort of existing bridge structure? I had very early on discovered the Atlas/Roco 'curved cord bridge'. It had a very nice open girder base structure that was also very stiff in strength, and it might be utilized with or without the upper portions of the superstructure.

I recently found one of those baking pan models I started out experimenting with, along with some photos of the Atlas/Roco bridge parts,...
 

Those base structures are stiff enough to be used 'unassisted' by any substructure. If so desired there could be a couple of steel rods of metal glued into the bottoms of this base structures lined up end-to-end. BTW, those bases are 9" long in HO size, so 2 of them give one an 18" long turntable bridge,...plenty long for most steam engines.

My next thoughts turned to how I might provide a more prototypical truss member for the TT bridge. The Atlas plate girder bridges, turned upside down sure looked to be the ideal item,...and they are the same 9" long,
 

BTW, I utilized that same bridgedeck construction for my first turntable experiment, but I did not use the baking pan here,..
https://forum.mrhmag.com/post/the-central-midland-layout-by-john-armstrong-atlas-plan-29-12207702

As you can see here there was no pit walls installed yet. The bottom of the pit was separated from the upper track deck by a series of long machine bolts around the perimeter (4 bolts I think is what I used).

My Damaged Diamond Scale Turntable

Somehow my story of building a new turntable (and the photos) got disjointed in this subject thread, so I thought I would try and reassemble it in a better format. It all began when I got out the old Diamond Scale one I had purchased used at a train show quite a few years ago, and put away for eventual use. The seller had cut out a section of the plywood deck of the original owners and brought it to the show like that.

I proceeded to separate the pit itself from the plywood deck, and that's when I discovered that it had warped. I'm quite sure this was NOT warped when I purchased it,...(looks like it got wet somewhere over the years),...
 

I first decided to see if I could bolt it up to a new solid piece of plywood and pull it back into shape. That only worked out half way,...not enough to convince me that it would work flawlessly enough required of a turntable.

What to do?

Build a New Pit

Bottom line, I'm better off constructing a new pit (and wall) for the very nice Diamond Scale bridge to work in, thus avoid any uneven wavy-ness for the turntable bridge to negotiate.

I spend a fair amount of time thinking about how I might construct a new pit for my turntable. The two primary pieces would be the pit floor itself, ...then the pit wall. One thing for sure I was NOT going to utilize plaster. So here is what I came up with.

Pit Floor:
Good grade ½” to 3/4” thick plywood was a top choice in my mind, painted to limit any moisture absorption and look like concrete.

I happen to have a nice square piece of 1/2” thick piece of black Sintra board (cellular PVC) I had collected up from the scraps being discarded by a local sign shop. Why not experiment with this first, ....then I can always return to the plywood alternative if this didn't work out?. I decided my pit wall was going to be 1/4” thick material ( several different options), and bent into a circle to fit the round trench I would cutting into the floor.

I got out the router, installed a ¼” dia bit, and cut my 'trench' into the flat floor board.
 

Here I will note that I had 2 options,..cut that 'trench' (partial depth into the floor pit),..or just just cut all the way thru the pit floor piece forming an inner disc, and an outward retaining hole in that square floor piece of PVC. Either way, both methods would support my ¼” thick vertical pit wall.
 

Pit Wall:
I had another scrap piece of the black PVC that was 1/4” thick 9” wide, and about 5' long. I needed a strip of it about 1+3/8” wide for the full length of 5' to bend around to form my 'pit wall'. At first I thought about cutting it with a saw, but on second thought, why not use a sharp razor/ box cutter type device making multiple passes,...much cleaner operation.

That relatively thin strip of plastic would be flexible enough to bend into a radius to fit into my trench, but I wanted to trial run it several times, so several in and outs, plus final gluing. I figured it was going to be easier if the strip of 'pit wall' material had somewhat of a 'preset bend'. I clamped it around my old pit wall casting, and got out my heat gun. Bravo, nice preset bend. Finally I glued in place with PVC cement.
 

 ....final pit wall installed

Once all glued into the trench everything became quit rigid, very happy with result.

Now I just have to cut some more of that 1/4” thick PVC (maybe the white stuff this time) to make a 'shelf' onto which to glue down the pit rail with its ties,...something like shown in the mock-up I did before I started the router cutting.

 

I can use the same router dimension plate to cut the hole in my main plywood deck that this turntable assembly fits into.

NOTE: I am now firmly convinced that I would NOT utilize that black sintra board as a pit floor,...it seems to be prone to warpage more readily than the white cellular PVC. And of course there is the good old reliable 3/4" plywood.

Pit Wall alternatives

There were several other materials I considered making the pit walls of:

a) 1/8" thick PVC window blinds (a neighbor had been ready to discard these items, and I saved them just in case). Two of them could have been bent around that trench, then had their tops sheared off level with the plywood train deck
 

b) I could have also utilized 2 pieces of 1/8" thick cellular PVC , rather than the 1/4" I used.

All of these materials were PVC, so cheap PVC glue was used.

Center Hole Correction

I was real upset with myself the other day when I found that my center hole was not centered properly. (have yet to understand how this happened,...but it needs correcting)

At first I though I might just scarf in a new piece of that black PVC material into the center area of the pit, then locate and redrill a proper center hole. But first I thought I would try drilling out the larger FULL size hole needed for that brass bushing at the pit level, and relocating the center of that larger hole to its proper center. This was NOT acceptable, as it left that hole too large to properly retain the brass bushing in a snug, secure manner (oblong hole).

But it did led to another idea,...why not some sort of small thin metal plate glued to my pit floor that would snugly keep the brass bushing in its centered position. After all this upper brass bushing's primary job is keeping the turntable deck in a perfectly centered position. And since the brass bushing has a flange on its one end, it will fit perpendicular to that metal plate, that will in turn sit flat on the pit floor. So the turntable shaft will end up very perpendicular, particularly when the lower shaft bushing is located on center 2-3 inches under  that upper bushing.

That led me to a drilled out 'fender washer' to support that upper bushing.

Here are a few photos of that fender washer/bushing centering piece. The white ring inside the pit wall is 1/4" thick piece of PVC that will support the pit rail track. The pale white appearance over the whole pit area is piece of white tracing paper that was cut and laid in there to help locate the exact center in which to glue down that fender washer. Bushing sitting there (upside down) waiting to be installed.
 

When I cut out those pit rail foundation rings I ended up with some circular sections that matched the radius of my pit wall. I decided to add those scrapes onto the outside of my pit wall, as I need an additional 1/4" pad/spacer between the bottom surface of my plywood train deck and the base plate of my turntable pit.
 
(turntable drive shaft waiting to be installed)

Building 75 Foot Turntable

Brian, 

This is also my to do.  I looked at several turntable and pit options and have found the only way is to build it myself.  There is/was some controversy over whether the Port Costa (California) turntable was 70 feet or 75 feet long. Personally I think it was 70 feet but I am tied to the Banta Models Round House kit for the Port Costa which was designed to fit the Diamond Scale 75 foot turntable and the 80 foot length of the building (drawings show 75 feet in length), spacing for the stalls and track angles adjusted accordingly.  I have just this past week mocked up the design for a 75 foot turntable including a mockup of the 80 foot roundhouse using the Banta kit as the guide. 

 Note that the white circle is the 70 foot diameter and the plastic sheet simulating the bridge on top is 75 feet long.  Port Cost only serviced small locomotives in the SP Consolidation and Mogul classes which fit on a 70 foot turntable.  The pit was quite shallow as the location beside the tidal Carquinez strait was close to sea level and the water table high. From photos I would estimate the pit was no more than 5 feet deep.  I had built a 70 foot bridge which will be scrapped and a 75 foot bridge constructed. 

The turntable was manually operated and I plan to emulate the prototype. 

I am most interested in the PVC sheet material you are using. Was this available at a building supply house and in what size sheets. I have some 1/4 inch thick styrene I was think about using but it is quite heavy. My actual baseboard under the Woodland Scenics 1/8 inch foam sheet  is a 1/2 inch foam board supported by horizontal poplar joists in an adaptation of an L girder.  It is very a stable platform for the small space I have for a layout. 

Again I am impressed and will use a lot of what you have done in my own turntable construction. 

Foamed PVC

Here is one definition of that material,...
https://en.wikipedia.org/wiki/Closed-cell_PVC_foamboard

I found it as throw out scraps at a local sign shop. Originally I thought is was 'gator board'. I did some testing with some of the scraps by placing pieces in a glass of water for several days,...never absorbed ANY water,..nor  changed shape.  Easy to work with using ordinary wood working tools. I have really grown to like this stuff.

Google it and I am sure you will find a lot more info.

Manual Drive Gear Box

This was my older 'drive box' from my Diamond Scale TT. I cut it off of the bottom of that pit floor, and sanded it level to mount on my new pit floor.
 


However I thought I might want a little extra height in there to both make getting at the set screws on the gears and bridge shaft a bit easier, AND to make the distance between the upper shaft bearing and the lower one a bit longer to help with any misalignment. So I fashioned a couple of 'PVC feet' (white feet) to space the drive box about 1/2" lower. (those feet also act as stiffeners for the pit floor plate),..
 

 (that's a plastic bottle under there to shelter the extra long drive shaft)

Used both epoxy cement and PVC cements to glue things together. Had to play around with getting that drive box and its spacer exactly positioned such that the bridge drive shaft was perfectly perpendicular and free to rotate the turntable bridge with NO friction.

Have not yet placed the gears back in the drive box yet.

 

I'm quite happy with my

I'm quite happy with my finished project, and it is so much more simply than the plaster pit walls of the original, And it is so much more light weight than the original.  I talked with the owner of Diamond Scale about my method, and he appeared to show a lot of interest over the phone. He thought the lt-weight would be particularly attractive in the O-scale size.

However he does not go on the internet, so I had to make zerox copies of my drawings and descriptions and snail mail them to him. Surprisingly I have not received a reply from him, either positive or negative ??

Finished project, but need a

Finished project, but need a new arch as I discovered when I went to clean the old one up, someone had repaired it via solder, and when I went to repair it it fell apart,...
 

PVC Sheet Material Source

Brian and anyone else interested.

TAP Plastics carries the 1/4 inch and 1/8 inch in 4X8 sheets.  Wondering if 2 thicknesses of 1/4 inch laminated together (or 3 thicknesses) might not be good on top of the L-girder joists as the underlay layer.  I used 1/2 inch foam board for the current layout with clear 1/16 thick polycarbonate sheet as a moisture barrier for scenery and ballast applied with a wet glue. I have Ballast Magic now if I ever finalize the track to the point that I can ballast.

I will have to build my turntable before that.

I think I would have to go to an industrial supply to get thicker sheets of the PVC material.

All of the  PVC I got came

All of the  PVC I got came from my sign shop source, including the 1/2" stuff. As I mentioned before it was scraps they were disposing of. They had some white and black stuff, but the black stuff warped to some degree when it set out in our Florida sun.  White would be my choice. I imagine sign shops would know of suppliers.

I don't think you would need any water barrier sheets, etc, as I have found it does not seem to absorb any water.

And I don't think you would need 3 layers of this PVC. The 1/2 stuff is quite stiff.

Installed Today

I used a router to cut the hold in my plywood deck and got my new turntable pit installed today. Rather messy as I cut the hold ever so slightly too close (too small), and had to file and sand the perimeter a little bit to get the pit wall up into the opening. I should have listened to my friend and cut the original hold just a little oversize.

 

underneath
 

I wanted to do this cutting while my staging tracks were absent,...keep all of the sawdust out of the tracks and turnouts.

Excellent construction

Excellent construction technique. It should hold up quite well for years to come. Cheap and durable. Easily modified to other diameters or depths. 

Keep us informed.

Pit-Wall Height Tweaking

Yesterday I decided to attack a little problem in the final 'adjustment' of my turntable installation. I had previously laid down most all the tracks that would be accessing the turntable,...as you can see quite a few...

When I first mounted my new pit up into its cut out hole I had noticed some slight discrepancies in the track heights between the bridge tracks and the adjoining tracks. At that time I just ignored the problem and figured I would attack it with shims when the final adjustments came due. 

Well those final adjustments are now due, and I decided that rather than correct each adjoining track with minute shimming, I should first check out the consistency of my pit-wall's height. I guess I shouldn't have been surprised that it was a little inconsistent around its circumference. No problem,.... loosen the four screws/bolts that hold the pit in place and take the whole pit out onto my outdoors work bench. At first I thought about making various measurements around the circumference, then attempt to sand the high spots down. Wait a minute, why can I just rig up some sort of fixture that I could move around the wall and file the wall down on the high spots.

At first I chose a relatively fine file, but I could see this was going to be VERY time consuming, and to what advantage? I got out my trusty course rasp and made relatively quick work of the job. Then I finished it off with some sand paper.

Here you can see that the pit wall is higher than the 3/4" plywood deck is. The pit wall needs to be filed down to be flush with the deck it is installed in,.....so the height of the turntable bridge tracks will match the height of the stub tracks off the turntable.

BTW, I am VERY happy with my turntable's pit construction. It is very stiff and EXTREMELY light weight. I tried weighing it on a bathroom scale and could not even get a reading. It must be only 1 lb or so,.....the whole pit !!

Weight of that Pit

Wish I had a better scale to establish the weight of the pit structure. These photos are the best example I can provide at the moment,..and they are self photos, so my arms are only so long,...
 

This shows the pit installed up under the deck,..
 

@Brian

Hi Brian,

That's a very cool turntable pit.  I like your ingenious idea to level out that pit rail.  I'm also intrigued by the turning mechanism.  Is that a gear box in there?  Is it a commercial part, or did you fabricate it?

Turning Mechanism

That turning mechanism is the stock Diamond Scale one I removed from the old pit board and glued onto the new one. I intend to just utilize a mechanical turning shaft rather than a powered one.

Notice I did glue it to two white strips of PVC foamboard. That was done for 2 reasons, those white strips add to the stiffness of the base board, and the extra depth assures a firm vertical post.

Details here:  https://forum.mrhmag.com/post/building-your-own-turntable-12215516