Modeling the Turntable Bridge

In previous posts I detailed the construction of the turntable pit and mechanism.  Now we turn to the construction of the turntable bridge, which will fit over the bridge turning plate in the center of the pit.  The bridge is assembled from laser ply sides that are laminated together to resemble the steel girders of the prototype.

The sides were laminated with contact cement, and pressed together with wooden clothespins.  The two sides are connected with cross pieces.  I used ACC to connect the two girder plates.

The cross beam assembly and deck is made of 1/8 inch laser board and rests on the tabs on the top side of the bridge cross pieces.  Here is a view from the bottom.

Here you can see how the bridge looks with the sides fully assembled.  The outside laminate for the girders is very delicate.  Be careful not to break anything, especially if you use contact cement (as I did) and have to make adjustments when putting the parts together!  Fortunately, if you happen to break something, the repairs are virtually invisible once the bridge is painted.

As I pointed out in the previous post, the turntable tracks rest on cross beams or sleepers that rest on the bridge girders.  The top of the bridge for the model has slots for the flex track to rest in.  Along with the laser ply board the kit also contained a pre-cut length of HOn3 code 70 flex track.  When I first tried to line up the flex track ties with the slots in the bridge, I couldn't understand how to fit them together.  Then I reread the instructions, which suggested cutting the tabs that join the rails on the flex track, then sliding the ties along the rails to fit in the slots.  This took some care, as the ties had a tendency to pop off the rail.  Here I am matching up the flex track to the top of the bridge.

Once I had the flex track ties where they needed to go, I put a drop of ACC on each tie where it contacted a rail, to keep the ties from moving.  Then I dropped the track in place.

I had already painted the sides of the track with Floquil rail brown.  I still had to stain the top of the bridge and the cross beams.  Here is how the unpainted bridge looked at this point.

Just for fun, I put one of my brass EBT mikado locomotives on the bridge.  If it looked this good now, just imagine how it will look when painted and assembled!

The next step was to stain the cross beams and top of the bridge.  Actually, I could have left the center of the bridge unpainted since it will be covered with plank decking, but it was easier to immerse the entire top in a bath of gray stain, then let it dry on a paper towel.  The results were pleasing to my eye

After test fitting the top to the girder assembly, I removed the cross beams and painted the girders with Floquil grimy black.  I then used rust colored weathering powders to give a rusty appearance to the girders, and fixed the entire paint job with a coat of Dullcoat.

The planking for the deck comes in three sections, one for each side and one for between the tracks.  I secured the deck with contact cement after staining it with Hunterline gray stain.  The slots in the sides of the deck are where the handrail supports will go.

The turntable revolves around a central bearing, which on the model consists of a turning plate that fits up inside the turntable bridge when the kit is fully assembled.  But the prototype also rests on wheels at each end that run on a circular track around the bottom of the pit.  The support wheel assemblies come as laser cut parts on a sheet of laser ply.

The support wheels are made from pairs of 6 mm N scale wheels joined face to face. The supports are cut from 1/32" laser ply.  Assembly is tricky as these are small parts and have to be cemented at the proper angle with ACC.  On one or two occasions they were also cemented to my fingers!  Be careful because the inside and outside wheel supports are different.  Follow the drawings in the instructions.

Here is what the bridge looks like when fully assembled.  Note that the wheel assembles actually lie on top of the bridge.  Be sure they are turned the correct way.  The top of the bridge then sits on top of the wheel assemblies and holds them in place..

Here is a close-up view of the end of the bridge with the support wheels in place.

The last structural detail to add is the air motor assembly.  On both the Durango and Rockhill (EBT) turntables there is a motor to drive the turntable powered by compressed air from the locomotive through a hose.  I have seen videos of the motor working at Durango, but to my knowledge the one at Rockhill was never used.  So you could leave it off altogther.  However, I chose to include it.  The motor, like the support wheels, is assembled from laser ply parts that include the deck.  Here is a view of the motor in place.

As you can see from the photo, I have begun to mount the handrails, which are the last step in constructing the turntable bridge.  The rails are made of steel wire (included in the kit) which is run through wooden stanchions cut from laser ply.  I painted the stanchions black, but at this point I have not painted the railings.

Following are two views of the deck with the handrails in place.  The wire is bent to follow the air motor control deck.  Simon Cox recommends gluing the end stanchions and one or two in the middle, then running the wire through while adding the additional stanchions as you go.  This is a bit tricky, since you need to line them all up together and the wooden stanchions are fragile.

On the Durango turn table the railings lean out.  On the EBT version, the railings are vertical, which is how I made them.

Here is a close-up of the handrails for the air motor platform.  Note the control handle made of a length of steel wire.

Just for fun, you can see how the finished model will look with one of the EBT mikes sitting on it.

In the next installment, I will illustrate how the completed turntable was installed on the module with the roundhouse, ready for installation on the layout.

The Turntable Bridge

The working part of a turntable is the bridge, which is exactly what it is -- a bridge connecting the rails on two sides of the pit.  In the case of the East Broad Top turntable in Rockhill, Pennsylvania, the bridge is composed of two steel girders held together by steel cross struts.  A few years ago, the EBT tore up the bridge tracks and ties and rebuilt the turntable.  Along the way, they also serviced the bearing on which the table turns.  Here is a photo of  the bridge partially disassembled:

The bridge turns on a central bearing that is so well balanced that two men can turn the turntable by hand.

Like its twin, the 65 foot Durango turntable in Colorado, the EBT turntable came equipped with a compressed air motor that could be operated by connecting a hose to the locomotive brake line.   However, there is no evidence that the motor was ever used.  During the restoration of the turntable, it was possible to see the compressed air motor.

From above, here is a view of the turntable bearing.

The rails sit on sleepers supported on the two main girders of the turntable.

Planks are laid over the sleepers on either side of the track and between the rails.   The handrails are made from iron pipe. Unlike the Durango turntable, the railings are not inclined away from the track, but are vertical.  In the following photograph, you can see one of the stout timbers used by the engine crew to push the turntable by hand. A similar lever is located on the opposite side.  Note also the stains on the sides of the pit made by rusty runoff from the tracks.  The circular rail that supports the ends of the bridge is ballasted with stone, while the interior of the pit floor is covered with gravel and weeds.

The turntable kit made by Kitwood Hill Models resembles both the Durango and Rockhill turntables.   The girders are made by laminating several layers of laser ply, and are joined by wooden spacers.   Rather than laying individual sleepers, the top of the turntable is a single unit which fits snugly over the girders.  Flex track fits into slots on the top, which is then covered by laser cut planking.  In the next installment I will detail the construction of the bridge and show its installation in the pit.

Assembling the Turntable Drive

In my last post, I detailed the assembly of the Kitwood Hill 65 foot turntable pit.  The next step was to mount the bridge turning plate on the upper side of the pit floor, and the gearbox and motor below.  The turning plate is a pre-assembled unit that uses two gears back to back, screwed onto 3.7 mm laser board with two machine screws.  A circular PCB board is held to the plate with doubled sided tape.  The PCB board is scored to allow two gold plated sprung plungers to route power to the bridge track as the bridge rotates.  Here is a picture of the bridge turning plate.

On the underside of the pit floor laser ply boards are assembled into the walls of the gear box.  The primary axle is inserted from the top side and two 60T gears are pressed back to back on the axle  Simon Cox has found that two large gears are needed to prevent slippage.

The idler gear is made up of a pinion gear and a large 60T gear.  It is inserted so that the pinion gear engages with the large gears on the primary axle.

Here is a view of the assembled gear train so far.  The two wires on the left are the power leads to the gold plated sprung plungers that make contact with the PC board on the turning plate, routing power to the track.

The final step is to insert the motor.  The motor is tiny -- about a half an inch long -- with an elaborate gear drive on one end.  In spite of its small size, the motor has plenty of power to turn the turntable bridge.

The motor is affixed to the motor plate with two machine screws.  In attaching the motor, one of the tiny 3 mm screws flew off into hyperspace, where it remains to this day.  I emailed Simon Cox and received an answer almost immediately.  He said he would send another set of screws at no cost.  I was impressed with the service and support provided by this small British manufacturer.  Kudos to Kitwood Hill Models!

Once the motor is secured to the plate, a small pinion gear is pushed onto the motor shaft.  This gear engages with the large idler gear, driving the turntable bridge plate.

The motor plate is secured to the sides of the gear box with small 3 mm machine screws.  Take note that the tabs that join the gear box to the motor plate are not symmetrical.  Be careful to follow the pictures in the instructions.  The parts must be assembled as per the instruction sheet or you will find yourself trying to unglue wooden tabs and slots without breaking anything .... a likely scenario, I can vouch for!

With the bridge plate, motor and gear train in place, it was time to test the mechanism to see if it all worked as promised.  The motor is low voltage (3 vdc) and the kit comes with a battery holder for two AA size batteries.  I powered up the motor, and with very little noise, the bridge plate began to turn slowly.  It worked!

With the completion of the pit and turntable drive, the next installment in this series will turn to construction of the turntable bridge, wiring the track, and inserting the model onto my roundhouse module.

Building the Turntable Pit

The EBT roundhouse and its turntable form a single unit.  Access to and from the roundhouse passes over the 65 foot steel girder turntable.  Some evidence suggests that the EBT purchased the bridge of the turntable from the New York Central Railroad.  A duplicate turntable serves the roundhouse of the Durango and Silverton Railroad in Colorado.

Whatever its history, the turntable has to be installed in the foam base at precisely 70 feet from the front wall of the roundhouse, and the eight tracks of the roundhouse must all line up with the center of the turntable.  I opted to purchase a laser cut kit for the turntable made by Kitwood Hill Models in the U.K.  The price, at slightly over $100 U.S., was certainly reasonable for a powered HOn3 table.  (The model is not indexed and must be lined up by eyeball, just like the prototype.)

I received a box of parts from Simon Cox, who owns Kitwood Hill Models, including laser cut parts for the pit and the bridge, and a separate bag for the motor and gear pack parts.  Also included were detailed instructions for assembly.  Be sure to read the instructions carefully and follow them exactly.  I failed to to this and found myself going back and redoing some of the work.

Many of the laser cut parts must be carefully removed from their carriers.  Usually there are a few small tabs that connect to parts to the carrier.  Use a hobby knife to cut the tabs and gently remove the parts.  Simon also gives good advice about glue (avoid water based glues so as not to warp the wood) and soldering.  The pit rail must be soldered to PC ties, as explained below.

Assembly begins by gluing a stiffener ring to the base plate.  Simon recommends using contact cement for large parts, and I found this to work well.  Smaller parts were glued in place with ACC.  The kit includes small "axles" that go through the parts being cemented to make sure they are properly aligned.

The first major step in building the pit is to construct the base for the running rail that encircles the pit and in the prototype supports the weight of the bridge.  A ring is provided marked with the locations for the 24 PC board ties to which the code 75 rail will be soldered.  The PC ties are glued to the base with ACC.  I glued two pieces of 1/16 inch square basswood into a simple jig to position the ties.

Once the PC ties were in place, wooden ties were glued between them using ACC.  The same jig was used to position the ties vis-a-vis the inner edge of the support ring.

The kit includes a length of code 75 nickle silver unweathered rail for the pit rail.  The rail is formed around a jig made from four pieces of laser board that assemble into a circle.  One of the PC ties is wider than the others, so that the ends of the rail can be soldered to it, as shown below.  Note the metal axles that properly position the jig to the base.

Once the rails are soldered to the PC ties looks like this:

The running rail support is then glued to the pit base.  Note the holes in the base, which will guide the installation of the gear box and motor.  At this point, Simon Cox recommends painting the ties, rail and pit base, as shown below.

The sides of the pit are made by attaching the pit base to the top ring, which supports the pit once it is installed on the layout.  In the preceding photo I have placed the top ring on the pit base.  The top ring rests on 24 laser cut uprights that are glued in place with ACC.

A word of warning.  It is critically important that these wooden uprights be fully inserted in the pre-cut holes in the top ring and the base.  Otherwise the pit will not be level and you will have all sorts of problems when you install the bridge.  I made this mistake, and as you will see.

In the above photo you can see how the pit base is attached to the top ring.  If you look carefully, you may be able to see that some of the wooden pegs that hold the two assemblies together are not fully inserted in the top ring.  Ooops.

Before I realized the problem, I followed the next step in the instructions, and installed the pit walls, which are made from four strips of 1/32 inch plywood.  The strips have to be gently formed into a curve to fit inside the pit against the uprights, and glued in place with ACC.  Be careful!  They break easily.  I know from experience.

It was about this point when I realized that the top ring of the pit was not level.  I turned the pit upside down on a sheet of glass and found that I could rock it back and forth by a good 1/16 inch!  I realized the error, but by then all the supports had been glued in with ACC and the entire assembly was solid as a rock!

The solution I came up with wasn't elegant, but it worked.  I found that the correct distance between the pit base and the top ring should be 1/2 inch when the uprights were fully inserted in their holes. But some of the uprights were not in all the way, making the height off by as much as 1/16 inch.  The only workable solution was to cut one end of the overly long supports with a Dremel tool, insert a 1/2 long inch piece of square basswood next to the original upright, clamp and glue it tightly to the original support post and the the top ring.  It may not be elegant, but the results were worth the effort.  Here is what the corrected pit looks like from the side.  Now the pit assembly rests flat on the sheet of glass, and there is no visible rocking or warping.  Whew!

At this point Simon recommends painting and detailing the kit.  I painted the ties with Floquil roof brown and the rail with rail brown.  The floor of the pit was painted a light brown.  It will eventually be covered with grass, weeds and gravel.  The sides of the pit were painted with Floquil aged concrete.  The holes in the top ring and the cracks between the pieces of plywood were filled with spackle and then painted.

In the next installment, I will detail the construction of the turntable bridge, the gear box and the motor that operates it.