This is the first turntable on the list as it is in the section of the layout I am working on. Here’s a prototype image of that turntable. You can see it in the background.
Here’s an image of the NPC turntable at Cazadero. Note both the Cazadero and the Sausalito Yard turntables are similar in design and are pit turntables. I suspect all the NPC turntables had similar designs. You can see that the cable rods are tightened using turnbuckles. They appear to be installed by drilling a hole through the gallows base and probably adding a bolt and washer to the underside. The bolts on the top of the gallows lateral elements appear to tie the turntable to the joists underneath. The pit is relatively shallow — just enough to allow the joists to rotate. The track appears to be nailed directly to the joists. The turntable rotates on a central pivot and is held on the two ends by trucks that ride on a circular rail. Rotation is accomplished by one or more trainmen pushing on the staves that protrude in either end. I assume some casting is on the end of the stave nearest the turntable as you can see the bolts that attach the stave protruding on the inside of the lateral supports.
The A frames themselves are stabilized by a lateral support about 1/3 of the way up the stave. I suspect a casting is used for attachment to the A frame as you can see the bolts on the other side of the A, placed too far apart to be directly attached to the cross piece. Here is a second image of the same turntable.
A cross piece at the top of the A’s keep them in alignment. That cross piece is supported by a 45 degree diagonal on each side. Once again it is likely castings are employed on the cross piece. In a second you will see there is a marked similarity of the Cazadero turntable to the turntable at the Nevada Railroad museum we will look view a bit further down the page..
The Nevada State Railroad Museum has a turntable based in Southern Pacific drawings.
The Laws Railroad Museum in California also has one.
This image shows the Owens gallows turntable. It is also based on the SP design.
The Model – Components and Design
While this post focuses on the Sausalito turntable, the approach to construction will be the same for all 3-4 turntables. This turntable will be constructed using the following components.
- Atlas Turntable and Motor Unit. About $70 new.
- NBWs were obtained from Grandt Line for $3.00,
- Other castings can be fabricated using styrene.
- Piano wire or heavy duty thread will be used to model the cables.
- Turnbuckles were obtained from Grant Line makes a turnbuckle cored for 0.015 inch wire. 0.015 wire translates to roughly 1.2″ in HO scale. 24 in styrene were purchased for $3.30.
- Track rails and the circular pit rail can be taken from a bit of HOn3 flex track.
- Pit trucks will be fabricated from small pulleys.
- Staves can be fashioned from stripwood.
- Turntable wood elements constructed from strip wood – detailed later.
Because I expect to assemble 3-4 turntables, the parts can be made on an assembly line basis. Those that are purchased can be purchased in quantity.
Turning the Atlas Turntable into a Pit Turntable
The Atlas is a flat turntable – no pit. Here is what is inside the turntable box for a 9″ Atlas Turntable. All components shown are included in the turntable box except the electric device that rotates the turntable. Note the raised castings outside the platter that separate the 21 indexed positions on this turntable. Users have the option of using a hand crank to rotate the turntable, which is included but is not very prototypical. Or they can purchase the electric device separately to replace the hand crank. The center pivot is a screw that takes a small square head screwdriver bit.
The turntable is 1/4″ thick in the area where the hand crank mounts, and a bit thinner elsewhere.
The reverse side of the turntable displays its electrical contacts. A small nut in the middle is the underside of the pivot. This next shot shows the electrical unit disassembled.
The turntable power is transferred from the can motor to the turntable using a small worm screw. What is really cool about this unit is that the turntable is self indexing. Turn the handle or run the motor until the unit reaches the next of the 21 indexed positions. It pauses there for a number of rotations then moves on to the next position. So how about that – a reliable motorized indexing turntable for around $70. The problem is the turntable is toy like and it is not a pit turntable. It is our job to take advantage of the turntable’s mechanical capabilities while turning this into a craftsman pit turntable. I was encouraged by the fact the pivot is screwed in place rather than riveted. A bit later, we will certainly use it to disassemble the turntable to see what is inside.
Additional Turntable Parts
I plan to support the turntable in the pit using 1/8″ lamp parts. The nipples are hollow in the middle allowing wires to be fed from below. Each turntable will require two cross bars and one nipple. A bottom crossbar will be mounted to the rotating platter of the Atlas turntable. A top crossbar will be mounted to the underside of the joists of the gallows portion of the turntable. The threaded rod will run between connecting the two.
- 1/8″ IP x 1/2″zinc plated pipe nipples – 12 ordered from e-Bay at around 60 cents apiece including shipping.
- 4″ Threaded Cross Bars – pack of 10 ordered from Amazon at around 79 cents apiece including shipping. Per unit costs drop dramatically with larger orders as the shipping is well over 50% of the cost of both nipples and cross bars.
Atlas Turntable Modifications
Judging from the prototype photos, the pit is likely to be 2-3 feet deep. Check out the Cazadero photo. The man in the background appears to be sitting on the top of the turntable wall. I used that to estimate the height of the wall. The floor joists rotate at the top of the pit with the top if the joists level with the top of the pit. Here is the sandwich I am considering. The pit height in this example is 40″.
The Atlas turntable is indexed to stop every 15 degrees. The Banta Sargents Roundhouse has a stall every 9 degrees. So a gearbox will be constructed that reduces rotation of the gallows from 15 degrees to 9 degrees. I ordered the following two gears. The left gear has 75 teeth, The right has 125 teeth. These gears are used in radio controlled race cars and are produced by Robinson Racing.
The left gear will be attached to the center of the Atlas turntable. The right gear will be attached to the underside of the gallows. When the Atlas turntable rotates the left gear 15 degrees, the right gear will rotate by 9 degrees. I’ll post photos of the gearbox and the under surface mount of the Atlas turntable once the gears are on hand and the design complete.
The idea for this modification came from an article found online. The following is a photo of the gearbox in the article. The left gear and shaft is mounted to the top of the Atlas turntable. The right gear and shaft rotates the gallows. His gear ratio is different as he used the old Atlas turntable with 30 degree rotation and wanted the rotation reduced to 10 degrees. Note that the left shaft is offset from the right. I suspect the distance between shafts in my case will be in the range of 1 1/2″ That means I can swing a longer bridge than 9″ without interfering with the motor unit on the Atlas. That is an important advantage in that some of the locomotives I would like to run are likely to be longer than 9″. I may go to a 10 1/2″ bridge.
This is the complete unit. Mine will need to take less space vertically because of my layout sandwich concept. But I have 3″ between the bottom of the layout surface joists and the top of the surface to work with, which should be plenty. I can save vertical space by fabricating the gear box out of aluminum or brass
This is the top of his turntable pit. Mine will be similar but will contain a gallows bridge.
This is the underside of the bridge that rotates. Note the use of brass pulley wheels and clevis pins to interface with the track on the previous photo.
Small brass pulleys are available from Proctor Enterprises. The pulley blocks below are $6.95 apiece and two would be needed per turntable. Width at top of rail is 0.03″ so the pulleys would ride nicely in the wheels of the block. Blocks would need to be disassembled to free up the wheels which could be mounted on clevis pins as in the above photo.
A Frame Modifications
If A frame vertical elements are end glued to the lateral element in the middle of the lateral element, the joint would be very weak. I am concerned about the strength of the end grain glue mount and the fact the thread, while providing compression to the joints, does’t provide any lateral structural strength. This drawing claims to be based on the same Southern Pacific prototype and would be much stronger than an end glued joint. The horizontal elements are three pieces, allowing the vertical elements to be pinched between the pieces. This design is consistent with the construction of Howe Truss bridges. Note also that the floor joists are deeper in the center section than on the outer portions.
These are the takeoffs from a printout of the drawing. The nine inch dimension in HO was used to derive all other measurements.
This photo of a completed model shows how the short vertical post in the middle of the A frame is used to support the angled portions of the A. Note also, the cables are mounted to the floor joists (small eye bolts?). The center post of the A is extended to the full width of the horizontal element above the cross support. this allows the cable to run on the inside and outside of the angled portion of the A.
I am also going to consider using a heavier duty sewing thread or music wire to provide reinforcement.
Strip Wood and Thread Required
I ordered the following strip wood from BlackBearCC.com
- 1/8 x 1/8 (6/bag) – Two bags – Used for outer floor joists, inner portion of horizontal members, A frame members, inner portion of short vertical member, horizontal braces.
- 1/8 x 1/4 (5/bag) – Two bags – Used for inner floor joists, diagonal braces.
- 1/16 x 1/4 (5/bag) – Two bags – Used for inside and outside of horizontal member, center vertical member, horizontal braces across track.
Thread Required – About 50 inches of heavy duty thread. My wife who is a sewer gave me a roll of buttonhole twist thread that comes in at 0.014″ which should work beautifully with the Grant line turnbuckles.
Track could be spiked to the joists or glued to the joists. It will need to be centered and gauged.
Building the Turntable
Now that the design is mostly finalized and materials on hand or on order, it is time to begin construction. See the links just below this one for step-by-step on various phases of construction.