TURNTABLE III    From the September 2001 CALLBOARD

The March & May THINK SMALL essays listed some advantages of having a turntable on the layout. My turntable slipped into the story. Then a reader asked some questions. A “some day I'm a gonna" project became more current as my answers tumbled out.

For over fifteen years a simple stick of wood with a carriage bolt poked through its center supported a section of four-rail track that functioned as my turntable. An "Ancient One" from the days of brass rail soldered to nails for track should do better than push a stick with his finger to let old number nine out of the roundhouse. I felt humbled. Then one of those electronic catalogs that seek out model railroaders came in the mail. A 6 RPM 12 volt DC gear-motor for just $12 caught my eye,

The burr under my saddle was active. The check book took the hit and an order was in the mail.

About a week later my mail order gear-motor arrived. The unit has a 3/8" square output shaft with lots of torque when four to twelve volts is supplied. The ammeter indicated less than 1/4 ampere of current as I tried to keep that square shaft from turning. I had no excuse. A suitable drive motor for my turntable was on site. The project provided plenty of hobby time - also a humbling dose of education.

The Bodega Western is a depression era short line. Naturally the design for the drive mechanism should conform. How would the V & T do it ? The old timers ran whole shops with belt drives. To power the belt we enlarged the hole in a wood sewing thread spool to 3/8" square. With a spool on the square shaft of the gear motor and a jar lid for a pulley on the turntable shaft, it was the Ancient One's style of model railroading. The belt connection between the two had to follow the pattern. Ma's sewing supplies had an offering to do the job - elastic tape. This drive belt material comes in various widths. My choice of the 3/4” width was fortunate. The arrangement was funky enough to make me smile. However, there was a problem. The lashup did not do the job. The motor turned the spool but the turntable just sat still! Finger poking was more powerful than I thought. Some motivated observation of my turntable revealed annoying defects. 1- Those brass slabs at each end of the bridge made good contact with the pit rail but they also provided lots of friction with a scale 41 foot lever arm from the pivot point. 2- The pivot was not at the exact center of the bridge. The long end of the bridge gently kissed the pit wall at several points. 3- At several spots, rail ends touched as the bridge revolved. (A good explanation for the solder joints coming loose from time to time.) These were minor annoyances during manual operation but big trouble if I expected to motorize the table.

The offending rail ends soon yield-ed to a few strokes with a file. Likewise, a trip to the sander made the long end of the bridge a better guest in the pit. Friction proved to be a persistent problem. An abrasive disk in the motor tool made quick work of reducing the area of slab sliding on the pit rail by 75% or more. This helped a little but not enough. Why not opt for a shorter lever arm? I added washers, common and shim, at the pivot bushing; just enough to give the support slabs a gnat's eyelash of clearance above the pit rail. (To assure electrical contact, small phosphor bronze wipers were added to the slabs at each end of the bridge.) The weight being transferred to the pivot bushing assured that adverse friction forces now worked via a much shorter lever arm (about 3 or less scale feet instead of 40 or so). A lazy susan bearing for the table is another option to reduce friction but a lot of work so I put that idea on the shelf.

The drive motor is robust. More of the useful friction effect between belt and pulleys would allow the motor to move the table and let engine nine out of her stall to pull the local. Thinking contact area, I made pulley sets for the 3/8" square drive and the carriage bolt. The 3/4" elastic tape is a tad underside. Just right for a 3/4" pulley face. Most of us can find scraps of 3/4" plywood and 1/8" masonite around the shop, Some time spent with jig saw and sander yielded pulleys for experiments. Their approximate diameters range from about 3/4" for the spool through 2", 2 3/4”, 6 "and 9 ". Bits of aluminum angle and a hole in the carriage bolt allow a simple wire pin to couple our homespun pulley to the table's carriage bolt pivot.

Friction can be friend, foe or both. With belt drives we need enough friction working for us to overcome the force working against us plus enough surplus to get the job done. More area increases the friction effect. Bigger pulleys provided more contact area between belt and pulleys. Today the pulley on the drive motor's square shaft is about 2 3/4" diameter and a 9" diameter pulley is pinned to the turntable's pivot bolt. The friendly little 12 volt motor provides ample torque and the pulleys contact enough area of belt to make good use of that torque. A combination of parts that worked took a while to find but learning isn't always easy. The increased area of contact between belt and pulleys coupled with the reduced force now needed to turn the table spelled victory for the shop force.

Applying depression era and short line thinking to my turntable

problem was fun. I learned that friction can be useful under special conditions. After all, if there was no friction between drive wheel and rail our trains would just sit still with their drive wheels spinning.

Next time we will look at the gear motor's electrical power and control circuit. How short line and depression era engineering does the job with a minimum of stuff.

Sweep the cobwebs off your workbench and enjoy some model railroading.

Bill Williams The Ancient One