The Design
(I included this second excerpt just to display the posting's professional approach beyond the opening pages that were not very interesting, ...Brian)
The Design
As I mentioned earlier, I was very inspired by the article "TURNTABLE AT
MALFUNCTION JUNCTION ON THE TETON SHORT LINE" by Wayne Roderick
( http://www.tslrr.com/turntabl.htm). I have basically used Wayne's design
with a few minor changes. Big kudos Wayne!
Having the bridge supported by only the dollies that bear on the ring
rail in the pit ensures good vertical alignment with the track on the
rim. A heavy engine moving onto the bridge will not cause any rocking
of the bridge. The shaft is affixed to the bridge with a long flat bar
of brass, flexible in the vertical direction and very rigid in the
horizontal direction. This allows the bridge to follow the shaft
rotation without any backlash while being very insensitive to any
vertical positioning error.
Underneath the pit bottom, affixed rigidly (and without backlash) is
the positioning (or indexing) disk. It has positioning vanes attached near
the rim, corresponding to the positions of the tracks to be indexed above
on the layout. The positioning vanes interrupt a light beam, which is
detected using 2 light sensors. I used an infrared LED as the light
source and IR Photosensing transistors as sensors. This makes the system
relatively immune to stray light. Using two sensors makes it easy to
determine when a positioning vane is in the exact middle point between
the sensors - just turn the table until the voltages from the
sensors are equal. This system is also relatively immune to variances
in the shapes of the vanes (although Wayne gives excellent hints
as to how to produce nearly identical pieces).
I deviated from Wayne's design by replacing the ring rail in the
positioning disk with inlaid ring of PCB board. The vanes are flat
and a circular rabbet cut around the perimeter of the disk provides
clearance for the sensors so that the vanes can pass between the
sensor and the light source.
The attached design picture will hopefully clarify what I am trying to
say above. The inlaid PCB board is shown in a deep green colour.
(In retrospect, Wayne's ring rail design would
probably have been easier to implement 
Another deviation from Wayne's design is the way the positioning
disk and shaft are supported. In his design a center bearing below
the shaft carries the weight. The motor presses on the rim of the
positioning disk and moves it by friction.
My design has three bearing wheels supporting the rim of the
positioning disk. They are positioned 120 degrees apart. Two of
the wheels are free-rolling wardrobe door rollers I found in my
junk box. The third wheel is the drive shaft from my salvaged
motor gearing. The drive shaft has a piece of silicone tubing
slipped over it to improve the grip. The 1/3rd weight of the
positioning disk is enough to provide sufficient grip for the
drive shaft with the tubing.
The center shaft is supported by the positioning disk. Below
the positioning disk is the shaft support bracket, which
keeps the shaft vertical.
The pit bottom is cut a couple of inches larger than the outside
diameter of the pit rim wall. This forms a flange that will be used to
attach the turntable from below to the bottom of the layout. Shims
will be used to get the pit rim to exactly the correct height
for the approach tracks.
In the next journal entry I will discuss the design of the electronics.
