THE IDEA: An externally mounted two gear set having a 3 to 1 tooth ratio driven by the Atlas HO Turntable's turning deck which has 30 degree indexing via its Geneva Movement results in a ten degree index.

The Walthers, Korber, and Heljan roundhouse kits each use 10 degree radial spacing for alignment of their engine stalls to the turntable. The roundhouse for this project is a Korber Models five stall making the 10 degree indexing a requirement. The longest steam locomotive for this project is 13 1/2 inches plus coupler over-hang so a 14 inch bridge is the requirement.

This project began in the fall of 1996 as a result of a discussion in the Atlas Model Railroad Company's On-Line Forum between this writer and another HO model railroader. Both of these individuals constructed this Atlas TT modification. This project takes advantage of the HO Atlas Turntable's (TT) intriguing and adaptable design which uses a Geneva Movement. This project was done for HO scale using the older 30 degree index model of the Atlas TT picked up at a swap meet for $5.00. The concept could be adaptable to the newer 15 degree index Atlas TT HO model or the old and new Atlas N scale turntables - all depending on what gear ratios are required. The concept might be adaptable to drive a commercial turntable kit. The Atlas TT's accurate indexing at low cost makes it an ideal component for model railroads. Using reduction gearing via two model race car gears, the Atlas TT's 30 degree indexing was easily "modified" to obtain 10 degree indexing. Also, a fourteen inch bridge for an open pit was constructed AND the Atlas drive motor was replaced.

Reduction Gears Attached to Atlas TT
(A "ladder" attached to the edges of the TT holds the gear shafts in place. The TT turning deck moves freely beneath the "ladder.")

A simple attachment of two gears - the smaller gear on a drive shaft affixed to the TT turning deck (moving, round disk with the embedded rails) of the Atlas TT and the larger gear on a drive shaft connected to a scratch built bridge in an open pit. Two matched pitch model race car gears with a 3 to 1 gear ratio (i.e. tooth ratio) were used. As the under the layout Atlas TT turns between Geneva Movement stops, the small gear moves in increments of 30 degrees, the large gear only turns 1/3 as much resulting in the scratch built turntable bridge indexing accurately (and very inexpensively) at 10 degrees.

The gears used in this project are for RC Model Race Cars and were purchased at a local hobby shop. The gears are 48 Pitch and are as follows (Note: The web sites below have changed quite a bit over the years and are not as "friendly" as in the past. It is suggested you visit a hobby shop that sells RC race car parts.):

  • The small gear has 29 teeth with a hub having an Allen Head setscrew and a shaft size slightly less than 1/8 inch. It was made by Robinson Racing.
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  • The large gear has 87 teeth with a 3/8-inch center shaft hole but no hub. Numerous patterned holes are on the face of the gear for attachment to different hubs. It was made by Kimbrough Products.
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For the two gears' teeth ratio, 87/29=3, the requirement is met. The project is simple, but does require effort to ensure that all features align properly, do not bind, and do not cause any free or "slop" motion in the turntable bridge movement. Conditions that might cause the turntable bridge, upon which a locomotive rides, to side to side wobble, end to end wobble, and/or rotational flex must be eliminated.

Small Gear on Shaft to Atlas TT & Large Gear on Bridge Drive Shaft
(The setscrew in the small gear is used to clock the bridge location to a roundhouse track location.)

Pit Floor with Edge Ring Rail and Bridge
(Ring Rail made by centerline sawing flex track segment.)

Bottom of Bridge
Pivot Axle Connects to Large Gear Drive Axle
Brass Pulley Wheels on Clevis Pins for Carriers

Connection of Atlas TT & Reduction Gears Thru Pit Floor to Bridge

The older design Atlas motor and drive gearbox is both noisy and relatively slow. The motor and drive gear assembly turns a series of gears in the Atlas TT which turns the Geneva mechanism. Below is a diagram and list of the gear drive items on the Atlas TT.

  1. Atlas TT gear box cover held on by 4 machine screws
  2. Atlas turning deck/Geneva primary movement
  3. large secondary drive gear
  4. worm on Atlas motor unit
  5. Geneva drive gear with perpendicular clocking pin
  6. small secondary (reduction) gear
  7. bottom of Atlas TT
  8. Geneva drive gear axle
  9. secondary drive gear axle (one piece integral with reduction gear)

In the above diagram/list, numbers 3, 4, 6, and 9 were removed. Plate number 1 was cut back so that the drive gear from the automobile antenna drive meshes directly with gear number 5 in the above diagram. The atenna drive gear box was also cut back to explose the primary gear. The result was a faster turning unit that still stops at 10 degree increments. Also, it is very quiet!

Replacement of the Atlas Drive Motor with an Automobile Antenna Drive
(The Pitch of the Atlas gear #5 was amazingly "close enough" to that of the antenna drive gear.)

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