MODIFIED ATLAS TURNTABLE
PROJECT

This project might make the reader say to himself, "Why didn't I think of that?"

"Gear Box" (its not really a box) Attached to Atlas TT

The HO Atlas Turntable (TT) isn't really modified. What is modified is the TT results - the indexing. That is, the turntable's actual indexing of 30 degrees is modified by at attachment of two gears to result in 10 degree indexing. The Atlas TT is an intriguing and adaptable design. Its accurate indexing at low cost makes it an ideal component for this project's model railroad turntable for steam locomotives.

In the early 1990s a used Atlas TT at a train show was purchased for $5.00. There was no plan for how to use it because it was too short for the steamers on the roster - but hey, there it was and it was cheap. The Atlas TT sat in a drawer for a few years and then Atlas Model Railroad Company began their Internet forum. Two forum members started a thread about changing the turntable indexing and then they began emailing back and forth - the project began. Realizing that the Geneva Mechanism of the Atlas TT is very simple and imagining it as "motor," it became very reasonable to think about the TT platform driving a reduction gearing setup. That is, while the Atlas TT platform turns 30 degrees, use reduction gears driven by the TT platform to get a lesser turn of degrees. It would take only two gears to do this. First a "drive" gear attached to the Atlas TT that turned exactly with the table and then a "driven" gear that had a larger diameter and thus turned less. It was practical and simple. The two forum members each built the Modified Atlas Turntable. Both were built out of parts in junk boxes, parts available at a local hardware store, and parts from a hobby shop resulting in 10 degree indexing.

The beginning of the project was the "gear box." It is not really a box but simply a support structure or fixture to hold the "drive" and "driven" gears between the Atlas TT and a scratch built turntable bridge.

This project was done for HO scale using the older 30 degree model of the Atlas TT; however, its concept is adaptable to either the newer 15 degree HO model or the old and new Atlas N scale turntables. Also, the concept described herein could be used to drive a commercial turntable kit. The concept is applicable to other degree indexing by using gears having the appropriate ratio.

This model railroad project added a simple gearbox to the turning plate (moving, round disk with the embedded rails) of the Atlas TT to change its 30 degree indexing to 10 degrees and coupled the gearbox to a scratchbuilt bridge and pit. Two matched pitch model racecar gears with a 3 to 1 gear ratio were used. The small gear is on a shaft fixed to the turning plate of the Atlas TT. The large gear is on a shaft coupled to the scratchbuilt turntable bridge. As the under the layout Atlas TT with gearbox turns, the small gear moves in increments of 30 degrees, the large gear only turns 1/3 as much resulting in the scratchbuilt turntable bridge indexing accurately (and very inexpensively) at 10 degrees.

Driven Gear (left) and Drive Gear (right)

This 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 into the turntable bridge. Conditions that might cause the turntable bridge, upon which a locomotive rides, to side to side wobble, end to end wobble, and/or rotationally flex must be eliminated.

HO TRACKSIDE STRUCTURES YOU CAN BUILD, Kalmbach Publishing, Page 73, "AN EASY TO BUILD, LOW COST HO TURNTABLE" by Al Turner.

The Walthers, Korber, and Heljan roundhouse kits each use 10 degree indexing for alignment of their engine stalls to the turntable. Commercial turntables such as the Walthers and Heljan models are too short for some steam locomotives and some diesels. The longest steam locomotive for this project is 13 1/2 inches plus coupler over-hang and the roundhouse is a Korber Models five stall. So, 10 degrees and 14 inches became requirements for this project.

This document was began in the fall of 1996 as a result of some discussions in the Atlas Model Railroad Company's On-Line Forum between this writer and another HO model railroader who were both constructing this Modified Atlas TT project.

The installation of this project's turntable into a model railroad layout to include the entry/exit tracks, the pit wall, the addition of details to the bridge and pit, and the integration with the roundhouse and steam locomotive service areas is left to another project. However, the "rough -in" of the scratch-built turntable bridge and pit floor is included to describe an element of completion for the working drive and index system.