MODIFIED ATLAS TURNTABLE
PROJECT
Drive Motor Replacement / RPM Change

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 turn the clockwork mechanism. The newer design Atlas motor and drive has a belt drive making it quieter but might still be labeled slow. The older Atlas motor and drive have often been replaced by model railroaders. One of the most frequently used replacements is a motor out of a VCR. Some gears on these motors are said to mesh correctly with the gears on the Atlas TT. This writer never throws anything away and had a lot of junk to consider in the motor replacement exercise including an old VCR. However, one of the items in the junk box was an automobile electric radio antenna assembly. The motor was still good - it was the broken Teflon cable drive that had failed. So, to make a long story short, this project's modified Atlas TT is driven by the motor, worm gear, and reduction gearbox from an automobile electric radio antenna. The antenna's gearbox was disassembled and use of motor and worm alone were considered; however, the gearing in the box proved to be very adaptable. The end of the plastic housing on the gear assembly of the antenna drive was cut off with the band saw exposing the large drive gear which meshed perfectly with the gear on the Atlas TT. An amazing result was that the large diameter drive gear from the antenna gear box that meshed with the Atlas gear resulted in a fairly quick translation - it turns faster. However, the smaller index of 10 degrees seems to mask this quick turn and it does not appear as a jerk. No additional description other than the picture will be included on this topic because no one, surely, would ever do this kind of replacement! Power radio antennaes can be found in just about any automobile junk yard at reasonable prices.

Note - This rpm change discussion relates to the older, 30 degree indexing Atlas TT. It may or may not apply to the newer model having 15-degree indexing because this writer has not had the opportunity to examine the interior of the newer Atlas TT gearbox.

The rpm of the Atlas TT is sometimes criticized as being too slow; however, its rpm may be okay when considering the rpm of a particular prototype. The Atlas TT's rpm can be changed with a little gear changing. If planning to change the rpm of this turntable, one must be mindful of torque. The Atlas motor is a high rpm, low torque. The torque required to turn the table and the locomotive on the table for the Atlas TT is achieved by reduction gearing. The rpm increase described below essentially decreases the reduction gearing and thus decreases the torque. So, this concept may not be applicable to all situations. Thus, the Atlas motor unit or its replacement may or may not have a problem with torque reduction if the rpm is increased as described below. In the motor change done in this project, the torque is very plentiful from both the motor and the reduction gears internal to the automotive antenna drive used. In short, a trip to an automobile junkyard can provide a great source for turntable motors.

The Atlas mechanism is a Geneva mechanism. Some people refer to it as a clockwork mechanism. The Atlas TT has two primary turning components: (1) the turning plate and (2) the Geneva drive gear. The Geneva gear in turn is driven by two secondary gears. The Geneva drive gear and the two secondary gears are in a flat gearbox under a cover plate held on by four machine screws. The larger secondary gear protrudes beyond the edge of this flat gearbox and is inturn driven by the worm gear of the Atlas motor unit. Underneath this large secondary gear and on the same axle is a very small gear. This small gear is the focus for rpm change. Below is a side view diagram depicting the Atlas TT gear box.

1. Atlas TT gear box cover held on by 4 machine screws
2. Atlas turning plate/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 diagram above, one revolution of the Geneva drive gear #5 turns the Atlas TT one index unit (i.e. 30 degrees). The small diameter of reduction gear #6 in relation to its drive gear #3 and the gear #5 which it drives is the point of focus when it comes to RPM. Increasing the RPM of gear #5 will increase the RPM and thus the index point to index point rotation "speed"of the turning plate #2; however, increased motor speed may produce increased motor noise. An alternate "way" to get the RPM increase is to increase the diameter of the small reduction gear #6.

Modification if using the Atlas Motorizing Unit:

1. Obtain a gear of larger diameter and matching pitch (say 1/2 inch) to replace gear #6 - lets call it gear #6A.
2. Make a new axle from a brass or steel rod that will match new gear #6A and call it #9A.
3. Redrill (may need to plug first) the square drive hole in gear #3 to match
4. Axle size.
5. Mesh gear #6A and gear #5 to locate a new hole for axle #9A in cover #1 and bottom #7 (it will be to the right of original #9 in the above diagram) and drill those holes.
6. Pin or epoxy gears #6A and #3 together
7. Put the joined gears on the axle #9A with #6A's set screw
8. Reassemble the Atlas TT gear box with the new components
9. Make a shim and base to attach the Atlas motorizing drive unit such that it will mesh with gear #3's new, more outward location.

Modification if using an Updated motor unit having its own reduction gearing:

1. Cut the cover plate #1 and base #7 to remove enough material to expose the gear teeth of gear #5 when the cover plate is reinstalled.
2. Position the drive gear of the new motorizing unit to mesh with gear #5 and affix the motorizing unit to the base of the turntable assembly.

In the photo below is a direct drive change to the Atlas Gear Box. The gears #3 and #6 along with axle #9 have been removed. Also, the gearbox covers #1 and #7 have been cut back exposing gear #5. Gear #5 now meshes directly with the drive gear on the motorizing unit. In this configuration, any amount of torque and speed can be delivered by the automobile antenna motor/gear box unit to the Modified Atlas Turntable. The variable voltage dictates the speed.