Robotics: Popular Science, April 1971: The Transmission They Said Wouldn't Work
Posted on 2012-03-25 @ 01:14:53 by r00t

I found this article while reading the issue on Google Books; it really shows an interesting and somewhat unusual application of a motor's torque both from the shaft and the motor casing!

The Transmission They Said Wouldn't Work - Popular Science, April 1971

The design uses a brushed DC motor (the inventor, Ed Kidera, used a car starter motor; in the article it also shows how he wanted to use a hydraulic system as well), modified to allow the power to be fed into the brushes via the output shaft and via a mount to the the case of the motor (obviously the shaft and the mount need to be insulated from one another); the mount is built so that it too has a shaft. One shaft (in the case of the article, the motor shaft) is connected to a wheel via a 1:1 spur-gearing, while the mount shaft is connected directly to another wheel. The following picture (from the article) shows this basic arrangement:

Ed Kidera - Motor Differential
Questions or Comments?

Now this is where it gets interesting: In normal operation (steering straight ahead), the shaft of the motor and the case move in opposite directions (which is where the gearing comes in), causing in an 2:1 reduction drive! When the wheel is turned, torque from the motor is evenly divided between each shaft, resulting in an automatic differential action! Strange, I know; but the article details how it all works.

Now, I know a lot of people when they build robots prefer to use two motors and a differential drive (to handle steering, instead of something like Ackermann steering arrangement), but I think this kind of system could even be used on a differential drive system: Just use a single motor in the same torque division arrangement, and brake the side you want to slow down instead.

You could also use this on a 4WD system; two possible arrangements come to my mind:

  1. Keep the front and rear differentials, but put this motor arrangement in place of the transfer case/center differential/transmission; the automatic differential action would take their place.
  2. Use one of these systems for the front and the back wheels; when steered, the automatic differential action would work for both, and any front/read differential action would also automatically be adjusted for, I think (this is potentially questionable, though).

The only "downside" that I see to the system (at least, in the case of the inventor's prototype) is the offset of one of the wheels caused by the 1:1 spur gear system; this is really only an aesthetic issue more than anything, though, and could possibly be avoided (definitely something for further development and improvement).

I think it is an interesting application of a motor, no matter what the power source; maybe someone could use a similar arrangement on their next robotic project?

UPDATE: The following "Reader's Talk Back" letter was published in June about this transmission:

RTB on Impossible Transmission
Questions or Comments?

The points brought up are interesting. The historical basis of this "new transmission" would be worth looking into. The issue about the offset wheels I already noted (in a robot this might not be that big of a concern, though). I'm not sure about the "unsprung weight" comment, either; if the motor is affixed to the frame via bearings on the shafts, then outer shafts to the wheels could be sprung via a traditional suspension system and CRV joints (indeed, even if the shafts passed through bearings on springs, like a regular axle, how would this be different than a regular differential?). The comment on brush lifting due to centrifugal force is pertinent, though: This could potentially be solved by using a brushless motor (which would also lessen any weight issues, especially if using a so-called "coreless" motor).

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