Lionel 2022 Volume 2 Strasburg Mogul

@Norton posted:

It has nothing to do with gear ratio. With worm and worm gears its a function of the lead angle of the drive worm. That is if you look at a worm a faster rising helix has a greater lead angle. Lead angles greater than 11 degrees are capable of being back driven or turning the output gear, the axle gear in this case, will turn the driving gear, the one on the motor.

Less than 11 degrees and the axle is locked and only rotates when the motor turns.

Ok, this makes a lot of sense. I stand corrected.

@gunrunnerjohn posted:

I was of the impression that the backdrive helped with MU consists since the faster locomotive could use that capability not to have them fighting.

It certainly helps in that arena …..no argument there,…

Pat

The MU consist was always the place where I thought the back drive helped.  Of course, I actually do like the fact that I can move a large steamer by hand at times...

@rplst8 Ryan, I've seen back-drivable locos with gear ratios as high as 22:1.  Particularly when the worm wheel is mounted on an intermediate shaft, shared with a spur gear that mates with another spur gear on the driving axle.  This setup affords a further opportunity to multiply the torque.  The Legacy light Mikados do it this way.  Many Legacy locos which drive the axle gear directly are also back-drivable, with a gear ratio of 17.5 or 18:1.  As Pete said, it has to do with the lead angle of the worm.  No matter what kind of motor you are using, it's generally a good thing!  Some aspects of Lionel's implementation have been called into question (i.e., excess side play in the intermediate shaft, mediocre choice of bearing and gear materials, etc.)  But that's already the subject of another extensive thread!