Armature torque, wheel quartering, and drive rod position ...

Alright guys ...

I've got my techno-bennie thinking cap on for this one, and I find myself getting stumped. And perhaps over something that makes very little difference, if any at all. However ...

In a recent thread on Magnetraction, ServoGuy noted this point:

servoguy said:

The 3 pole motors in the older locos have a torque ripple of 2x. The peak torque is double the minimum torque. This is why they don't run smoothly at low speed. The can motors have more poles on the armature and so are smoother. I will say again that I get better low speed running by oiling the commutator with 5W-20 engine oil. I know this sounds crazy, but it works great. I started doing this a year ago and have had no trouble with the oil getting gummy or burning.
Bruce Baker

Click to expand...

Furthermore, you seasoned guys have told me that it's critically important to set the "wheel quartering" position just right on the drive wheels of old steamers. As I understand it (which is limited!), one wants the "drive crank" on the left wheels to be EXACTLY offset 90-degrees in rotation from the drive crank on the right wheels. That makes sense to me ... especially if one were talking about REAL steam trains, where the drive rods were pushing the drive cranks and wheels, rather than the other way around (on our model trains), where the wheels get turned from the motor, and the drive rods pretty much go along for the ride.

But ... there is friction in our model drive rods, which must be overcome with work and motor power. And the motor power is a function of how much motor torque is available. Which gets us back to ServoGuy's initial point.

I'm asking / discussing all of this because I'm rebuilding a little Lionel 1681 loco. My drive wheels have been removed, and I need to reinstall them. I know that I need to "quarter" them properly, with the cranks offset 90-degrees of rotation. However ...

(And here comes the long-winded question ...)

If motor torque is non-constant, and the drive rods produce friction, then how exactly should I set the quartered position of my wheels with respect to the contact brushes and three-segment division of the spinning armature? Is there some ideal / preferred mapping or correlation of the two that would gain the max net power?

As I'm typing this, I realize (now, for the first time) that the RPM of the armature is not the same of the RPM of the drive wheels. Power (or torque, really) is transmitted through a reduction gear or two. And I have no idea if the reduction gear ratio (tooth-to-tooth ratio) is an integer number or not. If so, then wheel rotation position would always fall in the same place with respect to the armature rotation position. I.e., 4 rotations of armature equals exactly 1 rotation of wheels, or something like that. And if that were true, then the motor torque "harmonic" (per ServoGuy) would be directly translated to a harmonic power in the wheels. And if that were then true, would we want to locate the drive cranks (and drive rods) in a certain "ideal" position, as far as the armature / brushes were concerned.

I suspect that if there is any theoretical effect, it's probably miniscule in terms of real observed power. My guess is that the reduction gear ratio pretty much filters out any armature harmonic by the time it gets to wheel rotation.

But, I figured I'd babble here anyway, for no other reason than to put myself to sleep (it's late, and I gotta get to bed), and -- in the off chance that any of you are wearing your techno-bennie hats -- to see if you had any babble to add to the mix!

Sleep well, everyone!

TJ