Percy and the loose drivers

The more I drill Percy on the test track, the more the little engine mystifies me. Up until now, I’d cherished the theory that a shifting driver could cause it to lose quarter and ultimately bind the mechanism.

To probe this theory, I deliberately reamed one of Percy’s front drivers (the geared axle) so it could slip more easily on its axle. This configuration failed to cause any issues, so I reamed it some more, and then some more. Finally, the wheel was almost free to spurt right off the axle.

Even then, my efforts were fruitless. Indeed, when I deliberately slipped the loose driver out of quarter, it seemed to right itself before Percy had banged up against the end of the test track the first time. Certainly, it never got worse.

My efforts were fruitless again and again until finally there was a derailment. It’s always hard to say what causes this — I’m astonished 3D-printed Proto:87 wheels work at all, and the power pickups occasionally catch on the ties. Either way, Percy was on the ties and holding one connecting rod at an obscene angle. I fixed the power pickups and re-quartered the offending wheel, and it happened again. It kept on happening until I took the engine apart to examine it. This time, not only was the front axle out of quarter, but the back one was too: the rear wheels were about 120 degrees to one another rather than 90 degrees.

How the rear axle got to 120 degrees is a stumper. Certainly those loose front wheels were in no condition to shift the stiff rear ones. However, I’ve always thought that fidelity to 90 degrees was unimportant. So rather than fiddle with the rear axle, I matched the front axle to 120 degrees as well. Sure enough, Percy happily returned to grinding back and forth on the test track.

Other than the dalliance on the ties, there was no explanation for the newly angled rear axle. So, I reamed one of those wheels as well to see if I could replicate the problem. Now both front and back axles are able to move almost independently, and while they do occasionally bind, for the most part they actually work fine.

The stumping continues.

2 thoughts on “Percy and the loose drivers

  1. If it’s not related to the wheels, rods, or quartering, the only thing that it leaves is the worm gear and motor. Can you try running these same tests with the 622 motor and worm gear using Percy axles, wheels?

    This is really interesting to follow and makes me sure glad I don’t have these steamers in my fleet. 😉


    1. Thanks Craig. I’m glad you’re enjoying the process.

      The challenge with running with 622’s drive train is that it has a 1/8” axle, while Percy’s frame admits a 3mm axle. I still have some hypotheses that I can try out without building a new frame. So, I’ll do a few more experiments before going down this road.

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