Sharing the answer to a question I received this week:

Q: What effect will a mistake in traction motor gear ratios have on unit operating characteristics? i.e.: One motor with a different gear ratio or motors on one truck having a different ratio than motors on the other truck?

A: As the locomotive starts moving and accelerates it will generate a continuous wheelslip indication. Actual speed at which it appears depends on many factors: Speed, load, motor type, mismatch of the ratios, type of wheelslip system, which and how many motors have a different ratio, and also any wheel size mismatch.

To offer a very specific (1974) example I can tell you of a case of three 3000HP C-C locomotives with D77 motors and IDAC wheelslip detection that had trucks misidentified due to a stencil error at the factory. Two "freight" units supposed to have 62:15 motors each got one 60:17 truck and one 62:15 truck due to the stencil error. Both units behaved identically, alarming between 15 and 19 MPH, and attempting to push them faster resulted in 3rd stage IDAC actuation and load shedding around 25 MPH.

One of the "passenger" locomotives, supposed to have 60:17 ratio, got both the misidentified 62:15 trucks. It ran its motors over normal rated speed for a couple days while the problem with the freight units was being pinned down, with no problems evident. The motors on its misidentified trucks were inspected and found to have no damage.

Trucks were swapped between units to correct the errors. The customer was given an extended warranty on the 62:15 motors that had been running overspeed, but had no overspeed related failures and never applied any claims.

A bad enough wheel size mismatch will display very similar problem indications.

PC

Even though I have no practical need of this knowledge, I LOVE reading about stuff like this. Please keep it up!

I've never heard of that happening, but once upon a time, I had ten units off the pit at Argentine, on a train I was taking out to Newton. A WS light kept coming on and off on our way out to Turner to get on our train, After the air test, we departed and the WS light still kept coming on and off. I had a trainee, so I had him pull us slowly out of the yard out onto the main, while I walked the consist isolating units one at a time trying to figure out where the light was coming from. Of course it's always the rear unit, in R-1 or 2, I saw the load meter slowly peg it'self, turn on the light, drop the load back to zero, then build up to the peg again, turning the WS light on and on. So I just isolated it. It was a GP7, going back to Newton or Wichita for a switch engine, as were about half the units in the consist. We took the power to the house at Newton after yarding the train, we tied up and went to bed. Next day, we were going on duty at the roundhouse, and I saw the foreman there and asked him what was wrong with that unit. He said it had been in the shop, and they put a couple of re-built trucks under it, but the hooked the leads up wrong, one truck was wanting to go west, the other truck was wanting to go east. He also said it was a good career move for me that I spotted the problem, and isolated the unit.

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It's interesting, I've had several GE guys tell me that you can freely mix the 74-18 and 83-20 in the same locomotive. We were doing a wheel replacement in a U23B a couple years ago and when we broke down the 4 combos that came out it had 3 83-20 and 1 74-18.