Bachmann Hall - Surging Motion

[Right Away]

I'm sure a similar issue has been raised before but I'm danged if I can find the results.

 

I would like to identify the cause which has always, since new made a Bachmann Hall appear to move in a surging motion at low to medium speeds. It is most noticable in the effect it imparts on the coaches as they "to and fro". It is as if the drivers are not absolutely central to the axles, ie in a slightly eccentric fashion.

 

(Bachmann can't have intentionally incorporated the feature that can occur with long stroke 2 cylinder locomotives).

 

The loco, No 6988, is fitted with a Lenz Standard decoder and is otherwise a very good runner.

[34theletterbetweenB&D]

Take the decoder out and test on 'vanilla' DC for easiest analysis (the decoder will be attempting compensation and may mask some of the effect).

 

First question is what the repeat period of the surging is.

 

If it is synchronous with driving wheel position, then it has to be caused somewhere among the axle gear, wheelsets, crankpins and rods.*

 

If it isn't then you are looking at motor and gear train.

 

You could be really unlucky, and have problems in both of course, but that is unlikely as you say the model otherwise runs well. The implication of that is that the cause probably isn't a serious defect, it is just a matter of find and rectify.

 

*Have you ever had the driving wheels out of the chassis block? If one of the small brass collars in which the axles run wasn't seated correctly in the block at original assembly you can get exactly the effect described, and a simple removal and reseating can fix it 'just like that'. Also taking off the connecting rods, just in case a crosshead or piston rod is binding, usually at the forward end of stroke.

[Right Away]

Thanks 34C for your help - again!

 

As the problem is in step with wheel rotation, I'm probably looking at dropping the drivers; not exactly enamoured by that thought, having never had a Bachmann in pieces before.

 

I'll put her back to DC when I have time and work from there. Operating as a "visiting" engine, she only runs occasionally but I am determined to get to the bottom of this.

[Right Away]

Well, finally got around to dropping the driving wheels of 6988. The driving axles on this loco (31-780) have no brass collars.  They run in plain grooves in the chassis, held in place by the bottom; plate. All appears well in this area.

 

I have noticed that the left hand leading coupling rod is a somewhat tighter fit on the crank pin (big end-end) than the other side and will need to be taken down for inspection/rectification.

 

Not wanting to strip a thread, would any member know if the left hand crank pin threads on these models are "handed" to avoid loosening on rotation? 

 

Thanks for any confirmation.

Edited August 6, 2019 by Right Away

[34theletterbetweenB&D]

No, all regularly threaded, anti clockwise to remove.

[Right Away]

Thanks 34C.

I took down all the rods (which were not binding after all) and removed the drivers for inspection. The slots in which the driving axles sit look fine and the axles and wheels themselves appear OK when rolled - no perceptible wobble. On reassembly, the issue still remains. Next course of action looks to be replacing the wheelsets to rule out any possible minor imperfections in wheel alignment (when I get round to it).

 

Memo to self: Should have returned loco immediately after purchase and within warrantee instead of hoping that future running would eventually smooth things out!