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Thread: B'Mann Plymouth MDT Switcher runs better in reverse than forwards; why?

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    Default B'Mann Plymouth MDT Switcher runs better in reverse than forwards; why?

    I have recently received a brand-new example of the B'mann Plymouth MDT Switcher, and in the course of running it in, have noticed that it runs more smoothly in reverse (ie, Cab-leading). I've noticed this before with other examples of this loco from the same manufacturer, and would be interested in learning why this might be.

    Can anyone enlighten me please?

    Thanks.
    Komata "TVR - serving the Northern Taranaki . . . "

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    It wants to be a cabforward??
    Cheers,

    Russ

    CEO of Devil's Gate Mining Co.



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    Is the shell on backwards?

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    NZ is 'down under' eh? Just like water swirls down yours versus ours drains otherclockwise, similarly trains run better the other way.

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    Every one of these that I've seen has the worm directly on the end of the motor shaft so that the lateral force is directly transmitted to the motor bearings.

    That has the potential to do two things; first, push and pull the motor around in its own bearings, and if one end is better lubricated than the other - directly result in an RPM difference. If you do add some lubrication, go VERY easy, particularly on the commutator end because if you get oil in the commutator bad things happen; carbon buildup, fire rings, etc..

    The second thing is that it moves the brushes around on the commutator as well as reversing direction on the commutator, so that there's a different set of contacts in play going one direction or the other.

    So if you pull off the shell and run it, see if you can actually see the armature 'shucking around' horizontally between forward and reverse. Note which way runs better; i.e. is the motor all the way forward or all the way back; the OPPOSITE bearing end may need lubricated. And if you want to take it further... and you actually notice there's some slop in there of at least .005-.010, you can cut a motor washer to a letter "C" shape and see if you can stuff it in there to reduce the horizontal slop and keep the brushes on the same spot on the commutator. That only works on open frame motors instead of can motors. The can motors have such skimpy end bearings they really aren't made to take lateral forces on them.

    If that's not it, then it's all in the gearing, and the only way to determine if something weird is going on is to take out the motor so that you can roll the chassis around, spin the worm gear with your finger, and see if you can feel the forward vs. reverse differences. Sometimes it's design, sometimes its stuff in the gears, may not be fixable and you just have to live with it. Or you may discover something.

    But knowing Bachmann, I'm betting there's some horizontal shucking going on in the motor itself.
    Randgust N scale kits web page at www.randgust.com

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    Having watched a lot of switching operations, many crews like to have the cab forward. I've seen quite a few double headed units with the cabs out.

    They like to see the condition of the track. This is especially important during street operations or iffy track. The driver(engineer) and fireman/brakeman/crew can see what they are about to drive over and prevent derailments.

    It is not so important to watch the couplers.

    As Randgust said, many times it is how the gears are arranged, that a loco runs better in one direction than another. It may just need a lot of breaking in.
    Use what you know about the world to model…
    Learn from modeling what you don't know about the real world.



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    Agree with @randgust
    The tighter the tolerance the more precise the mechanism.
    The looser, the more slop is introduced.
    The second is why some mechanisms need more “break in time” than others.
    A precision made mechanism requires very little, to no, break in time.
    It’s just the motor brushes that need to “seat”.
    In my experience, Bachmann utilizes much looser tolerances than every other manufacturer.
    A well placed thrust washer, or three, go a long way to improving the running characteristics of many a Bachmann lokie(s).
    As suggested, take the shell off and look at it, see what’s moving and/or shifting forward to reverse.

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