Vibrations only in reverse

[Prime12]

I got started in the hobby not too long ago, so I only have two locos, Athearn RTR SD40-2 and Athearn Genesis MP15AC.

Both exhibit minute vibrations when running, but only in reverse. I don't find the vibration itself offensive; I can barely see it or hear it when the loco is running alone. However, with rolling stocks, the vibration sort of gets transmitted (and amplified) via couplers to the rest of the train, and the whole train rattles. I just cannot stand that.

The SD40-2 is a stock RTR with Digitrax dh163hd decoder. MP15AC is the newer run with Mashima motor and Tsunami decoder. Funny how 2 completely different locos with different mechanisms exhibit same symptoms.

I tried cleaning out the trucks, re-lubing, etc, but nothing helped so far. Help!!

Oh, and the vibration is a lot worse with BEMF turned on.

 

[Ronzzr11]

It could be that the motor is shifting slightly as it reverses direction,causing a vibration,maybe a little paper shim,between the motor and the frame could show if this is the case.
Ron

 

[Rico]

I'd put my money on the shift thing Ron mentioned too.
The worm gear will try to pull the motor shaft in one direction or the other before it starts turning the drive gears.
Another thing to note... I had a loco vibrating due to loose motor mounts that made the whole drivetrain out of alignment. Might be worth looking at as well.
Good luck!

 

[jeffrey-wimberly]

On the Athearn, the older ones have soft plastic motor mounts that are notorious for allowing the motor to shift position. On the newer ones the motor mounts are hard plastic and are held in place by screws. If one or more of the screws is a little loose the motor could shift. If the mounts are tight but you can move the motor with slight pressure then you could tighten it by putting some pieces of folded paper between the motor and the mounts.

 

[jackn2mpu]

I don't think it's so much the motor itself shifting position as the armature shifting position inside the motor. You may want to see if you have any excess play in the armature shaft as you hold the motor solidly down. If you do (the shaft moves fore & aft inside the motor shell) insert a thrust washer or two and see how it runs. Be careful taking the motor apart and you should be okay. Just don't put so many washers in that the motor shaft is too tight. You want a little play, but nothing much.