Fine Forward; Bad Backwards

[rab]

I have a couple of models, both bought second hand, which seem to have a similar problem.

They run fine forwards, but in reverse they run ok for a short distance, then seem to slow down a little.

After moving a similar distance, they speed up again.  This sequence then repeats.

 

One is a Farish DMU (Class 108), the other is a Tomix Thomas (yes I know),

so different manufacturer, similar problem.

 

Any suggestions as to likely source of problem would be welcome.

[Nearholmer]

Look for end-float on the motor armature, and see if it is excessive. Ideally there should be a very tiny amount of float in each direction, no more. Excessive float can cause parts within the motor to rub where they shouldn’t, and the ‘on/off’ behaviour you mention can occur as the armature ‘breaks free’ from a rub, then winds itself back to the same place.

 

Similar things can happen with excessive float in spur gear trains.

 

May not be the problem, but worth looking.