Hi,
I used to model in On30 and some of the Bachmann locos suffered this same problem. They were informed regularly and generally taken to task, here and in America, and, eventually, they acknowledged the problem and supplied new upgraded parts for free. They also said that they had tightened up on quality control as well. Only someone still modelling in that scale will be able to confirm whether this happened or not.
However, it does seem to me that Heljan should acknowledge this problem and supply upgraded replacement parts. I don't have any of their diesels but would not be a happy bunny if I did and this problem kept occurring.
They may already be aware but, from the number of posts, they don't seem to have done very much to improve the situation.
Regards,
Dave