Maintenance Detective

You have a standard DC brush motor on a DC speed control driving a production conveyor. One day, the operator tells you, "The motor is running at full speed and I can't turn it down." What's the most likely single cause and how could it have been prevented? Drop your answer in the comments before scrolling.  Answer: Brush dust buildup shorting the negative brush to ground. Carbon brush dust is conductive. As it accumulates on a brush grommet, it can create a short to ground. 

Suppose you look at weekly passdowns, and one of your plastic articulated conveyors has broken the chain multiple times in the last week. You have lots of these conveyors, but this one is breaking and getting repaired repeatedly. They're all about the same age. This one isn't all that different from the others. It does start and stop under load, but they all do. You go look at it and everything you can see while it's running seems fine. No jumping or bad mechanical components