The main motor's drive coupling is where the drum connects to the main motor which turns the drum. That junction and the connecting parts are very stable. The other end of the drum's axle rests in the dimple of the bearing plate. That little depression takes a good amount of force and over time is subject to wear. "So what?" is a reasonable question.
As the bearing plate wears rotations of the axle, (something like 1200 rotations for each roast), if you roast one batch a week, that is well over 60,000 rotations per year! The weight of the drum and the coffee inside it causes the plate to wear in one general direction.
If you examine the small inset image, the blue arrows indicate the worn area of the dimple of one of our customer's bearing plates. That wear allowed the drum to drop in the direction of the red arrow in the larger image. As it dropped, the general area where the red arrow points (around five o'clock in the photo) is where the drum would be further away from the bearing plate. The opposite area (between about seven o'clock and one o'clock) where most of the scrape marks are is where the drum would be closer to the bearing plate.
There are other factors in play here, but I offer this as a general explanation. The result of that rubbing is that a lot of stress is placed on the main motor's speed reduction gears. The effect was a damaged main motor which needed replacement.
So our advice to you is that If you hear any scraping or grinding sounds refer to our Roasting Drum Alignment page as this is the definitive guide to extending the life of the drum motor.