Dear Mr. Sonksen:
Without seeing the system, it is extremely difficult to give recommendations. Also, more information is required.
Is the conveyor a troughed system, or is it a flat belt feeder conveyor? Is the conveyor uni-directional, or does it run in both directions? Is the belt driven at the discharge end, on the return run, or is it tail driven? What are the enevironmental conditions (heat, dirt, wind, etc.)? How is the belt being loaded (in center, in-line, to one side, etc.)? What type of take-up system is used (gravity, pneumatic, screw, etc.)? How much take-up force is used? Is the conveyor aligned? Is the belt bowed?
Any of these can factors can through the belt tracking off.
From your description, it appears that the belt runs true when unloaded, but not when a load is applied. From this, the first two items that I would look at are loading geometry and belt tension.
Good luck in resolving it.
If you wish to discuss it in greater detail, feel free to contact me.
10668 Newbury Ave., N.W.,
Uniontown, Ohio 44685 USA
Tel: 001 330 265 5881
FAX: 001 330 494 1704