How can I fix my 2005 Troy Bilt Bronco Riding Mower drive problem?

When I engage the transmission in either forward or reverse the drive belt (from engine to rear pully) starts to smoke at the engine pully and there is rattling noise coming from the same area. The transmission won't engage. I have replaced the two bearings on the rear variable speed pully assembly (bearings were wasted) and I pulled the transmission, took it apart and inspected inside but found no problem, all the gears were in great shape. The model of this lawn tractor is 13AJ609G766. What the heck is going on with this thing?