Alternator belt keeps coming loose

This all started with a alternator going bad and the alternator belt snapped. Replaced both and then it was fine for a month. Then the belt starts squealing so I tightened and fine for a couple weeks or so. Started squealing again so bad that the belt was toast. Also found that the new alternator was bad. Replaced both again. Fine for another month and then started squealing again. This time it become so loose that the alternator stopped charging and the battery went dead. I can smell the burnt rubber from the belt too. Tightened up the belt again to check the alternator. Alternator is charging well at 14.6. This belt has a manual bolt style tensioner. I have even marked the bolt to see if it was moving and it has not. The only other thing the belt runs to is the water pump/fan clutch and the main pully. Water pump is not leaking and the car does not run hot. I have tried replacing the belt with the best quality I can find and still no difference. I am at a loss with this. Why do these alternator belts keep failing? Please help!