Craftsman 139.53973SRT drive gear issue
I am not absolutely sure, but I think that model may use a mechanical limit switch that is mounted on the trolley rail. The upper and lower limit switches are mounted on  brackets that are adjustable on the rail to fine tune the travel of the door. It is not uncommon for these brackets to get loose and move. If this switch moves too far, then your door will strike the floor or top end of travel and stop, but the switch will not activate stopping the motor. Result is motor continues to run, but chain will not move, so there goes your gear drives. Check the location of these limit switches, test the switches themselves to assure they are working. (Turn off power, unplug and test switch with an OHM meter.)