Some Schwinn Airdyne exercise bikes use a wedge pin for the crank arms. Over time and with use, the wedge pin will become loose and needs to be tightened. The wedge pin will need to be tightened more often as it and the mating surface become worn. Thus, allowing it to slide more into the hole. This will eventually allow the nut to bottom out on the threads. Now the nut will not turn anymore even if the wedge pin is still loose. When this happens, if you continue to tighten the wedge pin it will break, or the threads will strip.
This is why it is smart to replace the entire crank assembly with bottom bracket on the initial repair.
When you get ready to tackle this job check out our video on the subject.