[audio cassette player] Are both or only one "axis" supposed to drive the tape?
(piefed.cdn.blahaj.zone)
I always called them spindles or gears.
Anyway, you can test this by running it without a cassette in it. As others have pointed out, one side pulls and the other is idle.
I kinda miss tapes. K-Pop groups still release them. They are not cheap.