(beginner) Masking a gear behind a pinwheel

I have to animate a gear behind a half-pinwheel so that only about half of the gear is showing (the other half is behind the pinwheel). Right now the "hidden" half is still peeking out a lot and I wish to hide/mask it. My professor suggested that I render the animated gear seperately and mask it later in AdobeAE but I would like to avoid any external programs if possible. Is there any way to make a mask where the pinwheel is so the gear behind it wouldn't show but would still be animated?

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