In tape decks and mechanical drives, this idler wheel applies tension against the motorized capstan. The friction between the rubber, the tape, and the capstan pulls the tape across the magnetic heads at a precise, constant speed. Over time, the rubber can harden, crack, or turn to a sticky goo, causing the machine to eat tapes.
Heard on the job
The tape keeps slipping and chewing up the cassette because the pinch roller has completely dried out.
Insider tip
Cleaning a pinch roller with isopropyl alcohol can temporarily revive it, but often accelerates the drying and cracking of the rubber.