Here's a view of a service replacement swing plate assembly.
The assembly resides on the inboard side of the right side vertical chassis member, where it transmits rotation from the main drivetrain to the fuser when the toner cartridge access door is closed.
Its gears are in constant mesh with the main drivetrain, but opening the toner cartridge access door causes the black gear at its rear to disengage from the fuser to facilitate clearing a paper jam. The spring-loaded lever at the upper right is actuated when the toner cartridge access door is closed -- that's what forces the black gear to engage with the fuser's drive gear.
The two gears lead a hard life, and are prone to wear badly leading to noisy operation. Unit replacement is the only solution. See this post for the procedure.
There is a shortcut repair method of replacing only the black gear at the rear. That method is pretty much worthless, and is strongly advised against. The two gears tend to wear about equally; replacing only one is not a legitimate repair method.
The fit of the fuser in the 42X0/43X0 machines probably contributes to the wear of the swing plate assembly's two gears. The clip-fastened fuser has a sloppy fit in the chassis that can't be conducive to establishing correct gear mesh -- it's a design flaw that will never be corrected.
A complete, OEM swing plate assembly is P/N RM1-0043-060. When ordering from a vendor, specify that you want the complete OEM unit. Some outfits apply almost the same P/N to the black gear alone.
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