It is the drive gear that turns the transfer drum cleaning roller
(about 1/2 inch in diameter, 15 teeth, with an eccentric to poke the
toner down the waste chute).
Or...
If you have one of these religion killing printers (I take that back.
It might reinforce your belief in Satan) sitting around doing nothing,
I will make it worth your while to strip this part out for me.
It takes only a minute to remove one screw to get the gear out.
Thanks.
P.S. I think this mechanism is similar in all these printers:
Brother HL 3400CN, QMS magicolor 6100 / 6110, Tally T8306 / 8406 /