Xerox Phaser 6180 Service Manual page 79

Theory of Operation
However, when two sheets are fed concurrently, only the Feed Roller rotates.
The Retard Roller is coupled to a friction clutch that prevents the roller from
rotating due to the extra force from feeding two sheets; this process allows the
upper sheet to pass by as the lower sheet is stopped by the friction with the
Retard Roller at rest.
The Retard Roller is pushed toward the Feed Roller by spring pressure, and
controlled by the Torque Limiter (Friction Clutch Retard) with which it is
coupled.
Tray 1 (MPT) Feed Roller
Torque Limiter
Retard Roller
Retard Roller
Multiple Feed
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Phaser 6180 Color Laser Printer Service Manual 2-25