Brother MFC990CW Service Manual page 116

Normal SF and high-speed feed modes
Normal SF mode
1st stage The ASF motor rotates clockwise (when viewed from the output gear side) and its
rotational torque is transmitted to the ASF/maintenance drive gear that is always
connected via the idle gear 16 to the clutch gear L. When the mode switching lever
is placed in the left position, the clutch gear L also meshes with the ASF changeover
gear.
The rotational torque is further transmitted from the ASF changeover gear via the
two ASF idle gears 17, paper pull-in gear shaft and the gear train in the paper pull-in
roller holder. Consequently, the paper pull-in rollers turn in the forward direction to
pull in a sheet of paper loaded in the paper tray.
2nd stage After the pulled-in paper pushes the registration sensor actuator to turn the sensor
on, the ASF motor further rotates clockwise for the predetermined period to align
(register) the leading edge of the paper with the paper feed roller. (Registration)
3rd stage The ASF motor stops and the paper feed motor starts rotating clockwise to rotate the
PF roller gear L that rotates the paper feed roller in the forward direction to advance
the paper to the printing start position.
The rotational torque of the PF roller gear L is also transmitted via the ejection idle
gear and PF timing belt to the paper ejection roller that rotates in the forward
direction to eject the paper printed.
High-speed feed mode
1st stage Just as in the normal SF mode, the clockwise rotation of the ASF motor pulls in a
sheet of paper loaded in the paper tray.
The difference from the normal SF mode is that the paper feed motor also rotates
clockwise at the same time as the ASF motor. The paper feed roller rotates in the
forward direction.
2nd stage When the paper fed by the paper pull-in rollers reaches the paper feed roller, the
roller is already rotating in the forward direction; therefore, the paper continues to
advance to the printing start position.
Registration sensor activation has no effect on the paper feeding operation in the
high-speed feed mode.
Even during printing of the first sheet of paper, the ASF motor keeps rotating
clockwise to rotate the paper pull-in rollers in the forward direction. Immediately
after the first sheet leaves the paper tray, the paper pull-in rollers continuously pull
in paper sheet by sheet. After the last sheet of paper advances to the printing start
position, the ASF motor stops.
The paper feed roller always rotates faster than the paper pull-in rollers; therefore,
the trailing edge of the preceding sheet will not overlap with the leading edge of the
subsequent one
Just as in the normal SF mode, the rotational torque of the PF roller gear L is also
transmitted via the ejection idle gear and PF timing belt to the paper ejection roller
that rotates in the forward direction to eject the paper printed.
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