IBM Selectric I/O Manual Of Instruction page 97

When the lever is moved to the right, the feed pawl is piv-
oted into engagement with the right hand feed ratched (Fig.
193). When the detent lever is moved to the left, the feed
pawl engages to feed the left hand ratchet.
The ribbon feed pawl is powered to the rear by the action of
the ribbon feed cam located directly in the middle of the
print sleeve. The cam is keyed to the sleeve and rotates one
complete revolution on each operation. A sliding cam fol-
lower transfers the motion of the cam to the ribbon feed bell-
crank whi ch actuates the feed pawl to the rear (Fig. 194).
Sufficient motion is available from the cam to cause a two
teeth feed of the ratchet. The timing of the ribbon feed cam
causes the ribbon feed action to occur early in the cycle op-
eration. At the time the typehead prints, ,the ribbon has
completed its feeding operation except for the restoring of
the feed pawl. Care must be taken in replacing the feed cam
to insure that it isnot installed in a reverse position. A V-
shaped notch in the large part of the cam must be toward the
right. Reversing the cam wi II change the timing of the rib-
bon feed.
Ribbon Feed
Bellcrank
FIGURE 194. Ribbon Feed Mechanism - Side View
As the feed pawl restores to the front, it slides along the
teeth of the ratchet into the rest position. The drag of the
pawl along the teeth tends to rotate the ratchet backward and
unwind the ribbon. To prevent any backward rotation, a de-
tent pawl is spring-loaded into the teeth of the ratchet to al-
low feed in one direction only (Fig. 193).
77
In order for the ribbon to wind onto one spool it must be un-
wound from the other sppol. Each ratchet has a detent pawl
to prevent rotation in the unwinding direction. The detent
pawl must be disengaged from the supply side in order for
ribbon feed to occur. Each detent pawl has a long curved
extension resting against a roller on the ribbon feed detent
tever (Fig. 193). When one detent pawl is engaged with
its ratchet, the other is disengaged depending upon the posi-
ti on of the ribbon feed detent lever.
Two flat springs are mounted to the ribbon feed plate at the
rear so that they rest against the ribbon feed ratchets. The
slight drag applied by the springs prevents the jerk of the
ribbon feed operation from spinning the supply spool and
spi lIing off excessive ribbon.
2. Ribbon Reverse
The ribbon is fed from one spool to the other unti I the supply
spool is emptied. The ribbon feed pawl must then be moved
to the ratchet of the empty spool to begin feeding the ribbon
in the opposite direction. To achieve a ribbon reversing
operation, it is merely necessary to move the ribbon feed de-
tent lever from one position to another. The ribbon feed pawl
spring then pulls the pawl into the reversed position. The re-
versing operation is the same for both sides except that the
direction of parts movement is opposite.
The reversing operation requires a sensing mechanism to de-
termine when the reverse is to take place and a means of
powering the detent lever from one position to the other.
Each ribbon feed ratchet core contains a small bellcrank
called the ribbon reverse trigger (Fig. 195). As long as there
is ribbon around the spool, the ribbon holds the reverse trig-
ger into the core in the inactive positi on. When the ribbon
spool is emptied, a hairpin spring forces the reverse trigger
out of the core through a slot in the ribbon spool. A portion
of the reverse trigger pivots down through a hole in the rat-
chet into position below the ratchet.
Reverse Trigger
Reverse Trigger Spring
Ribbon Feed
Ratchet Assembly
FIGURE 195. Ribbon Reverse Trigger