IBM Selectric I/O Manual Of Instruction page 78

index pawl moves forward away from the index stop. The
spring is then allowed to rotate the pawl into engagement
with the platen ratchet. Further movement of the carrier
causes the index pawl to rotate the platen ratchet and pla-
ten.
The index detent lever located just below the platen has a
roller attached at the front that seats between two teeth of
the platen ratchet. The lever is spring-loaded to hold the
roller into the bottom of the teeth. The roller insures that the
platen wi II not creep out of position and that indexing wi II be
accurate. As the ratchet is rotated, the detent roller moves
from one tooth to the next. The detent lever spring forces the
roller to the bottom of the teeth to insure equal spacing on
each operati on.
The index is operated by a link that extends down from the in-
dex pawl carrier to the rear of the operational cam follower
(Fig. 151). When the single lobed cam rotates, the cam fol-
lower pulls down on the link to effect a line spacing opera-
tion.
Platen Ratchet
~
/
Index Pawl
C. R./lndexing Cam
\
Cam Follower
/
Index Pawl Carrier
.............. Pawl Carrier
Link
Index
Multiplying
Lever
\
FIGURE 151. Single Space Indexing Operation
Whether a single or a double space operation occurs is deter-
mined by how far the index pawl carrier link is pulled. Be-
cause the cam follower always moves the same distance on
each operation, a system of multiplying the leverage must be
uti lized. The index pawl carrier link is attached to the front
of the index multiplying lever that pivots at the rear of the
cam foil ower (Fig. 151). A lug at the rear of the cam fol-
lower prevents the multiplying lever from pivoting up in front.
During a single line spacing operation, the multiplying lever
moves down the same amount as the cam follower (Fig. 151).
When the index selector lever is pushed to the rear for a
double space operation, it pivots the multiply control lever
forward under the end of the multiplying lever blocking its
downward movement (Fig. 152). When the cam follower
operates, the multiplying lever pivots where it attaches to
the cam follower. This causes the front of the multiplying
lever to pull down on the index pawl carrier link sufficiently
to cause two line spaces of movement to the platen (Fig. 152).
60
Index Multiplyi,ng Lever
FIGURE 152. Double Space Indexing Operation
A double and sing Ie space operation require the same time
lapse, because both are operated by the same rotatj on of the
cam. The platen rotation must be accelerated for double
spacing, because of the added distance it is to be rotated.
This extra speed of rotation tends to cause the platen to space
too far because of the momentum developed. To prevent
overthrow, an index pawl stop is provided at the end of the
double spacing stroke (Fig. 153). Upon contact with the
stop, the index pawl is stopped and locked into mesh with the
platen ratchet to block further rotation of the platen.
FIGURE 153. Platen Overthrow Stop