IBM Selectric I/O Manual Of Instruction page 79

Late Style Indexing
The new indexing mechanism is designed to improve the ac-
curacy and reliabi lity of the indexing operation. It differs
from the earlier design mainly in the index pawl and the
method of selecting sing Ie or double spacing.
The index mechanism is operated by the cam follower by
means of the index pawl carrier link connected to the front of
the index multiplying lever. The rear of
thE~
multiplying lever
is always in contact with multiplying lever stop attached
solid Iy to the powerframe (Fig. 154). As the cam follower
operates, the multiplying lever pivots on the cam follower
and pulls down on the index pawl carrier link (Fig. 155).
Multiplying Lever
FIGURE 154. Index Mechanism - Rest Position
The index pawl carrier link always receives the same amount
of motion each time it operates regardless ()f whether the
mechanism is se~t for sing Ie or double spacing. The amount of
travel is sufficient to cause double space rc:>tation of the
platen.
During a double space operation, the index pawl is allowed
to enter the platen ratchet immediately. The index pawl then
forces the ratchet tooth forward two spaces unti I the pawl con-
tacts the platen overthrow stop. The overthrow stop wedges
the pawl into the ratchet teeth to lock the platen in position
(Fig. 155).
If only a single space operation is desired, the index pawl
must be prevented from entering the ratchet unti I it has passed
one tooth of the ratchet. The remaining travel after the index
pawl enters the ratchet is only sufficient to cause one tooth of
rotation to the platen. The index pawl contacts the platen
overthrow stop at the end of the stroke as on a double space
operation.
61
FIGURE 155. Index Mechanism - Active Position
The index pawl entry into the platen ratchet is controlled by
the line space cam lever attached to the index selection lever
(Fig. 156). The cam leve.r has two steps at the forward end in
position to contact a stud on the side of the index pawl. A
small spring between the index pawl and the pawl carrier
spring-loads the pawl toward the platen. With the index se-
lection lever tp the rear in the double space position, the in-
dex pawl stud contacts the lower step of the cam lever allow-
ing the pawl to rest near the platen ratchet (Fig. 156A). In
the sing Ie space position, the index pawl stud contacts the
upper step of the cam lever causing the index pawl to rest
farther from the platen (Fig. 1568). Thus in the sing Ie space
position, the index pawl stud maintains contact with the line
space cam lever longer and delays the entry of the pawl into
the platen ratchet.
Index Selector Lever
A. DOUBLE SPACE POSITION
FIGURE 156A. Index Selection Mechanism