IBM Selectric I/O Manual Of Instruction page 64

Contact Assembly
This assembly is comprised of five sets of contacts, each hav-
ing its own latch (Fig. 123).
LATCH
MOLDED BLOCK
FIGURE 123. Operational Contact Assembly
The operating strap of each contact set has three contact
points that make with three separate contacts {Fig. 124}. The
contacts are closed contacts held open by a bail. A block is
molded on the end of the operating straps to provide an insu-
lated operating surface for the bai I and a latching surface for
the contact latches.
Contact Latch
Latching Surface---",-","
FIGURE 124. Operational Contact Assembly
There are two contact actuating arms. One is operated by
the carrier return-index check pawl and called the right hand
contact actuating arm. The other is operated by the tab-
spacebar-backspace check pawl and called the left hand
actuating arm.
Contact Bai Is
Actuating Arm Pivot
FIGURE 125. Actuating Arm
50
The arms are mounted to the contact assembly by a shouldered
screw. The shou Idered screw provi des a pivot poi nt for the
arms. The top of the arm has a pin riveted to it, and the
lower end has two eccentric pins that serve as contact bai Is
(Fig. 125}.
The pin on the upper end of the arm rides underneath the op-
erational check pawl. As the cam rotates, the check pawl
rides out of the notch in the check ring {Fig. 126}. This
causes the check pawl to move down, pushing the top of the
actuating arm down. The actuating arm pivots on the shoul-
dered screw, raising the lower end. As the bail rises, the
contact that has its latch operated, wi II close. The other
contacts wi II be held inoperative by their latches (Fig. 127).
FIGURE 126. Actuating Arm Operated By Check Pawl
......tl..-_-
Interposer
Latch Spring
FIGURE 127. Contact Bail and Latch Operation
When the check pawl returns into the notch in the check ring,
the actuating arm will restore under its own spring tension.
As
the top of the actuating arm moves up, the lower end
moves down. The contact bai Is lowering wi II restore the
contact. With the actuating arm restored, the contact bai I
keeps the contacts from resting against their latches. The
purpose of this is to allow the latches to be moved away from
the contact by the interposer without any drag.
Contact Latches
The latches rotate on a shaft that is mounted to the contact
assembly by a bracket. The interposer moving to the rear,