IBM Selectric I/O Manual Of Instruction page 36

When the upper case magnet is energized two actions take
place. One, the upper case magnet armature moving to the
rear contacts a stud on the shift release arm pushing the arm
to the rear. Once the shift release arm is pushed to the rear,
t~e
shift takes place as normal. Two, the formed end of up-
per case armature moves down. The lower case armature
moves to the front, to its de-energized position. In this po-
sition, the" lower case armature mechanically latches the up-
per case armature in the energized position, by moving for-
ward into the path of the upper case armature's formed end
(Fig. 72).
Mounting Bracket
o
FIGURE 72. Shift Magnet Assembly
Attracting the lower case armature to the rear unlatches the
upper case armature. This a I lows the formed end to snap up
and the operating end to move forward, allowing the release
arm which is under spring load to return to the lower case
position.
Shift Contact Assembly
The shift contact assembly consists of a cam follower and two
contact assemblies mounted to a plate. The plate is mounted
to the right side of the power frame (Fig. 73).
TRANSMITTING CONTACT
CAM FOLLOWER MODE CONTACT
FIGURE 73. Shift Contact Assembly
27
Contact Cam Follower Actuating Arm
The cam follower arm ri des the peri phery of the shift cam. It
is spring loaded toward and follows the contour of the cam.
The contact cam follower arm is positioned on the cam to lead
the shift arm by 90
0
•
In upper case, the shift cam is detented
so the shift arm rests on the high point of the shift cam (Fig. 74).
In lower case, the cam is detented so the shift arm rests on the
low point (Fig. 75). Since the cam follower arm leads the
shift arm by 90
0
,
it wi \I rest midway between the high and
low point of the cam in either detent position. The contour
of the cam is such that Points A and B (Figs. 74 & 75) are in
the same plane. Therefore, the cam follower arm wi II be in
the same relative position when the shift cam is detented in
upper or lower case.
{
FIGURE 74. Shift Cam Detented Upper Case
Assume a shift operation from lower to upper case. The shift
cam rotates 180
0
and wi II detent with the shift arm on the
high point. As the high point moves past the cam follower
arm, the arm is moved to the right and back to its rest posi-
tion.
Returning to lower case, the shift cam rotates 180
0
and will
detent with the shift arm resting in the low dwell.
As
the
shift cam rotates, the cam follower arm moves to the left into
the low dwell and back to its rest position.
FIGURE 75. Shift Cam Detented Lower Case