IBM Selectric I/O Manual Of Instruction page 31

Shift Release Arm
ShIft Bail
FIGURE 61. Shift Release Mechanism
The shi ft arm must be forced to pivot outward for a shi ft op-
eration. A disc-shaped shift cam fits around an extension of
the operational shaft bearing outside the sideframe. The cam
operates between two rollers located at the rear of the cam
(Fig. 62). The roller to the left of the cam is in a fixed po-
sition on the power frame and serves as a back-up roller for
Shift Arm
--I~-+-t~
Adi.
Screw
Shift Cam Roll er
"*f--I-~~Shi
ft
Cam
a. LOWER CASE POSITION b. UPPER CASE POSITION
FIGURE 62. Shift Operation - lRear View
23
the cam. The roller on the right hand is attached to the
shi ft arm and ri des the cammi ng surface of the cam. The
camming surface is on the right side of the cam rather than
on the perimeter. When the low point of the cam is between
the two rollers, the shift arm is in lower case (Fig. 62). The
cam is operated 180
0
to the high point in order to force the
arm into upper case (Fig. 62b).
The shift cam rotates only during a shift operation and re-
ceives its motion from the operational shaft. Since the op-
erational shaft turns continuously when the motor is running,
a clutch mechanism is required to engage and disengage the
shift cam from the operational shaft when a shift operation is
desired. A spring clutch "makes" and "breaks" the driving
connection between the operational shaft and the shift cam.
The shift cam mounts on a shoulder on the right hand side of
the operational shaft bearing and is free to rotate about this
shou Ider (Fig. 63). The operati ona I shaft extends through
the center of this bearing beyond the right side of the shift
cam. An arbor (shift clutch arbor) is set screwed to the
shaft just to the right of the shift cam and turns with the
shaft continuously (Fig. 63). The driving force to the shift
cam comes from this arbor by means of a spring clutch that
is fastened to the cam and encircles the arbor. The spring
clutch is undersized and wound in the same direction that
the arbor is turning. Therefore, any time that the spring
clutch is allowed to collapse about the arbor, it will turn
with the arbor driving the cam. The cam is driven 180
0
each
time a shift operation occurs.