IBM Selectric I/O Manual Of Instruction page 34

The character interrupter mechanism must yield in order to
prevent parts damage. A torsion spring around the character
interrupter bai I loads the interrupter pawl against an adjust-
able stop on the bai I. If a collision occurs, the interrupter
pawl can yield by overcoming the torsion spring as the inter-
rupter bail rotates.
Shift Interlock
Operating the shift mechanism when the type head is in the
process of printing would result in parts damage. The rotate
detent would be engaged in a notch of the typehead and the
typehead cou Id be against the platen. At this time no rota-
tion of the typehead can be allowed; therefore the shift must
be prevented from operating once the typehead has started to-
ward the platen.
We have seen that the shift spring clutch remains disengaged
as rong as the shift clutch ratchet is prevented from rotating.
An interlock arm is operated by a cam on the right end of the
fi Iter shaft (Fig. 68). The interlock engages the teeth of the
shift clutch ratchet and prevents rotation of the ratchet.
When the cycle mechanism is at rest, a roller on the interlock
rests near the low point of the interlock cam allowing free
operation of the shift mechanism (Fig. 68a). As soon as a
cycle operation begins, the filter shaft rotates causing the
interlock cam to actuate the interlock into the teeth of the
shift clutch ratchet (Fig. 68b). This interrupts the shift
operation unti I the cycle operation is completed.
Shift Clutch Ratchet
Shift Interlock
@~@
Shift Interlock Cam
a. REST POSITION b. ACTIVE POSITION
FIGURE 68. Shift Interlock
If an operator shou Id operate the sh ift i mmedi ate I y after
striking a character, the shift cam could begin to rotate be-
fore the filter shaft had sufficient time to actuate the shift
interlock. This could cause an erroneous character to print
because the shift arm had already begun to move. This is
known as "beating the shift." This condition occurs mostly
in shifting from upper to lower case. Shifting from lower to
upper case is no problem because the shift arm does not rest
against the cam in lower case. The cam must rotate some-
what before it begins to move the shift arm, thereby allow-
ing the fi Iter shaft sufficient time to actuate th",e shift Inter-
lock.
Shift To Print Interlock (835 Printer)
The shift to print interlock provides a means for delaying a
print operation until the shift motion is complete. It mounts
on the cycle clutch trip mounting bracket.
Shift Cam
o
Input Cable
• FIGURE 68.1 Shift Cam Detent
When the shift cam starts to rotate, the shift cam detent roller
rides up onto the high surface of the sh ift cam (Fig. 68.1).
This rotation of the shift cam detent pulls on the input cable
wh ich transfers motion to the input interlock arm. As the in-
put interlock arm moves to the rear the shift interlock arm ro-
tates forward over the rear step on the cyc Ie clutch sleeve
(Fig. 68 .2).
.cycle Clutch
latch
Input Interlock Arm
I nJ?ut Cabl e
~J.a.
• FIGURE 68.2 Shift To Print Interlock (Shown Operated)
26
The shift interlock arm will remain engage<d until the shift cam
detent roller enters the detent of the shift cam. This insures
that the cycle clutch sleeve will not be released until a shift
operation is complete.
Shift Magnets Assembly
The shift magnet assembly is mounted to the right side of the
power frame below the shift cam. It consists of two magnets
and their armatures (Fig. 69).