IBM Selectric I/O Manual Of Instruction page 33

Note that the shift cam stop always follows behind the inner
lug of the ratchet when the ratchet and cam are operating.
Whenever the ratchet is stopped by the release arm (on either
lug) the momentum of the cam carries the stop further unti I
the stop strikes the rear side of the inner lug. The horse shoe
shaped release arm absorbs the shock of stopping the over-
throw of the shift cam through the shift clutch ratchet.
Mounted on a pivot stud below and to the front of the shift
cam is a small arm called the shift detent mm (Fig. 65). Al-
though the detent arms primary function is to ope?ate the
character interrupter mechanism, it is also used as a detent
for the shift cam. As the cam approaches either the upper or
lower case positi on a nylon roller mounted on the shift detent.
arm is spring loaded into corresponding detent notches located'
on the outside surface of the cam. This detenting acti on helps
to place the cam in its proper rest position for both upper and
lower case.
Cam overthrow is a greater problem in returning to the lower
case than in shifting to upper case. As the cam returns to
the lower case positi on, the roller of the shift arm is rolling
from the high point to the low point of the cam. The pressure
of the roller against the receding surface tends to accelerate
the movement of the cam. Excessive speed of the cam cou Id
cause a noisy operation and parts breakage. To prevent cam
acceleration, a braking action is necessar'y the same as an
automobile must be braked in descending
(1
hi II. A heavy
spring steel shift arm brake operates just in front of the shift
cam. A nylon brake shoe, attached to the end of the brake,
rides a raised surface on the circumference of the cam (Fig.
66). The friction applied by the brake prevents acceleration
of the cam and causes the shift action to be the same for both
upper and lower case. '
Character Interrupter
If the shift is already in process when a letter key is de-
pressed, the character must be delayed until the shift is com-
pleted. Otherwise the shift wou Id be interrupted in rnid-
Cycle Clutch Latch
Raised Braking Surface
Nylon Shoe
Shift Cam Brake
Shift Cam
FIGURE 66. Shift Cam Brake
operation and an erroneous character would result. The cha-
racter is delayed by blocking the cycle clutch release during
a shift operati on. When the shi ft cam starts an operati on, the
detent must move out of the recess in the cam. A forward ex-
tensi on of the detent lever operates a bai I under the right side
of the keyboard (Fig. 67). A small pawl on the left end of
the bail is rotated up into rhe path of the cycle clutch latch
link. The pawl blocks the forward movement of the link and
prevents the re lease of the cyc Ie clutch. This interlocking
action does not prevent the depression of the keylever nor the
interposer. The interposer is merely latched down into sto-
rage. When the shi ft acti on is completed, the detent enters
the recess in the cam. The bai I is rotated back to the rest
position and the pawl moves out of the path of the cycle
clutch latch link. The cycle clutch latch link is then pulled
forward by its extension spring and the stored character is
printed.
If the shift keylever and a character key lever are depressed
simultaneously, both will operate for an instant until the fil-
ter shaft has a chance to actuate the shift interlock. During
that time a collision wi II occur between the character inter-
rupter pawl and the bottom of the cycle clutch latch link.
Cycle Clutch Latch Link
FIGURE 67. Character Interrupt-er
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