IBM Selectric Manual page 46

The tab lever stop may be formed by inserting the T -bender
from the upper right side. It wi II be necessary to force
the tab torque bar to the rear in order to insert the
T -bender.
Thisadjustment insures that the backspace pawl will not
be prevented from bottoming in its rack during a back-
space operation. The tab lever rest position also directly
affects the adjustments of the tab mechanism. It deter-
mines howmuchmotion must be provided to the tab lever
to properly remove the backspace and escapement pawls
from their racks during a tab operation.
3. Backspace Rack - With the backspace rack in the rest
position, a clearance of .005
11
to .015
11
should exist be-
tween the working surfaces of the rack tooth and the
backspace pawl (Fig. 127). Adjust the hexagon headed
stud inthe backspace bellcrank to obtain this condition.
Backspace Bellcrank
Adjusting Stud
FIGURE
127.
Backspace Rack Adjustment
The adjustment minimizes lost motion in the mechanism
and insures that the backspace pawl will positively reset
into the next rack tooth at the completion of a backspace
operation. Excessive clearance can contribute to escape-
ment problems as well as backspace failures by allowing
the backspace pawl to hold the carrier against a back-
space rack tooth. Partial spacing will result if the car-
rier alternates holding on the escapement pawl and the
backspace pawl.
The adjustment may be gauged by feeling the motion of
the rack as it is manuall y moved from its rest position
into contact wi th the backspace pawl. The movement
should be equal to the adjustment clearance. The check
should be made at both extreme positions of the carrier
so as to include the difference in. mainspring tension in
the check. Check the resetting of the pawl at both
positionsbyoperating the backspace bellcrank manually
and releasing it slowly.
4. Intermediate Lever - With the backspace cam manually
operated to the high point, the escapement pawl should
just fail to drop into the preceding rack tooth causing
the manual backspace operation to fai I. Adjust the inter-
mediate lever pivot screw forward or back in its elongated
mounting hole to obtain this condition (Fig. 128).
45
Escapement Rack
FIGURE
128.
/ntermediate Lever Adjustment
(Late Level)
The rear portion of the intermediate lever is suppl ied
with the same amount of motion from the hexagon headed
screw on the bellcrank regardless of any change in the
front to rear position of the intermediate lever. There-
fore; the difference in throw to the backspace rack is
achieved, when changing the front to rear position of
the intermediate lever, by increasing or decreasing the
leverageordistance between the pivot point of the inter-
mediate lever and the point of contact between the in-
termediate lever and the backspace rack. Moving the
intermediate lever to the rear wi II increase the back-
space rack motion.
During a powered backspace operation the carrier de-
velopes enough momentum (allowing the escapement pawl
to properly overthrow and drop into the preceding rack
tooth) for a positive operation.
Too much motion to the backspace rack wi II cause double
backspacing. Check the operation at both ends of the
carriage so as to include the variation in the mainspring
tension.
CAUTION: The rest position of the backspace rack
should be checked and readjuste'd, if necessary, after
any change in the front to rear position of the intermediate
lever.
NOTE: On721 machines below serial number 4012015,
the contour or rise to the spacebar backspace operational
cam is sl ightl y di fferent than those found on later level
machines. This cam provided much less momentum to
the carrier, therefore the backspace rack required more
motion in order to produce a positive backspace opera-
tion. Adjust this early level backspace mechanism as
follows: With the backspace cam manually operated to
the high point, the escapement pawl mlJSt drop into the
preceding rack tooth and overthrow by .005" to .010
11
(Fig. 129). Obtain this condition by adjusting the in-
termediate lever forward or back in its elongated mount-
ing hole.