IBM Selectric Maintenance Manual page 68

This adjustment 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 direct-
ly affects the adjustments of the tab mechanism. It de-
termines how much motion 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" to .015" should exist be-
tween the working surfaces of the rack tooth and the .
backspace pawl (Fig. 138). Adjust the hexagon headed
screw in the backspace bellcrank to obtain this condition.
Backspace Bellcrank. _ _
c-----,
Adjusting S t u d - - - - -
Backspace Rack
FIGURE 138. Backspace Rack Adjustment
The adjustment minimizes lod motion in the mechanism
and insures that the backspace pawl wi II positively reset
into the next rock tooth at the completion of a back-
space operation. Excessive clearance can contribute to
escapement problems as well as backspace failures by
allowing the backspace pawl to hold the carrier against
a backspace racK tooth. Partial spacing wi II result if
the carrier alternates holding on the escapement pawl
and the backspace pawl. .
4. Intermediate Lever - With the backspace cam manually
operated to the high point, the escapement pawl should
just fail to drop int.o the preceding rack tooth causing
the manual backspace operation to fail. Adjust the in-
termediate lever pivot screw forward or back in its elon-
gated mounting hole to obtain this condition (Fig. 139).
The adjustment may be gauged by feeling the motion of
the rack as it is manually moved from its rest position
into contact with 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 po-
sitions by operating the backspace bellcrank manually
and releasing it slowly.
The rear portion of the intermediate lever is supplied
with the same amount of motion from the hexagaon head-
ed screw on the bellcrank regardless of any change in
the front to rear position of the intermediate lever.
Therefore, 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
2-42
Escapement Reck
FIGURE 139. Intermediate Lever Adjustment
leverage or distance between the pivot point of the in-
termediate lever and the point of contact between the
intermediate lever and the backspace rack. Moving the
intermediate lever to the rear will increase the back-
space rack motion.
During a powered backspace operation the carrier de-
velops enough momentum (allowing the escapement rack
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 readjusted, if necessary, after
any change in the front to rear position of the intermed-
iate lever.
,..." nnll-n "' ......... 1." .... I .... ,..,...., A 10.11'-&.. ,.. A ftl "
r""T"'"
r
\..,1-\1\1\11;1\ 1\1; 1 UI\I .... /V\I;\..,nl-\l .... I.)/V\ - I;I-\I\L I .) 1 1 LI;
Be sure that the print escapement and operational control ad-
justments are correct before attempting the carrier return ad-
justments.
1. Pawl Clearance - Adjust the clutch latCh eccentric sCi
that the escapement pawl will clear the rack teeth by
.005" to .020" when the latch is being held down by the
keeper (Fig. 140).
The adjustment insures that the escapement pawl wi II not
drag along the rack during a carrier return operation and
that the pawl wi II be a II owed to re-enter the rack qui ck-
Iy at the completion of the return operation.
2. Clutch Latch Overthrow - Use the following procedure
to obtain the correct clutch latch overthrow and oper-
ati ona I latch height.
a. Carrier Return Lever - Position the carrier return
lever laterally on the latch actuating arm pin,so
that the carrier return latch will hang vertical with-
out binding against its interposer. Be sure to tighten
the locking screw in the lever onto the flat portion of
the pin (Fig. 14l).