IBM Selectric I/O Manual Of Instruction page 82

Tab Set and Clear - Early Style
A rack of tab stops located just to the rear of the escapement
rack allows the operator to select the positions where the
carrier wi II stop when the tab keylever is depressed. The tab
stops operate friction tight in grooves of the tab rack -- one
corresponding to each escapement position. The tab set and
clear button located to the left of the keyboard may be rocked
forward or back to set or c lear a tab stop (Fig. 160). When
the front of the button is depressed (set), the set and clear
arm at the rear operates to rotate the front of the tab rack up.
Tab Set and Clear Bellcrank
Tab Set and Clear Keybutton
FIGURE 160. Tab Set And Clear Mechanism
A tab stop stri kes a proj ecti on on the escapement bracket
blocking the movement of the tab stop (Fig. 161A). As thetab
rack cont i nues to "rotate, it is forced to rotate i nsi de the tab
stop. When the set and clear button is released, the tab rack
is restored to the rest position leaving the set tab stop lower
than the others (Fig. 161 B). Depressing the rear of the button
(clear) I:auses the tab stop to strike a stud at the bottom of the
escapement bracket blocking its movement whi Ie the tab rack
is rotated further (Fig. 161 C). When the set and c lear lever
is released, the tab rack is restored to the rest position, leav-
ing the tab stop in the cleared position (Fig. 1610). Because
it
is
the escapement bracket that sets or clears the tab stop,
the carrier must be positioned to the desired tab stop before
th(:! set or c lear action can occur. The stop at the extreme
rifJht end of the tab rack is the tab final stop and remains in
the set position at a II times to disengage the tab mechanism
at the right hand I imit of trave I.
t
64
A. SET OPERATION
B. SET POSITION
C. CLEAR OPERATION
D. CLEAR POSITION
FIGURE 161. Tab Set And Clear Operation
Tab Set and Clear - "Gang Clear"
Both the tab rack and the tab clear mechanism has been re-
designed on late level machines so that the tab stops may be
"gang cleared". Gang clearing tab stops is the term given to
the procedure for clearing all of the set tab stops in one oper-
ation. This is accomplished by positioning the carrier at the
extreme right hand margin, depressing the tab clear button
and (while holding the tab clear button depressed) actuating
the carrier return mechanism.
As
the carrier travels toward
the left hand margin it wi II clear every set tab stop across
the entire length of the tab rack.
Figure 162 illustrates a cross section of the tab rack. Notice
that the tab stops encircle and operate freely about a round
shaft that runs through the center of the tab rack. The tab
rack is a slotted tube that mounts on shouldered bushings
which are set screwed to each end of the shaft. The slots i~
the tube are guide slots for each individual tab stop (Fig.
163). Sections of spring fingers mounted across the entire
length of the tab rack operate against sma II detent lugs (pro-
jections) on each tab stop. The function of the spring fingers
is to detent each tab stop in either its set or cleared position.
The tab set operation on the gang c lear mechanism remains
the same as on the old style." The tab stop strikes a projection
of the escapement bracket blocking the movement of the tab
stop as the tab rack rotates top to the rear (Fig. 162A). The
clear operation is slightly different. A gang clear finger
mounted to the top of the escapement bracket (Fig. 163) pro-
jects to the rear just below the detent lugs on the tab stops
(Fig. 162B). When the tab rack rotates top to the front during
a clear operation, the gang clear finger which is in the path
of the detent lug restri cts the tab stop from rotating with the
rack thereby causing the tab stop to rotate up to its cleared
position (Fig. 1620).