IBM Selectric I/O Manual Of Instruction page 56

A.
B.
c.
D.
FIGURE 107. Escapement Pawl Operation
Having the escapement rack stationary and the escapement
pawl movable is directly opposite to the arrangement on con-
ventional machines. The carrier and escapement pawl must
be moved to the left for a backspace operation. Because the
backspace pawl is mounted to the escapement bracket, move-
ment of the backspace pawl to the left forces the carrier and
escapement pawl to the left.
The backspace pawl is just above the escapement pawl, but
its tooth extends below the escapement pawl. It is held in
mesh with the backspace rack by a small extension spring
simi lar to that of escapement pawl. The backspace rack is
mounted to the rear of the power frame by shou Idered screws
through elongated holes in the rack. This mounting arrange-
ment a 1I0ws latera I movement of the rack. Movement of the
rack toward the left forces the backspace pawl to the left to
cause a backspace operation.
The backspace operation is pointed out here because of its
close association with the escapement mechanism. It is dis-
cussed more fully in the backspace section. The backspace
pawl is in mesh with its rack when in the rest position. This
means that both the backspace and escapement pawls must be
removed from their racks in order for the carrier to move to
the right.
42
Figure 107A illustrates a slight clearance between the work-
ing surfaces of the backspace pawl and a tooth on the back-
space rack when the escapement pawl is holding the carrier.
This clearance is necessary for proper operation of the back-
space mechanism. The clearance insures that the backspace
pawl wi II properly re-enter the backspace rack at the com-
pletion of each backspace operation during a repeat oper-
ation (this is when the escapement pawl is holding the carrier
and the backspace rack is restoring back to its rest position).
The backspace pawl requires an elongated mounting hole so
that it wi II operate in unison with the escapement pawl dur-
ing an escapement operation. Without the elongated motion,
the backspace pawl cou Id restore back into the same rack
tooth before the carrier began to move during an escapement
operation. This could cause partial or half spacing as the
two pawls would alternate holding the carrier during an es-
capement operation.
The backspace and escapement pawl are pinned together so
that they will always move together laterally but front to
rear independently. The reason for this is covered in the
backspace mechanism.
Torque Bar
The torque bar is a flat bar that pivots between the si des of
the powerframe just to the rear of the backspace and escape-
ment racks (Fig. 106). Its purpose is to trip the backspace
and escapement pawls out of their respective racks. The left
end pivots in a hole in the powerframe casting. A small C-
clip holds the right end in a large mounting plate on the
powerframe.
The rest positi on of the torque bar on early level machines is
determined by a lug on the left end of the torque bar that
contacts the tab rack. On late level machines, the rest po-
sition is controlled by an adjustable stop which mounts on the
right hand tab rack mounting plate and contacts a lug on the
right hand end of the torque bar.
The pivot point of the torque bar is near the bottom of the
bar. When the torque bar pivots, the top of the bar moves to
the rear. The escapement pawl and backspace pawl each have
a lug that extends down just behind the torque bar. As the
top of the torque bar pivots to the rear, it forces the lugs of
the pawls to the rear causi ng the ti ps of the pawls to be
tripped out of their racks.
Rotation of the escapement torque bar is instantaneous and
just sufficient to trip the pawls out of their racks. The torque
bar is immediately rotated back to the rest position by an ex-
tension spring located at the right end. This allows the pawls
to re-enter their racks to limit the carrier movement to one
space.
Because of the force required to trip the pawls out of their
racks, the torque bar tends to bow toward the front instead
of pushing the pawls to the rear. The tendency increases as
the carrier moves toward the middle of the torque bar. Bow-
ing of the torque bar could result in a fai lure of the escape-
ment trip to occur. To overcome this, the pawl pivot stud
has a large head that extends down in front of the torque bar
to stop any bowing toward the front (Fig. 106). The torque
bar actually pries against the pawl stud to trip the pawls