IBM Selectric I/O Manual Of Instruction page 49

When the machine is at rest the high point of the print cam
is facing toward the front of the machine. This makes the
print operati on occur late in the cycle which allows suffi-
dent time for the selection and differential mechanisms to
position the typing element before the print operation begins.
The larger camming surface on the left of the print cam is
called the restoring cam (Fig. 102). Its purpose is to power
the rocker back to the rest positi on after the typehead prints
thereby preventing any rebounding from occurring. This is
accomplished by the restoring roller on the upper arm of the
cam fol lower.
The camming surface of the print cam is designed so that the
typehead is powered to within a few thousands of an inch of
the platen. To prevent any choking action, the momentum
of the rocker carries the typehead the remaining distance.
The contour of the rebound cam is such that it allows "free
flight
II
of the rocker and typehead, when the typehead is
near the platen. Because the typehead is powered nearly
all the way to the platen, a II the characters are forced to
emboss the paper slightly, even those with a large surface
area.
Anvi I and Striker
Because the type head is powered nearly all the way to the
platen, it is necessary to restrict the amount of free travel of
th_e typehead so that the amount of embossing for all char-
acters wi II be the same. A heavy arm co I led the anvi I stri-
ker attached to the bottom of the rocker acts as a stop for the
rocker (Fig. 103). Just under the front of the carri er and
extending between the side frames is the anvi I. It is an
angle-shaped bar with the lip extending to the rear. As the
FIGURE 103. Anvil And Striker
40
type head embosses the paper to the correct depth, the stri ker
hits the bottom of the anvi I and prevents further travel of the
rocker and typehead. This means that the impression for each
character will be consistent with little variation between the
characters.
Whenever the carrier prints in the middle of the writing line
on long carriage machines, downward flexing of the print
shaft occurs resulting in a loss of impression. Carrier buffers
(Fig. 104) located on each side (at the front) of the carrier
casting limit this flexing action by operating against· the top
surface of the anvil.
Carri er Buffers
FIGURE 104. Carrier Buffers (Long Carriage)
Platen
The purpose of the platen is to feed the paper vertically and
to provide a solid backing for the paper during a typing op-
eration. The quality of type impression obtained is deter-
mined to a large extent by the condition of the platen.
Platen rubber may be adversely affected by numerous factors
such as light, heat, chemicals, etc. An old or worn platen
may be considerably harder than a new platen and may also
vary slightly in diameter. This machine is equipped with a
platen with Q....hardness density comparable to the number 2
platen.
The platen is held in position on the machine by a latch
pivoted at the front on each carriage plate (Fig. 105). The
platen may be removed by pressing the rear of the latches
down and lifting the platen out. It may be installed by snap-
ping it into position without depressing the latches. The
camming action of the latches causes them to remove all
verti co I as we II as hori zonta I p lay from the platen.