IBM Selectric Manual page 31

6. Velocity Control Plate - The velocity control plate must
be adjusted to satisfy the following two conditions.
a. With the cam follower held lightly against the low
point of the print cam, the center of the home char-
acter should clear the platen by .260" to .270".
b. With the cam follower held I ightly against the high
point of the print cam, the home character should
clear the platen by .020" to .030".
The copy control lever should
be
forward for both ad-
justments. These ad justments should be made with the
carrier positioned in the center of the writing line. On
long carriage machines the adjustments should
be
made
with the carrier at the extreme left hand position.
The print cam has a fixed amount of rise from its low
point to its high point. For this reason, the print cam
follower always receives the same amount of powered
travel ormotion from the print cam. However, the amount
of powered travel that the rocker and type head receive,
from the print cam follower, is directl.y dependent upon
the position ofthe velocity control plate pirv in the forked
slot of the follower (Fig. 75). Moving the pin to the
front of the slot decreases the po we red trave I of the type-
head as shown by dimension A (Fig. 75). Moving it to
the rear increases the powered travel, as shown by dimen-
sion B.
Print Cam
Print Cam Follower
FIGURE 75. Velocity Control Plate Adjustment
Because of the constant operational speed of the print
cam and follower, the typehead velocity must increase
or decrease proportional to the increase or decrease in
powered travel. An accompanying change in typehead
velocity occurs when the powered travel is changed be-
cause the type head must always travel from its rest posi-
tion to its active position in the same amount of time
regardless of what this distance may
be.
The proper impact velocity of the typehead can be
achieved by controlling the amount of powered travel of
the type head whi Ie maintaining at the same time the
correct amount of free flight. Since the position of the
platen has been previously fixed and the amount of free
flight is determined by the point at which the limit of
powered travel occurs relative to the platen, only the
rest position (beginning of powered travel) may change
when the amount of powered travel is changed. There-
29
fore, in order to maintain a fixed amount of free flight
to the typehead when the amount of powered travel is
changed, the velocity control plate pin must also be ad-
justed up or down relative to the rocker.
The eccentric shouldered nut (Fig. 76) on the velocity
control plate provides a means of controlling the vertical
position of the velocity control plate pin relative to the
rockerthereby controlling the free flight of the typehead.
The elongated hole in the velocity control plate (behind
the binding screw) allows the velocity control plate pin
to be adjusted front or rear in the forked slot of the fol-
lower thereby controll ing the amount of powered travel
that the type head wi II receive. Each adjustment affects
theotherand must be adjusted alternately until both are
correct .
The adjustments can
be
made easi Iy if the following pro-
cedure is used.
a. Raise the anvi I ad justing eccentrics and the restor-
ing cam follower eccentric to prevent any inter-
ference, and remove the ribbon feed plate for ac-
cessibility.
b.
If
the velocity control plate is loose or completely
outofadjustment, set the high part of the eccentric
forward and tighten it in place. Adjust the plate so
that about 1/32" ofthe adjusting slot is visible above
the binding screw and tighten the screw friction
tight. These settings provide a good starting point.
c. Hand cycle the machine using the letter liZ
II
until
the print cam follower is on the high point of the
cam. Do not hold the type head toward the platen.
With the mechanism in this position, adjust the
Type Head
Held Slightly
Forward
Slight Drag On Three Tab Cards
Approx. 1/.32" Of Slot
Visible Above Head Of
Screw
FIGURE 76. Velocity Control Plate
Eccentric Adjustment