IBM Selectric I/O Manual Of Instruction page 60

A cam rotates only when a particular operation is desired. In
order for the cam to rotate it must be "locked" to the opera-
tional clutch ratchet. Each cam has a pawl mounted to it in
such a manner that the pawl can be pivoted into engagement
with the rotating operational clutch ratchet (Fig. 113).
Whe~
ever the pawl is permitted to engage in the clutch ratchet,
the cam wi II rotate with the ratchet (Fig. 114).
Cam Pawl
CI utch Wheel
Clutch Release Arm
FIGURE 113. Operational Cams At Rest
FIGURE 114. Operational Cam Active
The operational cam pawl is prevented from engaging the
clutch ratchet bya disc called the clutch wheel (Fig. 113).
The clutch wheel is attached to and
forms
a part of the cam
assembly. The disc is mounted to the cam assembly so that
the cam pawl operates through an oversized hole in the disc.
The clutch wheel is free to rotate on the cam assembly, but
its movement is restricted by contact with the cam pawl. A
pin at the tip of the pawl fits into another hole in the clutch
wheel that has one side beveled (Fig. 114). Movement of
the cam whi Ie the clutch wheel is held stationary causes the
pin on the pawl to slide up the beveled side of the hole and
disengage the pawl from the ratchet (Fig. 115).
Movement of the clutch wheel is controlled by the clutch re-
lease arm pivoted on a shaft below and to the rear of the cam
assembly (Fig.
113).
The clutch wheel has a tooth that con-
tacts the end of the clutch release arm as the cam assembly
rotates. The clutch re lease arm stops the movement of the:
wheel. Further rotation of the cam
cau~es
the pawl to be ..
disengaged from the ratchet as described. When the clutch _
release arm is moved down, the clutch wheel is released
(Fig. 114). The cam pawl spring is then a II owed to pu II the
pawl into engagement with a tooth of the clutch ratchet by
46
Operational Control Bracket Lug
----~~~
FIGURE 115. Clutch Pawl Being Disengaged
merely rotating the clutch wheel out of the way. As soon as
the clutch release arm is allowed to restore, it is pulled by
its spring back up into position to contact a tooth of the
clutch wheel. There are only two such teeth for the double
lobed cam and on Iy one for the sing Ie lobed cam. The cam
assembly is driven 180
0
for the double lobed cam and 360
0
for the single lobed cam before the clutch release arm con-
tacts and stops a tooth of the clutch wheel. Rotation of the
double lobed cam through 180
0
and the single lobed cam
through 360
0
allows the high point of the cam to operate the
particular function involved.
The cam pawl spring tries to pull the pawl into engagement
with the clutch ratchet. Unless the cam is held in position
after the pawl is disengaged from the ratchet, the spring wi II
cause the cam to creep backward allowing the pawl to part-
ially engage the ratchet and create a loud buzzing sound. It
cannot fully engage the ratchet because it would be disen-
gaged immediately as previously described.
To prevent backward creep of the operati ona I cams and the
resu Iting noise, a cam check pawl is provided for each cam.
The check pawl engages a notch in the cam assembly as soon
as it has rotated far enough for the cam pawl to be disengaged
from the ratchet (Fig. 116). The check pawls extend to the
rear from a pivot pin located just in front of the cams. An
extension spring from each check pawl to the clutch release
arm maintains the two parts in the rest position. The notch
engaged by the cam check pawl is in a disc-shaped part
called the cam check ring. The ring is attached to the cam
by two screws. An eccentri c collar on one of the screws
faci litates the adjustment of the cam check ring.
Backspace/Spacebar Cam
Cam Check-i---+-
Ring
Cam Check Pawl
FIGURE 116. Operational Cam Check Pawl