IBM Selectric I/O Manual Of Instruction page 32

FIGURE 63. Shift Clutch
Usually a spring clutch is considered to be one that tightens
when its driving hub turns in the direction that the spring
clutch is wound and slips if the hub stops or rotates back in
the opposite directi on. This is not true of all spring clutches.
In the shift mechanism, the driving hub (the shift clutch
arbor) is rotating continuously in the direction that the
spring clutch is wound. In order to stop the shift cam, the
spring clutch must be allowed to slip whi Ie the arbor contin-
ues to rotate. The on Iy way to do this is to enlarge or ex-
pand the inside diameter of the spring clutch so that the
driving action will stop.
In the shift mechanism, the shift spring clutch must be held
in the unwound position so that it wi II be enlarged enough
to allow the arbor to slip freely inside it. To enlarge the
spring clutch, one end of the spring must be held while the
other is rotated in the unwinding direction of the spring. The
left end of the spring is formed out and fits into an adjustable
plate attached to the cam (Fig. '63). The right end of the
spring is formed to the right and fits into a hole in the shift
clutch ratchet (Fig. 63). The shift clutch ratchet is gear-
like part mounted on the end of the shift clutch arbor. A
large C-clip holds it in place. The arbor turns freely inside
the ratchet when the shift is not being operated. Rotation of
the ratchet counterclockwise causes the clutch spring to de-
crease in size. Conversely, if the ratchet is held stationary
whi Ie the cam is rotated counterclockwise, the spring dia-
meter is increased.
The shift clutch ratchet has two lugs 180
0
apart on its left
surface. One lug is nearer the center than the other and is
referred to as the inner lug. The other lug is called the
outer lug. The shift release arm pivoted just in front of the
cam blocks the movement of the ratchet lugs to stop to ro-
tation of the ratchet (Fig. 64).
The position of the shift release arm determines the lug of
the ratchet that wi II be stopped. The position of the shift-
release arm is controlled by a link from the arm to a lever
attached to the end of the shift bai I (Fig. 61). When a
shift keylever is depressed, the bail rotates to force the link
to the rear and operate the shift release arm. When the key-
levers are at rest, the release arm is in position to stop the
inner lug of the shift clutch ratchet (Fig. 64). Depressi on
of a keylever causes the release arm to rise out of the path
of the inner lug into the path of the outer lug (Fig. 64).
24
Inner Lug
Outer Lug
Shift Clutch Release Arm
Shift Clutch Ratchet
a. LOWER CASE POSITION
b. UPPER CASE POSITION
FIGURE 64. Shift Clutch Release Arm
When the inner lug is released, the clutch spring is allowed
to decrease in diameter by its own spring tension. It then
tightens around the rotating shift clutch arbor and the hub of
the cam to drive the cam • The shift clutch ratchet rotates
with the cam and clutch spring 180
0
•
The outer lug of the
ratchet then contacts the shi ft re lease arm, that has been
raised into its path, and its rotation is stopped. The momen-
tum of the shift cam causes it to rotate slight Iy farther. This
further rotation causes the clutch spring to unwind and in-
crease in diameter to allow the shift clutch arbor to slip free-
ly inside the spring.
Once the shift cams momentum has carried the cam for enough
to properly disengage the spring clutch the cam must be re-
stricted from overthrowing its rest position. Overthrow is
controlled by the shift cam stop which is attached to the cam
and operates against the inner lug of the shift clutch ratchet
(Fig. 65).
Shift Clutch Spring
f.iounting Plate
Shift Detent
FIGURE 65. Shift Arm and Detent
As long as the shift keylever is held depressed, the high
Shift Cam
point of the shift cam remains to the rear holding the shift
arm to the right. When the keylever is released, the shift
release arm moves down out of the path of the outer lug and
back into the path of the inner lug. This allows the clutch
spring to again tighten and drive the shift cam and ratchet
180
0
back to the lower case position. The inner lug of the
ratchet then contacts the re lease arm to disengage the spring
clutch as before. The shift cam stop again controls overthrow
of the cam.