Operational Cams And Control Mechanism - IBM Selectric I/O Manual Of Instruction

The center of the backplate contains a ball bearing assembly
that supports the rear end of the escapement shaft. The shaft
extends to the rear into the mainspring where a hub is set-
screwed to the shaft. The inner end of the mainspring is
rolled so that it fits into a groove in the hub and supplies a
rotary force to the hub and escapement shaft.
The escapement shaft extends forward through another ba
II
bearing assembly in the powerframe and has a drum attached
at the forward end. This drum is ca lied the cord drum gear
and is spirally grooved to accept the escapement/tab cord.
The grooves prevent the cord from piling up on the drum in-
suring uniform tensi on and minimum wear.
The escapement/tab cord is a small, round nylon-covered
linen cord. The drum end of the cord is knotted and fits into
a slot at the rear of the drum. The cord makes several turns
around the drum and rides up over a guide roller before pass-
ing through the right side of the machine (Fig. 111). Just
outside the machine the cord passes around a pulley and ex-
tends back to the left where it is attached to a hook on the
carrier assembly. As the mainspring turns the escapement
shaft, the drum winds up the cord to move the carri er to the
right.
More than one operati on is performed by the escapement
shaft. The gear teeth at the front of the cord drum are in-
volved in both the carri er return and tabu lator operati ons.
Between the powerframe and the backplate is another cord
drum with a cord attached to it simi lar to the escapement/tab
cord (Fig. 111). This cord is attached to the left side of the
carrier and exerts a pull to the left during carrier return.
These operations are fully covered in their particular sections.
However, the carrier return cord drum is significant in the
escapement mechanism, because it must payout cord in order
for the carrier to move to the right. Likewise, the escape-
ment/tab cord must be paid out from its drum in order for the
carri er to retu rn to the I eft.
The constant jerk and pull to which the cords are subjected
may tend to stretch them slightly. This could cause the cords
to become slack creating erratic movement of the carrier. A
method has been provided whereby the slack is automatically
removed from both cords. Outsi de the powerframe, the
pu Iley that gui des the escapement cord is mounted to the cord
tension arm (Fig. 111). A pair of spiral springs arched be-
tween the arm and the pulley mounting bracket apply a con-
stant pressure toward the right. The pressure is suffi ci ent to
keep the slack out of the escapement cord. This, in turn,
rotates the escapement shaft enough to keep the carrier re· ...
turn cord tight.
OPERATIONAL CAMS AND CONTROL MECHANISM
All powered service operations are driven by the operational
cam shaft located on the right side midway back in the ma-
chine. The powered operati ons are the spacebar, backspace,
tabulation, carrier return, indexing, and shift.
The spacebar, backspace, tabulator, carrier return, and in-
dexing mechanisms are operated by two cam assemblies
mounted on the operational cam shaft. The shift is driven
by a spring clutch at the right end of the shaft outside the
powerframe.
45
Operational Cams
The two operational cams are located on the right side of the
operational shaft just inside the powerframe (Fig. 112). The
left hand cam is a double lobed cam that requires only 180
0
rotation to complete one operation. Its purpose is to power
the spacebar, backspace and tabulation mechanisms. The
right hand cam is a single lobed cam requiring 360
0
rotation
to complete one operation. It powers the engaging of the
carrier return mechanism and operates the indexing mechan-
ism.
Each operational cam must power more than one function.
The mechanism to be operated is determined by a selection
system released by depressing the desired keylever. The se-
lection operation is discussed later in this section.
Both the single and the double lobed cams have the same
rise from the low point to the high point. The double lobed
cam completes an operation sooner than the single lobed
cam, because it requires only 180
0
rotation compared to
360 0
for the sing Ie lobed cam. The faster double lobed cam
is used in the spacebar mechanism because the spacebar act-
ion must be as fast as the print action in order to maintain
typing rhythm. The backspace and tabulation operations
must operate quickly in order to provide rapid positioning
of the carrier; therefore they also employ the double lobed
cam. Both the carri er return and indexing keylevers have a
repeat/non-repeat feature. When operated in the repeat po-
sition, they cause rapid indexing of the platen. Operating
too rapidly could create inaccurate indexing due to platen
overthrow and fai lure of the index pawl to restore quickly
enough; therefore the slower single lobed cam is used to
operate the carrier return and indexing mechanisms.
A ratchet, called the operational clutch ratchet, is set-
screwed to the operati onal cam shaft (Fig. 112). The cam
assemblies are C-clipped into position on shoulders of the
ratchet, one on each end. The operational cam shaft and
clutch ratchet are allowed to turn while the cam assemblies
remain stationary. Steel sleeves fitted into the cam bodies
act as bearings for the cams.
Operational Cam Shaft
Backspace/Spacebar/Tab
Cam
FIGURE 112. Operational Cams