IBM Selectric I/O Manual Of Instruction page 86

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ab Lever Imprisoned
Between Ears On Trigger
FIGURE 169. Tab Lever Trigger (Late)
Tab Lever Overthrow Stop
An overthrow stop is mounted on the escapement bracket. It
extends to the rear and down behind the trigger (Fig. 170).
The stop prevents the tab lever from being thrown into the tab
rack.
FIGURE 170. Tab Overthrow Stop
Tab Governor
The carrier speed during a tab operation must be controlled to
insure an accurate tab, reduce the noise, and prevent exces-
sive wear and shock on the components. During a tab opera-
tion, the carrier is pulled to the right by the tension of the
mainspring as during an escapement operation. The tab gov-
ernor operotes by limiting the speed with which the escal1e-
ment cord drum winds up the cord.
The beveled gear on the escapement cord drum meshes with
the tab governor pinion located on the operational shaft to
the right of the cord drum (Fig. 171). The pinion gear oper-
ates between two collars. The left collar and the pini on gear
have hubs enclosed by a clutch spring. The left collar is set-
screwed to the shaft and the pinion gear pivots freely on the
shaft. The spring is wound so that it slips when the pinion is
held stationary and the operational shaft is turning.
67
Pinion Tries To
Exceed Speed of
Collar and Causes
Clutch Spring to
Tighten
FIGURE 171. Tab Governor Mechanism
If the pinion gear is turned in the same direction as the oper-
ati ona I shaft but at a faster rate of speed, the fri cti on of the
clutch spring causes it to tighten around the two hubs locking
them together. During a tab operation, the cord drum drives
the pini on gear in the same directi on as the operati ona I shaft.
The mainspring tension causes the pinion to speed up and
tighten the clutch spring. The mainspring then tries to accel-
erate the operational shaft. The mainspring does not have
sufficient tension to drive the operational shaft, because of
the drag present in the system. The shaft must be driven by
the motor; therefore the speed of the tab governor pini on can
be no faster than the normal speed of the operational shaft.
The escapement cord drum can wind up the cord on Iy as fast
as the pinion will let it. The gear ratio between the pinion
gear and the escapement cord drum allows the carrier to be
moved at the proper speed during a tab operation. No gov-
erning action is obtained during approximately the first inch
of carrier travel, because a short distance is required to
tighten the tab governor clutch spring.
The tab governor pinion is the same size as the carrier return
pinion gear. This makes the speed of the carrier the same
for both tab and carrier return.
Tab Un latching
When the carrier reaches the desired stopping point, the es-
capement pawl must be a Ilowed to re-enter the escapement
rack and stop the movement of the carrier. The tab lever is
mounted in an elongated hole at its pivot point. An exten-
sion spring holds the tab lever to the right. As the carrier
moves toward the right, the tip of the tab lever contacts the
set tab stop and is prevented from movi ng further (Fig. 172A).
The carrier continues to the right carrying the pawls and the
tab latch with it. Movement is allowed by the elongated
hole at the tab lever pivot. As the escapement pawl moves
to the right in re lati on to the tab lever, a notch in the pawl
a Ilows it to drop off the lug of the tab lever and restore to
the escapement rack (Fig. 172B). Further movement of the
carri er moves the tab latch to the right out of the notch of
the tab lever (Fig. 172C). The tab lever then restores and
a Ilows the backspace pawl to re-enter its rack.