IBM Selectric I/O Manual Of Instruction page 17

Consider the positive rotation of the typehead first. Three
latches and a series of three levers are involved in positive
Ilotation (Fig. 26). The three latches are those farthest to the
right in the latch bai I. All the latches are spring loaded to
the rearunderthe latch bail and are operated by the bail unless
they are pulled forward.
Rotate Tape
Rotate Bellcrank
Ba lance Lever
FIGURE 26. Rotate Differential At Rest
The two latches on the extreme right are mounted by ball shoul-
dered rivets to a short lever similar to the tilt latch mounting.
A flat double link extends vertically from the lever to a second
lever above it. The connection is at the right end of the second
'lever and one-third of the distance from the right end of the
fi rst I ever.
The third latch is connected by a ball shouldered rivet to the
left end of the second lever. Because its mounting point is
higher than the other latches, the third latch is much longer
than the others to permit latching under the bai I at the bottom.
A second vertical I ink connects the second lever to the left
endofthethird lever in the series. The link is attached to the
second lever two-fifths of the distance from the right end.
The third lever is an adjustable lever connected at the middle
to the horizontal arm of the rotate bell crank. It is referred to
as the balance lever, because its ad justment balances the amount
of motion between positive and' negative rotation. The right
end of the balance lever is held stationary during positive ro-
tation. A downward pull at the left end causes the rotate bell-
crank to operate. A heavy link connects the bottom of the
rotate be II crank to the bottom of the rotate arm. The rotate
arm is sometimes referred to as the rotate multiplying arm be-
cause the movement of the rotate I ink is increased at the pulley
as a result of the leverage. Operation of the bellcrank counter-
clockwise causes the rotate arm to pivot about its fulcrum point
and exert a pull on the rotate tape.
In order to operate the ba lance lever for a positive rotate
cyc Ie, one or more of the latches must be pu lied down by the
latch bai I. Consider the latches one at a time starting with
the middle of the three rotate latches. When only the middle
latch is allowed to remain under the latch bail, it is the only
one pulled down when the bai I operates (Fig. 27). As the
middle latch is pulled down, its attached lever moves down
10
at the left and pivots at the right on a stop lug formed out
from the differential bracket. The vertical link is attached
to the lever one-thi rd of the distance from the pivot point to
the latch. This causes the link to be moved down one-third
as much as the latch.
This Point Stationary On
All Positive Selections
FIGURE 27. Positive - One Rotate Operation
The link exerts a pull on the right end of the second lever caus-
ing it to pivot on its stop lug at the left end. The second ver-
tical link is attached to the lever three-fifths of the distance
from the pivot point to the operating end of the lever. The
second link moves three-fifths as much as the right end of the
lever. The right end of the second lever moves one-third as
much as the latch. Multiplying the two together, the second
vertical link moves three-fifteenths or one-fifth as much as the
latch.
The second vertical link operates the left end of the balance
leverwhich, in turn, actuates the rotate bellcrank. This move-
ment is sufficient to cause a positive rotation of one character
on the type head. The movement is caused by operation of the
middle rotate latch, so it is referred to as the rotate-one latch.
When only the right hand latch is operated, the movement of
the rotate bell crank is doubled, and a positive rotation of two
characters is obtained on the type head. The increased move-
ment is obtained at the first lever. When the right hand latch
moves down, the lever pivots on a stop lug at the left. The
vertical I ink is attached two-thirds of the distance from the
pivot point to the latch; therefore the link moves down two-
thirds as much as the latch. This is twice as much as for the
rotate-one latch. The right hand latch is referred to as the
rotate-two latch.
When both the rotate-one and rotate-two latches are operat-
ing together, the first lever moves straight down without pivot-
ing at either end (Fig. 28). This gives the same amount of
motion to the first vertical link as is given to the two latches.
Movement of the link is three times as much as when operated
by the rotate-one latch alone. The additional movement is
transferred to the rotate bell crank to rotate the type head three
characters in the positive direction.
When additional rotation is required, the third rotate latch must
be used. The left hand latch is never used alone; but to clarify
its leverage in relation to the other latches, consider it to be
the only one in operation.