IBM Selectric I/O Manual Of Instruction page 45

ging slot. This causes the roller to move down in the slot
compensati ng for the wear.
In order to relax all of the pressure on the nylon roller when
the rotate arm contacts the machine sideframe, the sideframe
must oppose all of the rotate spring tension (plus the compen-
sator assist spri ng whi ch wi II be covered later). This causes
the spring load on the differential system to reduce to just
that of the compensati ng arm spri ng thereby causi ng the
system, from the nylon roller on down, to partially relax.
This partial relaxing of the differential system creates a
slight amount of motion to the rotate link in the opposite di-
rection that the link is moving, thus resulting in a reduction
of output of the rotate link to the bottom of the compensating
arm between a negative 4 and negative 5 rotate position.
The ratio change of the compensating arm overcomes this ef-
fect by producing more motion to the top of the arm for a re-
duced amount of motion from the rotate link, thus the rotate
arm reaches the negative 5 position and the pressure on the
nylon roller is relaxed.
Compensating Arm
FIGURE 92. Compensating Arm Pivot Hole
The compensating arm contains a large pivot hole (Fig. 92).
Fitted inside the large pivot hole is an eccentric shoulder
which is part of the rotate eccentric arm (Fig. 93). The
compensati ng arm and the rotate eccentri c arm act as one
solid piece as they rotate about the pivot pin for all positive
and negative rotate positions up to approximately the nega-
tive 4 position. If a negative five character is selected, the
rotate eccentri c arm wi II be restri cted from rotati ng (about
the pivot pin) beyond the negative 4 position and the com-
pensating arm will then begin to rotate about the eccentric
shou Ider on the rotate eccentri c arm.
Since the center of the eccentric shoulder is lower than the
pivot pin (closer to the rotate link) and the compensating arm
pivot point is shifted (between negative 4 and negative 5)
from the pivot pin to the eccentric shoulder, it is correct to
say that the compensating arm has undergone a ratio change.
36
FIGURE 93. Mounting Of Rotate Eccentric Arm
The rotate eccentric arm is spring loaded against the rotate
arm by means of the rotate eccentric arm spring (Fig. 94).
The rotate arm is spring loaded against the compensating arm
by means of the rotate pu Iley spring. Therefore, the rotate
Wear Com
Bracket
Stop Lug----
Rotate Eccentric Arm)
FIGURE 94. Zero Rotate Position
eccentri c arm, rotate arm, and compensating arm act as one
solid arm rotating about the pivot pin from a positive 5 to the
negative 4 rotate position. During a negative 5 selection,
the lower extension of the rotate eccentric arm (Fig. 95)
contacts the stop lug on the wear compensator bracket (at
the negative 4 positi on). The rotate eccentri c arm is stopped
and the compensating arm ratio change begins. The rotate
arm continues to follow the compensating arm because the
rotate pulley spring is stronger than the rotate eccentric arm
spring. The rotate arm is stopped by the machine sideframe
when it reaches the negative 5 position. The additional
motion given to the compensating arm (due to the ratio change)
is used up in partially relaxing the rotate differential system.
Thus, wear can be compensated for.