IBM Selectric I/O Manual Of Instruction page 42

Wear Compensator
A wear compensator is a device that senses a position and
makes a correction whenever it detects an)' change from this
position. The rotate arm assembly is constructed so as to
compensate for wear in the rotate differentia I system and re-
lated components. Because of this aspect of its operation,
the rotate arm assembly is referred to as the wear compensator.
Before going into the details of the operation of the wear
compensator, it is necessary to understand the alignment prob-
lem created by wear in the rotate system.
The rotate pulley spring beneath the typehead and the com-
pensating arm spring apply a constant pressure on the rotate
system in the negative direction. Whenever wear occurs at
any of the pivot points, bearings, stop pads, or linkage con-
nections that oppose the combined tension of the rotate pul-
ley spring and the compensating arm spring, the play (caused
by wear) wi II be removed by these springs in a negative di-
recti on.
Removing the play (which is due to normal wear) allows the
rotate pu Iley spring to turn the typehead (Fig. 89) slightly
in the negative direction. This is commonly referred to as
head "dri ft" .
~
Drift
) : - Drift __
1
Positive
~(
""-:-1
P
""'-)_-.._N_.e_ g _. _ _ _ _ _ _ -....
f_-_-_-_____ ...::....,,/
:1
rrs;=l---C
O
)
D r i f t - - - : . b - - - - - - - { ! }
FIGURE 89. Rotate System Drift
As wear increases, head drift could become excessive and
fai lure of coarse or fine detenting wou Id occur. Detenting
failure would necessitate a readjustment (re-homing) of the
typehead.
The wear compensator cannot prevent wear but it does prevent
head drift whi ch is due to wear in the system. The wear com-
pensator is composed of three basi c parts: the rotate arm, the
compensating arm, and a nylon roller between them (Fig. 90).
Other components are present that are indirectly involved in
the wear compensator action. At this time, consider only the
three parts menti oned.
The rotate arm (Fig. 90) pivots on a large pivot pin in a
bracket mounted to the left si de of the power frame. The
left-hand rotate pulley operates at the top of the rotate arm.
The compensating arm pivots at the same point as the rotate
arm and extends in two directi ons from the pivot. The rotate
link is fastened to the lower extension of the compensating
arm. The upper extensi on of the compensating arm fits be-
tween the side-frames of the rotate arm. The nylon roller
operates in a long vertical slot in the rotate arm. This slot
and the upper
E~xtensi
on of the compensati ng arm are at a
33
slight ang Ie to each other, thus forming a V-shaped wedging
slot. The roller is retained in position in the wedging slot
by the rotate tape tensi on •
"The Basic Compensator Action of the Wear Compensator
ll
V-Shaped
Wedging Slot
........ll.Ji,;,.;+--
Pivot Pin
FIGURE 90. Basic Components Of The Wear Compensator
Figure 91 -A shows the wear compensator in a zero rotate
position. Note the following in Figure 91-A:
a.
The compensating arm spring is applying ten-
sion on the upper extension of the compensating
arm in a negative direction.
b. The rotate tape is applying tension (by means
of the rotate pu Iley spring) to the top of the
rotate arm in a negative direction also.
c. The rotate link attached to the lower extensi on
of the compensating arm is opposing the com-
pensating arm spring tension. The rotate link
is a Iso opposi ng the rotate tape tensi on through
the compensating arm and the nylon roller.
d. The nylon roller is positi oned near the top of
the V-shaped wedging slot.
Figure 91-B shows the wear compensator in the negative 5
position. Note that the eccentric stud mounted on the top
of the rotate arm is just barely touching the machine si de
frame when the wear compensator is in the negative 5 posi-
tion.