IBM Selectric I/O Manual Of Instruction page 44

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Compensating Arm
FIGURE 91 D. Negative 5 Position With Drift
(Nylon Roller Drops)
spring. All the parts that are opposing this spring tension are
being stressed (flexed) slightly. Although steel parts appear
to be extremely strong and rigid, they do have a measurable
amount of e lasti ci ty when placed under a stress. Shou Id the
stress (spri ng tensi on) be reduced, the steel parts wi II tend to
return tothei r original shape and position.
During a negative 5 selection, the eccentric stud on the ro-
tate arm contacts the mach i ne si deframe • The si deframe now
starts to oppose the rotate pu Iley spring tensi
oni
and the
spring load on the rotate differential starts to reduce to that
of the compensating arm spring. When the spring load on the
rotate differential starts to reduce (due to the rotate arm con-
tacting the sideframe), the flexing of the rotate differential
begins to reduce also. Even though a portion of the rotate
pulley spring tension is being opposed by the sideframe, the
rest of the rotate pulley spring tension is still applying ten-
sion to the rotate differential through the nylon roller. This
keeps the system in a flexed condition maintaining pressure
against· the nylon roller. Under these conditions, a small
amount of wear in the rotate differential would not allow the
roller to drop. The wear would only be absorbed in reducing
the flexing of the rotate differential at the negative 5 rotate
position. The wear compensator would not compensate for
wear and "drift" would be apparent in all rotate positions
except negative 5 (sideframe would prevent drift at the neg-
ative 5 rotate position).
To summarize the above, let's simplify the situation by mak-
ing an assumption. Suppose that all the flexing in the rotate
differen.tial could be placed at one point. Let's assume that
the nylon roller is made out of soft rubber that can be com-
pressed easi Iy. With the wear compensator arm assembly in
35
the zero rotate position, the rotate pulley spring tension is
squeezing the rubber roller between the rotate arm and the
compensating arm. Pretend that the amount the rubber rol-
ler is squeezed is the flexing of the
differentia~
system. Now,
half cycle the machine to a negative 5 position. The ec-
centric stud just touches the sideframe and the rubber roller
remains compressed. At this time, let's assume that a large
amount of wear came into the rotate differential system.
(Wear shows up as play.) The compensating arm spring would
pull the top of the compensating arm in a negative direction,
removing the play from the differential system. The rotate
arm, restricted by the sideframe, could not follow the com-
pensating arm and the pressure on the rubber roller would be
relieved allowing it to expand. The play (caused by wear)
in the system was just enough to allow the rubber roller to
expand without dropping. Now, if the rotate link starts to
pull the arm assembly ba.ck to a zero rotate position, some of
the rotate link motion is going to be used to compress the
rubber roller before the rotate arm leaves the sideframe.
This means that the rotate arm wi II not receive sufficient mo-
tion to return to the zero rotate position. Wear has caused
the rotate arm to drift in a negative direction.
Conclusion:
a. Flexing in the system is inherent.
b.
Any lost motion from the rotate differential to
the rotate arm wi II cause "dri ft
II •
c. The pressure on the roller must be relaxed be-
fore compensati on takes place.
"Wear Compensator Rati
0
Change
II
In the wear compensator, there is a constant leverage ratio
between the amount of motion supplied to the bottom of the
compensating arm {by the rotate link} and that amount of mo-
ti on produced at the top of the rotate arm. (The movement
of the rotate arm directly depends upon the movement of the·
upper extension of the compensating arm through the nylon
roller.)
If the pivot point of the compensating arm is changed to a
lower position {closer to the rotate link} the leverage r¢lti
0
of
the compensating arm will increase. This will cause a great-
er amount of moti on to be produced to the top of the compen-
sating arm for a given amount of motion supplied to the bot-
tom of the arm. When this occurs, it is correct to say that
the compensating arm has. undergone a "ratio change ". The
ratio change is required in order to relax the pressure on the
nylon roller whi Ie maintaining the correct output to the rotate
arm at a negative 5 position.
The ratio change begins approximately at the negative 4 po-
sition and occurs as the arm sweeps through to a negative 5
position. It provides the upper extension of the compensating
arm with sufficient motion to relax the pressure on the nylon
roller {without allowing it to drop so that it is ready to drop
as soon as the slightest amount of wear is felt in the differ-
ential system. With the pressure relaxed on the roller, any
wear coming into the differential system allows-
tb~e
c;:ompen-
sating arm spring to pull the top of the
com~nsating'
arm
further in a negative direction opening up the V-shaped wed-