RE: [AV Media Matters] Re-Evaluating Tape Mastering Stock

[rohre@ARLUT.UTEXAS.EDU]

I will take a stab at the cinching question.
Finn Jorgenson's book was checked but this problem was not in his
basic index,
although it is covered with other problems in an illustration.

My take on cinching causing binder breakdown:  There are some large
forces as the tension is changed and the tape folds.  I would
suspect the forces of the cinching process are unequal on each side
of the tape.  If so, maybe there is shrinkage of the base film or
stretch, unequal to that of the coatings, and high stress of that
separates the material.  There is the pressure with likely shear
effects, along the surfaces, and this could be the mechanism leading
to spot failures.  Any temperature change that may lead to cinching
might have also contributed to the failure.

In our instrumentation stocks, cinching has usually been seen when
either a drive was not handling tape well, or an extreme change in
environment happened, and/or the tape was run before acclimating to
a new environment for 24 hours as suggested by most tape vendors.  I
believe I have seen "storage cinching", a case of the tape moving
from an apparent good pack to a cinched one, the next time you take
it out of the cabinet/box to play.  Of course, there was always the
question of did it happen on the transport before the tape was put
away and the operator just did not notice it. Luckily for our
purpose, cinching in instrumentation tape has not produced any
binder breakdowns that I am aware of.  However, cinching can happen
at the same time skew problems are being seen in a tape being
tested, or other failures, such as an edge riding a solid glass reel
flange.

That is another strange artifact, edge riding, or warping.  I have
seen different tapes that would ride up the flange of a reel when
moving in one direction, that could be rewound and "corrected" on
another transport.  IE, they would wind forward without edge riding.
Then, if you checked the first transport, other tapes would wind
without incident in the "problem" direction of the first tape.  All
guides would be in tolerance, and specification.  But, any attempt
to reuse the first problem tape, on the transport where it exhibited
the edge riding, would again produce the problem. We have felt this
was a skew problem of the tape, and would screen new stock intended
for Master use for this.  It was as if the plastic had forces or
stresses not parallel to the edges of the tape, and I wonder if
vendors can confirm that base film may have this problem, if not
slit parallel to an "optimum" direction of the film?

Glen Schulze reports that while he was at Martin Marietta Denver
Div., the Skylab program suffered an edge riding incident where the
tape moved the glass flange out enough distance that the glue line
at the hub gave way, and the flange detached, if I correctly
remember the story.

I did see another tape in his lab that moved a Corning glass flange
out audibly, but we stopped the playback, and rewound the tape to
not lose a glass reel.  There was an eerie creaking as the glass
distorted from the pressure of the tape wind against it. I don't
remember if we simply replaced the tape with another blank or what.
The incident was during a copy session, as I recall.

Another artifact we had to screen blank Master candidates for was
Ridging. That is where a "bump" forms somewhere in the pack of the
tape. It is almost always from something causing a thickness
aberration in a layer near the hub, which magnifies when more layers
build up, but some new tapes would possibly have a thickness delta
which would cause problems in the outer winding.  We had a few that
seemed to have no visible or measurable cause. You often could run a
gloved finger into the takeup reel and feel the bump increasing as
more wraps of tape built up.  This became the quick way to check for
this tendency early in the spooling.  You would look at the surface
of the pack, and feel it if need be, with lint free gloved finger.

All this care was of course on programs where the cost of the
experiment vehicles, manpower, etc. and the inaccessibility of the
recorder should a problem arise during the mission, made pre testing
vital to getting back a "good" tape.

Stuart Rohre
UTX ARL Recording Lab