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Winding tapes for long-term storage—a quandary

Filed under: archival practices,storage-care-handling — 2008-02-15 by Richard L. Hess — Last Edit 2011-02-17 by Richard L. Hess

In 2006, I wrote a blog post (here) called “Let Sleeping Tapes Lie: What to do with poorly wound tapes”. For years, tape experts have been suggesting that it is not as good an idea to rewind tapes as was originally thought. This was partially based on the fact that most rewinding in archives was done on the oldest, junkiest machines so as to not wear out the good machines. Unless rewinding is done on high-quality tape transports, it is indeed counter-productive.

We continue to receive poorly wound tapes and are able to play them successfully. So why the quandary now? The reason is that I read portions of another Bharat Bhushan book, Mechanics and Reliability of Flexible Magnetic Media, 2nd Edition, New York, Springer, 2000. Referring to several research papers he makes a compelling case that tapes should be rewound annually if subject to storage environment fluctuations and every 3.5 years if kept in a climate controlled storage area.

If we think about the two types of stresses on each layer in the pack, this becomes more clear. There are “hoop stress” which is the circumferential stress in that individual layer and the “radial stress” which is the stress in the direction of the centre hub.

Low interlayer pressures, which predispose the tape stack to ILS [Interlayer Slip], are created as follows: the winding of successive layers of tape onto a reel increases the radial compressive stresses in the wraps of tape at and near the hub. The continued inward radial deflection of the hub converts the circumferential tensile strain in the tape, originally caused by winding, to a compressive strain….The tape, hub, and winding parameters sufficient to obtain this condition can be determined using [the complex] analysis presented in Chapter 5. A highly compressible hub, high outer-wrap winding tension, low inner-wrap winding tension, the length of tape stored on a reel, and the entrapment of air during high-speed winding are strong contributors to low interlayer pressure

The interlayer pressure from winding is further reduced by temperature and humidity cycling and/or storage….The interlayer pressure can be reduced when the wound tape is subjected to a temperature of humidity change, and is dependent on the relative value of the coefficients of thermal and hygroscopic expansion of the hub and the tape. This effect is aggravated not only by the magnitude of this differential mismatch, but also by the mismatch in the tape’s radial and circumferential coefficients of thermal and hygroscopic expansion.

Bhushan also indicates that spoking can be triggered by impact forces to the tape reel. (Dropping it?)

One of the very interesting things which is discussed is that a tapered winding tension may produce a more archival tape pack than a constant tension winding tension. This raises many questions concerning current archival practices.

I suspect that more discussion of this will occur. Please leave your comments.

Peter Brothers left a lengthy response to this post in the Society of American Archivists list which I am reproducing here. I agree with him on all the major points, but I think Bhushan’s work was done in the late 1980s or early 1990s on single-reel data cartridges as part of the IBM proprietary tape systems like the 3480 (5.5-inch-square cartridge) and 3490 (same size as 3480). These are 1/2-inch wide serpentine recorded tapes and are the forerunner of the LTO tape.

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Date:      Tue, 15 Feb 2011 13:30:18 -0500
From:     Peter Brothers <peter (at)  specsbros (dot) com>
To:          Archives & Archivists (A&A) List <archives (at) forums (dot) archivists (dot) org>
Subject: RE: [archives] VHS Collections

Hello Richard:

I agree with you about not winding/rewinding tapes in storage unless there
is a compelling reason to do so. There is a significant chance of damaging
tape during the process. We wind and rewind tapes before transfer but that
is to check the tape condition, clean the tape for optimal signal response
and is done on special equipment.

In the Blog post you asked for comments, so here goes-

I can understand the findings you quoted from Bhushan as they might relate
to large reel-to reel tape. I was indicating in my earlier e-mail that
there are no studies I’ve seen that show a net benefit to periodically
exercising (winding) VHS tapes.

I don’t know what type of data tapes Bhushan was testing but if we compare
2″ video tapes (similar to the early data tapes we processed for the
government that exhibited the problems he highlights) to VHS, there are a
significant number of differences that make the conditions he was testing
much less relevant for VHS than for large reel-to-reel.

In the section you quoted from Bhushan’s study, he was suggesting exercising
tapes to counter the loosening of the tape pack due to tension stresses, hub
compression and expansion and contraction of the tape due to environmental
changes in storage. VHS is wound at a much lower tension than 2″ data tape
so that variations in the winding tension are not as critical to the tape
pack in storage. Similarly, some 2″ data tape we processed had rubber
wrapping around the hub that allowed for hub compression. This wrapping is
not used on VHS so hub compression is not critical. Finally, the main
problem we have seen with loose packs is due to expansion and contraction of
the tape that, as Bhushan points out, is due to heat and moisture changes in
the environment. The primary vector for thermal and hygroscopic
expansion/contraction is in the thickness of the tape. VHS is approximately
1/3 the thickness of 2″ data tape so there is significantly less material
per strand of tape to contract. In addition Bhushan indicates the
expansion/contraction effects are related to the circumference and radius of
the tape pack. A standard 120 minute VHS is 812 feet long and is on a 3 1/2
inch reel. A standard 60 minute 2″ video (similar to the 2″ data) is 4,950
feet long and is on a 12 1/2 inch reel. Considering the thickness, width
and length of the tape, there is nearly 72 times as much tape material on a
2″ 60 minute reel to be affected by expansion/contraction than on a 120
minute VHS and the 2″ reel is nearly 4 times the diameter of the VHS.
Expansion/contraction effects that might cause serious damage to 2″ tapes
are negligible on VHS.

Over the last 27+ years we have processed over 350,000 tapes at our facility
and we have definitely seen the negative effects of loose tape packs from
expansion/contraction during storage on large reel formats (2″ video, 1/4″
audio, etc.). Where these effects are extreme and allowed to remain
uncorrected, they can result in unrecoverable damage to the tape in
long-term storage. On the other hand, unless a tape has been exposed to
flood, fire or machine malfunction; we have never seen a VHS or other small
reel cassette with a hub lock that has suffered significantly from
expansion/contraction in storage.

All that being said, I do not believe there is any net benefit to
periodically winding and rewinding VHS tapes in storage. There may be some
benefit to exercising large format/large reel tapes but the practice, as you
pointed out, exposes the tapes to possible machine damage.

Peter Brothers
SPECS BROS., LLC
973-777-5055
peter (at)  specsbros (dot) com

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Doug Nishimura of the Image Permanence Institute of the Rochester Institute of Technology responded with a recollection of discussions “back in the day” concerning the issue of winding tape and stresses–this more from his experience working with film than tape, but some of the same basics should apply. I wanted to include the point about polymer flow as I do not think that Bhushan addressed that in the studies (but I could be wrong) and it presents yet another property of polymers to consider when deciding how to approach the issue.

I am still open to further discussions on this which can be appended here.

=======================================

Date:      Tue, 15 Feb 2011 14:52:18 -0500
From:     D NISHIMURA <dwnpph (at) rit (dot) edu>
To:          Archives & Archivists (A&A) List <archives (at) forums (dot) archivists (dot) org>
Subject: RE: [archives] VHS Collections

Back in the 1980s there was a rather circular concern (which came first?). If left alone, tape packs would loosen and therefore periodic winding was considered to be good to “retension” the pack and prevent damage due to a loose pack. I don’t recall ever looking for test data on the problem, but a number of tape manufacturers raised the issue. On the other side was polymer flow that said that if you put a polymer under constant stress, the polymer molecules will move (relative to each other) so as to remove the stress (and this is the problem that we have with films wound on small spools). (This is also known as creep or stress-relaxation in materials science, although technically, creep is more general and stress-relaxation applies to materials with constant strain and not constant load.) The demonstration was to hang a ribbon of some polymer by one end and hang a weight from the other end. If you left it over time, the ribbon would stretch and would get longer (under constant load.) So if you “retension” a tape and leave it, the polymer molecules should move relative to each other so as to remove the stress. In theory, the tape stretches and the tape pack gets loose again (requiring retentioning) and so and so on.

That then caused the whole question about whether it was better to leave the tape (and potentially cause damage to the tape) or retension periodically and risk destroying the magnetic information through tape stretching.


	


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