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.