CAUSES OF MAGNETIC TAPE FAILURE
Tape failure happens when a magnetic tape's performance degrades to a point where it's no longer usable. The exact point where failure occurs will vary, depending on the type of tape and how it is used.
There are four main causes for tape failure:
Normal wear occurs because the tape must come in contact with fixed surfaces, such as a recorder's magnetic heads, rollers, and guides. Over time, this repeated contact with the fixed surfaces causes excessive dropout errors and makes the tape unusable.
Accidental tape damage that causes tape failure is any damage that wouldn't normally occur under ideal operating and handling conditions. It can be caused by either a human operator or the tape recorder itself. Accidental tape damage caused by human operators can range from accidentally dropping a reel of magnetic tape to improperly threading a magnetic tape recorder. Accidental tape damage caused by recording equipment can occur if the recorder is poorly designed or if the tape transport mechanism is adjusted improperly.
The negative effect of environmental extremes on tape can also cause tape failure. Magnetic tape is very flexible and can be used in a wide range of environmental conditions. It's designed for use in a temperature range of about 2 to 130 degrees Farenheit (-20 to 55 degrees Celsius), and in a relative humidity range of about 10 to 95%. Of course, these numbers are the extreme. Ideally, magnetic tape should be used and stored at a temperature of about 60 to 80°F (room temperature), and in a relative humidity of about 40 to 60%.
Large changes from the ideal relative humidity cause tape to expand or contract and thus affect the uniformity of a tape's oxide coating. High relative humidity causes the tape to stretch and increases the tape's friction. The increased friction causes increased head wear, head clog by oxide particles, and head-to-tape sticking. Low relative humidity encourages oxide shedding and increases static build-up on tape surfaces, causing the tape to collect airborne contaminants.
The effects of exceeding the ideal temperature and humidity ranges described above can cause the following environmental damage to magnetic tape: tape deformation, oxide shedding, head-to-tape sticking, layer-to-layer sticking, dirt build-up, and excessive tape and head wear.
Magnetic tapes are wound onto tape reels with tension applied. This tension causes great layer-to-layer pressure within the reel pack. Changes in temperature and humidity can cause the backing material to expand or contract, creating even more pressure. All of this pressure causes the tape to become deformed or warped.
At temperatures above 130°F, a tape's oxide coating tends to become soft. At temperatures below 2°F, the oxide coating tends to be brittle. In both cases, the oxide coating will shed, flake off, or otherwise become separated from the base material. These free pieces of oxide will then stick to parts of the tape transport, to the magnetic heads, or back onto the tape and cause dropout or level errors.
At higher temperatures, the tape binder glue can soften to the point where it will stick to the recorder's magnetic head. This head-to-tape sticking causes jerky tape motion.
When reels of magnetic tape are stored at higher temperatures, the tape's binder glue may melt and cause the layers of tape to stick to one another. In very severe cases, layer-to-layer adhesion can separate the oxide coating from the base material and completely destroy a tape.
Dirt build-up happens when the relative humidity level is less than 10%. The low humidity causes static electricity that attracts dirt and dust which builds up on the magnetic tape and other parts of the magnetic tape recorder.
Excessive Tape and Head Wear
When the relative humidity is more than 95%, the high humidity causes increased friction as the tape passes over the heads. This, in turn, causes excessive tape and head wear.
Q.12 What is tape failure?
Winding errors are another cause of tape failure. They happen when improper winding practices create an excessive or uneven force as the tape is being wound onto a tape reel. The form taken by the tape after it is wound onto the reel is called the tape pack. Winding errors can cause a deformed tape pack that will prevent good head-to-tape contact.
In most cases, a deformed tape pack can be fixed simply by rewinding it onto another reel at the proper tension and at the right temperature and humidity. The four most common types of tape pack deformation are:
Cinching happens when a tape reel is stopped too quickly. The sudden stop causes the outer layers of magnetic tape to continue to spin after the inner layers have stopped. This causes any loosely wound tape within the pack to unwind and pile up. Figure 2-2 shows an example of a cinched tape pack (note the complete foldover of one tape strand).
Figure 2-2. - Example of cinched tape pack.
Pack slip happens when the tape is loosely wound on the reel and is exposed to excessive vibration or too much heat. This causes the tape to shift (side-to-side), causing steps in the tape pack. When a tape reel with pack slip is used, the magnetic tape will unwind unevenly and rub against the sides of the tape reel or the recorder's tape guides. This can damage the magnetic tape and cause oxide shedding. Figure 2-3 shows an example of pack slip.
Figure 2-3. - Example of pack slip.
Spoking happens when magnetic tape is wound onto the tape reel with the tension increasing toward the end of the winding. The higher tension on the outside of the tape pack causes the inner pack to buckle and deform. Spoking is also caused by the uneven pressures created when a tape is wound on a reel that has a distorted hub, or when the tape is wound over a small particle that is deposited on the hub. Figure 2-4 shows a spoked tape pack.
Figure 2-4. - Example of spoked tape pack.
Windows are voids or see-through air gaps in the tape winding. They happen when magnetic tape is loosely wound onto a tape reel, and especially when the loosely wound reel is later exposed to extreme heat or humidity. Figure 2-5 shows a windowed tape pack.
Figure 2-5. - Example of windowed tape pack.
Q.22 Tape winding errors can cause a deformed tape pack. What are four common types of
tape pack deformation?