Handbook for Sound Engineers

1048 Chapter 28

voltage, generating a constant torque from the motor. To
convert motor torque to tape tension, divide the torque
by the radius of the tape pack (the lever arm).
However, the radius of the pack on the supply reel
decreases as the tape plays off. By the end of a 10½ inch
NAB reel, the radius has dropped to half the starting
value, causing the tape tension to double. (Some plastic
7 inch reels have outside to inside diameter ratios of
more than 3:1.)
The tape tension is further altered to some degree by
every component that comes into contact with the tape.
When tape slides over any stationary guide or head
surface, the tape tension changes slightly due to the fric-
tion between the tape and the stationary surface. (The
bearing friction and viscous drag of rotating guides is
usually negligible.) The relative contribution of friction
tension to the total tape tension ranges from a low of 5%
for transports with only rotating guides to over 50% for
transports with numerous fixed guides and/or large tape
deflection angles around fixed guides.
The amount of drag tension generated by a cylin-
drical post is shown in Fig. 28-7. The tension and fric-
tion build up as the tape moves around the guide. The
true expression for the total drag is an exponential func-
tion, but for tape paths with only small amounts of
wrap, we can approximate the tension change with the
expression:

(28-2)

Note that although the diameter of the guide does not
appear in the tension expression, the pressure exerted by
the guide against the tape surface increases as the diam-
eter decreases. This increased pressure makes small
guides wear faster and accumulate dirt more quickly.
Since a speck of dirt trapped on the surface of a small
guide would also be more prone to scratch the tape
surface, small-radius fixed guides must be kept very
clean.

The coefficient of friction depends not only on the
type of tape, but also the condition of the roll of tape.

Older tapes may lose the surface lubricants that allow

the tape to slide freely across the stationary surfaces.

This may result in a squealing sound as the tape runs

through the recorder. Even worse, when sticky shed

debris from breakdown of the urethane binders in the

tape collects on the tape guides, the tape may be

dragged to a dead stop. These problems occur

commonly when dealing with older archived tapes.

Some transport designs are more sensitive than

others to changes in tape tension resulting from tape

problems. Tape paths with either high amounts of drag

tension or no pinch rollers may require a readjustment

of the tape tension to maintain acceptable performance

and avoid tape slippage.

28.2.3.1 Capstan-Derived Tensioning

Because of conflicting tension requirements at various

point along the tape path, it is useful to break the total

path into segments with different tape tensions. The

classic tape transport, for example, has two distinct

tension zones, one to the left of the capstan and another

to the right of the capstan. One common approach is to

use a driven capstan and pinch roller as an isolation

device, with the capstan motor supplying the power

needed to overcome the tension differential across the

capstan, see Fig. 28-5. This isolation can then be used,

for example, to achieve low head tension and high

takeup spooling tension.

Carrying this strategy one step further, if one capstan

can be used to isolate the head from the takeup system,

then it should be possible to use a second capstan to

isolate the heads from supply reel disturbances. This

would provide the desired isolation on both sides as

shown in Fig. 28-8. The difficulty is in generating a

controlled tension between the capstans where the tape

passes over the heads.

The solution is to have slightly different surface

velocities on the two capstans. If we need a 0.1% stretch

Figure 28-7. Tension increase due to guide friction.

Tension change Kutape tension

uangle of wrapucoefficient of friction

=

Tape direction

Tension

Angle of warp

Tension and

tension change

Figure 28-8. Dual capstan transport showing multiple dis-

crete tension zones.

Supply reel Takeup reel

Slower

capstan

Supply

tension

zone

Head tension zone

Takeup

tension

zone

Faster

capstan

Tape direction