Handbook for Sound Engineers

1046 Chapter 28

stiffness and bandwidth of the servo loop, requiring
either an improvement in the inherent errors that are to
be corrected by the servo or a decrease in the expected
level of performance.

A further extension of the free-running idler concept
is to eliminate the drive capstan completely, as in
machines manufactured by John Stephens. These
machines relied on two high-performance spooling
motors to perform all the speed control tasks. The
Stephens recorders provided excellent speed control
under normal conditions, but the large inertia of a
6 pound roll of 2 inch tape limited the responsiveness of
such systems, rendering them vulnerable to abrupt
disturbances such as tape splices or layer-to-layer adhe-
sion of the tape.

28.2.1.1 Tape-to-Capstan Contact Enhancement

Constant tape speed requires that the capstan driving the
tape must have enough traction on the tape due to fric-
tion to exert positive control of the tape. If we just wrap
the tape around the capstan, the traction force due to
friction will usually be too weak to exert full control of
the tape. To maintain control, the capstan’s drive force
must be at least equal to the difference in the tape
tensions on the ingoing and outgoing side of the
capstan, as shown in Fig. 28-5.

Active contact enhancement devices such as the
rubber pinch roller push the tape against the capstan
surface to maintain firm contact. Unfortunately, the
pinch roller also produces numerous undesirable side
effects, including:

1. Heavy side loads on the capstan that produce
   bearing wear and can even cause small diameter
   capstans to bend or tilt.


2. Speed errors due to the elastic deformation of the
   rubber roller at the point of contact.
   3. Increased variations in speed created by imperfec-
   tions of the rubber, eccentricities of the roller, and
   bearing rattle.

One way to avoid the problems of pinch rollers is to

clamp the tape to the capstan using air pressure and a

vacuum pump. Computer tape drives have frequently

used hollow vacuum capstans to achieve rapid tape

start/stop and shuttling. The capstan must be of porous

material or have machined passageways so that air can

be sucked from the surface of the capstan. The ambient

air pressure will then push the tape firmly against the

surface of the capstan. Since the air pressure differential

will be somewhat lower than the maximum 14.7 pounds

per square inch (psi) of nominal atmospheric pressure,

there will need to be a substantial tape contact area to

generate the required traction force.

Passive contact enhancement methods concentrate

on maximizing the traction between the tape and

capstan surface. Roughening of the capstan surface by

sandblasting or coating the surface with urethane rubber

or diamond-impregnated grit yields an improvement in

the coefficient of friction. After heavy usage, however,

the roughening will be polished away by the abrasive

surface of the tape, or the urethane surface will glaze

and harden, requiring reconditioning to avoid slippage.

Other passive techniques concentrate on eliminating

any loss of contact due to air being trapped between the

tape and capstan. This air bearing effect, which

becomes evident at tape speeds as low as 30 in/s

(78 cm/s), can be minimized by cutting bleed slots in

the surface of the capstan. These slots are similar to the

tread grooves on an automobile tire, providing escape

paths for the trapped air.

28.2.1.2 A Word of Caution Regarding Urethanes

The standard roller rubber is neoprene, a fairly stable

rubber compound that can resist ozone and smog. Many

newer compounds, especially various urethanes, have

also been tried with some success. Sometimes the new

roller will give excellent results when new, but then it

will glaze over and lose its adhesion to the tape. In other

cases the roller’s elastomer will turn into a gummy ooze

with the consistency of taffy.

The urethane is affected by temperature and

humidity conditions, and by any solvents used to clean

the tape path. Always check the cleaning pad after you

clean the pinch roller. If the pad has just tape residue,

you are providing proper cleaning. If, on the other hand,

you see a residue that looks suspiciously like the surface

of the roller, you may be dissolving your pinch roller!

Figure 28-5. Forces at the capstan.

4 oz

4 oz 4 oz

4 oz

Holdback

tension

Holdback

tension

Takeup

tension

Takeup

tension

8 oz

Beginning of reel End of reel

Capstan force Capstan force

Capstan pulling Capstan retarding

8 oz