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Measuring tape tension and torque in VCRs
By Wayne Graham
VCRS are complex devices from an electronics standpoint, but they also
contain complex mechanical systems for driving the tape. It is these mechanical systems
that cause the majority of VCR failures. Proper diagnosis of problems with VCR mechanical
components such as clutches, brakes, belts, pinch rollers, solenoids and motors requires a
good comprehension of the terms torque and tension. Many service technicians treat these
terms almost equally, but in fact they are quite different, although they are related.
Tension
Tension is simply the pull force exerted and is expressed in units of grams, ounces, pounds or, for the really scientific, newtons. The few purists and Ph.D.s among us are concerned with the terms grams of force vs. grams of mass, or pounds of force vs. pounds of mass. Suffice it to say that here on earth, the rest of us can merely use the terms grams or ounces and we'll all know what we mean. (The purists will be the ones talking about newtons.) Useful conversion factors are:
28.35 grams = 1 ounce
16 ounces = 1 pound
453 grams = 1 pound
1,000 grams = 1 kilogram and to allow us to communicate with the purists: 102 grams = 1
newton
It is easy to simulate tension just hang a weight on a
string or piece of magnetic tape and the value of the weight equals the amount of tension
in the string or tape. (See Figure 1.)
Torque
Torque is a little more complex than tension. Torque is a rotational force; it is the
result of a force (tension) acting at a point some distance from a center of rotation.
(See Figure 2.) Torque is therefore a computed value. Torque is equal to the tension
multiplied by the perpendicular distance between the axis of rotation and the applied
force (for tape transports this is equal to the radius of the tape pack): torque =
tension x radius
Torque is a function of both the force applied and the distance to this force, so the
units of torque are:
g-cm (gram-centimeters), in-oz (inch-ounces), and ft-lb (foot-pounds).
Other convenient conversion factors are:
72g-cm= lin-oz, 192in-oz= lft-lb, 1152g-cm= lin-lb.
Relating this to a VCR, the torque (in gram-centimeters) exerted by a reel is equal to the
tape tension (in grams) multiplied by the radius of the tape reel (in centimeters).
Remember that the radius equals the diameter divided by two.
Torque = tension x radius
Radius = diameter . 2
2.54cm = lin
Measuring tension and torque
Let's compare some of the common methods used for measuring torques and tensions in VCRs:
1. The hope-and-pray method
2. The tension cassette
3. The spring (fish) scale
4. The Tentelometer tension
gauge
5. The dial-torque gauge
1.) The hope-and-pray method is by far the easiest but the least effective. Spray some oil or clean with a Q-tip and hope and pray it keeps working-no tools or gauges required. This method is not recommended for building a large base of satisfied customers.
2.) Tension cassettes look like actual tape cassettes but have a meter built in. They are available from a number of VCR manufacturers. Although they are configured like a standard cassette, they are not interchangeable from one brand of VCR to another. Because they work by measuring the torque (tension times radius) exerted by the holdback reel inside the cassette, as the tape guidance system between the cassette and video head changes, different torques are required for providing the proper tape tension going into the scanning video heads. Keep in mind that consistency in tape tension going into the video heads is required between VCRs to maintain interchangeability from one machine to another. Care must also be taken to keep tension cassettes properly calibrated, because a faulty tension cassette would cause VCRs with proper tension to be "corrected" to a faulty tension. Because service technicians are usually working on malfunctioning machines, chances are good that the tape inside a tension cassette will become damaged and will require replacement. After tape replacement, it is recommended that the readings be checked against another new tension cassette to make certain that the rebuilt gauge is reading correctly. This procedure would automatically double your tension cassette investment, and tension cassettes are not available for all brands and models of VCRs.
3.) Spring scales, sometimes called fish scales because they are similar to the scales used to weigh fish, can be used to pull on tape. For measuring supply holdback tension, a reel (obtained by taking apart a cassette) is placed onto the supply spindle, the tape is threaded out into the machine, simulating the tape path during PLAY, and the back tension is determined by measuring the tension exerted on the tape by the holdback torque of the supply reel. It is usually necessary to remove at least one side of the VCR to gain access to the tape so that you can connect the spring scale to the tape's end. The tape and spring scale must be pulled smoothly and at the approximate tape speed that the tape would encounter during normal play. The various sensors on the machine would have to be "fooled" to allow the VCR to go into the play mode or into other modes where torques or tensions need to be measured.
4.) The Tentelometer tape tension gauge consists of three tiny probes that protrude from one side of a small case. The case contains a meter scale and pointer that provides readings in grams and ounces of tension. The gauge provides a method of measuring the supply holdback tension by sliding the probes of the gauge over the tape while the VCR is moving the tape. VCRs can move tape smoothly and at the correct speed quite easily, so these variables are removed from the measurement. Calibration can easily be checked in the field, allowing service technicians to be confident of their readings. Because the tension measurement is made out in the tape path next to the inlet idler (on a VHS), the correct specification is the same on all VHS recorders. Measuring a specification of + 30g (25g to 35g) will assure proper back tension for skewing interchange. The device can also be used for checking the functioning of the holdback tension servo. By com paring the holdback tension on a full supply reel to that of a nearly empty reel, the device tells you that the servo requires adjustment if the readings change by more than five grams. The Tentelometer is referenced in many of the factory service manuals, including Panasonic, GE, Gold Star and Hitachi, but will work on any brand of VCR.
5.) The dial torque gauge is an excellent, fast method of determining the torques (usually in gram-centimeters) exerted by both the supply spindle and the take-up spindle during the various modes of the VCR. These desired torques are specified in both the clockwise and counter-clockwise direction for fast-forward, rewind and stop modes. Torques are also associated with take-up during play as well as between modes such as "play to stop" and "stop to play." In all, there are approximately eleven torques that could contribute to tape damage or an inoperable VCR and should be checked with a dial torque gauge. Dial torque gauges are available from the individual VCR manufacturers and from independent suppliers. All units measure directly in gram centimeters. No matter how you do it, there are mechanical measurements that should be made when servicing a VCR. Problems with supply back tension can cause tape damage or hooking (flagging) tape interchange problems. Problems with fast forward or rewind can cause slow FF or REW times, tape cinching (tight pack), edge damage and subsequent tape destruction. Problems with various take-up torques can cause tape destruction because the tape is not drawn back into the cassette and is crinkled in the cassette door when the tape is ejected. With the proliferation of low cost VCRs, it is imperative that the service technician choose a fast, reliable, repeatable, field verifiable method of performing the various mechanical tests. To do otherwise is to invite problems and complaints.
Measuring VCR tape tension
By Wayne Graham
One of the most important parameters for proper, interchangeable video is
hold-back tape tension. Improper tape tension has been demonstrated to cause several
problems that may be improperly diagnosed as electronic problems. Low tape tension may
cause:
• intermittent video signal.
• increased video dropouts.
• intermittent control track and subsequent intermittent picture break up.
• tape guidance problems and subsequent tape-edge damage.
• skewing errors with video hooking.
• intermittent audio.
• tape destruction due to tape being pulled by the rotating video drum. High tape
tension may cause:
• skewing errors with video hooking.
• premature video-head tip wear.
• tape-edge damage due to high tension at tape guides.
• tape destruction due to stretching at high stall tensions.
Improper tape tension is the most probable cause of distorted and intermit tent video.
Virtually every experienced service technician knows the importance of proper hold-back
tape tension.
On VHS transports, there are only two accepted methods of measuring this hold-back
tension: the back-tension cassette and the in-line tape-tension gauge. Only one of these
methods, the in-line tape-tension gauge, may be used with any brand and model of VCR; the
other method, if not used carefully, can give misleading readings and may actually cause
a good machine to be set up incorrectly.
Tension cassettes
For checking tension, tension cassettes appear to be the ideal solution. They go into a
VCR as easily as a cas sette, and they're moderately priced. The drawback is that they
aren't a universal test tool. Purchasing one of these tension cassettes from one VCR
manufacturer (or supply house) will not provide a gauge that will work on other brands or
even different models of the same brand. Tension cassettes can only measure the force
exerted at the spindle; they have no way of detecting any tensions beyond the cassette
housing. Because they can't measure tensions at or near the entrance to the video drum,
they will provide different readings each time the tape-path design differs in various
brands and even different models of the same brand of VCR. Tension cassettes are actually
measur ing the hold-back torque exerted on the supply reel by the supply-spindle assembly.
There is no standard for this hold-back torque. The tape tension standard is specified at
the inlet to the scan ning video drum. Tension cassettes would be a good way to measure
tape tension if the number of tape guides, angle of wrap around these guides and angle of
wrap around the erase head were the same for all VCR brands and models. If these
parameters were standard, a given hold back torque would always result in the same
corresponding tape tension. Unfortunately, VCRs with properly adjusted tape tension
according to the specification can peg a tension cassette from another manufacturer and
actually stall during its use. In fact, some manufacturers produce and sell two different tension cassette models—one for their old-style VCRs and one for their new
style. It's important not to assume that any tension cassette will work in any particular
brand or model of VCR unless that particular version of the tension cassette is
referenced in the service manual for the machine.
In-line tension gauges
The SMPTE (Society of Motion Picture and Television Engineers) and EIAJ (Electronic
Industries Association of Japan) standards provide specifications for tape tension
entering the video drum, so you can obtain a universal reading by measuring tape tension
out in the tape path. The in-line tape tension gauge (for example, the Tentelometer) has
three probes that slide directly onto the video tape during the normal play mode.
On VHS machines, even the
in-line gauge can't be inserted directly at the inlet to the video drum, which is the
ideal point at which to measure tape tension. It can, however, be inserted at the inlet
impedance roller location, which is upstream by a rotating guide (P2) and the final
non-rotating slant guide. Because these two guides are virtually the same on all VHS
recorders, you can compute the proper tape tension at their entrance. The correct tension
at the inlet impedance roller can be computed from the formula for tape drag around a non
rotating guide post:
T(out)/T(in) = e(µo)
where:
µ = the coefficient of friction of tape against the guide
O = the wrap angle, expressed in radians (2¼ radians = 360°; thus, 45° = ¼l8 radians =
0.785 radians)
e = natural log 2.72
T(out) = high tension going away from the guide
T(in) = low tension going into the guide.
Don't let the formula scare you—all it says is that when the tension at the inlet to
the video drum is the proper 31 to 46 grams, the corresponding tension at the inlet
impedance roller location is 23 to 35 grams. This specification can be used on all VHS
machines that have a rotating guide next to the non-rotating slant guide leading into the
video drum. Another benefit to this method is that, because tension is being measured in
the tape path, not within the cassette, it's easy to check the performance of the VCR's
tension servo system. The tension servo system has been designed to maintain the
back-tension relatively constant throughout the diameter change during the playing of a
T120 tape. Without this servo system, the tension would change by more than a 3 to 1
ratio; thus a tension reading of 25 grams at the inlet impedance roller with a full T120
supply reel would have risen to more than 75 grams when it's empty at the end of a 2-hour
movie. This high tension would cause significant video problems or videotape destruction,
and could lead to premature head-wear problems. The tension should remain within the 23 to
35 gram range during this supply-reel diameter change. This tension variation can easily
be measured with an in-line tape-tension meter. The closer the tension is maintained to a
nominal 30 grams, the better the tension servo is designed and is operating. Although an
in-line tape-tension meter can be used on any VCR, do not use a tension cassette on VCRs
for which it was not intended. If you do, you will reset some properly operating
transports to be outside the correct tension specifications. Consult with the factory
service technicians if you are unsure of the application for tension cassettes you already
have.
