True-Tension Stringing Machines (TTSM)
TENSION CONTROL: THE KEY TO BETTER STRINGING
Written by John Balaban
in approximately 1980
Since the beginning of the tennis sport, the most misunderstood part of a tennis racquet (or any strung racquet, for that matter) has been the stringing process.
Yet, correct stringing is by far the most critical element affecting racquet performance. A properly strung racquet increases playing efficiency and accuracy, assures the largest possible sweet spot, and improves the life of the racquet by eliminating distortion caused by irregular and improper forces. On the other hand, if the stringing is done poorly, even the best frame with the best string is of little value.
Since the invention of stringing machines over 50 years ago, the racquet sports world has generally assumed that almost anyone could string a racquet frame using almost any kind of stringing machine.
Nothing could be further from the truth. The reality proven by tests with Stringmeters, is that string tension generally varies by at least 30% from the actual requested tension on most racquets. Tests prove that even experienced, professional players are not immune to the problem. At one recent tournament, three racquets were tested for each pro. The average tensions varied as much as 6-15 lbs., and not one player had less than a 3 lb., average variance from one of his racquets to another.
Why all the inaccuracies? Only recently have engineers begun to investigate the scientific forces at play in the stringing process - and to incorporate their discoveries into stringing machine designs. Today, for the first time, we have begun to remove the "black magic" from racquet stringing. With an understanding of the physical processes which affect stringing, we are now able to control the accuracy and consistency of racquet stringing to a degree never before possible.
Tension Control - The Key To Better Stringing:
Tension control is the critical element in proper stringing. Tension control means uniform tension from string to string, and balanced tension between the main strings and the cross strings.
In addition to the playing benefits described above, tension control assures reliable and repeatable stringing results, even from relatively novice stringers. It permits correct stringing of oversized racquets, without the risk of breakage and means the tension requested by a stringer or player will be the same tension actually achieved accuracy that is provable with a Stringmeter test.
Major Problems In Controlling Tension:
The first step in achieving accurate string tension is to understand what happens to the racquet frame and string during the stringing process - and why it happens. The major areas of difficulty are:
1. Racquet frame distortion
▪ Between racquet frame and string
▪ At 180 degree turn of string
▪ Between the main and cross strings as cross strings are tensioned
3. String clamp slop
▪ Clamp on the glide bar rod
▪ String through the string clamp
▪ String into the string clamp
▪ String in the tensioning clamp or string holding device
5. String stretch
6. Pulling rate of the string while tensioning
All of these factors contribute to tension inaccuracies and tension losses. They result in a significant difference between the tension applied on the string outside the racquet frame and the final tension left inside the racquet frame.
Racquet Frame Distortion:
Avoiding frame distortion is the most difficult problem in controlling string tensions. Frame distortion can alter the final tension on strings by as much as 20 lbs. (as well as stressing the frame) because of these three factors:
1. During the stringing process, hundreds of pounds of force are exerted on the racquet frame. For example, when only the main strings are in place, the force at the top throat support is about 600-1000 lbs., while the pressure on the outside of the sides is about 400-750 lbs. Such pressure can obviously bend the frame. Each and every time you add a string and tension it, the frame changes shape, thereby altering the tension of each string already in the frame.
2. Because racquet frames are not completely round, the natural tensions of main and cross strings will not be equal. Instead, they fall into a fixed ratio, determined by the exact shape of any given racquet. If the racquet is not firmly supported and strung to a balanced tension which approximates the natural ratio, the strings will seek their natural tension ratio and distort the racquet.
3. Most of the frame distortion takes place in small increments which are not readily visible, particularly when the frame is mounted on most traditional stringing machines. The resultant tension inaccuracies are likewise subtle - only Stringmeter tests (or particularly sensitive players) can reveal the extent of the problems.
Solving the Distortion Problem:
Almost all stringing machines have some form of support system designed to prevent frame distortion. Most, however, are only two point holding systems (or improperly designed four or more point holding systems) which cannot withstand the pressures mentioned earlier. Therefore, each time an additional main or cross string is added, the tension on the previously installed strings changes.
Some stringers try to adjust the tension by plucking the strings and listening to the tone. The problem with this method is that tension is not the only factor which determines tone. String mass (or diameter) and length combine with tension to establish what the tone will be, so the tone method is only partially effective.
Other experienced stringers can determine the tension of a string by a finely developed sense of touch. The difficulty here is that no matter how accurately a string is installed, the tension cannot be retained as the stringing continues.
By now, it should be clear that the only reasonable solution to frame distortion and its negative impact on string tension is to use a highly effective frame support system during the stringing process. This is more easily said than done. To withstand the forces of stringing, we need either an extremely heavy structure, or an annular design in which the forces are exerted in the direction of the plate's greatest strength. There must also be a minimum of four support points (with inside support at throat and top and outside support at the sides), with all clamps mounted to the same plate as the supports. Lastly, the racquet supports must have very short lever arms to maximize resistance to bending when force is applied.
Designed in this manner, the frame support system permits applied tensions to closely approximate final tensions.
Tension loss due to friction is a particularly troublesome problem. Any single miscalculation may seem minor, but the effects of even a small error become multiplied by the stringing process and can make a major impact on the ultimate accuracy of the stringing job.
Furthermore, because there are many points at which friction occurs, there are many opportunities for tension loss - frequently adding up to a total reduction of tension ranging from 5-20 lbs.
There are three major categories of friction-induced tension loss. All are correctable:
1. Up to 5 lbs. overall tension loss can occur due to friction between the racquet frame and the string at the point where the string leaves the racquet frame as it is being tensioned. The amount of loss will depend on the degree (angles) of directional change in the string as it leaves the frame, as well as the coefficient of friction of the materials involved.
2. The 180 degree change of direction to install the next string is a particularly vulnerable area. It can account for 8-20 lb. overall tension loss. Again, the exact impact will also depend on the coefficient of friction of the materials.
3. A third area of tension loss, up to 5 lbs., occurs due to the friction between the cross string and the main string as the cross string is tensioned.
Solving Friction Problems:
Although tension loss due to friction cannot be totally eliminated with existing stringing machine designs, the effects can be substantially reduced.
The friction loss described in #1 and #2, will be minimized by pre-stretching each string as it is tensioned.
To eliminate the cross string friction problem, you need to use either a stringing aid or a main string reverser as the cross string is tensioned.
A Word About Strings And Stretching:
Most stringers are aware that string stretching is an inevitable and unpredictable characteristic that varies depending on the material and the manufacturing processes of the string.
No matter what type of string is used, stretching and therefore tension loss will occur, particularly when strings are under stress.
Experienced stringers know the value of pre-stretching as a means to stabilize string elasticity and improve playing characteristics. However, it is a somewhat delicate process. Too much stretching can damage strings, while neglecting to pre-stretch can undermine consistent tension.
Few existing stringing machines include pre-stretch systems. On most machines, the strings are stretched only to the desired tension. This is not sufficient to eliminate tension loss and uneven results.
Those stringers who appreciate the need for pre-stretching often use "home made" systems, pulling the strings prior to tensioning. The problem here is time lapse. Within about ten minutes, a pre-stretched string will return to about 90% of its original length - often unevenly because every string has a different "recovery time".
The ideal solution is a stretching system which is built into the string machine...one which pre-stretches the strings with precision, just as they are being tensioned. Such a system stretches the strings past the desired tension and then drops back down to the desired tension. In this way, the string elasticity is stabilized, consistent and carefully controlled.
Speed Of String Stretching:
The rate at which a string is stretched in tensioning effects the resultant tension. The faster you pull the string to a predetermined tension, the lower the resultant tension will be. This problem can be minimized by pre-stretching strings before tensioning, as well as by maintaining a steady, even pace of pulling (about 2-3 inches per second).
String Clamp Slop:
String clamp slop is caused by improper engineering and design of the stringing machine. If the string clamp bends, twists or slips when the string tension load is transferred to it (by releasing the tensioning clamp) then inconsistent tension losses occur (2-5 lbs.). Unfortunately, string clamp problems are often hard to see-their effects, however, are significant.
String clamp problems are minimized by choosing a design which uses short ridged string clamps that are firmly supported on the same plate which supports the racquet frame.
Various types of slippage in the string process are a fairly common cause of undesired tension loss.
1. If the string clamp slips on the glide bar - and cleaning does not correct the slippage - the cause may be improper adjustment or poor engineering design.
2. If the string slips through the clamp, then the clamp is too loose and should be tightened.
3. An often undetected problem occurs if the string creeps into the string clamp between the first and second prongs when the tensioning clamp is released. This condition is hard to identify and cannot be corrected by pre-stretching. The best solution is to adjust the string clamp to make sure it grips the string quite tightly.
4. If the string slips in the tensioning clamp or string holding device, tension loss can also be anticipated.
String slippage problems are greatly compounded when oils are used either on or in the string or when wax is applied to the string. Wax often causes more problems than it solves. In general, you can avoid many difficulties by keeping the tensioning clamp clean and free from oils or wax.
Correct and controllable tension is essential to accurate and consistent racquet stringing. Because most stringers use non-scientific string methods and less than adequate equipment, the "reference tension" frequently differs from the achieved tension by about 30%.
Improved tension control permits better playing, as well as repeatable results. With an understanding of the forces at play in the stringing process, most stringers can improve their technique.
The essential elements in improving tension control are (1) strongly supporting the frame to avoid distortion of the racquet and changes in the tension of previously installed strings, (2) minimizing friction losses by pre-stretching strings and using stringing aids, (3) improving the string clamps to avoid bending and other types of clamp slop, and (4) keeping the clamps clean and precisely adjusted.
Above all, the concerned stringer will choose equipment which is designed to help avoid the common areas of tension loss - and, of course, attempt to understand and keep alert to the many subtle problems which can undermine his best efforts.
About The Author:
John Balaban, the inventor of the True-Tension Stringing Machine, has been devising creative solutions to engineering problems for many years. Formerly an Air Force pilot, Balaban spent over 20 years in engineering research, development, design and testing for Sandia Laboratories, a contractor of the Atomic Energy Commission.
John's long-time interest in racquet stringing has been encouraged by his son - a tennis pro. After stringing more than 3,000 racquets, and more than five years of research and testing, John developed the True-Tension Stringing Machine. Recognized as state of the art in stringing technology, this machine incorporates a patented pre-stretching system, as well as a four point annular frame support.
Note: The above "Tension Control: The Key To Better Stringing" was written by John Balaban in approximately 1980. John's creative genius holds true today!
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