Tennis Racket

Background

The game of tennis dates back officially to 1873, when the first book of rules was published by Major Walter Clopton Wingfield of north Wales. But tennis has antecedents in ball games played with the hand that evolved in Europe before the Renaissance. These games were played first with the bare hand, later with gloved hands, then with hands wrapped in rope. Later, a wooden bat was introduced, and the first rackets seem to have showed up during the fifteenth century. These early rackets were smaller than modern tennis rackets, and were strung in various patterns. When rules of tennis were standardized by Wingfield and others following him, the shape and size of the court was specified, and the kind of ball that could be used. There were, however, no rules governing the racket size, shape, or material makeup.

Until 1965, all professional tennis rackets were made of wood. A steel tennis racket was patented in 1965 by the French player Rene Lacoste, and in 1968 the Spalding company marketed the first aluminum rackets. These metal rackets caught on gradually. What the metal rackets made possible was a change in design to allow a broader head. Wooden rackets could not be made wider or longer in the head without causing problems with the stringing: if the head was too broad, string tension became too great, and the racket did not play well. But the greater strength of metal frames could accommodate greater string tension. An oversized aluminum racket developed by Howard Head in the mid 1970s was at first scoffed at by professionals, but amateurs quickly discovered that they could hit better with it. The prime hitting area, or so called "sweet spot," was doubled in size in the the new, larger rackets, and so for most people, it was easier to use. The larger rackets became the standard at all levels of play by the early 1980s.

The International Tennis Federation finally adopted rules defining acceptable tennis rackets in 1981. The Federation had banned a racket introduced in 1977 that used an innovative stringing technique. Players using "spaghetti string" rackets scored huge upsets over high-ranked opponents, and after only five months, these rackets were not allowed in professional play. The first racket rules allowed the racket and strings to be made of any materials, and did not limit the size, weight, or shape. Strings were required to interlace or be bonded at cross points at least a quarter inch (0.64 cm) and not more than a half inch (1.3 cm) apart. No attachments were allowed that might alter the flight of the ball, and the weight distribution along the longitudinal axis of the racket must not change in play. Later the maximum length of rackets was limited to 32 in (81 cm). This was modified again in January 1997, bringing the length back down to 29 in (74 cm).

The average racket is now about 28 in (71 cm) long, and weighs from 10-14 oz (284-397 g). There have been many recent innovations in racket technology, not all of which have caught on with players. One maker markets a hexagonal racket, while others are making rackets with extra wide bodies. A racket made of a new material—graphite fiber-reinforced thermoplastic viscoelastic polymer—was designed to have variable flexibility, depending on how hard the ball is struck. A design to alleviate tennis elbow employs small lead bearings enclosed in plastic chambers inside the head frame. The movement of the bearings as the racket connects with the ball is supposed to cushion the vibrations that might cause pain to the player's arm. But the most common rackets are now made of aluminum or of a composite of graphite, fiberglass, and other materials.

Raw Materials

Aluminum rackets are usually made of one of several alloys. One popular alloy contains 2% silicon, as well as traces of magnesium, copper, and chromium. Another widely used alloy contains 10% zinc, with magnesium, copper, and chromium. The zinc alloy is harder, though more brittle, and the silicon alloy is easier to work. Composite rackets may contain many different materials. They usually consist of a sandwich of different layers around a hollow core or a polyurethane foam core. The typical layers of a composite racket are fiberglass, graphite, and boron or kevlar. Other materials may be used as well, such as ceramic fibers for added strength.

Other materials found in tennis rackets are nylon, gut, or synthetic gut for the strings, and leather or synthetic material for the handle grip. Nylon is probably the most common string material, and only a few professionals still use gut, which is made from twisted cow or sheep intestine. Synthetic gut is made from nylon which has been twisted to achieve the same effect as natural gut. Old wooden rackets usually used a leather handle grip, but modern rackets generally use a leather-like replacement such as vinyl. Rackets may have plastic parts too, such as the yoke at the base of the head and the cap at the bottom of the handle.

The Manufacturing
Process

Most rackets sold in the United States are mass produced at one of several large factories in Japan or elsewhere in Asia. So regardless of the brand, chances are the racket was made by one of the methods described below. Rackets with unusual features might be exceptions. Also, top of the line rackets are often sold unstrung, and the buyer has it strung to his or her specifications at a pro shop. So in this case, the stringing step at the factory would be skipped.

Aluminum racket

• 1 Forming the frame. There are two methods for forming aluminum rackets. The aluminum may be melted and forced through a die in the shape of the racket frame. Or the metal may first be melted and extruded into a tube, and then the tubing drawn through a die.
• 2 Drilling and sanding. The rough racket is then placed in a drilling machine, and holes are drilled for the yoke—the throat piece that holds the bottom of the strings—on the sides for the strings, and at the base of the stick. The drilling machine uses multiple spindles, each holding a drill bit in position for each string hole. The racket is held in place horizontally in the center of the machine. The drills are then activated, and all the holes are drilled simultaneously. The frames are then placed in a sander to smooth out sharp edges left from the drilling.
• 3 Tempering. At this stage, the rackets are O tempered, that is, subjected to heat and rapid cooling. This process hardens the aluminum, giving the racket additional strength. The rackets are placed on a tray in an oven and heated to white-hot. Then the tray is removed from the oven and the rackets are immersed in water. After tempering, the rackets may also be anodized. They are immersed in a mild sulfuric acid solution, and an electric current is passed through the bath. This treatment changes the surface of the aluminum, and gives the rackets a shiny finish.
• 4 Stringing. A grommet strip is inlaid in the groove around the edge of the head. The flexible grommet strip, usually plastic, has been predrilled so that its holes fit over the string holes in the frame head. Then the yoke is fitted into the base of the racket head. Now the racket is ready for stringing. Each racket is strung individually, by a worker seated at a stringing machine. The worker first clamps the racket into the machine, which holds it horizontally. The worker forces the strings through the holes using a powerful threader mounted on a movable bar above the racket. The length-wise strings are pulled through first, then
  
  the cross strings are woven through, and the tension is adjusted.
• 5 Finishing. To finish the racket, a worker cuts the end of the handle and inserts a cap called the butt cap. Next the worker wraps strong double-stick tape around the handle, followed by vinyl grip tape. After this, the strings may be imprinted with a logo, and the frame may be stamped with a decal. Inspectors check the racket for nicks and mars, and make sure it conforms to size and weight specifications. Rackets may then go through a final cleaning stage. Then workers place them in protective covers, the rackets are packaged, and finally sent to a warehouse for distribution.

Composite racket

• 6 Forming the frame. Composite rackets are made out of layers of different materials, usually graphite and fiberglass, and perhaps other layers containing boron, kevlar, or a material similar to fiberglass that contains ceramic particles. The racket manufacturer begins by assembling the layers as a flat sandwich. The sandwich is then cut into strips, and the strips rolled around a hollow, flexible tube. The wrapped tube is then placed into a racket-shaped mold. The tube extends all the way through the racket, and is connected to a pump. Then the mold is heated, and air pumped into the tube. The pressure of the air in the tube, along with the heat, bond the layers of the sandwich. Alternately, the hollow tube may be filled with polyurethene foam. The foam expands as the mold is heated, consolidating the materials.
• 7 Drilling and sealing. Workers release the rackets from the molds and carry them to an inspection area, where any defective ones are removed. The end of the frame is cut, and then the rackets are placed in a drilling machine and the string holes are drilled, as above. After drilling, the rackets are brushed with a polymer coating and placed in a dryer. This step is repeated several times, and then the rackets are sanded. Before the final coating, the brand name decal is applied.
• 8 Stringing and finishing. The next steps are the same as for the aluminum racket previously described. A grommet strip and yoke are fixed in the appropriate grooves, and workers string the rackets one at a time on stringing machines. A logo or brand name may be screen printed on the strings. Workers insert the butt cap, then wind double-stick tape and grip tape around the handle. Then the rackets are cleaned, inspected, packaged, and sent to a warehouse.

Quality Control

Inspectors check the rackets at many points in the manufacturing process. When the frames are first taken from the molds, they are inspected visually. Defective rackets are set apart, and passing rackets may be roughly graded for quality. Aluminum rackets are subjected to stress tests to determine

if the frames are the proper hardness. Composite rackets are also tested for stiffness. Inspectors weigh both types of racket, usually before and after stringing, to make sure they meet specifications. They also check the balance, as this is extremely important to how well the racket plays. It should not be too heavy at the head or in the handle, but balance close to the mid-point (though some models are designed to be deliberately head-heavy). The grommet holes are inspected. If these are not smooth or even, string tension is affected, and strings may break against rough edges. The finishing details are also subjected to visual inspection. The butt cap should fit snugly, and the printing on the frame and strings should be even and clear. The grip should be wound smoothly, and there should be no nicks or scratches. Some rackets may be play-tested, especially if it is a new design.

The Future

The science of tennis rackets is surprisingly complex—not the manufacturing process but the physics of string and frame vibration as the ball connects with the racket. Rackets are now being designed by laboratory scientists who use mathematics to calculate the effects of weight, size, and material changes. Since the rules governing acceptable rackets are very broad, innovators have a lot of leeway. New rackets are also being made with computer-aided design (CAD) and computer-aided manufacturing (CAM), which allows precise calculation of material rigidity and center of gravity. As such advanced science is being lavished on the tennis racket, doubtless new models with eccentric features will continue to be developed. The trend today is toward lighter, bigger rackets, and these are viable because of advanced materials engineering.