Tennis ball
The tennis ball is the play equipment with which tennis is played. It is played with the help of tennis rackets .
history
Even in the direct forerunner of today's tennis, the so-called lawn tennis , which spread from Great Britain from 1870, balls made of solid natural rubber were used, onto which a layer of flannel was sewn. Later it turned out that balls with a hollow core filled with compressed gas had better playing characteristics. For this purpose, the outer skin was punched out of rubber in the shape of a clover , placed in spherical containers and filled with a chemical. This chemical formed gas when it was subsequently heated in the oven, which caused excess pressure to develop in the ball core. However, since the balls produced in this way differed greatly in their properties, the modern production method was gradually developed.
In the past, tennis balls were mostly black or white, depending on the surface of the field. The color common today did not gain acceptance until 1972, after it was found that it was best seen on color television sets . At the Wimbledon Championships until 1986 only white balls were used.
specification
The tennis rules of the International Tennis Federation (ITF) prescribe the following for balls:
- The outer shell of the ball must be uniform and seamless, its color white or yellow / green.
- The ball must have a mass greater than 56.7 g and less than 58.5 g.
- The diameter must be more than 6.54 cm and less than 6.86 cm. For slow balls (type 3) it is 7.00 cm to 7.30 cm.
- There are several specified ball types. Each ball must have a jump height of more than 135 cm and less than 147 cm when falling from a height of 254 cm onto a flat, hard surface, e.g. a ball. B. concrete , is dropped.
Then there are rules for the types of balls on different types of courts.
production method
The ball is surrounded by a tightly woven and hard-wearing layer of felt . This consists of a mixture of sheep's wool and nylon that is spun into a yarn and then woven with a cotton thread . The felt is created by shortening the fibers after impregnation . In order to obtain the desired strength , the felt is dried under tension . After the back has been coated with a hot glue , two dog-bone-shaped pieces are cut out of the felt for each ball, glued to the rubber bladder of the ball and dried. The final steaming ensures that the felt bulges.
The rubber bladder located under the felt, also known as the ball core , consists of natural rubber and up to eleven different chemicals , including the like. a. Alumina , quartz , sulfur , zinc oxide and magnesium carbonate . These basic materials are mixed in so-called extruders with constant kneading and at high temperature, so that a homogeneous mass is created. This leaves the machine in the form of a strand , which is cut into the portions necessary for a ball. These so-called pellets are pressed into hemispherical shells in molds and vulcanized to make the mass elastic . The hemispheres are then glued together under heat to form a hollow sphere , the ball core. In the case of pressure balls , the core halves are joined together under overpressure - nitrogen is often used - whereas pressureless balls are glued together under normal pressure.
Around 240 million tennis balls are manufactured in this way worldwide every year.
Pressure balls
When producing a small pressure ball, the rubber bladder is filled with gas (1.6 to 2.2 bar overpressure) in order to improve the jumping properties. Over time, the gas escapes and the jumping strength decreases, so the balls must be replaced frequently. In order not to let the gas escape before the first game, the balls are stored in an airtight can, in which the same pressure prevails as inside the ball.
With a few exceptions, pressure balls are used in the competition area. In professional tournaments, the balls are replaced according to a set rhythm of 7 to 9 games within a match. If a ball is lost at an early stage, it is not replaced by a new ball, but by one with which the number of games played since the last exchange was played before the match. This ensures that all the balls in the match at any one time have the same number of games and thus similar jumping properties.
Pressureless balls
When producing the pressureless balls, the two rubber halves are simply put together. The air composition and pressure inside the ball therefore correspond to the ambient conditions during production. In contrast to the pressure ball, the jumping properties of the pressureless ball are determined solely by the rubber composition, which determines the elasticity, and by the wall thickness of the ball. Since there is no loss of pressure, the jumping and playing properties are retained much longer than with pressure balls. Pressureless balls only have to be replaced when the rubber material is tired from the heavy deformation loads and the resilience decreases. By the time the felt is worn out, the ball has not been playable in the usual way for a long time. The disadvantage of pressureless balls is that they are usually harder to bounce.
Brands
Well-known tennis ball brands are Dunlop , Wilson , Head , Tretorn . In the former Eastern Bloc , people played with Kobold from the GDR, Optimit from Czechoslovakia or Stomil from Poland.
See also
Individual evidence
- ↑ a b Ball history. International Tennis Federation , accessed September 10, 2014 .
- ^ Rules for equipment. Appendix I - The Ball. ITF, 2012, accessed September 10, 2014 .
- ↑ Everyday life: sentence and victory - the tennis ball In: spectrum of science. 7/07, No. 7, 2007, ISSN 0170-2971 , p. 42f.
- ^ Deutscher Tennis Bund eV: "Tennis in Germany - From the Beginnings to the Present, Hundred Years of the German Tennis Federation" , p. 186.