A key function of the Technical Centre is tennis ball approval – any ball used in tournament play must be approved by the ITF, based on the tests conducted in this laboratory.
Failures can occur in all tests, but the most common are the rebound and deformation tests. Least common are the size and mass tests. If a brand fails any of the tests, then approval cannot be granted. The manufacturer can re-apply by sending another sample, which is then tested in the same way.
The regulations for testing of balls to obtain ITF Approval are specified in Appendix I of the Rules of Tennis. Testing is carried out in the following order:
- Pre-compression (and conditioning)
Before testing, the balls are acclimatised for 24 hours in our climate controlled laboratory. The lab operates at a temperature of 20 +/- 2ºC and at 60 +/- 5% relative humidity. Studies have shown that temperature and humidity can affect tennis ball properties. High humidity can increase the mass of the ball, by increasing the moisture content of the felt, and high temperatures can increase bound height.
Before any testing takes place, all balls are pre-compressed to remove any “set” in the ball. Set is a property that occurs in some materials when they are not subjected to varying forces. This is true for tennis balls between the time they are packaged and when they are opened for testing.
The pre-compression machine compresses each ball 2.54 cm (1 inch) three times from three directions using pneumatic actuators.
The weighing scales used in the laboratory are calibrated to +/- 0.01g. The acceptable mass range is 56.0-59.4 grams. This last changed in 2000 (from 56.7-58.5 grams).
Diameter and mass are the oldest ball specifications, having been in the Rules of Tennis since 1880. Although it would appear to be an easy task, accurate and repeatable measurement of ball diameter is difficult.
Whether a ball is of an appropriate size is determined using two ring gauges in a “go/no-go” manner. A ball must pass through the larger one under its own weight, but not through the smaller one. Each ball is rotated on the gauges to check its concentricity.
The internal diameters of the ring gauges are calibrated to +/- 0.0064 cm and the two rings are calibrated to the acceptable range for ball diameter (6.54 cm to 6.86 cm respectively for a Type 2 ball). This specification last changed in 1966 (from 2.575-2.675 inches).
The behaviour of tennis balls after impact with the racket and/or court surface is partly determined by the ball’s deformation characteristics, which describe how the ball shape changes under a known load.
The original device for testing ball deformation is the Stevens Machine, named after its inventor. This is a mechanical device which applies a known force to a ball placed between two plates. The operator compresses the ball by turning a handle and the amount of deformation under a known force is measured from a scale. While accurate, the Stevens Machine is a time-consuming process.
The automated deformation testing device can work about twice as quickly as a human operator using the Stevens Machine. This machine is controlled by computer, which specifies how many balls to test and what values to record.
A key advantage over the Stevens machine is elimination of operator error due to factors such as speed and timing of testing. In a year, this machine will compress over 5,000 tennis balls.
Deformation measurements are made under a load of 80.07 N (18 lbf), after a small initial load of 15.57 N (3.5 lbf) has been applied to the ball. The procedure is:
• Apply a force of 80.07 N
• Measure the deformation under this load (called “forward deformation”)
• Continue to compress the ball until its initial diameter has been reduced by exactly one inch.
• Remove this compression until the load on the ball is again 80.07 N
• Measure the deformation under this load (called “return deformation”)
The deformation rule has been constant ever since it was introduced, although it was introduced in two parts; forward deformation in the 1930s; return deformation in 1967.
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The rule specifying how the ball should bounce was set in 1925 and is still in use today. The test involves dropping a ball vertically from a height of 254 cm (100 inches) and measuring the rebound, which should (for all Type 2 balls) be 135-147 cm (53-58 inches). The range for balls for use at high altitude is 122-135 cm (48-53 inches).
The equipment used in the ITF laboratory consists of a vacuum pipe that holds the ball at the correct height before being released, a smooth, granite block onto which the ball bounces, and a video camera and light source. Each rebound is recorded and the rebound height measured from the shadow of the ball that is cast against a scale. This is all done with software that analyses the video of the bounce in real time and calculates the height using the contrast between dark (the ball) and light (the background) over a series of frames either side of the peak.
The durability test is the most recent development in ball approval and was introduced in 2009. The changes in mass, size, deformation and rebound of six balls are measured following standardised wearing regime. Laboratory equipment is used to simulate the effects of nine games play.
Wearing begins in the wear rig, which contains an air cannon that fires the balls onto a rigid surface at approximately 90º. The impact speed of the ball is 40 m/s (144 km/h); each ball is subjected to 20 impacts. The outer surface (cloth) of the ball is then worn by putting two balls at a time in an abrasion box for 2 minutes. The abrasion box is lined with rough emery paper and has three rotating wooden paddles that knock the balls around inside the box.
The effects of nine games of play (i.e. a typical ball change) were established by measuring thousands of new and used balls from tournaments played on a variety of surfaces.
In general, balls softer (deformation increases) and weigh less after they have been used. However, the mass of a ball used on clay often does not change because particles of clay stick to the cloth.