| Deformation |
 | | Stevens deformation testing device |  | | Automated deformation testing device |
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 8.165 kg (18 lbs). The procedure is:
• Apply a force of 8.165 kg
• 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 8.165 kg
• 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 2 parts; forward deformation in the 1930s; return deformation in 1967.
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