New balls please

Wimbledon begins on 2 July. Over the course of the Championships some 2,500 tennis balls will be used. To ensure balls have the required elasticity for play, they are routinely changed every six games.

A simple and convenient way to test a ball court-side doesn’t exist, so many balls are rejected unnecessarily. Until now.

A team of mechanical engineering students at the University of Nottingham has developed a portable impact tester to accurately gauge the rebound of a tennis ball.

The prototype is affordable, lightweight, can test a ball every 20 seconds and is designed to minimise waste.

“Tennis balls begin at two bar of pressure when they start life on the tennis court, but the pressure dissipates out of the ball over time because it’s made out of rubber, which is permeable,” explains third-year engineer student, Megan Schofield.

Cameron Wilson, also in the team, said: “The International Tennis Federation do an official test which involves taking a tennis ball up to 100 inches and dropping it onto a solid surface below. It should bounce between 53 and 57 inches for it to be a good ball to use. It’s quite an inconvenient and difficult way to test a ball. You need things like slow-motion cameras to really get an accurate recording. There are other tests out there that compress the ball, but they are very expensive.

“We decided to make a little compact rig, that cost around 100 to produce. The ball is held in a holder and there is a pendulum which has been calibrated to match the amount of kinetic energy of the ITF drop test when it hits the ball.”

The idea for the impact tester came from materials analysis. It’s the same principle for impact testing as would be used in aerospace, to replicate the force of bird strikes on aeroplanes.

A potentiometer is connected to the pendulum shaft to measure the angle that the pendulum rebounds at - if it’s above a certain angle it gives a green light result, and below it, a red light.

Fellow student, Graeme Duffy added: “We worked out that the minimum acceptable bounce back is roughly 68 degrees of the 90 degree rotation, so we just include that in the Arduino code and say that if is 68 or greater than then it’s acceptable; less than that it gets rejected.”

Project supervisor, Professor Mike Clifford, said: “At the moment tennis balls are routinely changed after six games - no matter whether a ball has been used or not. Professional players use their perception and experience to test the state of a ball, but how precise is that? Looks can be deceptive and can offer no correlation. Older looking balls we tested had the best re-bound whereas some newer looking balls were flat.

“Temperature is a really key factor in tennis, particularly if you play on clay courts. The balls get too bouncy if they get hot. Our machine can test them and accurately state if they are overly bouncy and need changing or putting in the fridge to bring them back down to temperature.

“After Wimbledon there will be a surplus of balls, some of which are going to be okay for amateur competitions, others are going to be completely spent. The code inside the machine is very variable so you can programme different parameters. You could change the calibration so that balls had to be 68 degrees for professional competitions or could be dropped to 60 degrees for amateur or junior tournaments or 55 degrees for a casual knock about. That way wastage is vastly reduced.”

At club level, match officiators could in principle use the machine to guarantee the quality of the tennis balls. Future developments could include adaption for testing golf or cricket balls.

Ball manufacturers could also use the machine to track what makes their products alter over time to prevent more balls failing regulation standards at the point of production.

They could examine the impact of temperature or wear on the bounce. If they wanted to change the material or the manufacturing process they could use the impact tester to compare different standards.