Why Are Tennis Balls Pressurised And How Are They Tested?
I had an email from a customer enquiring why tennis ball manufactures use so much packaging with their tennis balls. A tube or tin of balls usually contain 3 or 4 balls and these are sealed with a removable metal lid and a further plastic lid to keep the balls in the tin once the metal seal has been broken. One of life’s simple pleasures is breaking the seal on a new tin of tennis balls.
Given the lengths supermarkets are having to go to, to reduce the packaging and waste on many of their products, it will only be a matter of time before tennis ball manufacturers are having to consider more environmentally friendly products to store their balls.
Aside from this issue though, it did beg the question why are tennis balls sold in pressurised cans? A new tennis ball has an internal ball pressure of around 14psi so to ensure the ball retains the new ball feel on the shelf, the cans also have the same pressure applied to the can. This ensures the shelf life of the ball is the same from day on manufacture to the day it is opened.
Once the tin is opened though the balls do start to lose their pressure and are recommended for use for around 9 games. After this time a good player should notice the ball doesn’t bounce in the same way and have a flat feel. This is because the internal ball pressure has now equalised with the natural air pressure of it’s surroundings.
There are around 200 different ball brands that have tennis balls approved for use, by the ITF (International Tennis Federation). Dunlop for example has 38 brands approved in the Type 2 band of balls. Some are the same ball, such as the Dunlop Fort All Court Tournament Select but manufactured in 3 different countries – Philippines, Thailand and Indonesia. Each one though will have to go through a vigorous testing procedure to get final ITF approval.
There are 7 different types of tennis ball and each one has to be tested to conform the the ITF regulations for each type of ball. The costs for testing are also quite expensive with a different tariff for each type. If the balls fail the test then a new fee applies for further testing. I was surprised to find that approval was only valid for 12 months and then the balls must be tested again with fees due once again. This can be an expensive process and maybe a reason tennis balls cost so much. Although it does mean the quality is consistent year after year.
As mentioned there are 7 types of tennis ball:
- Type 1 – A slightly harder and fast speed ball for slower court surfaces
- Type 2 – Most common tennis ball for standard courts surfaces
- Type 3 – Slightly larger for faster courts
- High Altitude – As stated for use in conditions which have higher than average altitude
- Stage 1 – 25% slower than a traditional tennis ball
- Stage 2 – 50% slower than a traditional tennis ball for play on a smaller court
- Stage 3 – Larger than a traditional tennis ball and much slower too. For use on 1/4 size courts.
Each ball is tested for the following:
- Acclimatisation – The balls are acclimatised for 24 hours. The aim is to get the balls to a temperature of 20°C and 60% humidity. High temperatures can increase the bound height and a higher humidity can increase the moisture content of the felt. So all balls are tested at the same humidity and temperature in strict lab conditions.
- Pre-Composition – A pre-compression machine compresses each ball by 25.4mm, 3 times in 3 different directions. This is to ‘wake’ the ball up from a set condition, which is likely to occur during storage.
- Mass – Each ball is weighed to ensure they are within the weight range of 56g to 59.4g. This was a much tougher test years ago as the weight range was smaller at 56.7g to 58.5g.
- Size – This is the oldest test having been set in tennis rules since 1880. A tennis ball is passed through 2 metal rung gauges, it must pass through the larger one but not through the smaller one. The ball is rotated many times to ensure the test is passed from various angles. The acceptable ball diameter is 65.4mm to 68.6mm – this was updated in 1966 from 2.575 – 2.675 inches.
- Deformation – A compression machine called the Stevens machine is used to compress each ball to a force of 80.07n. This machine can compress up to 5000 balls per year which removes the risk of human error. This test is to measure the deformation of a ball when hit in both forward deformation and return deformation.
- Rebound – The rule specifying how a tennis ball should bounce was first introduced in 1925 and is still in use today. This test measures the bounce of a ball when dropped from a specific height of 2.54 metres. The rebound for type 2 balls should be 135cm to 147cm and for high altitude balls 122cm to 135cm.
- Durability – This test is the most recent and was introduced in 2009. An abrasion box is used to test the tennis balls for wear of cloth and is set to simulate 9 games of play. This is 2 phases – the first is at 144kph and has 20 impacts into a 90° surface without abrasion. The 2nd phase introduces an additional ball into a abrasion box which is lined with emery paper (sand paper) and 3 rotating wooden paddles, which knock the balls around the box and simulate play and through hitting the paddles and bouncing of the rough surface. The cloth is then assessed in comparison to other approved balls from various surfaces.
So balls are tested and stored in the optimum conditions, so the balls are perfect for use every time. Sadly at this time this does mean there is a vast amount of materials used for packaging. Over time I am sure this will change. If you cannot bear the wastage though maybe you should consider using a bucket of tennis balls instead. Although these are generally used for training and social play rather than matches.