tennis, anyone?

A portable, battery-powered machine allows for four hours of practice time on the court in one stretch.

You'd think that serving a tennis ball over a net would be easy. But creating a machine capable of delivering four hours worth of balls at a constant velocity, for a reasonable price, isn't as easy as you'd suspect.

Sports Tutor of Burbank, Calif., tackled just this problem with the assistance of design engineering company Keck-Craig of nearby Pasadena. Keck-Craig Inc. has spearheaded engineering and design projects for pharmaceutical companies, sports medical groups, and aircraft builders. It was involved in the redesign of the aircraft flight recorder—also known as the "black box," the recording device that is vital to analysis of the cause of a crash—and more recently, a battery-powered gadget that chills a can of beer in just minutes.

Bill Greene, founder of Sports Tutor and a former electrical engineer with Lockheed-Martin, was developing a tennis ball machine that would automatically project tennis balls over the net on a regulation-size tennis court. His working model incorporated a unique battery-powered design that uses counter-rotating wheels to propel balls at up to 85 miles per hour over the tennis court net. Since most tennis courts do not have power outlets, a conveniently sized, lightweight, yet durable machine was essential. Unfortunately, the working model, made of metal parts, was far too costly to manufacture in volume.

Greene knew that he had to redesign the machine so that it could be manufactured cost-effectively, but wasn't sure where to start. He contacted Keck-Craig after learning about the company from a local plastics molder.

Keck-Craig was presented with the challenge to reconfigure Greene's working prototype into a compact, lightweight, battery-powered machine that could be manufactured in high-production runs. It was essential for the machine to weigh less than 40 pounds, house a rechargeable battery that could provide at least four hours of playing time, and hold a sizable number of tennis balls. Other challenges included ensuring that the tennis ball machine continued to be lightweight, yet durable, and would fit into any car trunk.

Another design and engineering challenge for Keck-Craig was "uniform ball delivery," complicated because the tennis balls tended to cluster and become undeliverable to the ball chute.

Keck-Craig decided that in order to eliminate a proliferation of small parts, a structural foam, molded plastic housing would be an inexpensive, long-lasting solution, along with an internal metal structure to provide motor and control support.

In addition, by hinging four side plates to form a trough on the top of the machine, Keck-Craig designed a tennis ball hopper for the machine that opens easily and is capable of holding 150 tennis balls. The company also designed and developed a hinged handle and built-in external wheels that allow for easy movement of the machine. These improvements were incorporated into the original design, which included a battery that could be charged overnight, providing up to six hours of playing time and a built-in remote control to allow users to conveniently stop ball delivery and control side-to-side ball placement.

To eliminate the tennis ball cluster problem, Keck-Craig designed a ball-feeding trough that had to go through several modifications.

Greene and Keck-Craig worked closely together for several months to prepare a working prototype that not only looked just like the production model, but was also a high-quality machine with rugged features that would withstand the tough conditions of constant use. Keck-Craig also located a manufacturer that could make the parts, as well as a banker who helped the client with initial start-up costs.

Initially, it was decided to make the housing white, so that it had an appropriate tennis court look. However, the white finish showed an unsightly mass of dirty smudges from the returned tennis balls, and it was necessary to change the color to black.

Battery-powered Tennis Tutor was designed to be portable and durable.

Tennis Tutor was the first portable tennis ball machine to use counter-rotating wheels, like major league pitching machines. Now, most high-end tennis ball machines use counter-rotating wheels. In this system, two wheels turn in opposite directions (hence, counter-rotating), and have a small space between them. The tennis ball rolls down a chute, and is squeezed by the counter-rotating wheels, which then eject the ball. Previously, the majority of machines used air pressure to propel a tennis ball through a tube.

Counter-rotating machines offer the advantage of being quieter than pneumatic machines, and of being able to deliver balls at a steady rate and consistent velocity. Pneumatic machines require a lot of electrical current and don't adapt easily to battery power. The advantage of pneumatic machines is their cost, which tends to be lower than that of wheel-driven machines.

The Tennis Tutor has an integrated electronic panel for choosing ball speed, trajectory, and ball feed rate. The ball return speed was calculated to be electronically variable from a gentle toss for children to hard-hitting groundstrokes that can challenge top amateur players. This feature also gives tennis players the option of setting Tennis Tutor to repeat the shots they need to practice or to direct random shots across the court.

Finally, an anti-jam ball feeding mechanism and a 10-second start-up delay were integrated to allow users to position themselves on the other side of the net before Tennis Tutor begins serving balls.

The Tennis Tutor has appeared on the cover of the Hammacher-Schlemmer catalog and is now one of the top-selling tennis ball machines in the world. A version of the machine is used by tennis pros at the Association of Tennis Professionals Training Center in Ponte Vedra, Fla., and is considered one of the fastest ways to improve your tennis skills because you can actually hit more balls in one hour with Tennis Tutor than you would in a three-set match, according to a Tennis Tutor spokesperson.

Today, Sports Tutor offers a variety of sports equipment, including volleyball, baseball, and soccer machines, as well as seven different models of the Tennis Tutor, and a handful of other tennis ball machines for amateurs and pros.

This article was prepared by staff writers in collaboration
with outside contributors.

Return to Index

© 2005 by The American Society of Mechanical Engineers