Gee Tim

That sounds like a conundrum wrapped in an enigma.

Can you rephrase your question, do you have a picture?

Regards
John

Sounds like college/ school project.

Are you thinking of a "Ball Machine"? Lots of examples on the net.

Obviously he's looking to rig the next World Snooker tournament.

Surely, if this is a "fun" project, you are taking the fun out of it by asking someone else to solve the problem for you, aren't you? Smacks of plagiarism to me!

John

Two quick thoughts…
Would your Mechanism. .have to survive the experience of being “served”…..and it would be challenging to meet the tennis rules for ball weight….

And then there’s the challenge if getting your gubbins into a ball leaving the ball intact.

.the proccess of examination of methods evenif rapidly discarded as impractical. .is still part of the project.

If you make the balls self-returning ..as in powered and directed to 'home'.. then you cannot so do without destroying their playing characteristics.. weight for a start.

The only solutions here are either to have a playing surface that tilts or a ball-getter. To design a ball getter you need some identifier on/in the ball and a collecting device with a homing function. Such homing functions exist in most quadcopters and indeed in those robot lawn mowers that know their designated mowing areas and where to recharge.

Personally I'd go with a dog.or orphan rental.

A big fan at one end of the tennis court and blow them in the direction you want. Niko.

Or one of these in each ball:

And one of these:

Not quite as simples, but more fun