Things are not meant to last forever.  Eventually your Eye Coach Junior arm will need replacing, and this is the part you'll need.  This is for the junior version and will not work with the Pro/adult model.

Billie Jean King's Eye Coach Junior Replacement Arm Details:

• Arm includes replacement ball
• Replacement ball colour may vary
• Spring and Aluminum stem included/attached
• Exclusively for Eye Coach Junior
• Product Code: EYECOACHARMJR