Technic Ball Steel 18mm D. : Part 99948 | BrickLink
Importance of the caster ball on the LEGO Mindstorms robot | FLLCasts
Amazon.com: MOC Universal Wheel 92911 bb607 Stainles Steel Ball Compatible with Robot EV3 6023956 4610380 99948 Building Blocks Technical Parts Power Functions Pack of 4PCS : Toys & Games