Pololu ボールキャスター（1/2インチ金属ボール入り）

These worked well for my application. Plastic seems plenty strong.

Distance between mounting hole centers was 15mm. Overall height was about 13.6mm. Two 2mm x 12mm pan head Philips screws & nuts were included for mounting along with 2 plastic spacers - 1.62mm & 2.95mm. The ball is very free running and held up well running the robot on concrete floor. Tip: Remove ball from plastic housing before installing screws then pop it back in.

It can be hard to find a ball caster this small that works well. These were perfect for mini-bot projects with 2 wheeled drives.

Good product for my follow lines robot

Very small, Did not work for my idea, Which was to have a movable table