This is a metal ball caster, that can load max 15kg. You can also adjust the height by adding the spacers. The mount holes diameter is 4 mm. The Caster is strong and lightweight. It has 20mm high round body, with a total weight of only 40g; and large steel ball around a small ball around as much as 40 to rotate the main scroll ball much more smoothly, which make the robot walk or move at a more flexible and comfortable way.