Grip ring stem casters come in two sizes and are designed to fit into tubing or a socket that is securely attached to the equipment.  The grip ring, also called a compression ring, is a ring on the stem that compresses as it is pressed up into the tubing or socket. It is squeezed and then expands against the tubing walls which creates the force that keeps the casters from falling out should the equipment come off of the ground or floor.  Load capacities range from 80lbs to 300lbs.