Rhombus 391 Series casters meet ISO Standard 22882: 2004 Casters & Wheels – Requirements for Casters for Hospital Beds. Precision raceway bearings provide excellent swivel motion. Ideal for hospital beds, operating room tables, and stretchers, 391 casters have stems that meet industry standards to integrate with existing central locking brake systems. The central locking selection mechanism is factory set and maintenance-free for the life of the caster.