The Industry's Most Dependable Hex Dumbbell. Dura-Bell dumbbells have a patented design that makes them maintenance free. Urethane encased heads minimize damage to floors and equipment. Our 35 millimeter ergonomic handle features a hardened chrome finish and is secured to the head with a quarter inch roll pin. Our US Patent # is 6099443. The only patented urethane hex dumbbell in the industry.