Rubber back-up ring is STU support ring, Commonly used in hydraulic systems, especially hydraulic valves and pumps, but also to seal the bottom of the cylinder and the cylinder head (used with an O-ring). However, STU support rings are also used in many other areas throughout the industry, such as for sealing ball valves.The concave section of the supporting ring provides a larger contact surface for the O-ring, which has the effect of keeping the O-ring almost completely round under high pressure and resistant to deformation. This means that the shape of the O-ring is better supported and therefore can withstand higher pressure loads. This can produce better sealing effect and extend the service life of the seal. Because of its enclosed design, the O-ring cannot be damaged by the sharp edge at the open joint under high pressure (the same is true for the open support ring).