Backup Rings Back-up rings or Anti Extrusion Rings do not have a sealing function. They are designed to prevent elastic sealing elements of gap extrusion.ApplicationsDynamic and static applications in reciprocating pistons and rods in hydraulic cylinders, plungers, etc.Unfavorable tolerances and wide gap dimensions. Intensely pulsating pressures and pressure change. ST08 ST09 ST11 ST12