ERIKS nv - O-ring Technical Handbook

Sealing ElementsTechnical Handbook O-rings12. O-ring Gland Design12 F. Groove Design for Static and Dynamic Applications whenusing Back-up RingsThe use of Back-up ringsExtrusion occurs when part of theO-ring material is forced through the gap between mating metalparts because of system pressure.Extrusion can be prevented in several ways:• Reducing the gap will help prevent extrusion.• A harder O-ring material i.e. NBR 90 Shore, FKM 90 or 95Shore A, AU, EU, PUR (Polyurethane) can be used to preventextrusion. (FKM 95° shore is also excellent for explosivedecompression applications, see page 96).• An O-ring can be installed with a back-up ring of a harder materialto close the gap and support the O-ring.The use of back-up rings is related to O-ring hardness, mediapressure, and whether it is a static or dynamic application. Ingeneral the guidelines are:Pressure static application:up to 1000 psi (70 bar, 7 MPa) without back-up ring, up to 6000psi (400 bar, 40 MPa) with back-up ring, up to 30000 psi (2000bar, 200 MPa) with special construction. Dynamic application:reciprocating up to 750 psi (50 bar, 5MPa) without back-up ring,higher pressures with back-up ring.Speedreciprocating up to 0.5 m/sec.Backup ringPressureFig. 1-38Different shapes of Back-up rings:Spiral Split (skarf-cut) Solidb 2b 3PressurePressurePressure147All the information in this documentation has been compiled with the greatest of care.Despite this we can bear no responsibility whatsoever for any errors present in the documentation. The recommendations are intended as guidelines.www.eriks.info