ISO 3601-4:2008

Overview

ISO 3601-4:2008 - "Fluid power systems - O-rings - Part 4: Anti-extrusion rings (back-up rings)" specifies the form, dimensions and tolerances for anti-extrusion (back-up) rings used with O-rings in fluid power systems. The standard covers five types of back-up rings (spiral T1; angle cut T2; solid T3; angle cut concave T4; solid concave T5) and links their use to selected O-ring sizes (ISO 3601-1) and housing dimensions (ISO 3601-2). It also gives guidance on materials, positioning in housings and a standardized designation code for procurement and specification.

Key topics and technical requirements

• Types of anti-extrusion rings: Definitions and intended use for T1–T5, including assembly considerations and suitability for pressure/temperature ranges.
  • Spiral (T1) typically for ~10–20 MPa (100–200 bar) or where assembly ease is needed.
  • Angle cut (T2) and solid (T3) preferred for higher pressures (>20 MPa); T3 is recommended for extreme conditions (e.g., >25 MPa and >135 °C).
  • Concave variants (T4, T5) are designed to accommodate and retain O-ring shape under high pressure.
• Dimensions and tolerances: Detailed requirements for axial and radial widths, inside/outside diameters, spiral gap dimensions and tolerances tied to O-ring cross-section and housing geometry.
• Positioning: Guidance on placement of one or two anti-extrusion rings in piston and rod housings depending on pressure direction and application (static vs dynamic).
• Materials: Preferred materials (unfilled PTFE, PTFE filled with glass/carbon/bronze, polyamide, thermoplastics, polyurethane) and advice to consult suppliers for material selection based on operating conditions.
• Designation code: A clear format for naming/back-up ring parts for unambiguous ordering and documentation.

Applications and practical value

• Used in hydraulic and pneumatic equipment to prevent O-ring extrusion between mating metal parts (e.g., piston-to-bore, rod-to-housing), improving seal life and system reliability.
• Applicable to designers, sealing-device engineers, OEMs and maintenance teams specifying seals for cylinders, valves and actuators in high-pressure or high-temperature fluid power systems.
• Helps standardize procurement, reduce fitment errors and ensure interchangeability across suppliers by defining dimensional and material criteria.

Who should use this standard

• Seal designers and manufacturers, hydraulic/pneumatic system engineers, OEMs, procurement specialists, quality/control engineers and maintenance personnel involved in specifying or installing O-ring sealing systems.

Related standards

• ISO 3601-1 (O-ring sizes and designation codes)
• ISO 3601-2 (Housing dimensions for general applications)
• ISO 3601-3 (Quality acceptance criteria)
• ISO 3601-5 (Elastomer suitability for industrial applications)

Keywords: ISO 3601-4:2008, anti-extrusion rings, back-up rings, O-rings, fluid power systems, PTFE, piston sealing, rod sealing, dimensions and tolerances.