ISO 3601-4:2008

INTERNATIONAL ISO
STANDARD 3601-4
First edition
2008-06-01
Fluid power systems — O-rings —
Part 4:
Anti-extrusion rings (back-up rings)
Transmissions hydrauliques et pneumatiques — Joints toriques —
Partie 4: Bagues anti-extrusion

Reference number
©
ISO 2008
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ii © ISO 2008 – All rights reserved

Contents Page
Foreword. iv
Introduction . v
1 Scope .1
2 Normative references .1
3 Terms and definitions .1
4 Symbols .2
5 General requirements related to O-ring housings .2
6 Types of anti-extrusion (back-up) rings .3
6.1 Spiral type, T1 .3
6.2 Angle cut type, T2.5
6.3 Solid type, T3.5
6.4 Angle cut concave type, T4 .6
6.5 Solid concave type, T5 .7
7 Positioning of anti-extrusion rings in housings.8
8 Anti-extrusion ring designation code and materials.8
8.1 Designation code.8
8.2 Materials .9
9 Dimensions and tolerances .9
9.1 Axial and radial widths of anti-extrusion rings.9
9.2 Outside diameters of anti-extrusion rings for piston sealing applications.10
9.3 Inside diameters of anti-extrusion rings for rod sealing applications.11
10 Identification statement (reference to this part of ISO 3601) .11

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 3601-4 was prepared by Technical Committee ISO/TC 131, Fluid power systems, Subcommittee SC 7,
Sealing devices.
ISO 3601 consists of the following parts, under the general title Fluid power systems — O-rings:
⎯ Part 1: Inside diameters, cross-sections, tolerances and designation codes
⎯ Part 2: Housing dimensions for general applications
⎯ Part 3: Quality acceptance criteria
⎯ Part 4: Anti-extrusion rings (back-up rings)
⎯ Part 5: Suitability of elastomeric materials for industrial applications
iv © ISO 2008 – All rights reserved

Introduction
In fluid power systems, power is transmitted and controlled through a fluid (liquid or gas) under pressure within
an enclosed circuit. Where high pressures are encountered, it is recommended that an anti-extrusion ring
(back-up ring) be incorporated within the O-ring housing to limit extrusion of the O-ring between the metal
parts (e.g. cylinder bore and piston or rod and housing).

INTERNATIONAL STANDARD ISO 3601-4:2008(E)

Fluid power systems — O-rings —
Part 4:
Anti-extrusion rings (back-up rings)
1 Scope
This part of ISO 3601 specifies dimensions and tolerances for five types of anti-extrusion rings, which are also
known as back-up rings:
a) spiral type (T1);
b) angle cut type (T2);
c) solid type (T3);
d) angle cut concave type (T4);
e) solid concave type (T5).
These anti-extrusion rings are for use with selected O-ring sizes as specified in ISO 3601-1 and the relevant
housing dimensions specified in ISO 3601-2.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
1)
ISO 3601-1 , Fluid power systems — O-rings — Part 1: Inside diameters, cross-sections, tolerances and
designation codes
ISO 3601-2:2008, Fluid power systems — O-rings — Part 2: Housing dimensions for general applications
ISO 5598, Fluid power systems and components — Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 5598 apply.

1) To be published. (Revision of ISO 3601-1:2002)
4 Symbols
The following letter symbols are used in this part of ISO 3601:
b width of O-ring housing without anti-extrusion ring
b width of O-ring housing with one anti-extrusion ring
b width of O-ring housing with two anti-extrusion rings
b width of single anti-extrusion ring, angle cut types T2 and T4, and solid types T3 and T5
b width of anti-extrusion ring, spiral type T1
b total width (b + h) at apex of single anti-extrusion ring, angle cut concave type T4 and solid concave
7 5
type T5
d O-ring cross-section diameter
d housing diameter for piston seal housings
d bore diameter for piston application
d rod diameter
d housing diameter for rod seal housings
d piston diameter
d bore diameter for rod application
d internal diameter of anti-extrusion ring (spiral type T1 only)
d external diameter of anti-extrusion ring (spiral type T1 only)
d inside diameter of anti-extrusion ring (angle cut types T2 and T4, or solid types T3 and T5)
d outside diameter of anti-extrusion ring (angle cut types T2 and T4, or solid types T3 and T5)
h extension of concave anti-extrusion ring beyond b base width
l gap of anti-extrusion ring, spiral type T1, on mandrel diameter d
R radius of cavity in concave anti-extrusion ring
t radial section of anti-extrusion ring
5 General requirements related to O-ring housings
For general requirements of O-ring housings, such as surface roughness, corners and edges of undefined
shape, lead-in chamfers, housing diameter tolerances and housing widths, see the related clauses in
ISO 3601-2.
2 © ISO 2008 – All rights reserved

6 Types of anti-extrusion (back-up) rings
6.1 Spiral type, T1
6.1.1 The spiral type, T1, anti-extrusion ring (see Figure 1) is typically associated with applications in which
the system pressure is between 10 MPa (100 bar) and 20 MPa (200 bar). Continuous rings, such as the angle
cut type (T2) or the solid type (T3), are preferred for use at pressures higher than 20 MPa (200 bar), but
assembly problems can be encountered with closed housings or at small diameters.

a
This angle may be increased to 45° ± 5° when d is less than 7,0 mm.
NOTE Direction of the spiral is optional.
Figure 1 — Spiral type (T1) anti-extrusion ring
6.1.2 At higher temperatures, typically in excess of 100 °C, anti-extrusion rings can be necessary even
when pressures are less than 10 MPa (100 bar). Independent of temperature and pressure, special operating
conditions can also require the use of anti-extrusion rings. These operating conditions should be discussed
with the seal supplier at the design stage.
6.1.3 Dimensions shall conform to the requirements given in Table 1.
Table 1 — Dimensions of gap, l, of spiral type (T1) anti-extrusion rings
in relation to mandrel diameter, d
Dimensions in millimetres
O-ring nominal Mandrel diameter Gap
cross-section
d d l
2 14
d u 10 1,2 ± 0,4
10 < d u 20 1,4 ± 0,6
1,78
20 < d u 60 1,8 ± 0,6
d
...