ISO 6621-2:2020

Internal combustion engines — Piston rings — Part 2: Inspection measuring principles

This document defines the measuring principles to be used for measuring piston rings; it applies to piston rings up to and including 200 mm diameter for reciprocating internal combustion engines. This document can be used for piston rings for compressors working under analogous conditions.

Moteurs à combustion interne — Segments de piston — Partie 2: Principes de mesure pour inspection

General Information

• Status: Published
• Publication Date: 02-Mar-2020
• ICS: 43.060.10 - Engine block and internal components
• Technical Committee: ISO/TC 22/SC 34 - Propulsion, powertrain and powertrain fluids
• Drafting Committee: ISO/TC 22/SC 34/WG 4 - Piston rings
• Current Stage: 6060 - International Standard published
• Start Date: 03-Mar-2020
• Due Date: 13-Nov-2019
• Completion Date: 03-Mar-2020

Relations

• Revises: ISO 6621-2:2003 - Internal combustion engines — Piston rings — Part 2: Inspection measuring principles
• Effective Date: 21-Nov-2015

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 6621-2
Third edition
2020-03
Internal combustion engines —
Piston rings —
Part 2:
Inspection measuring principles
Moteurs à combustion interne — Segments de piston —
Partie 2: Principes de mesure pour inspection
Reference number
ISO 6621-2:2020(E)
ISO 2020
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ISO 6621-2:2020(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2020

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

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below or ISO’s member body in the country of the requester.
ISO copyright office
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Website: www.iso.org
Published in Switzerland
ii © ISO 2020 – All rights reserved
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ISO 6621-2:2020(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Measuring principles ....................................................................................................................................................................................... 5

4.1 General measuring conditions ................................................................................................................................................... 5

4.2 Characteristics and measuring principles ....................................................................................................................... 6

4.2.1 Ring width ............................................................................................................................................................................. 6

4.2.2 Radial wall thickness, a .......................................................................................................................................... 8

4.2.3 Total free gap m, p .......................................................................................................................................................... 8

4.2.4 Closed gap, s ..................................................................................................................................................................... 9

4.2.5 Tangential force, F (in Newton) .....................................................................................................................10

4.2.6 Diametral force, F (in Newton) .....................................................................................................................15

4.2.7 Ovality, U (in millimetres) ....................................................................................................................................16

4.2.8 Point deflection, W (in millimetres) ............................................................................................................17

4.2.9 Light tightness (percentage of ring circumference) ......................................................................17

4.2.10 Taper on peripheral surface (in micrometres or degrees) ......................................................18

4.2.11 Barrel on peripheral surface, t , t (in millimetres) ......................................................................18

2 3

4.2.12 Land width, h , h (in millimetres) ..............................................................................................................20

4 5

4.2.13 Land offset (in millimetres) ...............................................................................................................................21

4.2.14 Plating/coating thickness (in millimetres) ...........................................................................................21

4.2.15 Nitrided case depth (in millimetres) ..........................................................................................................22

4.2.16 Keystone angle (in degrees) ...............................................................................................................................22

4.2.17 Obliqueness (in degrees) ......................................................................................................................................24

4.2.18 Twist (in millimetres) .............................................................................................................................................25

4.2.19 Unevenness Te , Te ...................................................................................................................................................26

r u

4.2.20 Helix (axial displacement of gap ends) (in millimetres) ...........................................................27

4.2.21 Free flatness (in millimetres) ...........................................................................................................................27

4.2.22 Surface roughness Ra, Rz (in micrometers) .........................................................................................28

4.2.23 Circumferential waviness — Bottom side face (in micrometres) .....................................28

Bibliography .............................................................................................................................................................................................................................30

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ISO 6621-2:2020(E)
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.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).

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. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/ patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso .org/

iso/ foreword .html.

This document was prepared by Technical Committee ISO/TC 22, Road vehicles, Subcommittee SC 34,

Propulsion, powertrain and powertrain fluids.

This third edition cancels and replaces the second edition (ISO 6621-2:2003), which has been technically

revised.
The main changes compared to the previous edition are as follows:

— Oil ring diameter range for ring widths 3,0 mm and 3,5 mm increased up to 160 mm.

A list of all parts in the ISO 6621 series can be found on the ISO website.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO 2020 – All rights reserved
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ISO 6621-2:2020(E)
Introduction

This document is one of a number of International Standards dealing with piston rings for reciprocating

internal combustion engines. Others are ISO 6621-1, ISO 6621-3, ISO 6621-4, ISO 6621-5, ISO 6622,

ISO 6623, ISO 6624, ISO 6625, ISO 6626 and ISO 6627 (see Bibliography for details).

The common features and dimensional tables presented in this document constitute a broad range of

variables, and the designer, in selecting a particular ring type, should bear in mind the conditions under

which it will be required to operate.

It is also essential that the designer refer to the specifications and requirements of ISO 6621-3 and

ISO 6621-4 before completing their selection.
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INTERNATIONAL STANDARD ISO 6621-2:2020(E)
Internal combustion engines — Piston rings —
Part 2:
Inspection measuring principles
1 Scope

This document defines the measuring principles to be used for measuring piston rings; it applies to

piston rings up to and including 200 mm diameter for reciprocating internal combustion engines.

This document can be used for piston rings for compressors working under analogous conditions.

2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements 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.

ISO 4287, Geometrical Product Specifications (GPS) — Surface texture: Profile method — Terms, definitions

and surface texture parameters

ISO 4288, Geometrical Product Specifications (GPS) — Surface texture: Profile method — Rules and

procedures for the assessment of surface texture
ISO 6621-1, Internal combustion engines — Piston rings — Part 1: Vocabulary

ISO 6624-1, Internal combustion engines — Piston rings — Part 1: Keystone rings made of cast iron

ISO 6624-2, Internal combustion engines — Piston rings — Part 2: Half keystone rings made of cast iron

ISO 6624-3, Internal combustion engines — Piston rings — Part 3: Keystone rings made of steel

ISO 6624-4, Internal combustion engines — Piston rings — Part 4: Half keystone rings made of steel

3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 6621-1 and the following apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1

distance between the sides, at any particular point perpendicular to the reference plane measured in

millimetres
Note 1 to entry: See Figures 1 and 2.
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ISO 6621-2:2020(E)
3.2

distance between the sides at a specified distance a from the peripheral surface.

Note 1 to entry: See Figure 4.

Note 2 to entry: Alternatively, the ring width is controlled by a at a specified width h (see Figure 6).

6 3
3.3
radial wall thickness

radial distance between the peripheral surface and the inside surface of the ring measured in

millimetres
Note 1 to entry: See Figure 7.
3.4
total free gap

chordal distance between the gap ends of the ring in a free unstressed state, measured at the centre

line of the radial wall thickness (3.3)
Note 1 to entry: See Figure 10.

Note 2 to entry: For rings with an internal notch for a peg, the total free gap is defined by the chordal distance

marked as p in Figure 11.
Note 3 to entry: The total free gap is measured in millimetres.
3.5
closed gap

distance between the gap ends of the ring measured at the narrowest point, which the ring would have

when fitted in a gauge of nominal cylinder bore size
Note 1 to entry: See Figure 12.
Note 2 to entry: The closed gap s is related to the nominal diameter d .
1 1
3.6
tangential force

force necessary to maintain the ring at the closed gap (3.5) condition by means of a tangential pull on

the ends of a circumferential metal tape or hoop
Note 1 to entry: See Figures 13 to15.
Note 2 to entry: Tangential force is measured in Newtons.

Note 3 to entry: For single-piece rings, it is not recommended for rings d < 50mm; for these rings, see 4.2.6.

Note 4 to entry: For multi-piece rings, vibration is used to reduce friction during or prior to measurement.

3.7
diametral force

force, acting diametrically at 90° to the gap, necessary to maintain the ring at the nominal diameter

condition measured in the direction of the force
Note 1 to entry: See Figure 20.
Note 2 to entry: This method is only applicable to single-piece rings.
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ISO 6621-2:2020(E)
Note 3 to entry: Diametral force is measured in Newtons.
3.8
ovality

difference between the mutually perpendicular diameters d and d when the ring is drawn to a closed

3 4
gap (3.5) within a flexible tape

Note 1 to entry: It can be either positive (d > d ) or negative (d < d ) (see Figure 21).

3 4 3 4
Note 2 to entry: This method is only applicable to single-piece rings.
Note 3 to entry: Ovality is measured in millimetres.
3.9
point deflection

deviation of the butt ends from the true circle when restrained in a gauge of nominal cylinder bore

diameter
Note 1 to entry: See Figure 22.
Note 2 to entry: Point deflection is measured in millimetres.
3.10
light tightness

ability of the peripheral surface of a ring when mounted in a gauge of nominal cylinder bore diameter to

exclude the passage of light
Note 1 to entry: See Figure 23.

Note 2 to entry: Areas of the ring showing pinpoint, burry or fuzzy light shall be considered as light tight.

Note 3 to entry: Light tightness is measured in percentage of ring circumference.

3.11
taper on peripheral surface

intentional angular deviation of the peripheral surface from a line perpendicular to the reference plane

Note 1 to entry: See Figure 24.

Note 2 to entry: In the case of the taper faced peripheral surface with partly cylindrical area both measuring

points shall be placed on the taper area.

Note 3 to entry: Taper on peripheral surface is measured in micrometres or degrees.

3.12
barrel on peripheral surface

intentional convex deviation of the peripheral surface from a line perpendicular to the reference plane

Note 1 to entry: See Figure 26 for a symmetrical barrel and Figure 28 for an asymmetrical barrel.

Note 2 to entry: t is used for lower barrel face and t for upper barrel face.
2 3
Note 3 to entry: Barrel on a peripheral surface is measured in millimetres.
3.13
land width

width of the land which theoretically should be in contact with the cylinder bore

Note 1 to entry: See Figure 29.
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ISO 6621-2:2020(E)
Note 2 to entry: Land width is measured in millimetres.
3.14
land offset

displacement of the two peripheral surfaces of a slotted or drilled oil control ring in relation to each

other in a radial direction
Note 1 to entry: See Figure 31.
Note 2 to entry: Land offset is measured in millimetres.
3.15
plating/coating thickness

distance between the outer surface of the plating/coating and the surface of the base ring material

connected with the different configurations of platings/coatings
Note 1 to entry: See Figure 33.
Note 2 to entry: Plating/coating thickness is measured in millimetres.
3.16
nitrided case depth

thickness of the surface layer with a hardness value ≥700 HV 0,1 measured perpendicular to the ring

peripheral surface or side faces.
Note 1 to entry: See Figures 34 and 35.
Note 2 to entry: Nitrided case depth is measured in millimetres.
3.17
keystone angle

angle enclosed by the two sides of the ring or alternatively, the sum of both side face angles, i.e.

included angle
Note 1 to entry: See Figure 36.
Note 2 to entry: Keystone angle is measured in degrees.
3.18
obliqueness

unintentional deviation of the bisector of the keystone included angle from parallelism with the

reference plane
Note 1 to entry: Not applicable to rings with designed twist (3.19).
Note 2 to entry: See Figure 42.
Note 3 to entry: Obliqueness is measured in degrees.
3.19
twist

intentional torsional deviation of the section of the ring from the reference plane when the ring

is restricted to nominal diameter, as in the case of asymmetrical rings such as those internally or

externally stepped or bevelled
Note 1 to entry: See Figure 43.
Note 2 to entry: Twist is measured in millimetres.
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ISO 6621-2:2020(E)
3.20
unevenness

unintentional deviation of the sides of the ring from parallelism to the reference plane, i.e. twisted or

dished rings
Note 1 to entry: See Figures 46 and 48.

Note 2 to entry: Not applicable to rings with designed twist (3.19), as covered by 4.2.18.

3.21
helix
displacement of the gap ends perpendicular to the reference plane
Note 1 to entry: See Figure 51.
Note 2 to entry: Helix is measured in millimetres.
3.22
free flatness

relationship between the ring in the free state and a plane parallel to its reference plane

Note 1 to entry: Free flatness is measured in millimetres.
3.23
circumferential waviness

three or more lobes showing a continuous pattern of peaks and valleys spaced at a set frequency

circumferentially around the bottom side face of the ring
Note 1 to entry: Circumferential waviness is measured in micrometres.
4 Measuring principles
4.1 General measuring conditions

The following general requirements are applicable to all measuring principles unless otherwise

specified:

a) the ring shall rest on the reference plane in the free or open condition. No additional force shall be

applied to load the ring on the reference plane; except when measuring "unevenness" in accordance

with 4.2.19 or "helix" in accordance with 4.2.20;

b) certain measurements are made with the ring in the closed condition in a gauge of nominal cylinder

bore diameter. When orientated rings are measured in this way, they shall be so placed that the top

side of the ring is towards the reference plane;

c) measurements shall be made using instruments with a resolution not to exceed 10 % of the

tolerance of the dimension being measured.
For further terms and definitions see ISO 6621-1.
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ISO 6621-2:2020(E)
4.2 Characteristics and measuring principles
4.2.1 Ring width
4.2.1.1 Parallel sided rings, h (in millimetres)
Measuring principle

Measure with spherical measuring probes each of radius 1,5 mm ± 0,05 mm, exerting a measuring force of

approximately 1 N (see Figure 3).

In the case of slotted oil rings, the measurement shall be made between the slots and not across them (see

Figure 2).
Key
h ring width
Figure 1
Key
h ring width
Figure 2
Figure 3
4.2.1.2 Keystone rings, half keystone rings, h
Measuring principle
Method A
This method determines h for a specified value of a (see Figure 4).
3 6
Key
h keystone ring width
a keystone ring depth
Figure 4
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ISO 6621-2:2020(E)
Measuring principle

Measure with spherical measuring probes each radius 1,5 mm ± 0,05 mm exerting a measuring force of ap-

proximately 1 N (see Figure 5).

If the measuring equipment is calibrated with parallel gauges instead of keystone gauges, the use of spherical

measuring probes for the measurement according to Figure 4 will give rise to an error as follows:

— for 6° keystone angle: 0,004 mm (Ring types: T, TB, TBA, and TM)
— for 7° keystone angle: 0,012 mm (Ring types: HK, and HKB)
— for 15° keystone angle: 0,026 mm. (Ring types: K, KB, KBA, and KM)

To obtain the correct measured width of the keystone ring, the above values shall be deducted from the meas-

ured values.
Values of a are given in ISO 6624-1 to ISO 6624-4.
Key
a keystone ring depth
Figure 5
Method B
This method determines a for a specified value of h (see Figure 4).
6 3

Measure with a flat face probe exerting a measuring force of approximately 1 N. The ring shall be placed between

two sharp-edged (radius ≤0,01 mm) circular discs which are spaced apart at the specified gauge width h (see

Figure 6).
Values of h are given in ISO 6624-1 to ISO 6624-4.
Key
h keystone ring width
a keystone ring depth
Figure 6
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ISO 6621-2:2020(E)
4.2.2 Radial wall thickness, a
Measuring principle

1. Measure radially between a flat measuring surface on the peripheral surface and a special measuring

surface of the radius approximately 4 mm on the bore, and using a measuring force of 3 N to 10 N (see

Figure 8).
Key
a radial wall thickness
Figure 7

2. Measure radially between cylindrical inserts or rollers of radius approximately 4 mm and with a measuring

force of 3 N to 10 N. The peripheral surface of the rollers shall be perpendicular to the reference plane.

Figure 8

The length of the rollers shall be greater than the axial ring width (see Figure 9).

Figure 9
4.2.3 Total free gap m, p
Measuring principle
Measure with a steel rule to the nearest 0,25 mm.
Optionally, this feature can be measured with callipers.
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ISO 6621-2:2020(E)
Measuring principle
Key
m free gap
a radial wall thickness
Figure 10
Key
p free gap
a radial wall thickness
Figure 11
4.2.4 Closed gap, s
Measuring principle

Measure in a bore gauge of nominal diameter using a wedge gauge or feeler gauges and using a measuring force

of approximately 1 N (see Figure 12).

For closed gap tolerances lower than values in the ISO standard tables (ISO 6622, ISO 6623, ISO 6624, ISO 6625,

ISO 6626, ISO 6627) measuring methods with improved accuracy are required. One example would be to use

an optical technique. The gap area shall be thoroughly cleaned to obtain an accurate measurement.

+0,001 xd
The diameter of the bore gauge shall comply with the following: d

Correction shall be made for any deviation of the bore gauge from the nominal ring diameter.

Key
d nominal diameter
s closed gap
Figure 12
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ISO 6621-2:2020(E)
4.2.5 Tangential force, F (in Newton)
4.2.5.1 For single-piece rings
NOTE Not recommended for rings d < 50mm; for these rings, see 4.2.6.
Measuring principle
1. Tape method (see Figure 13)

The encircling steel tape of thickness 0,08 mm to 0,10 mm is carried around 10 mm diameter rollers set

20 mm apart (see Figure 13). In tightening the tape, the ring is closed to the point where the gap ends

touch and then open to the closed gap dimension previously measured. The ring force is then read off from

the precision measuring scale. The gap of the ring shall be symmetrically disposed between the rollers.

An alternative method to set up the tangential loading of the force measuring instrument is using a solid

disc of nominal bore diameter ±0,005 mm to set up the length of the tape. The gauge disc is inserted into

the tape and the tape length adjusted until the specified mean limit tangential force is indicated.

Key
1 measuring scale
2 diameter of rollers 10 mm
s closed gap
F tangential force
Figure 13
2. Hoop method (see Figure 14)

The ring is placed in a correctly sized hoop with its gap aligned to the gap of the hoop. The hoop is then

closed in a precision loading machine until the loading pins are at a predetermined distance apart at

which point the hoop is precisely at the cylinder bore diameter appropriate to the ring (see Figure 14). The

force is then read off from the display.
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ISO 6621-2:2020(E)
Measuring principle
Key
1 measuring scale
2 loading pins pacing to suit machine
d nominal diameter
s closed gap
F tangential force
Figure 14
3. Encircling tape method

A steel tape 0,08 mm to 0,1 mm in thickness encircles the ring crossing at the gap (see Figure 13).

The tape is tightened until the ring is closed to the closed gap previously measured. The ring force is then

read off the precision measuring scale.

NOTE No vibration when measuring single-piece rings according to methods 1), 2) and 3).

4.2.5.2 For multi-piece rings
Measuring principle

For the measurement of coil spring loaded rings or similar rings where the spring is supported in the bore of

the ring, the gap of the spring shall be positioned at 180° to the gap of the ring.

For the measurement of expander/segment oil control rings, the ring assembly shall be mounted in a carrier

simulating the piston ring groove. The gap of the spring element is placed at 180° to the gap of the rails, both of

which shall be in line. Choice of ring carrier type (see Table 1) shall be decided between manufacturer and client.

For the measurement of a ring provided with a wavy spring, or other spring which is groove root supported,

the ring assembly shall be mounted in a carrier simulating the groove, the root diameter of which is equal to

the mean diameter of the piston ring groove in which the ring will be used.

Tolerance on carrier root diameter ±0,02 mm. The gap of the wavy spring shall be at 180° to the gap of the ring.

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ISO 6621-2:2020(E)
Measuring principle
Key
1 measuring scale
s closed gap
F tangential force
Figure 15
Table 1 — Alternative ring carriers
Dimensions in millimetres
Types Carrier grove width
Tolerance
I h +0,01
+00, 2
II h +0,02
III h +0,03
h = nominal ring width
1. Tape method with circumferential vibration

Identical procedures are used as for single piece rings but an appropriate vibration shall be applied to the

tape loading mechanism to relieve forces of friction (see Figure 16). A suitable level is 40 Hz to 50 Hz at an

amplitude of ±0,15 mm.
Key
1 measuring scale
2 diameter of rollers 10 mm
3 vibration
s closed gap
F tangential force
Figure 16
2. Encircling tape method with axial vibration
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ISO 6621-2:2020(E)
Measuring principle

Identical procedures are used as for the single piece rings (encircling tape method) except that a carrier

may be used and vibration (slapping) is applied to the encircled ring or encircled ring with carrier to re-

duce friction (see Figure 17). A suitable level of slapping is 1 to 3 times per second.

Key
1 slapping
s closed gap
F tangential force
Figure 17
3. Hoop method with circumferential vibration

Identical procedures are used as for single-piece rings but an appropriate vibration shall be applied to the

hoop loading mechanism to relieve all forces of friction (see Figure 18).
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ISO 6621-2:2020(E)
Measuring principle
Key
1 measuring scale
2 loading pin spacing to suit machine
3 circumferential vibration
d nominal diameter
s closed gap
F tangential force
Figure 18
Tape- or Hoop-method with axial vibration

Identical procedures are used as for single-piece rings but an appropriate axial vibration shall be applied to the

carrier which is simulating the ring groove to relieve forces of friction.

A suitable level of vibration is 420 Hz (≈ equal to 25 000 cycles per minute). The axial vibration shall have an

amplitude that the exerted force on the carrier will reach approximately ±18 N. (see Figure 19).

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ISO 6621-2:2020(E)
Measuring principle
Key
1 measuring scale
2 diameter of rollers 10 mm
3 axial vibration
d nominal diameter
s closed gap
F tangential force
Figure 19

Before tangential force measurements are made, rings shall be degreased and optionally lightly coated with

thin mac
...