Internal combustion engines -- Piston rings

ISO 6621-4:2015 specifies the general characteristics of piston rings for reciprocating internal combustion engines for road vehicles and other applications (the individual dimensional criteria for these rings are given in the relevant International Standards). It also provides a system for ring coding, designation, and marking. It is applicable to all such rings of a nominal diameter from 30 mm up to and including 200 mm.

Moteurs à combustion interne -- Segments de piston

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Published
Publication Date
17-Mar-2015
Current Stage
9060 - Close of review
Start Date
04-Jun-2020
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INTERNATIONAL ISO
STANDARD 6621-4
Third edition
2015-03-15
Internal combustion engines — Piston
rings —
Part 4:
General specifications
Moteurs à combustion interne — Segments de piston —
Partie 4: Spécifications générales
Reference number
ISO 6621-4:2015(E)
ISO 2015
---------------------- Page: 1 ----------------------
ISO 6621-4:2015(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2015

All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form

or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior

written permission. Permission can be requested from either ISO at the address below or ISO’s member body in the country of

the requester.
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Tel. + 41 22 749 01 11
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Published in Switzerland
ii © ISO 2015 – All rights reserved
---------------------- Page: 2 ----------------------
ISO 6621-4:2015(E)
Contents Page

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

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

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

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

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

4 Piston ring codes .................................................................................................................................................................................................. 1

5 Designation of piston rings ........................................................................................................................................................................ 4

5.1 Designation elements and order ............................................................................................................................................. 4

5.2 Designation examples ....................................................................................................................................................................... 5

6 Marking of piston rings ................................................................................................................................................................................. 6

6.1 General ........................................................................................................................................................................................................... 6

6.2 Mandatory topside identification marking ..................................................................................................................... 6

6.3 Additional marking ............................................................................................................................................................................. 7

7 General characteristics .................................................................................................................................................................................. 7

7.1 Ring shape ................................................................................................................................................................................................... 7

7.2 Light tightness ......................................................................................................................................................................................... 8

7.3 Closed gap ................................................................................................................................................................................................... 8

7.4 Tangential force, F , and diametral force, F , of single piece piston rings ............................................ 8

t d

7.5 Tangential force F of multipiece oil control rings as specified in ISO 6626-1, ISO 6626-2,

and ISO 6626-3 ....................................................................................................................................................................................12

7.6 Tangential force F of expander/segment oil control rings as specified in ISO 6627 .............13

8 Notches for preventing ring rotation ...........................................................................................................................................13

8.1 Ring joint with internal notch (only for compression rings as specified in ISO 6622 and

ISO 6624) ..................................................................................................................................................................................................13

8.2 Ring joint with side notch (only for compression rings as specified in ISO 6622) ...................15

9 Machining of surfaces ...................................................................................................................................................................................16

9.1 Peripheral surfaces ...........................................................................................................................................................................16

9.2 Side faces ...................................................................................................................................................................................................16

9.3 Other surfaces .......................................................................................................................................................................................16

10 Plated, coated, and treated surfaces .............................................................................................................................................17

10.1 Chromium plating on peripheral surfaces ...................................................................................................................17

10.2 Spray coated peripheral surfaces .........................................................................................................................................20

10.3 Nitrided surfaces ................................................................................................................................................................................24

10.4 Treated surfaces ..................................................................................................................................................................................25

10.5 Physical vapour deposition coating (PVD) ..................................................................................................................26

11 Miscellaneous .......................................................................................................................................................................................................27

11.1 Cleanliness ...............................................................................................................................................................................................27

11.2 Corrosion protection .......................................................................................................................................................................27

11.3 Packaging ..................................................................................................................................................................................................27

12 Traceability (optional) ................................................................................................................................................................................27

Bibliography .............................................................................................................................................................................................................................28

© ISO 2015 – All rights reserved iii
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ISO 6621-4:2015(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 on the meaning of ISO specific terms and expressions related to conformity

assessment, as well as information about ISO’s adherence to the WTO principles in the Technical Barriers

to Trade (TBT), see the following URL: Foreword — Supplementary information.
The committee responsible for this document is ISO/TC 22, Road vehicles.

This third edition cancels and replaces the second edition (ISO 6621-4:2003), which has been technically

revised.

ISO 6621 consists of the following parts, under the general title Internal combustion engines — Piston

rings:
— Part 1: Vocabulary
— Part 2: Inspection measuring principles
— Part 3: Material specifications
— Part 4: General specifications
— Part 5: Quality requirements
iv © ISO 2015 – All rights reserved
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ISO 6621-4:2015(E)
Introduction

ISO 6621 is one of a series of International Standards dealing with piston rings for reciprocating internal

combustion engines. Others are ISO 6622-1, ISO 6622-2, ISO 6623, ISO 6624-1, ISO 6624-2, ISO 6624-3,

ISO 6624-4, ISO 6625, ISO 6626-1, ISO 6626-2, ISO 6626-3, and ISO 6627.
© ISO 2015 – All rights reserved v
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INTERNATIONAL STANDARD ISO 6621-4:2015(E)
Internal combustion engines — Piston rings —
Part 4:
General specifications
1 Scope

This part of ISO 6621 specifies the general characteristics of piston rings for reciprocating internal

combustion engines for road vehicles and other applications (the individual dimensional criteria for

these rings are given in the relevant International Standards). It also provides a system for ring coding,

designation, and marking. It is applicable to all such rings of a nominal diameter from 30 mm up to and

including 200 mm.
2 Normative references

The following documents, in whole or in part, are normatively referenced in this document and are

indispensable for its application. For dated references, only the edition cited applies. For undated

references, the latest edition of the referenced document (including any amendments) applies.

ISO 6507-3, Metallic materials — Vickers hardness test — Part 3: Calibration of reference blocks

ISO 6621-1, Internal combustion engines — Piston rings — Part 1: Vocabulary

ISO 6621-2, Internal combustion engines — Piston rings — Part 2: Inspection measuring principles

3 Terms and definitions

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

4 Piston ring codes

Codes used for piston rings shall be as given in Table 1, corresponding to their explanatory descriptions.

Table 1 — Codes and descriptions in alphabetical order
Relevant International
Code Description
Standard
B Barrel-faced rectangular ring ISO 6622-1, ISO 6622-2
BA Asymmetrical barrel-faced rectangular ring ISO 6622-1, ISO 6622-2
CR1E ... CR2E Peripheral surface chromium plated semi-inlaid design ISO 6621-4
CR1F ... CR2F Peripheral surface chromium plated inlaid design ISO 6621-4
CRF ... CR4 Peripheral surface chromium plated fully faced design ISO 6621-4
CSN, CSG, CSE Type of coil spring ISO 6626-1, ISO 6626-2
D Bevelled edge oil control ring ISO 6625
D22 Radial wall thickness for “d1/22” ISO 6622-1, ISO 6623

Material mark (for alternative materials) at the discretion of the manufacturer.

Any other additional marking on customer’s request, which shall be quoted clearly in the order, shall be agreed upon

between manufacturer and customer.
© ISO 2015 – All rights reserved 1
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ISO 6621-4:2015(E)
Table 1 (continued)
Relevant International
Code Description
Standard
DSF Coil spring loaded bevelled edge oil control ring ISO 6626-1, ISO 6626-2
DSF-C Coil spring loaded bevelled edge oil control ring,
ISO 6626-1, ISO 6626-2
chromium plated, and profile ground
DSF-CNP Coil spring loaded bevelled edge oil control ring,
ISO 6626-1
chromium plated, not profile ground
DSF-NG Coil spring loaded bevelled edge oil control ring (face
ISO 6626-1
geometry similar type DSF-C or DSF-CNP)
DV Bevelled edge V-groove oil control ring ISO 6625
E Scraper ring (stepped) ISO 6623
EM1 ... EM4 Scraper ring (stepped), taper-faced ISO 6623
ES-1 ... ES-4 Expander/segment oil control rings ISO 6627
FE Ferro oxided on all sides ISO 6621-4
G Double bevelled oil control ring ISO 6625
GSF Coil spring loaded double bevelled oil control ring ISO 6626-1, ISO 6626-2
HK Straight faced half keystone ring 7° ISO 6624-2, ISO 6624-4
HKB Barrel faced half keystone ring 7° ISO 6624-2, ISO 6624-4
HKBA Asymmetrical barrel-faced half keystone ring ISO 6624-2, ISO 6624-4
HOL Openings in oil rings in form of holes ISO 6626-2
IF Internal bevel (top side) ISO 6622, ISO 6624-1, ISO 6624-3
IFU Internal bevel (bottom side) ISO 6622
IFV Variable internal bevel (top side) for defined twist ISO 6622-1
IFVU Variable internal bevel (bottom side) for defined twist ISO 6622-1
IW Internal step (top side) ISO 6622-1, ISO 6624-1
IWU Internal step (bottom side) ISO 6622-1
K Straight-faced keystone ring 15° ISO 6624-1, ISO 6624-3
KA Peripheral edges chamfered ISO 6622
KB Barrel-faced keystone ring 15° ISO 6624-1, ISO 6624-3
KBA Asymmetrical barrel-faced keystone ring 15° ISO 6624-1, ISO 6624-3
KG Reduced size of peripheral edges at the gap of chromium ISO 6621-4
plated/spray coated/nitrided/PVD coated rings
KI Inside edges chamfered ISO 6622
KM1 ... KM5 Taper-faced keystone ring 15° ISO 6624-1, ISO 6624-3
KU Reduced peripheral bottom edge chromium plated fully ISO 6621-4
faced design
LF Uncoated ring peripheral surface or uncoated land ISO 6621-4
peripheral surface, fully lapped
LM Taper-faced piston ring with partly cylindrical machined ISO 6621-4
peripheral surface

Material mark (for alternative materials) at the discretion of the manufacturer.

Any other additional marking on customer’s request, which shall be quoted clearly in the order, shall be agreed upon

between manufacturer and customer.
2 © ISO 2015 – All rights reserved
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ISO 6621-4:2015(E)
Table 1 (continued)
Relevant International
Code Description
Standard
LP Taper-faced piston ring with lapped land over the whole ISO 6621-4
circumference but not over the whole width of the
peripheral surface
M1 ...M5 Taper-faced rectangular ring ISO 6622-1, ISO 6622-2
MC11 ... MC68 Material subclasses ISO 6621-3
MM Manufacturer’s mark ISO 6621-4
MR ISO 6621-4
Reduced ratio m da− for reduced load
MU Any other additional mark ISO 6621-4
MX Material mark ISO 6621-4
MY Mark for required ring shape “negative ovality” ISO 6621-4
MZ Mark for required ring shape “round” ISO 6621-4
N Napier ring (undercut step) ISO 6623
NB030 ... NB130 Nitrided surface, case depth specified on peripheral ISO 6621-4
surface and bottom side
NE1 ... NE3 Ring joint with lateral stop ISO 6621-4
NH1 ... NH3 Ring joint with internal stop ISO 6621-4
NM1 ... NM4 Napier ring (undercut step), taper-faced ISO 6623
NP030 ... NP130 Nitrided surface, case depth specified on peripheral ISO 6621-4
surface only
NS 010 ... NS 050 Nitrided surface, case depth on segments ISO 6627
NT030 ... NT130 Nitrided surface, case depth specified on peripheral ISO 6621-4
surface and side faces
NX Nitrided surface of expanders/spacers ISO 6627
PC001…PC040 Physical vapour deposition coating (PVD) thickness ISO 6621-4
PN 10…PN 25 Nominal contact pressure classes ISO 6626-3
PNE, PNL, PNR, Contact pressure class ISO 6626-1, ISO 6626-2, ISO 6627
PNM, PNH, PNV
PO Phosphated on all sides ISO 6621-4
PR Phosphated on all sides (for rust protection purposes) ISO 6621-4
R Straight-faced rectangular ring ISO 6622-1, ISO 6622-2
RU Napier ring with reduced undercut or step ISO 6623
S Slotted oil control ring ISO 6625
S005 ... S100 Closed gap (minimum values) ISO 6621-4
SC1 ... SC4 Peripheral surface spray coated fully faced design ISO 6621-4
SC1E ... SC4E Peripheral surface spray coated semi-inlaid design ISO 6621-4
SC1F ... SC4F Peripheral surface spray coated inlaid design ISO 6621-4
SSF Coil spring loaded slotted oil control ring ISO 6626-1, ISO 6626-2

SSF-L Coil spring loaded slotted oil control ring with 0,6 mm ISO 6626-1, ISO 6626-2

nominal land width

Material mark (for alternative materials) at the discretion of the manufacturer.

Any other additional marking on customer’s request, which shall be quoted clearly in the order, shall be agreed upon

between manufacturer and customer.
© ISO 2015 – All rights reserved 3
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ISO 6621-4:2015(E)
Table 1 (continued)
Relevant International
Code Description
Standard
T Straight-faced keystone ring 6° ISO 6624-1, ISO 6624-3
TB Barrel-faced keystone ring 6° ISO 6624-1, ISO 6624-3
TBA Asymmetrical barrel-faced keystone ring 6° ISO 6624-1, ISO 6624-3
TM1 ... TM5 Taper-faced keystone ring 6° ISO 6624-1, ISO 6624-3
TT Seating tab angle θ ISO 6627
WF Reduced heat set ISO 6621-5, ISO 6626-1,
ISO 6626-2
WK Reduced slot length ISO 6626-1, ISO 6626-2
WTL Radial wall thickness (large) ISO 6626-3
WTS Radial wall thickness (small) ISO 6626-3
Y Ring shape negative ovality ISO 6621-4
Z Ring shape round ISO 6621-4

Material mark (for alternative materials) at the discretion of the manufacturer.

Any other additional marking on customer’s request, which shall be quoted clearly in the order, shall be agreed upon

between manufacturer and customer.
5 Designation of piston rings
5.1 Designation elements and order
5.1.1 General

When designating piston rings complying with the relevant International Standards, the following

details shall be provided in the order given, using the codes according to Table 1.

5.1.2 Mandatory elements

The following mandatory elements shall constitute the designation of a piston ring:

— designation (i.e. piston ring);
— number of International Standard;
— type of piston ring (e.g. R);
— hyphen;
— size of piston ring, d × h ;
1 1
— radial wall thickness “regular” without code;

— Code D22 if the selected wall thickness, in accordance with ISO 6622-1 and ISO 6623, is d1/22;

— hyphen;
— material code (e.g. MC11).
4 © ISO 2015 – All rights reserved
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ISO 6621-4:2015(E)
5.1.3 Additional elements

The following optional elements may be added to the designation of a piston ring and, if so added, shall

be positioned on a second line beneath or separated by a slash (/) from the mandatory elements given

in 5.1.2:
— reduced ratio md −a , MR;
— ring shape (e.g. Z);

— selected nominal closed gap if it differs from the closed gap specified in the dimension tables

(e.g. S05);
— the selected coating (e.g. CR3);

— uncoated rings with fully lapped peripheral surface [e.g. for LF taper faced rings with partly

cylindrical peripheral surface, LM (machined), or LP (lapped)];
— selected surface treatment (e.g. PO);
— selected inside edge feature (e.g. KA);
— inside step of bevel (e.g. IWU);
— selected notch to prevent ring rotation (e.g. NH1);
— reduced slot length, if required (e.g. WK);
— coil spring with reduced heat set, if required, (e.g. WF);
— selected type of coil spring, (e.g. CSG);
— selected pressure class (e.g. PNM).
5.1.4 Elements for additional marking

Any additional marking shall be the following with the additional elements of 5.1.3:

— manufacturer’s mark, if required (e.g. MM);
— marking of required ring shape (e.g. MZ);
— material, MX (see Table 1, Footnote a);
— code for any other marking, MU (see Table 1, Footnote b).
5.2 Designation examples
5.2.1 Designation example of a piston ring in accordance with ISO 6622-1

For a straight-faced rectangular ring (R) of nominal diameter d = 90 mm (90) and nominal ring width

h = 2,5 mm (2,5), made of grey cast iron, non heat treated material subclass 11 (MC11):

Piston ring ISO 6622-1 R - 90 × 2,5 - MC11.
Parameters in parentheses are used in the ISO ring designation.
5.2.2 Designation example of a piston ring in accordance with ISO 6624-1

For a keystone ring 6°, taper faced 60′ (TM3) of nominal diameter d = 105 mm (105) and nominal ring

width h = 2,5 mm (2,5) made of spheroidal graphite cast iron, martensitic type, material subclass 51

© ISO 2015 – All rights reserved 5
---------------------- Page: 10 ----------------------
ISO 6621-4:2015(E)

(MC51), ring shape round (Z) with a selected closed gap of 0,3 mm (S030), inside edges chamfered (KI),

and peripheral surface chromium plated, with plating thickness 0,1 mm minimum (CR2):

Piston ring ISO 6624-1 TM3 – 105 × 2,5 – MC51 / Z S030 KI CR2
Parameters in parentheses are used in the ISO ring designation.
5.2.3 Designation example of a piston ring in accordance with ISO 6626-1

For a coil spring loaded, bevelled edge oil control ring, chromium plated and profile ground (DSF-C),

with nominal diameter d = 125 mm (125) and nominal ring width h = 5 mm (5) made of grey cast

1 1

iron, non heat treated, material subclass 11 (MC11) with a selected closed gap of 0,2 mm (S020) having

a chromium plating thickness on the lands of 0,15 mm minimum (CR3), phosphated on all cast iron

surfaces to a depth of 0,002 mm minimum (PO) with reduced slot length (WK), a coil spring with reduced

heat set (WF) having a variable pitch with coil diameter, d ground (CSE), tangential force, F , according

7 t

to the medium nominal contact pressure class (PNM) marked with manufacturer’s mark (MM):

Piston ring ISO 6626-1 DSF-C – 125 × 5 – MC11 / S020 CR3 PO WK WF CSE PNM MM

Parameters in parentheses are used in the ISO ring designation.
6 Marking of piston rings
6.1 General

The requirements and recommendations for piston ring marking according to 6.2 and 6.3 apply to piston

rings of 1,6 mm radial wall thickness and above. Marking of piston rings of less than 1,6 mm is at the

discretion of the manufacturer.
6.2 Mandatory topside identification marking

All rings requiring orientation shall be marked to indicate the top side only (i.e. the side nearest the

combustion chamber).

In the absence of any other mark agreed upon between manufacturer and customer, the mark “TOP”

should be used.
Marking of the top side applies to all the following types of ring:
— taper faced rings;
— asymmetrical barrel-faced rings;
— rings with reduced peripheral bottom edge;
— internally bevelled or stepped rings;
— semi-inlaid rings;
— scraper rings;
— half keystone rings;
— directional oil control rings.

All such rings requiring marking are specified in the relevant International Standards (see Foreword

and Bibliography).
6 © ISO 2015 – All rights reserved
---------------------- Page: 11 ----------------------
ISO 6621-4:2015(E)
6.3 Additional marking

6.3.1 Additional marking of piston rings is optional or at the customer’s request. Such additional

marking may comprise of the following:
— manufacturer’s mark;
— mark for required ring shape;
— oversize rings;
— material mark (for alternative materials);
— any other additional mark agreed upon between manufacturer and customer.

6.3.2 Colour marking on the running surface is in common use. Colour marking may be used to identify

the following:
— assembling orientation;
— groove assignment;
— oversize diameters;
— supplier identification;
— engine identification.

The choice of colour and acceptance conditions may be agreed upon between ring manufacturer and

customer.
7 General characteristics
7.1 Ring shape
[1] [2] [3] [4] [5] [6] [7]

Degrees of ovality only apply to rectangular rings, scraper rings, and keystone rings.

The following are forms of ovality:
— positive ovality, without code;
— round, Code Z;
— negative ovality, Code Y.
Values are given in Table 2.
Table 2 — Ovality
Dimensions in millimetres
a b
Nominal diameter Positive ovality Round Negative ovality
d Code: Z Code: Y
30 ≤ d < 60 0 ... +0,60 −0,30 ... +0,30 −0,60 ... 0
60 ≤ d < 100 +0,05 ... +0,85 −0,35 ... +0,35 −0,70 ... 0
100 ≤ d < 150 +0,10 ... +1,10 −0,45 ... +0,45 −0,95 ... −0,05
150 ≤ d ≤ 200 +0,15 ... +1,35 −0,50 ... +0,50 −1,10 ... −0,10

For taper-faced coated and uncoated rings with lapped land, the recommended ring shape is round.

Not applicable for material Class 10 of ISO 6621-3.
© ISO 2015 – All rights reserved 7
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ISO 6621-4:2015(E)
7.2 Light tightness

At least 90 % of the circumference of the piston ring peripheral surface shall be light-tight.

At least 95 % of the circumference on the peripheral surface of a taper-faced ring with plated/coated or

nitrided and ground peripheral surface shall be light-tight.

100 % of the circumference on the peripheral surface of the following piston ring designs shall be light-

tight:

— piston rings with machined land over the whole circumference of the peripheral surface;

— taper-faced piston rings with machined land over the whole circumference of the peripheral surface.

In the case of piston rings with a treated surface, the light tightness is normally measured prior to

surface treatment. When it is checked after treatment, rotation of the ring in the gauge will be required.

In the case of rings with negative point deflection, visible light is permitted at the butt ends but should

be confined to the angle θ as defined in ISO 6621-2.
7.3 Closed gap

Whenever the selected closed gap differs from that given in the dimensional tables of the relevant

International Standard, Table 3 shall apply and the tolerances shall remain the same.

Table 3 — Closed gap
Dimensions in millimetres

Code S005 S010 S015 S020 S025 S030 S035 S040 S045 S050 S055 S060 S070 S080 S090 S100

Closed gap 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40 0,45 0,50 0,55 0,60 0,70 0,80 0,90 1,00

7.4 Tangential force, F , and diametral force, F , of single piece piston rings
t d

NOTE The individual types of piston rings are given in Reference [1] to Reference [8]. The definitions of F

and F are given in ISO 6621-2.
7.4.1 Calculation of F and F values in dimension tables of dimensional standards
t d

The tangential and diametral forces of piston rings are tabulated in the dimension tables of the

dimensional standards.
The values are calculated for the following:
— the basic feature of each piston ring type;
— nominal radial wall thickness a , and mean ring width h or h ;
1 1 3
— piston rings made of cast iron with a modulus of elasticity of 100 GN/m
2 2
(100 GN/m = 100 000 MPa = 100 000 N/mm );
— piston rings made of steel with a modulus of elasticity of 210 GN/m ;
— ratio of total free gap to nominal diameter [md −a ], according to Table 4.

NOTE The calculation of tangential forces and diametral forces of rectangular rings made of steel (ISO 6622-2)

is based on a theoretical contact pressure of 0,16 ± 0,01 N/mm . The calculation of tangential forces and diametral

forces of keystone rings (ISO 6624-3) and half keystone rings (ISO 6624-4) made of steel is based on the same

ratio md()−a used for rectangular rings (ISO 6622-2). The ratio md()−a for rings made of steel is quite

11 11

different from the values given in Table 5 for rings made of cast iron and depends on the nominal diameter and the

special radial wall thickness. This radial wall thickness is not in a constant ratio to nominal diameter because

there are steps of wall thickness which belong to a range of nomi
...

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