ISO 6281:2020

INTERNATIONAL ISO
STANDARD 6281
Second edition
2020-02
Plain bearings — Testing under
conditions of hydrodynamic and
mixed lubrication in test rigs
Paliers lisses — Essai des paliers lisses dans les conditions de
lubrification hydrodynamique et mixte dans des machines d'essai
pour paliers
Reference number
ISO 6281:2020(E)
©
ISO 2020

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ISO 6281:2020(E)

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© 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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Published in Switzerland
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ISO 6281:2020(E)

Contents Page
Foreword .v
1 Scope . 1
2 Normative references . 1
3 Terms and definition . 1
4 Symbols . 1
5 Test objectives for bearing properties . 2
6 Test rigs . 3
6.1 General recommendations. 3
6.2 Generic types of test rig . 4
7 Test procedures . 6
7.1 General . 6
7.2 Running-in ability . 6
7.3 Wear resistance . 7
7.4 Compatibility between bearing and journal material (resistance to adhesion) . 7
7.5 Embeddability (foreign particles absorption) . 7
7.6 Resistance to journal scoring and abrasion . 7
7.7 Conformability . 7
7.8 Deformability (compressive strength) . 7
7.9 Resistance to erosion (cavitation erosion, fluid erosion, particle erosion) . 7
7.10 Static load carrying capacity . 8
7.11 Dynamic load carrying capacity (fatigue strength) . 8
7.12 Friction characteristics . 8
7.13 Lubricant flow rate characteristics. 8
7.14 Temperature increase characteristics . 8
8 Testing and test report . 8
8.1 General . 8
8.2 Test rig . 9
8.3 Measuring equipment . 9
8.3.1 General. 9
8.3.2 Measuring equipment for independent variables (which can also be time-
dependent) . . 9
8.3.3 Measuring equipment for dependent variables (which can also be time-
dependent) . . 9
8.4 Test bearing and journal .10
8.4.1 General.10
8.4.2 Test bearing .10
8.4.3 Journal . .11
8.5 Lubricant and lubricant supply method .11
8.5.1 Lubricant .11
8.5.2 Lubricant supply method .12
8.6 Operating conditions (test conditions) .12
8.6.1 General.12
8.6.2 Bearing load .12
8.6.3 Sliding velocity.12
8.6.4 Lubricant supply condition .12
8.6.5 Ambient conditions .13
8.7 Test results.13
8.7.1 General.13
8.7.2 Hydrodynamic parameters (including estimated results) .13
8.7.3 Assessment of bearing performance .14
8.8 Discussion of test results and remarks .14
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Bibliography .15
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ISO 6281:2020(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 123, Plain bearings, Subcommittee SC 2,
Materials and lubricants, their properties, characteristics, test methods and testing conditions.
This second edition cancels and replaces the first edition (ISO 6281:2007), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— editorial changes;
— reference to ISO 9045 has been replaced by ISO 4378 (all parts);
— Clause 1 has been edited to be in line with ISO drafting rules;
— the unit of the dynamic viscosity in Clause 4 has been modified.
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.
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INTERNATIONAL STANDARD ISO 6281:2020(E)
Plain bearings — Testing under conditions of
hydrodynamic and mixed lubrication in test rigs
1 Scope
This document defines requirements for the testing of lubricated plain journal bearings in test rigs,
running under conditions of hydrodynamic or mixed lubrication, during bearing and/or material
development. It deals with both static and dynamic loading in solid and multi-layer journal bearings. It
is not applicable to the testing of dynamic characteristics of lubricant film in journal bearings applied in
calculation of vibration and stability of turbo-rotors.
NOTE It is intended that further details of test procedures be established when carrying out testing
based on this document.
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 4378 (all parts), Plain bearings — Terms, definitions, classification and symbols
3 Terms and definition
For the purposes of this document, the terms and definitions given in ISO 4378 (all parts) 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
wear rate
ratio of wear extent to the time interval during which it has developed
3.2
wear intensity
ratio of wear extent to the specified distance on which wear developed or to the volume of the work done
4 Symbols
See Table 1.
Table 1 — Symbols
Symbol Description Unit
a length of period s
B bearing width mm
F bearing load N
F* bearing load per unit bearing width N/mm
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Table 1 (continued)
Symbol Description Unit
f coefficient of friction of journal bearing —
t time s
U sliding velocity m/s
β direction of bearing load °
η dynamic viscosity of lubricant Pa ⋅ s
ω angular velocity rad/s
5 Test objectives for bearing properties
The test objectives for plain journal bearing test rigs operating under conditions of hydrodynamic or
mixed lubrication are to obtain information, among others, on the following bearing properties, which
can serve as critical variables when designing and applying the bearing (see the ISO 4378 series):
a) the running-in ability;
b) the wear resistance;
c) the compatibility between bearing and journal materials (resistance to adhesion);
d) the embeddability (foreign particles absorption);
e) the resistance to journal scoring and abrasion;
f) the conformability;
g) the deformability (compressive strength);
h) the resistance to erosion (cavitation erosion, fluid erosion, and particle erosion);
i) the static load carrying capacity;
j) the dynamic load carrying capacity (fatigue strength);
k) the friction characteristics;
l) the lubricant flow rate characteristics;
m) the temperature increase characteristics.
Of these bearing properties, the first group, a) to h), depends primarily on the mechanical and
tribological properties of sliding materials under specified conditions. The second group, i) to m),
depends primarily on hydrodynamic variables, and therefore also on
— viscosity as a function of temperature, pressure and shear rate,
— energy dissipation in the lubricant film (shear heating and heat dissipation), and
— elastic and thermal deformation of the bearing and journal, and hence change of lubricant film
thickness (thermo-elastohydrodynamic lubrication).
The determination of these bearing properties, or test objectives, requires lubrication conditions that
can involve boundary, mixed or hydrodynamic lubrication — the three modes of lubrication regime. In
certain cases, a repeated, time-dependent change between mixed and hydrodynamic lubrication can be
required.
NOTE It is possible that specific test methods do not yet exist for all of the above-mentioned bearing
properties.
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ISO 6281:2020(E)

Figure 1 depicts the typical relation between the dimensionless number ηU/F* representing a
normalized speed, and the coefficient f of friction of the journal bearing, where η, U and F* denote
dynamic viscosity of the lubricant, sliding velocity and bearing load per unit bearing width (F* = F/B),
respectively. It shows the three regimes of boundary, mixed and hydrodynamic lubrication and
qualitatively indicates the dependence between these important parameters.
Key
a
Boundary lubrication.
b
Mixed lubrication.
c
Hydrodynamic lubrication.
Figure 1 — Three modes of lubrication regime
6 Test rigs
6.1 General recommendations
It is often more practical and efficient to investigate the bearing in a test rig than in an actual application.
The design of the bearing test rig should be such as to simulate as far as possible all the relevant
characteristic parameters (e.g. geometric, dynamic, hydrodynamic, thermal, and thermodynamic) of
the actual application.
In addition, the following is recommended for the test rig.
a) A simple mechanical construction.
b) Simple dismantling and assembly procedures for the test objects; with well-defined positioning
of the bearing and housing; preferably it should be possible to inspect the test bearing in situ.
In addition, the test rig should be equipped with an emergency stop mechanism, both for safety
reasons and to allow the inspection of the sliding surface before the onset of catastrophic damage.
c) A well-defined dimensions for the test bearing.
d) A high dimensional stability with little shaft deflection. The test rig should be as rigid as possible,
with a high natural frequency. In special cases, however, it can be necessary to vary the dimensional
stability or the shaft deflection in order to simulate the operating condition of the actual application.
e) An appropriate lubricant supply condition. When the lubricant flow within the bearing clearance
needs to be simulated exactly, the circumferential and axial position of the lubricant supply in the
test rig should be the same as in the actual application.
f) Well-defined and experimentally verifiable lubrication conditions.
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g) The regime of laminar or turbulent flow should be the same in the test rig and in the actual
application.
h) The rig should replicate as far as possible the temperature and stress range that can occur in
practice.
i) Appropriate measuring techniques or equipment should be employed.
6.2 Generic types of test rig
Generic types of test rig for plain journal bearings are shown in Figure 2 and Figure 3. Figure 2 a) and
Figure 2 b) depict the rotational motion of the journal, where a combination of both is also possible.
In practice, many more patterns of journal motion other than rotation can occur, such as inclination,
bending, axial, conical and their combinations. In addition, the bearing itself can rotate, oscillate or
even move in space instead of, or together with, the journal, as with a crank-pin bearing. In any case, the
relative motion of the journal to the bearing shall be known (measurable) exactly. However, constant
rotational speed of journal and the parallel movement of journal to bearing are the simplest and most
preferable for testing.
Figure 3 shows patterns of the bearing load. In the case of statically loaded journal bearing [Figure 3 a)],
the magnitude, F, and the direction, β, of the bearing load are constant. In a special case of dynamically
loaded bearing, F is constant, but β increases or decreases with time [Figure 3 b)]. In the general case of
dynamically loaded bearing [Figure 3 c)], both or at least one of F and β change(s) with time, while the
remaining variable can be constant. The change of form of F (also β) is then arbitrary, such as sinusoidal
with or without constant offset, curving steeply up and downwards, as, for example, in engine bearing
loading.
With regard to the loading of the test bearing, it is often more practical to load the test bearing directly
supported by the journal [Figure 4 a)], than to load the test bearing indirectly through the journal
[Figure 4 b)]. For static loading, a dead weight system, with or without lever, or hydraulic or pneumatic
actuation can be used. For dynamic loading, a rotating or vibrating mass system, with or without lever,
an electromagnetic exciter, and hydraulic actuation, can be applied. Dynamic loading by means of a
mass fixed to the journal seems to be simple, but the amplitude of the bearing load is then determined
primarily by the rotational speed of the journal. Therefore, it is not easy to change the load amplitude
independently of the rotational speed. Furthermore, the magnitude and direction of the bearing
load shall be precisely measured, and it is important to let the journal move freely inside the bearing
clearance without hindrance from the loading mechanism.
Besides such bearing test rigs operating under hydrodynamic or mixed lubrication, as described above,
many other kinds of test apparatus and test methods may be used to investigate the tribological or
mechanical properties of bearing materials, including coefficient of friction, mechanical strength,
hardness, elasticity, plasticity and bond strength. The study of the tribological properties of boundary
films has also led to the development of other test apparatus and methods; these are, however, outside
the scope of this document (see ISO 4384-1, ISO 4384-2, ISO 4385, ISO 7148-1, ISO 7148-2, ISO 7905-2,
ISO 7905-3, and ISO 7905-4).
NOTE The testing of the resistance to corrosion of bearing materials by the lubricant is the subject of
ISO 10129.
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a)  Rotation b)  Oscillation
Figure 2 — Rotational motion of a journal
a)  Static load
b)  Dynamic load (rotating load)
c)  Dynamic load (arbitrary pattern)
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Key
a length of period
F bearing load
t time
β direction of bearing load
ω angular velocity
Figure 3 — Examples of bearing load patterns
a)  Load on bearing b)  Load on journal
Key
F bearing load
ω angular velocity
1 test bearing
2 journal
Figure 4 — Two modes of load application
7 Test procedures
7.1 General
The actual test procedure depends on the property to be determined. It is important to establish the
test conditions in order to ensure that test results obtained on test rigs are applicable in practice and
that results obtained on different test rigs are mutually compatible.
In the following, examples of test procedures for obtaining the bearing properties according to Clause 5,
a) to m) are described together with the evaluation of the results. Bearing properties given in 7.2 to 7.9
depend primarily on mechanical and tribological characteristics of the bearing material itself, and in
some cases, may be determined qualitatively by proper material testing. However, they can be evaluated
quantitatively only by testing in bearing test rig. When stepwise increase or decrease of bearing load
or severity of operating condition is prescribed, thermal equilibrium shall be achieved in the test object
at each step to ensure reproducibility of the results. During the test, it is important to be aware of the
eventual change of the test object itself, even under seemingly constant operating conditions, such as
through wear, foreign particles embedding, diffusion, chemical reaction, and lubricant degradation.
This should be checked and documented in the test report.
7.2 Running-in ability
The change of surface topography, roughness, friction torque, wear rate or wear intensity of the
bearing, or the temperature of the lubricant and/or bearing should be measured from the initial state
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of the sliding surfaces under the specified operating condition. From the characteristic change of these
variables with time, the completion of running-in process can be detected. The shorter the time until
running-in is completed, the higher the running-in ability.
7.3 Wear resistance
The severity of the operating condition of the bearing should be increased until wear occurs. Wear can
be mechanical or mechano-chemical in nature. The former can be adhesive wear, seizure, scoring or
scratching, abrasion, fatigue wear, spalling, cavitation wear, erosive wear, or fretting wear. The latter
can be oxidative wear, fretting corrosion, or electro-erosive wear. The more severe the operating
condition under which wear begins to occur and the smaller the wear rate and/or the wear intensity,
the higher the wear resistance.
7.4 Compatibility between bearing and journal material (resistance to adhesion)
The frictional torque and/or the temperature of the lubricant and bearing should be measured
during the stepwise increase in the severity of the operating condition (i.e. increase in inlet lubricant
temperature, specific bearing load, sliding velocity), and the occurrence of adhesion should be detected.
The more severe the operating condition under which the adhesion begins to occur, or the less the
sliding surface suffers adhesion damage, the higher the compatibility and the resistance to adhesion.
7.5 Embeddability (foreign particles absorption)
Foreign particles of known hard material (i.e. hardness, quantity, and size) should be mixed with the
lubricant, and the quantity and depth to which the foreign particles have embedded into the bearing
surface in a specified time, together with the grade of damage of the journal surface, should be
measured under the specified operating condition. The larger the quantity and the greater the depth to
which the foreign particles have embedded, or the less the damage of the journal surface by the foreign
particles, the higher the embeddability.
7.6 Resistance to journal scoring and abrasion
The severity of the operating condition of the bearing should be increased stepwise and the occurrence
of the journal scoring (severe scratches) or abrasion checked. The more severe the operating condition
under which the damage begins to occur and the less the scoring and abrasion caused to the journal
(or the smaller the wear rate and the wear intensity), the higher the resistance to journal scoring and
abrasion.
7.7 Conformability
The bearing load should be increased stepwise under a specified operating condition such that a high
specific local load or edge load is applied to the bearing, which in consequence deforms elastically and
plastically towards the form of the journal. The more the bearing deforms without showing any other
bearing damage, or the higher the grade of similarity of form of the sliding surfaces reached, the greater
the conformability.
7.8 Deformability (compressive strength)
The specific bearing load should be increased stepwise under a specified operating condition until
the compressive strength of the bearing material is almost reached. The higher the deformation of the
bearing, the greater the deformability.
7.9 Resistance to erosion (cavitation erosion, fluid erosion, particle erosion)
The bearing should be run under a specified erosive operating condition until a predetermined quantity
of damage by erosion is detected. The longer the time or sliding distance until damage is detected and
the more severe the operating condition (i.e. higher specific bearing load, temperature, and sliding
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velocity), the greater the resistance to erosion
...