ISO 15113:1999
(Main)Rubber — Determination of frictional properties
Rubber — Determination of frictional properties
Caoutchouc — Détermination des propriétés frictionnelles
General Information
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Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 15113
First edition
1999-05-01
Rubber — Determination of frictional
properties
Caoutchouc — Détermination des propriétés frictionnelles
A
Reference number
ISO 15113:1999(E)
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ISO 15113:1999(E)
Contents
Page
1 Scope .1
2 Normative references .1
3 Terms and definitions .1
4 Principle.2
5 Apparatus .3
6 Test surfaces.3
7 Preparation.4
8 Conditioning.6
9 Test parameters .6
10 Cleaning or renewal of the test track.7
11 Procedure A (initial friction measurements).7
12 Procedure B (service behaviour).7
13 Procedure C (tests with added lubricants or contaminants) .8
14 Stick-slip.8
15 Presentation of results.8
16 Test report .11
Annex A (informative) Design principles .12
Annex B (informative) Ball-on-flat geometry .14
Annex C (informative) Static friction and "stiction".15
Annex D (informative) Other parameters .16
Bibliography.19
© ISO 1999
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ISO ISO 15113:1999(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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.
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.
International Standard ISO 15113 was prepared by Technical Committee ISO/TC 45, Rubber and rubber products,
Subcommittee SC 2, Physical and degradation tests.
Annexes A to D of this International Standard are for information only.
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Introduction
Various geometrical arrangements can be used when measuring friction, but each is likely to give a different value
for m, the coefficient of friction. Each may be appropriate in particular circumstances, but it is desirable that some
standard method utilizing specified test conditions be employed when comparisons between materials are
undertaken.
Rubber samples are most readily available in sheet form, and for many practical applications measurement
between two planar surfaces most nearly approaches service behaviour. Consequently, this is the most widely used
geometry. For this geometry, the apparatus used needs careful design in order to ensure reproducible contact
between the surfaces, and this is discussed in annex A.
Where rubber moulding facilities are available, some workers prefer to use a hemispherical rubber slider and a
planar test track. This gives a more definable contact area and minimizes the errors involved if the friction plane
does not contain both the line of action of the load cell and that of the towing force. However, when this geometry is
used, the frictional force is not proportional to the normal load (see annex B), and the contact area is estimated from
a knowledge of the modulus of the rubber. Hence care should be taken when quoting values for coefficients of
friction. The big disadvantage of the method is that special test pieces need to be moulded from unvulcanized
rubber, and rubber products cannot be accommodated. Finally, since some degree of wear is inseparable from
friction, extended testing will produce a "flat" on the hemispherical test piece. Frequent inspection of the test surface
is recommended, therefore, to ensure that the initial contact geometry is maintained.
The alternative "ball on flat" geometry where a hard ball slides on a flat rubber surface is not an exact equivalent.
The ploughing action of the ball through the rubber results in an energy loss by hysteresis which gives a higher
measured coefficient of friction. However, in some circumstances this may be an appropriate test procedure.
Although there may be some uncertainty in the contact area using plane-on-plane geometry, this International
Standard is based on this geometry because of its wide practical applicability. However, it is emphasized that it is
necessary to have a well designed apparatus with the line of action of the load cell included in the plane of contact
of the test pieces (see annex A). The method can be adapted to cover other contact geometries to suit particular
products, including the ball-on-flat geometry set out in annex B.
This International Standard is based on linear motion, and guidance on the experimental arrangement is given in
annex A. Because friction generates heat, it is usual to restrict testing to velocities typically below 1000 mm/min in
order to avoid a large temperature rise at the interface. If service conditions involve high speeds, then an entirely
different method based on rotary motion is more appropriate as discussed in annex A. The method of test set out
here enables kinetic friction to be measured at a number of fixed velocities. It can be arranged that the lowest
velocity is such that movement is barely discernible, and this gives an approximation to frictional behaviour close to
zero velocity (static friction). This may be different from the starting friction, which may involve some element of
adhesion (stiction) as discussed in annex C. This method is suitable for measuring the initial friction only if the
machine has a constant-rate-of-load facility and a sufficiently compliant load cell. A discussion on static friction and
the correct approach to its measurement is given in annex C.
Rubber friction is complex, and the coefficient of friction is dependent on the contact geometry, normal load, velocity
and temperature, as well as the composition of the rubber. A discussion of the influence of these parameters and
some other factors which affect measurement is presented in annex D.
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INTERNATIONAL STANDARD ISO ISO 15113:1999(E)
Rubber — Determination of frictional properties
1 Scope
This International Standard outlines the principles governing the measurement of coefficient of friction and
describes a method suitable for measuring the coefficient of friction of a rubber against standard comparators,
against itself, or against any other specified surface.
2 Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of
this International Standard. For dated references, subsequent amendments to, or revisions of, any of these
publications do not apply. However, parties to agreements based on this International Standard are encouraged to
investigate the possibility of applying the most recent editions of the normative documents indicated below. For
undated references, the latest edition of the normative document referred to applies. Members of ISO and IEC
maintain registers of currently valid International Standards.
NOTE Those documents to which reference is made in the text but which are only informative in nature, are listed in the
bibliography.
ISO 471:1995, Rubber — Temperatures, humidities and times for conditioning and testing.
ISO 3383:1985, Rubber — General directions for achieving elevated or subnormal temperatures for test purposes.
ISO 5893:1993, Rubber and plastics test equipment — Tensile, flexural and compression types (constant rate of
traverse) — Description.
3 Terms and definitions
For the purposes of this International Standard, the following terms and definitions apply:
3.1
coefficient of friction
the ratio of the frictional force opposing motion between two surfaces to the normal force between the surfaces
under specified test conditions
NOTE Coefficient of friction is dimensionless and its value is not restricted to numbers less than unity.
3.2
area of contact
the whole of the apparent area made between the two test surfaces (test track and test piece)
NOTE The real area of contact (see 3.3) may well be less than this.
3.3
real area of contact
the sum total of the minute contact areas at which the two test surfaces touch
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3.4
velocity of test
the velocity with which one surface is driven relative to the other
NOTE If stick-slip (see 3.5) occurs, this will then be the mean velocity with which one surface moves relative to the other.
3.5
stick-slip
a condition in which the actual velocity between the surfaces oscillates between two extremes about the test
velocity, resulting in corresponding oscillations in the measured frictional force
3.6
test track
the surface against which the rubber is to be tested
NOTE The test track may be made of the same material as the rubber under test or it may be different.
3.7
temperature of test
the temperature of the test apparatus and its environment
NOTE Since friction generates heat, this may differ from the actual temperature of one or both of the test surfaces.
3.8
lubricant
a substance introduced between two surfaces to lower the coefficient of friction
NOTE A lubricant is usually a liquid, but in some circumstances solid powders are used, e.g. talc. Usually, lubricants are
introduced deliberately.
3.9
contaminant
any substance present on either test surface not of the same composition as that surface
NOTE A contaminant may act as a lubricant. Usually, in service, contaminants are introduced inadvertently.
3.10
stiction
the force needed to move one surface over another when the external normal load is reduced to zero
NOTE This is an apparent frictional force, but no coefficient of friction can be calculated since the normal force is zero. See
annex C.
3.11
static friction
the frictional force needed to start motion (i.e. the frictional force at zero velocity)
NOTE Where there is an external normal load, a coefficient of static friction can be calculated. Static friction often involves
some element of stiction. See annex C.
4 Principle
Two test surfaces are brought together under the action of a measured normal load. A mechanism slides one of the
surfaces over the other at a measured velocity, and the force opposing motion is monitored and recorded. The ratio
of this frictional force to the normal load at any instant is the coefficient of friction at that time. Since the test itself will
alter the surfaces and may change the temperature at the interface, the measured coefficient of friction may change
as the test proceeds.
In an ideal apparatus, the line of action of the force-measuring equipment will lie in the plane of the two contacting
surfaces. This may be either a horizontal or a vertical plane.
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5 Apparatus
5.1 Device, with provision for attaching two friction surfaces and capable of providing linear motion between the
surfaces for a distance of typically 100 mm at a number of fixed velocities, typically between 0,5 mm/min and
1000 mm/min. This may be a dedicated device or, alternatively, a tensile-testing machine may be adapted for the
purpose.
5.2 Means of providing several measured normal loads between the surfaces within the range 1 N to 200 N.
When the test track is horizontal, suitable weights may be used directly to provide the normal load, but on a
machine with a vertical test track it will be necessary to use a bell crank lever system to convert the vertical
gravitational force into a horizontal normal force.
5.3 Series of load cells or, alternatively, a load cell with multiple ranging, conforming to grade A as defined in
ISO 5893:1993, fitted with a means of recording the output and fastened to one of the friction surfaces, with ranging
or other means of indicating the frictional force to an accuracy of – 1 % throughout the range of measurement.
NOTE Corresponding to the range of normal loads stated in 5.2, the measured frictional forces are likely to be within the
range 0,1 N to 1 kN.
5.4 Environmental cabinet (if the effects of temperature are to be studied), to contain the apparatus and the two
surfaces under test (but not the load cell), with a means of measuring and recording the temperature to an accuracy
of – 0,5 °C. The environmental chamber shall not make physical contact with any moving parts.
NOTE 1 The exclusion of a condensation-forming atmosphere from the test environment is extremely difficult, and the
formation of ice crystals or particles or films on the test surfaces can only be assessed visually.
NOTE 2 To avoid the formation of ice when testing at temperatures at or below 0 °C, a very dry atmosphere (e.g. 5 % to
10 % r.h.) is needed.
5.5 Means of avoiding stick-slip, as the whole apparatus (including the load cell) needs to be as stiff as possible.
All connections shall be made with rods and not with wire. Where an apparatus is designed to be attached to a
tensile-testing machine, then a machine with a high degree of stiffness shall be chosen. In practice, this means a
tensile-testing machine with a load capacity some 20 times greater than the maximum frictional force being
measured.
5.6 Means of separating the surfaces under test, for use when the apparatus is reset to its initial position after
each measurement. This is necessary because friction is very dependent on the history of the surfaces.
NOTE Separation of the surfaces may be carried out manually or automatically.
6 Test surfaces
6.1 General
For each test, two prepared surfaces shall be used, one manufactured from the rubber under test and the other (the
test track) made either from this same rubber or, alternatively, from a specified material.
6.2 Test track
The test track shall be approximately planar but may have a patterned surface.
The material used to form the test track shall be larger in both linear dimensions than the test pieces (see 6.3). The
longer dimension shall be sufficient to allow a linear travel of at least 50 mm.
The test track may be any surface agreed between the interested parties, but where comparisons have to be made
it may be more appropriate to select one of the following:
a) The rubber under test with the surface moulded, split or buffed.
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b) Float glass with the surface either polished or ground.
c) A specified stainless steel with the surface either polished or ground.
d) Cast iron with the surface ground to a specified finish.
e) Resin-bonded paper of specified grit size.
NOTE The measured coefficient of friction will depend not only on the material chosen but also on the surface finish of the
test track (see clause D.1).
6.3 Test pieces
Either moulded test pieces or test pieces cut from products may be used. Three test pieces shall be tested.
When planar test pieces are used, they shall be of smaller dimensions than the test track selected (see 6.2), so that
it is possible to obtain linear motion between the two surfaces for at least 6 s, while maintaining contact (apparent
contact) over the whole of the rubber surface throughout the test.
Test pieces shall normally be of a thickness between 1 mm and 8 mm. When the test surface is thinner than this, it
shall be mounted on a support of adequate thickness using adhesive.
NOTE 1 In some circumstances, the contact area may be affected by the modulus of the underlying support, and it is then
advisable to match as closely as possible the modulus of the test surface to that of the support.
The test piece shall not be stretched during mounting.
Any adhesive used shall not unduly swell or otherwise adversely affect the test piece.
Round off the leading edge of all planar test pieces to avoid buckling or digging in of this front edge.
NOTE 2 To reduce the possibility of stick-slip, it is advisable to keep the thickness of low-modulus test pieces below 4 mm.
When a test piece is made from a product, it may not be possible to cut a planar piece of adequate size. A suitable
test piece may than be fabricated by mounting a number of small pieces cut from the product (for example, lengths
of a windscreen-wiper blade may be mounted so that the wiping surfaces define a plane). Three small pieces,
mounted at the corners of a triangle, will always define a plane. A greater number than this will need more careful
mounting or perhaps additional preparation by buffing or abrasion. Alternatively, it may be better to use a different
test geometry as discussed in annex A.
7 Preparation
7.1 General
Materials may be tested as received, but where comparisons are to be made it is advisable to bring the surfaces to
some standard condition. Texture is important since, in general, rough surfaces have a lower coefficient of friction
than smooth surfaces when dry and a higher coefficient of friction than smooth surfaces when wet. Thus different
coefficients of friction will be observed depending on the method of preparation used.
7.2 Surface texture
A test track made of float glass or mirror-finished metal shall be cleaned without other treatment (see 7.3). Other
surfaces that need to be abraded shall be machine-ground, buffed or abraded by hand against resin-bonded paper
of specified grit size.
Generally, test pieces prepared by splitting on a leather-splitting machine will need no further preparation other than
cleaning.
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7.3 Surface cleaning
Where contamination occurs in service and forms part of the agreed test conditions (see clause 13), then it may be left,
but where contamination is the result of the preparative procedure it shall, as far as possible, be removed.
It has to be recognized that complete removal of contaminants is not always possible, and sometimes the coefficient
of friction is permanently altered by the residual contamination. For example, complete removal of silicone oil is
rarely possible. For this reason, preparative techniques shall be chosen with great care. Where lubricants are
needed for any abrasive, these should preferably be water-based rather than oil-based. Similarly, when mounting
test pieces with adhesive great care shall be taken to keep the test surface free from adhesive. Care shall be taken
not to contaminate the test surface with finger grease.
Where contamination has occurred, proceed as follows:
Blow all loose debris from the surface using a jet of clean, dry air or similar gas.
NOTE 1 A compressed-air line is not suitable as the air is usually wet and contaminated with oil.
Alternatively, brush debris away using a clean, dry, soft brush.
When the contaminants, such as grease, cling to the surface, select a suitable solvent from the following list:
a) distilled water plus a small amount of detergent;
b) distilled water only;
c) tap water;
d) ethyl alcohol;
e) isopropanol;
f) acetone;
g) butanone;
h) perchloroethylene;
i) toluene.
The chosen solvent shall not dissolve or unduly swell the surface being cleaned.
NOTE 2 Health and safety regulations apply to the use of some of these solvents.
In general, high-purity solvents are needed since it is only too easy to spread further contamination by using impure
solvents. Similarly, any cloth or paper used for cleaning shall not be allowed to contact the neck of the storage
vessel in order to avoid contaminating the solvent in the vessel.
Wet, with a little of the solvent, a piece of lintless cloth or tissue (which shall be unaffected by the solvent) and wipe
the test surface in one direction only. Discard the cloth or tissue. Repeat this procedure twice more using a fresh
tissue and fresh solvent.
When distilled water plus detergent has been used, remove the detergent by wiping the surface three times more
with distilled water only.
In locations where the tap water is known to be very pure, rinsing under the tap may alternatively be used.
NOTE 3 Where contaminants are water-soluble, this is undoubtedly the most efficient cleaning procedure.
Drain off any excess solvent by holding the surface in a vertical plane, and allow the surface to dry in the air.
Handle the test pieces by the edges only, wearing gloves to avoid contamination with finger grease. Do not place
the prepared surface in contact with any surface other than that against which it is to be tested.
Condition the test pieces in accordance with the procedure given in clause 8.
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8 Conditioning
8.1 Time lapse between vulcanization and testing
For all test purposes, the minimum time between vulcanization and testing shall be 16 h. For non-product tests, the
maximum time between vulcanization and testing shall be 28 days and, for evaluations intended to be comparable,
the tests shall, as far as possible, be carried out after the same time interval. For product tests, whenever possible,
the time between vulcanization and testing shall not exceed 90 days. In other cases, tests shall be made within
60 days of the date of receipt of the product by the customer.
8.2 Protection of samples and test pieces
Samples and test pieces shall be protected as completely as possible from all external influences likely to cause
damage or contamination during the interval between vulcanization and testing, e.g. light, heat, dust.
[1]
NOTE Additional guidance is given in ISO 2230 .
8.3 Conditioning of test pieces
Condition prepared test pieces for a minimum of 3 h at 23 °C – 2 °C.
If preparation includes buffing, the time interval between buffing and testing shall be not less than 16 h and not more
than 72 h.
For tests at temperatures other than 23 °C – 2 °C, condition the test pieces at the temperature at which the test is to
be conducted for a period sufficient to enable test pieces to attain substantial equilibrium in accordance with
ISO 471.
9 Test parameters
NOTE For guidance on the effect of test conditions on frictional behaviour, see annex D.
9.1 Temperature of test
The test shall be carried out at a temperature of 23 °C – 2 °C or at any other temperature(s), selected from those
listed in ISO 3383, agreed between the interested parties.
NOTE At temperatures below the dew-point, moisture will condense on the test surfaces, and this may seriously affect the
measured frictional force. To avoid this, it is necessary to conduct these tests in conditions of low humidity in an environmental
chamber. Similarly, at 0 °C ice may form and to avoid this a very dry atmosphere is needed.
9.2 Velocity of test
Unless otherwise agreed or specified, the test shall be carried out at a velocity of 50 mm/min.
NOTE Doubling or halving the velocity gives little change in friction. Test velocities should therefore preferably be chosen a
factor of 10 apart if a range is being studied. Frictional heating is greatest at higher velocities and, at values greater than
1000 mm/min, cannot be ignored.
9.3 Normal load
The normal load shall be chosen to suit the application.
NOTE In the absence of an application, the preferred normal loads are 10 N, 50 N and 100 N.
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10 Cleaning or renewal of the test track
The test track shall be cleaned or renewed each time a new test piece is used.
NOTE As testing proceeds, the surfaces change and friction may alter. It is not practicable to renew the test pieces after
each test in a series involving changes in load, velocity and temperature. However, it is advisable to renew or clean the test
track whenever the test piece is changed. This is the minimum procedure specified. The test track may be cleaned or renewed
more frequently than this in cases where this seems appropriate.
11 Procedure A (initial friction measurements)
11.1 If the test temperature is other than 23 °C – 2 °C, set the environmental chamber to the specified temperature
and allow the apparatus to reach this temperature.
11.2 Mount the test piece(s) in the apparatus, fixing them in place either mechanically or using adhesive. If tests
are to be conducted over a range of temperatures, the same method of fixing shall be used throughout the
temperature range.
11.3 Using packing pieces, or whatever means of adjustment is provided, adjust the test piece(s) so that the line of
action of the load cell and the line of action of the drive mechanism both lie in the plane of contact of the test piece
with the test track. If this is not possible, record the degree of offset to the nearest 0,5 mm.
11.4 Set the test piece(s) to the start position and select the velocity of test.
11.5 Select and apply the normal load.
NOTE With a horizontal apparatus, the normal load includes the weight of the test piece and its mounting.
Operate the mechanism which separates the surfaces.
11.6
11.7 Set the force-recording equipment to the required range (a force range approximately equal to the normal
load applied) and check the zero.
11.8 After the conditioning period has passed (see 8.3), bring the two test surfaces together and within 5 s start the
machine.
11.9 At the end of the test run, separate the surfaces again and return the machine to its starting position.
NOTE The return velocity need not be the same as the velocity of test.
11.10 Repeat steps 11.8 and 11.9 twice, making three measurements in all.
11.11 If the test track or test piece has a directional surface finish, then reverse the test track or test piece and
repeat steps 11.1 to 11.10.
11.12 If further measurements are to be made, then select the appropriate velocity, temperature and normal load
and repeat steps 11.1 and 11.10.
12 Procedure B (service behaviour)
12.1 As testing proceeds, the condition of the surface of the test piece and the test track change and the
coefficient of friction alters. If this corresponds with service behaviour, the later values of the coefficient of friction
may be more relevant than the early ones. Under these circumstances, proceed as given in 12.2 to 12.4.
12.2 Follow the procedure from 11.1 to 11.10.
12.3 Switch on the recording mechanism and repeat 11.8 to 11.9 50 times or as agreed between the interested
parties.
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12.4 Switch on the recording mechanism and repeat steps 11.8 to 11.10,
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