ISO 19973-1:2015
(Main)Pneumatic fluid power — Assessment of component reliability by testing — Part 1: General procedures
Pneumatic fluid power — Assessment of component reliability by testing — Part 1: General procedures
ISO 19973-1:2015 provides general procedures, the calculation method for assessing the reliability of pneumatic fluid power components and the methods of reporting. These procedures are independent of the kinds of components and of their design. ISO 19973-1:2015 also provides general test conditions and a method for data evaluation. NOTE Because the service life of any component is subject to variations, a statistical evaluation assists the interpretation of the test results. The methods specified in this part of ISO 19973 apply to the first failure without repairs (see IEC 60300‑3‑5), but exclude outliers; however, because outliers can be highly significant, information about how to deal with them is given in Annex F.
Transmissions pneumatiques — Évaluation par essais de la fiabilité des composants — Partie 1: Procédures générales
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INTERNATIONAL ISO
STANDARD 19973-1
Second edition
2015-08-01
Pneumatic fluid power — Assessment
of component reliability by testing —
Part 1:
General procedures
Transmissions pneumatiques — Évaluation par essais de la fiabilité
des composants —
Partie 1: Procédures générales
Reference number
ISO 19973-1:2015(E)
©
ISO 2015
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ISO 19973-1:2015(E)
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ISO 19973-1:2015(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and units of measurement . 3
5 Concept of reliability . 3
6 Strategies for conducting testing . 4
6.1 Assumptions . 4
6.2 Test stand and measurement of parameters . 4
6.3 Test planning . 4
7 Statistical analysis . 4
8 Test conditions . 4
9 Sample size and selection criteria. 5
10 End of test . 6
10.1 Minimum number of failures required . 6
10.2 Termination time of a test unit. 6
10.3 Termination life . 6
10.4 Suspended test unit . 6
10.5 Censored test . 6
11 Evaluation of reliability characteristics from the test data . 6
12 Test report . 8
13 Identification statement (reference to this part of ISO 19973) . 9
Annex A (normative) Determination of the termination life .10
Annex B (informative) Determination of threshold values for leakage rates.14
Annex C (informative) Calculation procedures for censored data without suspensions .21
Annex D (informative) Calculation procedures for censored data with suspensions .24
Annex E (informative) Verification of minimum life at a specified reliability and one-sided
confidence level .28
Annex F (informative) Dealing with outliers in test data .33
Annex G (informative) Examples of test results .39
Bibliography .44
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ISO 19973-1:2015(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 131, Fluid power systems.
This second edition cancels and replaces the first edition (ISO 19973-1:2007) which has been
technically revised.
ISO 19973 consists of the following parts, under the general title Pneumatic fluid power — Assessment of
component reliability by testing:
— Part 1: General procedures
— Part 2: Directional control valves
— Part 3: Cylinders with piston rod
— Part 4: Pressure regulators
— Part 5: Non-return valves, shuttle valves, dual pressure valves (AND function), one-way adjustable flow
control valves, quick-exhaust valves
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ISO 19973-1:2015(E)
Introduction
In pneumatic fluid power systems, power is transmitted and controlled through a gas under pressure
within a circuit. Pneumatic fluid power systems are composed of components and are an integral part
of various types of machines and equipment. Efficient and economical production requires highly
reliable machines and equipment.
It is necessary that machine producers know the reliability of the components that make up their
machine’s pneumatic fluid power system. Knowing the reliability characteristic of the component,
which can be determined from laboratory testing, the producers can model the system and make
decisions on service intervals, spare parts inventory and areas for future improvements.
There are three primary levels in the determination of component reliability:
a) preliminary design analysis: finite element analysis (FEA), failure mode and effect analysis (FMEA);
b) laboratory testing and reliability modelling: physics of failure, reliability prediction, pre-
production evaluation;
c) collection of field data: maintenance reports, warranty analysis.
Each level has its application during the life of a component. A preliminary design analysis is useful to
identify possible failure modes and eliminate them or reduce their effect on reliability. When prototypes
are available, in-house laboratory reliability tests are run and initial reliability can be determined.
Reliability testing is often continued into the initial production run and throughout the production
lifetime as a continuing evaluation of the component. Collection of field data is possible when products
are operating and data on their failures are available.
Specific component test procedures and exclusions are provided in ISO 19973-2, ISO 19973-3,
ISO 19973-4 and ISO 19973-5.
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INTERNATIONAL STANDARD ISO 19973-1:2015(E)
Pneumatic fluid power — Assessment of component
reliability by testing —
Part 1:
General procedures
1 Scope
This part of ISO 19973 provides general procedures, the calculation method for assessing the reliability
of pneumatic fluid power components and the methods of reporting. These procedures are independent
of the kinds of components and of their design.
This part of ISO 19973 also provides general test conditions and a method for data evaluation.
NOTE Because the service life of any component is subject to variations, a statistical evaluation assists the
interpretation of the test results.
The methods specified in this part of ISO 19973 apply to the first failure without repairs (see IEC 60300-
3-5), but exclude outliers; however, because outliers can be highly significant, information about how to
deal with them is given in Annex F.
2 Normative references
The following referenced 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 3534-1, Statistics — Vocabulary and symbols — Part 1: General statistical terms and terms used
in probability
ISO 5598, Fluid power systems and components — Vocabulary
ISO 6358 (all parts), Pneumatic fluid power — Determination of flow-rate characteristics of components
using compressible fluids
ISO 10099, Pneumatic fluid power — Cylinders — Final examination and acceptance criteria
ISO 19973-3, Pneumatic fluid power — Assessment of component reliability by testing — Part 3:
Cylinders with piston
ISO 80000-1, Quantities and units — Part 1: General
IEC 60050-191, International Electrotechnical Vocabulary, chapter 191: Dependability and quality of service
IEC 61649, Goodness-of-fit tests, confidence intervals and lower confidence limits for Weibull distributed data
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 3534-1, ISO 5598 and
IEC 60050-191 and the following apply.
3.1
catastrophic failure
failure of an item that results in its complete inability to perform all required functions
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ISO 19973-1:2015(E)
3.2
confidence coefficient
confidence level
value (1 − α) of the probability associated with a confidence interval or a statistical coverage interval
Note 1 to entry: See also 3.6.
Note 2 to entry: See ISO 3534-1 for notes related to this term and definition.
3.3
confidence limit
either of the limits, T or T , of the two-sided confidence interval, or the limit, T, of the one-sided
1 2
confidence interval
Note 1 to entry: See ISO 3534-1 for notes related to this term and definition.
3.4
failure
termination of the ability of an item to perform a required function
Note 1 to entry: In the ISO 19973 (all parts), the reaching of a threshold level for statistical calculation is also
considered a statistical failure (see Annex A).
[SOURCE: IEC 60050-191]
3.5
one-sided confidence interval
T
interval estimator for a parameter, Θ, comprised of the interval from the smallest possible value of the
parameter, Θ, up to T or the interval from T up to the largest possible value of Θ, where the probability
p(T ≥ Θ) or p(T ≤ Θ) is at least equal to (1 − α), where (1 − α) is a fixed number, positive and less than 1
Note 1 to entry: See ISO 3534-1 for notes related to this term and definition.
3.6
relevant failure
failure that should be included in interpreting test or operational results or in calculating the value of a
reliability performance measure
[SOURCE: IEC 60050-191]
3.7
reliability
probability that an item can perform a required function under given conditions for a given time interval
[SOURCE: IEC 60050-191]
3.8
sample
one or more test units taken from a population and intended to provide information on the population
Note 1 to entry: A sample can serve as a basis for a decision on the population or on the process that produced it.
3.9
sample size
number of test units in the sample
Note 1 to entry: In a multi-stage sample, the sample size is the total number of test units at the conclusion of the
final stage of sampling.
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ISO 19973-1:2015(E)
3.10
three-point moving average
3PMA
arithmetic average of three consecutive measured component’s test data
3.11
threshold level
value of a performance characteristic (for example, leakage, shifting pressure, stroke time, etc.) against
which the component’s test data is compared
Note 1 to entry: This is an arbitrary value defined by the experts as the critical value for performance
comparisons, but is not necessarily indicative of a component failure.
4 Symbols and units of measurement
4.1 The symbols used in this part of ISO 19973 are given in Table 1.
Table 1 — Symbol list
a
Symbol Definition
B Expected time at which 10 % of the population is predicted to fail (10 % of the lifetime
10
distribution)
(B ) B life at the one-sided 95 % confidence level
10 95 % 10
η Scale parameter (characteristic life) of the Weibull distribution
F(t) Probability of failure, expressed in percent
β Shape parameter (slope) of the Weibull distribution
R(t) Reliability of a component at time t ; 1-F(t)
t Life time expressed in time, cycles, or distance
a
Other symbols can be used in other documents and software.
4.2 Units of measurement are in accordance with ISO 80000-1.
5 Concept of reliability
For the purposes of this part of ISO 19973, reliability is the probability that a component does not have
a relevant failure for a specified interval of time, number of cycles or distance when it operates under
stated conditions.
A relevant failure occurs when
— component data, determined using the three-points moving average (3PMA), exceeds a threshold
level for the first time (see 10.2), or
— a component experiences a catastrophic failure (burst, fatigue or functional failure, etc.).
Threshold levels of the components covered by ISO 19973 (all parts) are specified in the component-
specific parts of this International Standard.
This probability can be determined by analysing the results of a series of tests and describing the
population failure by statistical methods. There are many different statistical distributions that
describe the population of failures that result from testing.
It is also possible to verify the minimum life of a component by the one-sided confidence estimation at a
specified reliability level. Examples are given in Annex E.
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ISO 19973-1:2015(E)
6 Strategies for conducting testing
6.1 Assumptions
The reliability of pneumatic components in an application depends on many environmental factors,
including pressure, temperature, dew point and contamination level of the compressed air, externally
imposed loads, duty cycle, etc. Any prediction of the reliability of an individual component, therefore,
shall take all of these environmental factors into consideration.
This part of ISO 19973 is based on a prescribed level of stress, test conditions and duty cycle that
reflects the best judgement of its developers to represent typical industrial conditions. It also includes
conditions that provide consistency in the test method. Thus, the results can be used as a reference that
a user can apply to judge against any other set of conditions.
In particular applications, the requirements of this part of ISO 19973 may be modified to suit a specific
stress level, test condition or duty cycle. However, such testing shall follow all of the other requirements
for test methods and data analysis specified in this part of ISO 19973.
6.2 Test stand and measurement of parameters
Two other important factors are the test stand and measurement of parameters. The test stand shall be
designed to operate reliably within the planned environmental conditions. Its configuration shall not
affect the results of the test being run on the component. Evaluation and maintenance of the test stand
during the reliability test program is critical. The accuracy of parameter measurement and control of
parameter values shall be within the specified tolerances to ensure accurate and repeatable test results.
6.3 Test planning
Proper test planning is essential in order to produce results that accurately predict the component’s
reliability under specified conditions. The goals and objectives of the test program shall be clearly
defined if a supplier and user agree to apply ISO 19973 (all parts).
7 Statistical analysis
The resulting test data shall be evaluated for assessing the reliability. One of the most commonly used
methods is the Weibull analysis because of its versatility in modelling various statistical distributions.
This method shall be used for the analysis of the test data to ensure comparability of the results.
Examples of applying Weibull analysis are given in the Annex C and Annex D.
NOTE Commercial software can be helpful for this purpose.
8 Test conditions
8.1 Testing shall be carried out in accordance with the provisions defined in the part of ISO 19973 that
relates to the component tested, including the test parameters that are measured and threshold levels
specified for each test parameter.
8.2 No repairs are permitted on the test units during the reliability test.
8.3 Unless otherwise specified in the relevant part of ISO 19973 that relates to the component being
tested, or when agreed between the user and supplier, all tests shall be carried out under the conditions
specified in Table 2.
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ISO 19973-1:2015(E)
Table 2 — General test conditions
Parameter Value
Test pressure 630 kPa ± 30 kPa (6,3 bar ± 0,3 bar)
Ambient temperature 23 °C ± 10 °C
Temperature of the medium 23 °C ± 10 °C
Filtration: nominal filtration rating 5 µm
a
Dryer: maximum inlet or test pressure dew point +7 °C
Lubrication None
a
Testing at dew points of less than −20 °C could result in shorter lifetimes.
8.4 Temperature changes due to thermodynamic processes while pressurizing and depressurizing test
units should be considered during the setup and initial running period of the first day. If the temperature
change of the test unit’s body exceeds ±20 °C during the initial running period, the test frequency should
be adjusted. Later adjustments of the test frequencies are not permitted.
8.5 During the endurance test, test units shall be operated continuously, and the measuring intervals
for recording data shall be determined taking into account the experience and judgement of the people
conducting the test. A measuring interval of one week is recommended.
8.6 Except for cylinders, the volume at outlet ports depends on the component’s sonic conductance, C,
as determined in accordance with ISO 6358 (all parts). The volumes shall meet or exceed the minimum
values given in Table 3.
NOTE During testing, the volumes at the outlet ports can become hot. It is necessary to take care to
protect personnel.
Table 3 — Minimum volume at the outlet ports, based on component’s sonic conductance
Sonic conductance Minimum volume
C at the outlet ports
3 3
dm /(s·kPa)(ANR) cm
C ≤ 0,004 2
0,004 < C ≤ 0,04 10
0,04 < C ≤ 0,12 25
0,12 < C ≤ 0,2 50
0,2 < C ≤ 0,4 100
C > 0,4 200
9 Sample size and selection criteria
9.1 The samples shall be representative of the population and shall be selected randomly.
9.2 The minimum sample size shall be seven test units.
NOTE It is important that the sample has at least seven test units in order that the first data point on the
Weibull graph is below the 10 % cumulative-failure point. This allows a more accurate projection of the lower
confidence limit lines to intersect the 10 % cumulative-failure point and determine a B life.
10
9.3 For a product series with the same design principle, it is not necessary to test all types or sizes.
However, the test program shall include the type with the most critical conditions, for example, highest
stress caused by velocity or load.
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ISO 19973-1:2015(E)
10 End of test
10.1 Minimum number of failures required
The minimum number of test units that are required to fail (e.g. reach a threshold level) is described in
Table 4. This number does not include suspensions, which are not considered failures.
NOTE It is desirable to achieve at least 10 failures in accordance with IEC 61649. Fewer failures result in a
wider confidence interval and a shorter (B ) life at the lower confidence limit.
10 95 %
Table 4 — Minimum number of failures for evaluation of the characteristic life
Sample size 7 8 9 10 >10
Minimum numbers of failures 5 6 7 7 70 % of the sample size
10.2 Termination time of a test unit
A test unit shall be terminated from testing when its life reaches the first failure, calculated as follows:
First, determine a three-point moving average of the test data on a continuous basis (an example is
shown in Annex A). When the three-point moving average exceeds the threshold value, the test unit
shall have reached a first failure and shall be terminated from testing.
10.3 Termination life
The termination life shall be the last time at which the three-point moving average did not exceed
the threshold, or the time preceding a catastrophic failure. If a more precise determination of the
termination life is desired, performance of a test unit can be monitored with limit switches or other
suitable means to detect failures.
10.4 Suspended test unit
Testing on an individual test unit may be stopped before a relevant failure occurs. This is known as a
suspension. Some examples of suspensions include
— a unit which has been disassembled for inspection, or
— a unit which has been accidentally crushed.
Suspensions have an influence on the result of calculating the statistical parameters and should
therefore be considered. See Annex D.
10.5 Censored test
If the test is stopped after the minimum number of failures specified in Table 4 is reached but the
remaining test units are still operating, the test shall be considered censored. If the censored test does
not include any suspensions, the method specified in Annex C should be used to calculate the statistical
parameters. If the censored test includes one or more suspensions, the method specified in Annex D
should be used to calculate the statistical parameters.
11 Evaluation of reliability characteristics from the test data
11.1 To improve the interpretation of the calculation results, the failure mode shall be specified and
recorded.
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ISO 19973-1:2015(E)
11.2 The Maximum Likelihood Estimation or Median Rank Regression shall be used to determine
the best fit of the Weibull curve to the test data, and the Fisher Matrix shall be used to determine the
confidence bounds.
11.3 Calculations shall be made from the test data to determine the following:
— characteristic life, η: relative location of the straight line in the Weibull plot relative to the x-axis
(time or scale parameter);
— Weibull shape parameter, β: slope of the straight line in the Weibull plot.
11.4 Calculate the B life at the best fit line (see Figure 1, key b).
10
NOTE See Annex D for information on how to deal with censored data with suspensions.
11.5 Calculate the confidence limit of the (B ) life at the lower 95 % confidence level (see
10 95 %
Figure 1, key a).
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ISO 19973-1:2015(E)
Y
99,0
90,0
c
4
63,2
50,0
3
a b
1
10,0
5,0
2
5
1,0
X
100E+6
1.00E+6 10.0E+6
Key
a
X number of cycles to failure, t (B ) life at the lower, one-sided 95 %
10 95 %
confidence level.
b
Y probability of failure, expressed as a percentage B life.
10
c
1 best fit line, determined by the Maximum Characteristic life, η.
Likelihood Estimation
2 lower confidence limit at 95 %, obtained by Fisher Matrix
3 10 % failure probability line
4 63,2 % failure probability line
5 upper confidence limit at 5 %
NOTE Commercial software can be useful in constructing the graphs.
Figure 1 — Example of how a B life value is determined from a Weibull curve
10
12 Test report
The test report shall include at least the following data:
a) number of the relevant part of ISO 19973, including the component-specific part number (for
example, ISO 19973-2 for valves);
b) date of the test report;
c) component description (manufacturer, type designation, series number);
d) sample size;
e) test conditions (test pressure, temperature, air quality, frequency, load, etc.);
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ISO 19973-1:2015(E)
f) threshold levels;
g) type of failure for each test unit;
h) B life at the median rank, and confidence limit of (B ) life at one-sided 95 % confidence level;
10 10 95 %
i) characteristic life, η, and shape parameter, β;
j) number of failures considered and test interval used;
k) method used to calculate the Weibull data (for example, maximum-likelihood, median rank
regression, Fisher Matrix);
l) Weibull plot;
m) other remarks, as necessary.
13 Identification statement (reference to this part of ISO 19973)
It is recommended that manufacturers use the following statement in test reports, catalogues and sales
literature when electing to comply with this part of ISO 19973:
“General procedures for assessing pneumatic component reliability by testing performed in accordance
with ISO 19973-1, Pneumatic fluid power — Assessment of component reliability by testing — Part 1:
General procedures.”
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ISO 19973-1:2015(E)
Annex A
(normative)
Deter
...
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