ASTM E2428-08

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Designation: E2428 − 08
StandardPractice for
Calibration of Torque-Measuring Instruments for Verifying
the Torque Indication of Torque Testing Machines
This standard is issued under the fixed designation E2428; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2.2 American National Standard:
B46.1Surface Texture
1.1 This practice is to specify procedure for the calibration
of elastic torque-measuring instruments.
ELASTIC TORQUE-MEASURING INSTRUMENTS
NOTE 1—Verification by deadweight and a lever arm is an acceptable
method of verifying the torque indication of a torque testing machine.
3. Terminology
Tolerances for weights used are tabulated in Practice E2624; methods for
3.1 Definitions:
calibrationoftheweightsaregiveninNISTTechnicalNote577,Methods
of Calibrating Weights for Piston Gages.
3.1.1 elastic torque-measuring device—a device or system
consisting of an elastic member combined with a device for
1.2 The values stated in either SI units or inch-pound units
indicating the measured values (or a quantity proportional to
are to be regarded separately as standard. The values stated in
the measured value) of deformation of the member under an
each system may not be exact equivalents; therefore, each
applied torque.
system shall be used independently of the other. Combining
3.1.2 primarytorquestandards—adeadweightforceapplied
values from the two systems may result in non-conformance
throughaleverarmorwheel,withacalibratedlengthorradius
with the standard.
of a known uncertainty, that is traceable to national standards.
1.3 This practice is intended for the calibration of static or
3.1.3 secondary torque standard—an instrument or
quasi-static torque measuring instruments. The practice is not
mechanism, that has been calibrated by a comparison with a
applicable for high speed torque calibrations or measurements.
primary torque standard(s).
1.4 This standard does not purport to address all of the
3.1.4 torque—a vector product of force and length, ex-
safety concerns, if any, associated with its use. It is the
pressed in terms of N-m, lbf-in., etc.
responsibility of the user of this standard to establish appro-
3.2 Definitions of Terms Specific to This Standard:
priate safety and health practices and determine the applica-
3.2.1 calibration equation—a mathematical relationship be-
bility of regulatory limitations prior to use.
tween output of the unit under test and the applied standard
torque, sometimes referred to as the calibration curve.
2. Referenced Documents
3 3.2.2 continuous-reading device—a class of instruments
2.1 ASTM Standards:
whose characteristics permit interpolation of torque values
E29Practice for Using Significant Digits in Test Data to
between calibrated torque values.
Determine Conformance with Specifications
3.2.2.1 Discussion—Such instruments usually have torque-
E2624Practice for Torque Calibration of Testing Machines
to-deflection relationships that can be fitted to polynomial
and Devices
equations. Departures from the fitted curve are reflected in the
uncertainty (see 8.4).
3.2.3 deflection—the difference between the readings of an
ThispracticeisunderthejurisdictionofASTMCommitteeE28onMechanical
instrument under applied torque and the reading with no
Testing and is the direct responsibility of Subcommittee E28.01 on Calibration of
Mechanical Testing Machines and Apparatus.
applied torque. The definition of deflection applies to output
Current edition approved Feb. 15, 2008. Published March 2008. DOI: 10.1520/
readings in electrical units as well as readings in units of
E2428-08.
torque.
Available from National Institute of Standards and Technology (NIST), 100
Bureau Dr., Stop 1070, Gaithersburg, MD 20899-1070, http://www.nist.gov.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
the ASTM website. 4th Floor, New York, NY 10036, http://www.ansi.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2428 − 08
3.2.4 torque range—a range of torque values within which
D = density of the weight in the same units as d, and
the uncertainty is less than the limits of error specified for the
9.80665 = the factor converting SI units of force into the
instrument application.
customary units of force. For SI units, this factor
is not used.
3.2.5 reading—a numerical value indicated on the scale,
dial, electrical output or digital display of a torque- measuring
6.1.2 The masses of the weights shall be determined by
instrument for a given torque.
comparison with reference standards traceable to the national
3.2.6 resolution—the smallest change in reading or indica- standards of mass. The local value of the acceleration due to
gravity, calculated within 0.0001 m/s (10 milligals), may be
tion appropriate to the scale, dial, or display of the torque
measuring instrument. obtained from the National Geodetic Information Center,
National Oceanic and Atmospheric Administration.
3.2.7 specific torque device—an alternative class of instru-
ments not amenable to the use of a calibration equation.
NOTE 2—If M, the mass of the weight, is in pounds, the force will be
in pound-force units (lbf). If M is in kilograms, the force will be in kilo
3.2.7.1 Discussion—Such instruments, usually those in
gram-force units (kgf). These customary force units are related to the
which the reading is taken from a dial indicator, are used only
newton (N), the SI unit of force, by the following relationships:
at the calibrated torque values.
1 kgf = 9.80665 N (exact)
3.2.8 uncertainty—astatisticalestimateofthelimitsoferror
1 lbf = 4.44822 N
in torque values computed from the calibration equation of a
The pound-force (lbf) is defined as that force which, applied to a 1-lb
torque-measuring instrument when the instrument is calibrated
mass, would produce an acceleration of 32.1747 f/s/s.
in accordance with this practice.
6.1.3 The lever arm or wheel shall be calibrated to deter-
mine the length or radius with a known uncertainty, that is
4. Significance and Use
traceable to national standards of length. The expanded uncer-
4.1 Testing machines that apply and indicate torque are in
tainty with a confidence factor of 95% (K=2) for the torque
general use in many industries. Practice E2624 has been
calibrator shall not exceed 0.012% .
writtentoprovideapracticeforthetorqueverificationofthese
6.2 Secondary Standards—Secondary torque standards may
machines.Anecessary element in Practice E2624 is the use of
be either elastic torque-measuring instruments used with a
devices whose torque characteristics are known to be traceable
machine for applying torque, or a mechanical or hydraulic
to national standards. Practice E2428 describes how these
mechanism to apply or multiply a deadweight force.
devices are to be calibrated.The procedures are useful to users
6.2.1 The multiplying ratio of a force multiplying system
of torque testing machines, manufacturers and providers of
used as a secondary torque standard shall be measured at not
torque measuring instruments, calibration laboratories that
less than ten points over its range with an accuracy of 0.06%
provide calibration services and documents, and service orga-
ofratioorbetter.Somesystemsmayshowasystematicchange
nizations using devices to verify torque testing machines.
in ratio with increasing force. For these cases the ratio at
5. Reference Standards
intermediate points may be obtained by linear interpolation
betweenmeasuredvalues.Deadweightsusedwithmultiplying-
5.1 Torque-measuring instruments used for the verification
type secondary standards shall meet the requirements of 6.1
of the torque indication systems of torque testing machines
and 6.1.2. The force exerted on the system shall be calculated
may be calibrated by either primary or secondary standards.
from the relationships given in 6.1.1. The force multiplying
5.2 Torque-measuring instruments used as secondary stan-
system shall be checked annually by elastic force measuring
dardsforthecalibrationofothertorque-measuringinstruments
instrumentsusedwithintheirclassAAloadingrangestoverify
shall be calibrated by primary standards.
the forces applied by the system are within acceptable ranges
definedbythisstandard.Changesexceeding0.06%ofapplied
6. Requirements for Torque Standards
force shall be cause for re-verification of the force multiplying
6.1 Primary Standard—Torque, with traceability derived
system.
from national standards of length and mass, and of specific
6.2.2 Elastic torque-measuring instruments used as second-
measurement uncertainty, that can be applied to torque mea-
arystandardsshallbecalibratedbyprimarystandardsandused
suring devices. Weights used as primary mass standards shall
only over the Class AA loading range (see 8.5.2.1).
be made of rolled, forged, or cast metal. Adjustment cavities
6.2.3 Other types of torque standards may be used and shall
shall be closed by threaded plugs or suitable seals. External
be calibrated. The expanded uncertainty with a confidence
surfaces of weights shall have a finish of 3.2m (Ra) or less as
factor of 95% (K=2) shall not exceed 0.06% of the applied
specified in ANSI B46.1.
torque.
6.1.1 The force exerted by a weight in air is calculated as
follows:
7. Calibration
Force 5 Mg/9.80665 1 2 d/D (1)
~ !~ ~ !! 7.1 Basic Principles—The relationship between the applied
torque and the deflection of an elastic torque-measuring
where:
M = mass of the weight,
2 5
Available from National Oceanic and Atmospheric Administration (NOAA),
g = local acceleration due to gravity, m/s ,
3 14th St. and Constitution Ave., NW, Room 6217, Washington, DC 20230, http://
d = air density (approximately 1.2 kg/m ),
www.noaa.gov.
E2428 − 08
instrument is, in general, not linear. As the torque is applied, torque for every 10% interval throughout the range. It is not
theshapeoftheelasticelementchanges,progressivelyaltering necessary, however that these torque values be equally spaced.
its resistance to deformation. The result is that the slope of the Calibration torque values at less than one tenth of capacity are
torque-deflection curve changes gradually and uniformly over permissible and tend to give added assurance to the fitting of
the entire range of the instrument. This characteristic of the calibration equation. If the lower limit of the loading range
full-scale non-linearity is a stable property of the instrument of the device (see 8.5.1) is anticipated to be less than one tenth
that is changed only by a severe overload or other similar of the maximum torque applied during calibration, then the
cause. torquevaluesshouldbeappliedatorbelowthislowerlimit.In
no case should the smallest torque applied be below the
7.1.1 Localized Non-linearities—Superimposed on this
theoretical lower limit of the instrument as defined by the
curve are localized non-linearities introduced by the imperfec-
values: 400 × resolution for ClassAloading range and 2000 ×
tions in the torque indicating system of the instrument. Ex-
resolution for Class AA loading range.
amples of these imperfections include voltage and sensing
7.2.2 Resolution Determination—The resolution of a digital
instabilities. Some of these imperfections are less stable than
instrument is considered to be one increment of the last active
the full-scale non-linearity and may change from one calibra-
number on the numerical indicator, provided that the reading
tion to another.
does not fluctuate by more than plus or minus one increment
7.1.2 Curve Fitting—A second degree polynomial fitted to
when no torque is applied to the instrument. If the readings
theobservedcalibrationdatausingthemethodofleastsquares,
fluctuate by more than plus or minus one increment, the
predictsthedeflectionvaluesforappliedtorquethroughoutthe
resolution will be equal to half the range of fluctuation.
loading range of the elastic torque measuring instrument (see
7.2.3 Number of Calibration Torque Values—A total of at
8.4). Such an equation compensates effectively for the full-
least 30 torque applications per mode, clockwise or counter
scale non-linearity, allowing the localized non-linearities to
clockwise,isrequiredforacalibrationand,ofthese,atleast10
appear as deviations. A statistical estimate, called the
must be at different torque values.Apply each torque value at
uncertainty, is made of the width of the band of these
least twice during the calibration in both the clockwise and
deviationsaboutthebasiccurve.Theuncertaintyisanestimate
counter clockwise direction, as applies.
ofthelimitsoferrorcontributedbytheinstrumentwhentorque
7.2.4 Specific Torque Devices (Limited Torque Devices)—
values measured in service are calculated using the calibration
Because these devices are used only at the calibrated torque
equation. Actual errors in service will vary for torque values
values, select those torque values which would be most useful
applied under loading and environmental conditions differing
in the service function of the instrument. Coordinate the
from those of the calibration.
selection of the calibration torque values with the submitting
7.1.3 Curve Fitting for High Resolution Devices—Annex
organization.Apply each calibration torque at least three times
A1 recommends a procedure for obtaining the degree of the
in order to provide sufficient data for the calculation of the
best-fit calibration curve for these devices.
standard deviation of the observed deflections about their
NOTE 3—Experimental work by several calibration laboratories in
average values.
fitting higher than second degree polynomials to the observed data
7.3 Temperature Equalization:
indicated that, for some devices, use of a higher degree equation may
7.3.1 Allow the torque measurement system sufficient time
result in a lower uncertainty than derived from the second degree fit
(ASTM RR:E28-1009). Over-fitting should be avoided. Equations of
to adjust to the ambient temperature in the calibration machine
greater than 5th degree should not be used due to the limited number of
prior to calibration in order to assure stable instrument re-
torqueincrementsinthecalibrationprotocol.Numericalerrorsmayoccur
sponse.
if calculations are performed with insufficient precision.Adevice used to
7.3.2 The recommended value for roo
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