1、Designation: E74 13aStandard Practice ofCalibration of Force-Measuring Instruments for Verifying theForce Indication of Testing Machines1This standard is issued under the fixed designation E74; the number immediately following the designation indicates the year of originaladoption or, in the case of
2、 revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscriptepsilon () indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the Department of Defense.1. Scope1.1 The purpose of thi
3、s practice is to specify procedures forthe calibration of force-measuring instruments. Procedures areincluded for the following types of instruments:1.1.1 Elastic force-measuring instruments, and1.1.2 Force-multiplying systems, such as balances and smallplatform scales.NOTE 1Verification by deadweig
4、ht loading is also an acceptablemethod of verifying the force indication of a testing machine. Tolerancesfor weights for this purpose are given in Practices E4; methods forcalibration of the weights are given in NIST Technical Note 577, Methodsof Calibrating Weights for Piston Gages.21.2 The values
5、stated in SI units are to be regarded as thestandard. Other metric and inch-pound values are regarded asequivalent when required.1.3 This practice is intended for the calibration of staticforce measuring instruments. It is not applicable for dynamicor high speed force calibrations, nor can the resul
6、ts ofcalibrations performed in accordance with this practice beassumed valid for dynamic or high speed force measurements.1.4 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-
7、priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:3E4 Practices for Force Verification of Testing MachinesE29 Practice for Using Significant Digits in Test Data toDetermine Conformance with Specificati
8、onsE1012 Practice for Verification of Testing Frame and Speci-men Alignment Under Tensile and Compressive AxialForce Application2.2 American National Standard:B46.1 Surface Texture4ELASTIC FORCE-MEASURING INSTRUMENTS3. Terminology3.1 Definitions:3.1.1 elastic force-measuring instrumenta device or sy
9、s-tem consisting of an elastic member combined with a devicefor indicating the magnitude (or a quantity proportional to themagnitude) of deformation of the member under an appliedforce.3.1.2 primary force standarda deadweight force applieddirectly without intervening mechanisms such as levers, hy-dr
10、aulic multipliers, or the like, whose mass has been deter-mined by comparison with reference standards traceable tonational standards of mass.3.1.3 secondary force standardan instrument ormechanism, the calibration of which has been established bycomparison with primary force standards.3.2 Definitio
11、ns of Terms Specific to This Standard:3.2.1 calibration equationa mathematical relationship be-tween deflection and force established from the calibration datafor use with the instrument in service, sometimes called thecalibration curve.3.2.2 continuous-reading instumenta class of instrumentswhose c
12、haracteristics permit interpolation of forces betweencalibrated forces.3.2.2.1 DiscussionSuch instruments usually have force-to-deflection relationships that can be fitted to polynominalequations.1This practice is under the jurisdiction ofASTM Committee E28 on MechanicalTesting and is the direct res
13、ponsibility of Subcommittee E28.01 on Calibration ofMechanical Testing Machines and Apparatus.Current edition approved May 1, 2013. Published May 2013. Originallyapproved in 1947. Last previous edition approved in 2013 as E74 13. DOI:10.1520/E0074-13A.2Available from National Institute for Standards
14、 and Technology, Gaithersburg,MD 20899.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.4Available from Americ
15、an National Standards Institute (ANSI), 25 W. 43rd St.,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 States13.2.3 creepThe change in deflection of the force-measuring instrument under const
16、ant applied force.3.2.3.1 DiscussionCreep is expressed as a percentage ofthe output change at a constant applied force from an initialtime following the achievement of mechanical and electricalstability and the time at which the test is concluded.Valid creeptests may require the use of primary force
17、 standards to maintainadequate stability of the applied force during the test timeinterval. Creep results from a time dependent, elastic deforma-tion of the instrument mechanical element. In the case of straingage based load cells, creep is adjusted by strain gage designand process modifications to
18、reduce the strain gage response tothe inherent time-dependent elastic deflection.3.2.4 creep recoveryThe change in deflection of the force-measuring instrument after the removal of force following acreep test.3.2.4.1 DiscussionCreep Recovery is expressed as a per-centage difference of the output cha
19、nge at zero force followinga creep test and the initial zero force output at the initiation ofthe creep test divided by the output during the creep test. Thezero force measurement is taken at a time following theachievement of mechanical and electrical stability and a timeequal to the creep test tim
20、e. For many devices, the creepcharacteristic and the creep recovery characteristic are approxi-mate mirror images.3.2.5 deflectionthe difference between the reading of aninstrument under applied force and the reading with no appliedforce.3.2.5.1 DiscussionThis definition applies to instrumentsthat h
21、ave electrical outputs as well as those with mechanicaldeflections.3.2.6 loading rangea range of forces within which thelower limit factor is less than the limits of error specified forthe instrument application.3.2.7 readinga numerical value indicated on the scale,dial, or digital display of a forc
22、e-measuring instrument under agiven force.3.2.8 resolutionthe smallest reading or indication appro-priate to the scale, dial, or display of the force measuringinstrument.3.2.9 specific force devicean alternative class of instru-ments not amenable to the use of a calibration equation.3.2.9.1 Discussi
23、onSuch instruments, usually those inwhich the reading is taken from a dial indicator, are used onlyat the calibrated forces. These instruments are also calledlimited-force devices.3.2.10 lower limit factor, LLFa statistical estimate of theerror in forces computed from the calibration equation of afo
24、rcemeasuring instrument when the instrument is calibratedin accordance with this practice.3.2.10.1 DiscussionThe lower limit factor was termed“Uncertainty” in previous editions of E74. The Lower LimitFactor is used to calculate the lower end of the loading range,see 8.5. Other factors evaluated in e
25、stablishing the lower limitof the loading range of forces are the resolution of theinstrument and the lowest non-zero force applied in thecalibration force sequence, The Lower Limit Factor is onecomponent of the measurement uncertainty. Other uncertaintycomponents should be included in a comprehensi
26、ve measure-ment uncertainty analysis. See Appendix X1 for an example ofmeasurement uncertainty analysis.4. Significance and Use4.1 Testing machines that apply and indicate force are ingeneral use in many industries. Practices E4 has been written toprovide a practice for the force verification of the
27、se machines.A necessary element in Practices E4 is the use of deviceswhose force characteristics are known to be traceable tonational standards. Practice E74 describes how these devicesare to be calibrated. The procedures are useful to users oftesting machines, manufacturers and providers of force m
28、ea-suring instruments, calibration laboratories that provide thecalibration of the instruments and the documents oftraceability, and service organizations that use the devices toverify testing machines.5. Reference Standards5.1 Force-measuring instruments used for the verification ofthe force indica
29、tion systems of testing machines may becalibrated by either primary or secondary force standards.5.2 Force-measuring instruments used as secondary forcestandards for the calibration of other force-measuring instru-ments shall be calibrated by primary force standards. Anexception to this rule is made
30、 for instruments having capacitiesexceeding the range of available primary force standards.Currently the maximum primary force-standard facility in theUnited States is 1 000 000-lbf (4.4-MN) deadweight calibra-tion machine at the National Institute of Standards and Tech-nology.6. Requirements for Fo
31、rce Standards6.1 Primary Force StandardsWeights used as primaryforce standards shall be made of rolled, forged, or cast metal.Adjustment cavities shall be closed by threaded plugs orsuitable seals. External surfaces of weights shall have a finishof 125 or less as specified in ANSI B46.1.6.1.1 The fo
32、rce exerted by a weight in air is calculated asfollows:Force 5Mg9.80665S1 2dDD(1)where:M = mass of the weight,g = local acceleration due to gravity, m/s2,d = air density (approximately 0.0012 Mg/m3),D = density of the weight in the same units as d, and9.80665 = the factor converting SI units of forc
33、e into thecustomary units of force. For SI units, this factoris not used.6.1.2 The masses of the weights shall be determined within0.005 % of their values by comparison with reference stan-dards traceable to the national standards of mass. The localvalue of the acceleration due to gravity, calculate
34、d withinE74 13a20.0001 m/s2(10 milligals), may be obtained from the NationalGeodetic Information Center, National Oceanic and Atmo-spheric Administration.5NOTE 2If M, the mass of the weight, is in pounds, the force will bein pound-force units (lbf). If M is in kilograms, the force will be inkilogram
35、-force units (kgf). These customary force units are related to thenewton (N), the SI unit of force, by the following relationships:1 lbf 5 4.44822N (2)1 kgf 5 9.80665N exact!The Newton is defined as that force which, applied to a 1-kg mass,would produce an acceleration of 1 m/s/s.The pound-force (lb
36、f) is defined as that force which, applied to a 1-lbmass, would produce an acceleration of 9.80665 m/s/s.The kilogram-force (kgf) is defined as that force which, applied to a1-kg mass, would produce an acceleration of 9.80665 m/s/s.6.2 Secondary Force StandardsSecondary force standardsmay be either
37、elastic force-measuring instruments used inconjunction with a machine or mechanism for applying force,or some form of mechanical or hydraulic mechanism tomultiply a relatively small deadweight force. Examples of thelatter form include single- and multiple-lever systems orsystems in which a force act
38、ing on a small piston transmitshydraulic pressure to a larger piston.6.2.1 Elastic force-measuring instruments used as second-ary force standards shall be calibrated by primary forcestandards and used only over the Class AA loading range (see8.6.2.1). Secondary force standards having capacities exce
39、ed-ing 1 000 000 lbf (4.4 MN) are not required to be calibrated byprimary force standards. Several secondary force standards ofequal compliance may be combined and loaded in parallel tomeet special needs for higher capacities. The Lower LimitFactor (see 8.5) of such a combination shall be calculated
40、 byadding in quadrature using the following equation:LLFc5 = LLFo21LLF121LLF221.LLFn2(3)where:LLFc= Lower Limit Factor of the combination, andLLFo, 1,2.n= Lower Limit Factor of the individual instru-ments.6.2.2 The multiplying ratio of a force-multiplying systemused as a secondary force standard sha
41、ll be measured at not lessthan three points over its range with an accuracy of 0.05 % ofratio or better. Some systems may show a systematic change inratio with increasing force. In such cases the ratio at interme-diate points may be obtained by linear interpolation betweenmeasured values. Deadweight
42、s used with multiplying-typesecondary force standards shall meet the requirements of 6.1and 6.1.2. The force exerted on the system shall be calculatedfrom the relationships given in 6.1.1. The force-multiplyingsystem shall be checked annually by elastic force measuringinstruments used within their c
43、lass AA loading ranges toascertain whether the forces applied by the system are withinacceptable ranges as defined by this standard. Changes exceed-ing 0.05 % of applied force shall be cause for reverification ofthe force multiplying system.7. Calibration7.1 Basic PrinciplesThe relationship between
44、the appliedforce and the deflection of an elastic force-measuring instru-ment is, in general, not linear. As force is applied, the shape ofthe elastic element changes, progressively altering its resis-tance to deformation. The result is that the slope of theforce-deflection curve changes gradually a
45、nd continuouslyover the entire range of the instrument. This characteristiccurve is a stable property of the instrument that is changed onlyby a severe overload or other similar cause.7.1.1 Superposed on this curve are local variations ofinstrument readings introduced by imperfections in the forcein
46、dicating system of the instrument. Examples of imperfectionsinclude: non-uniform scale or dial graduations, irregular wearbetween the contacting surfaces of the vibrating reed andbutton in a proving ring, and instabilities in excitation voltage,voltage measurement, or ratio-metric voltage measuremen
47、t in aload cell system. Some of these imperfections are less stablethan the characteristic curve and may change significantly fromone calibration to another.7.1.2 Curve FittingTo determine the force-deflectioncurve of the force-measuring instrument, known forces areapplied and the resulting deflecti
48、ons are measured throughoutthe range of the instrument. A polynomial equation is fitted tothe calibration data by the least squares method to predictdeflection values throughout the loading range. Such an equa-tion compensates effectively for the nonlinearity of the cali-bration curve. The standard
49、deviation determined from thedifference of each measured deflection value from the valuederived from the polynomial curve at that force provides ameasure of the error of the data to the curve fit equation. Astatistical estimate, called the Lower Limit Factor, LLF, isderived from the calculated standard deviation and representsthe width of the band of these deviations about the basic curvewith a probability of 99%. The LLF is, therefore, an estimateof one source of uncertainty contributed by the instrumentwhen forces measured in service are calculated by m
