1、Designation: E74 13Standard 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 this
3、 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 deadweigh
4、t 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 s
5、tated 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 result
6、s 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-p
7、riate 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 Specificatio
8、nsE1012 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 devicea device or systemc
9、onsisting of an elastic member combined with a device forindicating 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-draulic m
10、ultipliers, 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 Definitions of T
11、erms 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 devicea class of instrumentswhose characteris
12、tics 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 responsibilit
13、y of Subcommittee E28.01 on Calibration ofMechanical Testing Machines and Apparatus.Current edition approved March 1, 2013. Published May 2013. Originallyapproved in 1947. Last previous edition approved in 2012 as E74 12. DOI:10.1520/E0074-13.2Available from National Institute for Standards and Tech
14、nology, 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 American Nation
15、al 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 constant appli
16、ed 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 standard
17、s 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 reduce th
18、e 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 change at ze
19、ro 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 time. For ma
20、ny 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 have elect
21、rical 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 force-measuri
22、ng 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 DiscussionSuch in
23、struments, usually those inwhich the reading is taken from a dial indicator, are used onlyat the calibrated forces. These instruments are also calledlimited-load devices.3.2.10 lower limit factor, LLFa statistical estimate of theerror in forces computed from the calibration equation of aforcemeasuri
24、ng 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 establishin
25、g the lower limitof the loading range of forces are the resolution of theinstrument and the lowest non-zero force applied in thecalibration load sequence, The Lower Limit Factor is onecomponent of the measurement uncertainty. Other uncertaintycomponents should be included in a comprehensive measure-
26、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 these machines
27、.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 mea-suring i
28、nstruments, 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 indication system
29、s of testing machines may becalibrated by either primary or secondary standards.5.2 Force-measuring instruments used as secondary stan-dards for the calibration of other force-measuring instrumentsshall be calibrated by primary standards. An exception to thisrule is made for instruments having capac
30、ities exceeding therange of available primary standards. Currently the maximumprimary force-standard facility in the United States is1 000 000-lbf (4.4-MN) deadweight calibration machine at theNational Institute of Standards and Technology.6. Requirements for Force Standards6.1 Primary StandardsWeig
31、hts used as primary forcestandards 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 force exerted by a weight in air is calculated
32、 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 force into thecustomary units of force. For SI u
33、nits, 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, calculated within0.0001 m/s2(10 milligals), may be ob
34、tained from the NationalE74132Geodotic 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-force units (kgf). These customary force unit
35、s 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 (lbf) is defined as that force which, applied to
36、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 StandardsSecondary force standards maybe either elastic force-measuring instruments used in conjunc-
37、tion with a machine or mechanism for applying force, or someform of mechanical or hydraulic mechanism to multiply arelatively small deadweight force. Examples of the latter forminclude single- and multiple-lever systems or systems in whicha force acting on a small piston transmits hydraulic pressure
38、 toa larger piston.6.2.1 Elastic force-measuring instruments used as second-ary standards shall be calibrated by primary standards and usedonly over the Class AA loading range (see 8.6.2.1). Secondarystandards having capacities exceeding 1 000 000 lbf (4.4 MN)are not required to be calibrated by pri
39、mary standards. Severalsecondary standards of equal compliance may be combinedand loaded in parallel to meet special needs for highercapacities. The Lower Limit Factor (see 8.5) of such acombination shall be calculated by adding in quadrature usingthe following equation:LLFc5 = LLFo21LLF121LLF221.LL
40、Fn2(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 standard shall be measured at not less thanthree points over its range with an accuracy of 0.05 % of
41、 ratioor better. Some systems may show a systematic change in ratiowith increasing force. In such cases the ratio at intermediatepoints may be obtained by linear interpolation between mea-sured values. Deadweights used with multiplying-type second-ary standards shall meet the requirements of 6.1 and
42、 6.1.2. Theforce exerted on the system shall be calculated from therelationships given in 6.1.1. The force multiplying system shallbe checked annually by elastic force measuring instrumentsused within their class AA loading ranges to ascertain whetherthe forces applied by the system are within accep
43、table rangesas defined by this standard. Changes exceeding 0.05 % ofapplied force shall be cause for reverification of the forcemultiplying system.7. Calibration7.1 Basic PrinciplesThe relationship between the appliedforce and the deflection of an elastic force-measuring instru-ment is, in general,
44、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 and continuouslyover the entire range of the instrument. This characteristiccurve is a stable p
45、roperty 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 forceindicating system of the instrument. Examples of imperfectionsinclude: non-uniform scale or dial
46、 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 measurement in aload cell system. Some of these imperfections are less stablethan the characteristic cur
47、ve 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 deflections are measured throughoutthe range of the instrument. A polynomial equation is fitted tothe
48、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 deviation determined from thedifference of each measured deflection value from the valuederive
49、d 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 means of thecalibration equation. Actual errors in service are likely to bedifferent if forces a
