1、Designation: E 74 06Standard Practice ofCalibration of Force-Measuring Instruments for Verifying theForce Indication of Testing Machines1This standard is issued under the fixed designation E 74; the number immediately following the designation indicates the year of originaladoption or, in the case o
2、f revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.Asuperscriptepsilon (e) 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 t
3、his 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 deadwe
4、ight 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 value
5、s 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 res
6、ults 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 appr
7、o-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 Specifica
8、tions2.2 American National Standard:B46.1 Surface Texture4ELASTIC FORCE-MEASURING INSTRUMENTS3. Terminology3.1 Definitions:3.1.1 elastic force-measuring devicea device or systemconsisting of an elastic member combined with a device forindicating the magnitude (or a quantity proportional to themagnit
9、ude) 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 multipliers, or the like, whose mass has been deter-mined by comparison with reference standards traceable tonational stand
10、ards of mass.3.1.3 secondary force standardan instrument or mecha-nism, the calibration of which has been established by com-parison with primary force standards.3.2 Definitions of Terms Specific to This Standard:3.2.1 calibration equationa mathematical relationship be-tween deflection and force est
11、ablished from the calibration datafor use with the instrument in service, sometimes called thecalibration curve.3.2.2 continuous-reading devicea class of instrumentswhose characteristics permit interpolation of forces betweencalibrated forces.3.2.2.1 DiscussionSuch instruments usually have force-to-
12、deflection relationships that can be fitted to polynominalequations. Departures from the fitted curve are reflected in theuncertainty (8.4).3.2.3 deflectionthe difference between the reading of aninstrument under applied force and the reading with no appliedforce.1This practice is under the jurisdic
13、tion ofASTM Committee E28 on MechanicalTesting and is the direct responsibility of Subcommittee E28.01 on Calibration ofMechanical Testing Machines and Apparatus.Current edition approved March 1, 2006. Published March 2006. Originallyapproved in 1947. Last previous edition approved in 2004 as E 74 0
14、4.2Available from National Institute for Standards 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 Summ
15、ary page onthe ASTM website.4Available from American National Standards Institute, 25 W. 43rd St., 4thFloor, New York, NY 10036.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.2.4 loading rangea range of forces within which theunce
16、rtainty is less than the limits of error specified for theinstrument application.3.2.5 readinga numerical value indicated on the scale,dial, or digital display of a force-measuring instrument under agiven force.3.2.6 resolutionthe smallest reading or indication appro-priate to the scale, dial, or di
17、splay of the force measuringinstrument.3.2.7 specific force devicean alternative class of instru-ments not amenable to the use of a calibration equation.3.2.7.1 DiscussionSuch instruments, usually those inwhich the reading is taken from a dial indicator, are used onlyat the calibrated forces. These
18、instruments are also calledlimited-load devices.3.2.8 uncertaintya statistical estimate of the limits oferror in forces computed from the calibration equation of aforcemeasuring instrument when the instrument is calibratedin accordance with this practice.4. Significance and Use4.1 Testing machines t
19、hat apply and indicate force are ingeneral use in many industries. Practices E4has been writtento provide a practice for the force verification of thesemachines. A necessary element in Practices E4is the use ofdevices whose force characteristics are known to be traceableto national standards. Practi
20、ce E 74 describes how thesedevices are to be calibrated. The procedures are useful to usersof testing machines, manufacturers and providers of forcemeasuring instruments, calibration laboratories that provide thecalibration of the instruments and the documents of traceabil-ity, and service organizat
21、ions that use the devices to verifytesting machines.5. Reference Standards5.1 Force-measuring instruments used for the verification ofthe force indication systems of testing machines may becalibrated by either primary or secondary standards.5.2 Force-measuring instruments used as secondary stan-dard
22、s for the calibration of other force-measuring instrumentsshall be calibrated by primary standards. An exception to thisrule is made for instruments having capacities exceeding therange of available primary standards. Currently the maximumprimary force-standard facility in the United States is1 000
23、000-lbf (4.4-MN) deadweight calibration machine at theNational Institute of Standards and Technology.6. Requirements for Force Standards6.1 Primary StandardsWeights used as primary forcestandards shall be made of rolled, forged, or cast metal.Adjustment cavities shall be closed by threaded plugs ors
24、uitable 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 asfollows:Force 5Mg9.80665S1 2dDD(1)where:M = mass of the weight,g = local acceleration due to gravity, m/s2,d = air density (approximatel
25、y 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 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
26、 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 obtained from the NationalGeodotic Information Center, National Oceanic and Atmo-spheric Administration.5NOTE 2If M, the mass of the weight,
27、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 units are related to thenewton (N), the SI unit of force, by the following relationships:1 lbf 5 4.448 22 N (2)1 kgf 5 9.806 65 N exact!The Newton is
28、 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 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, wo
29、uld produce an acceleration of 9.80665 m/s/s.6.2 Secondary StandardsSecondary force standards maybe either elastic force-measuring instruments used in conjunc-tion with a machine or mechanism for applying force, or someform of mechanical or hydraulic mechanism to multiply arelatively small deadweigh
30、t force. Examples of the latter forminclude single- and multiple-lever systems or systems in whicha force acting on a small piston transmits hydraulic pressure toa larger piston.6.2.1 Elastic force-measuring instruments used as second-ary standards shall be calibrated by primary standards and usedon
31、ly over the Class AA loading range (see 8.5.2.1). Secondarystandards having capacities exceeding 1 000 000 lbf (4.4 MN)are not required to be calibrated by primary standards. Severalsecondary standards of equal compliance may be combinedand loaded in parallel to meet special needs for highercapaciti
32、es. The uncertainty (see 8.4) of such a combinationshall be calculated by adding in quadrature using the followingequation:Uc5 = Uo21 U121 U221 .Un2(3)where:Uc= uncertainty of the combination, andUo, 1,2.n= uncertainty of the individual instruments.6.2.2 The multiplying ratio of a force multiplying
33、systemused as a secondary standard shall be measured at not less thanthree points over its range with an accuracy of 0.05 % of ratioor better. Some systems may show a systematic change in ratio5Available from National Oceanic and Atmospheric Administration (NOAA),14th St. and Constitution Ave., NW,
34、Room 6217, Washington, DC 20230.E74062with 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 6.1.2. Theforce exerted on t
35、he 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 acceptable rangesas defined by thi
36、s 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, not linear. As force is appli
37、ed, 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 uniformly overthe entire range of the instrument. This characteristic full-scalenonlinearity is a stable property of the
38、 instrument that ischanged only by a severe overload or other similar cause.7.1.1 Localized NonlinearitiesSuperposed on this curveare localized nonlinearities introduced by the imperfections inthe force indicating system of the instrument. Examples ofimperfections include: non-uniform scale or dial
39、graduations,irregular wear between the contacting surfaces of the vibratingreed and button in a proving ring, and voltage and sensinginstabilities in a load cell system. Some of these imperfectionsare less stable than the full-scale nonlinearity and may changesignificantly from one calibration to an
40、other.7.1.2 Curve FittingIn the treatment of the calibrationdata, a second degree polynomial fitted to the observed datausing the method of least squares has been found to predictwithin the limit of the uncertainty (8.4) deflection values forapplied force throughout the loading range of the elastic
41、forcemeasuring instrument. Such an equation compensates effec-tively for the full-scale nonlinearity, allowing the localizednonlinearities to appear as deviations. A statistical estimate,called the uncertainty, is made of the width of the band of thesedeviations about the basic curve. The uncertaint
42、y is, therefore,an estimate of the limits of error contributed by the instrumentwhen forces measured in service are calculated by means of thecalibration equation. Actual errors in service are likely to begreater if forces are applied under loading and environmentalconditions differing from those of
43、 the calibration.7.1.3 Curve Fitting for High Resolution DevicesThe useof calibration equations of the 3rd, 4th, or 5th degree isrestricted to devices having a resolution of 1 increment ofcount per 50000 or greater active counts at the maximumcalibration load. Annex A1 recommends a procedure forobta
44、ining the degree of the best fit calibration curve for thesedevices.NOTE 3Experimental work by several force calibration laboratories infitting higher than second degree polynomials to the observed dataindicates that, for some devices, use of a higher degree equation mayresult in a lower uncertainty
45、 than that derived from the second degree fit.(ASTM RR: E28-1009) Overfitting should be avoided. Equations ofgreater than 5th degree cannot be justified due to the limited number offorce increments in the calibration protocol. Errors caused by round-offmay occur if calculations are performed with in
46、sufficient precision.A force measuring device not subjected to repair, overloading, modifi-cations, or other significant influence factors which alter its elasticproperties or its sensing characteristics will likely exhibit the same degreeof best fit on each succeeding calibration as was determined
47、during itsinitial calibration using this procedure. A device not subjected to theinfluence factors outlined above which exhibits continued change ofdegree of best fit with several successive calibrations may not havesufficient performance stability to allow application of the curve fittingprocedure
48、of Annex A1.7.2 Selection of Calibration Forces A careful selection ofthe different forces to be applied in a calibration is essential toprovide an adequate and unbiased sample of the full range ofthe deviations discussed in 7.1 and 7.1.1. For this reason, theselection of the calibration forces is m
49、ade by the standardizinglaboratory.An exception to this, and to the recommendations of7.2.1 and 7.2.4, is made for specific force devices, where theselection of the forces is dictated by the needs of the user.7.2.1 Distribution of Calibration Forces Distribute thecalibration forces over the full range of the instrument,providing, if possible, at least one calibration force for every10 % interval throughout the range. It is not necessary, how-ever that these forces be equally spaced. Calibration forces atless than one tenth of capacity are permissible and tend
