A GERMAN CALIBRATION GUIDELINE FOR TORQUE WRENCH CALIBRATION DEVICES REVISED

Transcription

1 XXI IEO World Cogress easuremet i Research ad Idustry August 30 September 4, 05, Prague, Czech Republic A GERAN CAIBRATION GUIDEINE FOR TORQUE WRENCH CAIBRATION DEVICES REVISED Dirk Röske Physikalisch-Techische Budesastalt, Brauschweig, Germay, dirk.roeske@ptb.de Abstract Torque wreches are commoly used measurig istrumets i may applicatios. They are calibrated o torque wrech calibratio devices. I Germay, at least i the accredited laboratories, the latter are calibrated usig precisio torque trasfer wreches ad applyig the correspodig DD calibratio guidelie. This guidelie was recetly revised. The mai ew ideas ad modificatios will be preseted i this paper. eywords: torque wrech calibratio device, calibratio guidelie, DD-R 3-8, DAkkS-DD-R 3-8, DD-R 0-8. INTRODUCTION I the past, prior to 00, the Deutscher alibrierdiest (DD, Germa Calibratio Service) was resposible for the accreditatio of calibratio laboratories as well as for the draftig ad issuig of calibratio guidelies alog with other stadardizatio bodies i Germay. I the field of torque, the techical committee (TC) Torque (committee o. 0) of the DD elaborated the three calibratio guidelies show i Table. Table. DD torque calibratio guidelies i 009. Name DD-R 3-5 DD-R 3-7 DD-R 3-8 Title Calibratio of torque measurig devices for static alteratig torque Static calibratio of idicatig torque wreches Static calibratio of torque tool calibratio devices [] I the year 00, the DD became a part of the Deutsche Akkreditierugsstelle (DAkkS), the Germa Accreditatio Body. At that time, all guidelies were trasferred to the DAkkS ad their ames chaged to DAkkS-DD-R. Soo it tured out that the accreditatio body DAkkS could ot be resposible for techical documets such as calibratio guidelies. I additio, the status of the still existig techical committees of the former DD was ot clear. Therefore, the decisio was made by PTB to reestablish the ew DD o 3 rd ay, 0 as a PTB pael for cooperatio betwee the Germa atioal metrology istitute ad the accredited laboratories. The ew DD is the home of the old techical committees ad oe of their mai tasks is the maiteace of the DD calibratio guidelies. Due to cotractual reasos, the DAkkS-DD-R documets, which are at preset the basis for may accreditatios, still exist i this form, but begiig with the year 05, all guidelies will be replaced by ew guidelies with the former title DD-R. Guidelies that require a revisio will be discussed i the TCs ad issued as ew versios. Oe importat amedmet is that the first umber i the ame of the guidelie (3) will ow be the umber of the TC which deals with this guidelie. For torque this umber is 0; therefore, i future all guidelies from the torque TC will have the ame DD-R 0-XX with a secod umber XX. I order ot to chage too much ad to cofuse the users it was decided to keep the secod umber of the old ame. This meas, the old DD-R 3-8, which later became DAkkS-DD-R 3-8, will have the ew ame DD-R 0-8. This guidelie deals with the calibratio of torque wrech calibratio devices ad it was revised i the TC 0 Torque committee of the DD. I the followig, the mai ew ideas, improvemets ad amedmets will be explaied i detail.. TORQUE TRANSDUCERS VS. TORQUE WRENCHES Torque trasducers are calibrated usig torque without ay additioal compoets, i.e. with pure torque. This ca be doe o a torque calibratio machie with a supported lever ad deadweights or agaist a referece torque trasducer i a referece calibratio machie. I the first case, the torque is geerated by a force couple (the weight ad the reactio force of the support, which should be a air bearig if smallest ucertaities are aimed at). I the secod case, the torque is geerated as pure torque by a electric drive ad a gearbox. The mai differece betwee torque trasducers ad torque wreches is the cross force that acts at the ed of the wrech arm. This force is either used to geerate the torque (just like whe the worker applies this force to tighte a screw) or it is the reactio force agaist a support whe the torque is geerated i the measurig axis, for example, usig a drive. For the measuremet, both methods are comparable. Now we have to cosider the questio as to how the torque with superimposed cross force (ad possibly a additioal bedig momet due to the fact that the cross force also acts at a certai distace i the directio of the measurig axis) ca be traced back to torque stadards that were established for pure torque. This problem ca be solved

2 whe the cross force (ad additioal bedig momets) is compesated by a bearig betwee the torque wrech ad the referece torque trasducer. The bearig must have sufficietly low frictio, ad air bearigs are used for this purpose at PTB. This method is applied to the calibratio of torque trasfer wreches (Figure ), which are later used for the calibratio of torque wrech calibratio devices (Figure ). Fig.. Torque trasfer wrech with electroic display (example). 3. CAIBRATION OF TORQUE WRENCH CAIBRATION DEVICES The mai purpose of the majority of torque wrech calibratio devices is the calibratio of settig or idicatig torque wreches accordig to ISO Such torque wreches are commoly used i may applicatios for safety-related bolted coectios, for example i the productio ad maiteace of cars. Ufortuately, ISO 6789 i its preset versio (003) does ot distiguish betwee pure torque tools (screwdrivers) ad tools workig at torque superimposed with cross force ad bedig momet (torque wreches). Together, the two are called torque tools. Therefore, there are o special requiremets for the calibratio equipmet for the differet kids of these tools. The scope of applicatio of the ew DD-R 0-8 is clearly limited to torque wrech calibratio devices. For this reaso, the title was chaged to Static calibratio of torque wrech calibratio devices. Fig.. Torque wrech calibratio device (by courtesy of CEH). 3.. Types of torque wrech calibratio devices There are may differet types of torque wrech calibratio devices available o the market. Some are maually operated i the sese that the torque is geerated by a force applied to a torque wrech by had. These devices lack the ecessary reproducibility ad, therefore, they caot be accredited. The miimum requiremet for a accreditatio is that the torque is geerated by a motor drive or by a had-operated spidle drive both usig a suitable gearbox. That meas that the force is ot directly applied by had. Two workig priciples are possible here: a) the drive turs the torque trasducer ad the torque wrech coected to the trasducer is supported at a certai distace alog the such that it geerates the reactio force; b) the torque trasducer is fixed ad the support of the moves usually alog a straight lie due to the actig drive such that it geerates the force. Both priciples are comparable, there are o great techical advatages of oe agaist the other. I both cases, care has to be take to allow the wrech its self-adjustmet due to the iteractio betwee the coectig profiles (ofte square drives) ad due to its elastic deformatio. Torque wrech calibratio devices with mechaical measuremet of the torque (ofte equipped with dial gauges) are usually ot suitable for the calibratio of torque wreches accordig to ISO 6789 due to their iadequate ucertaity. A large cotributio is due to the resolutio which is quite limited. Nevertheless, such devices ca be calibrated usig the guidelie discussed here. Aother differece i torque wrech calibratio devices is the cross force ad bedig momet reactio. Very precise devices use bearigs (i the best case air bearigs) to compesate these additioal mechaical compoets. This results i a better reproducibility of the idicatios of the torque trasducer used as referece. By desig, these devices are ot affected by the value or the directio of the cross force or the bedig momet actig i a torque wrech setup. 3.. New ideas ad modificatios of the guidelie I additio to the chaged title of the guidelie, the attempt was made to take the differet types of torque wrech calibratio devices ito accout ad to allow a ecoomical calibratio of the device, depedig o the requiremets for the target ucertaity. Oe major issue is the hysteresis. I the former guidelie this parameter was used to determie the ucertaity of the device. I cotrast to this procedure, there is o applicatio kow where torque wreches are used i a decremetal measuremet series, whe the torque is reduced from a higher value dow to zero. Screws are tighteed with icreasig torque. To loose a screw, a icreasig torque i the opposite directio must be applied. Whe the tighteig was achieved with clockwise torque, the for looseig ati-clockwise torque must be applied. Oce the ecessary torque value has bee reached, the there is usually o eed for a measuremet of the decreasig torque. Therefore it was decided ot to iclude the hysteresis cotributio i the ucertaity evaluatio aymore. This parameter must be measured, recorded ad evaluated like a quality parameter of the device. A applicatio of this is devices with bearigs where a low hysteresis value idicates

3 the proper fuctioig of this bearig. Nevertheless, this parameter is ot ecessary for the calibratio of torque wreches accordig to ISO Depedig o the target ucertaity, the umber of measuremet series ca be reduced by oe. A limit value of 0.5 % expaded relative ucertaity (k = ) was defied. The etire set of series must be measured whe the target is lower tha 0.5 %. If 0.5 % or more is requested, the the reduced program ca be applied. The maximum umber of measuremet series is six, five icremetal ad oe decremetal series: two icremetal series i the stadard positio of the device s torque trasducer () for repeatability, oe series with decremetal torque for hysteresis as metioed above, oe series with reduced lever arm legth for measurig the effect of lever legth, oe series with a chaged moutig positio of the device s torque referece trasducer (rotated by a agle α) ad a last series with a chaged positio of the coector (which ca be chaged i the device as well as o the trasfer wrech usually by a agle of 45 or 9 for square drives). Depedig o the desig of the torque wrech calibratio device ad the trasfer wrech that is used for the calibratio, it may ot be possible to chage the positio of the device s referece trasducer or the coector i the device or o the wrech. I the last case of fixed coector positios, the maximum umber of series ca be reduced to five whe the iformatio about the ifluece of the coector is obtaied by other meas, for example from a former calibratio with a suitable wrech where the positio of the coector could be chaged. I the first case with a fixed positio of the torque referece trasducer, the measuremet series with chaged trasducer positio is replaced by a series with uchaged trasducer positio after the trasfer wrech has bee dismouted ad mouted agai. If the target ucertaity is 0.5 % or higher, the the measuremet of the chaged positio of the referece trasducer - if it caot be chaged, the additioal series with uchaged trasducer positio - ca be omitted. For the legth, the icreased measurig rage of trasfer torque wreches is ow cosidered. For torque wreches with a rage from 000 N m to 3000 N m, the omial legth is show to be 800 mm ad the reduced legth is 300 mm. It is advised that the first measurig series i the calibratio should be the series with reduced legth because the larger mechaical iteractios (cross force, bedig momet) are associated with the reduced lever legth. The termiology has bee slightly chaged, the former medium legth is ow amed omial legth ad miimum legth was replaced with reduced legth. Figures 3 ad 4 show examples of the measuremet sequece depedig o the target ucertaity ad the possible rotatioal positios of the torque sesor ad the coector betwee sesor ad torque trasfer wrech. Fig. 3. Calibratio sequece of a torque wrech calibratio device with fixed torque sesor ad fixed coector ad a target ucertaity of 0.5%. Table. iimum umber of measurig series (icr. = icremetal, decr. = decremetal series). Target ucertaity i % Nomial legth Nomial legth Sesor ad coector turable Reduced legth Rotated sesor Rotated coector < 0.5 icr. decr. icr. icr. icr. 0.5 icr. decr. icr. - icr. Sesor turable ad coector fixed < 0.5 icr. decr. icr. icr icr. decr. icr. - - Sesor fixed ad coector turable < icr. decr. icr. - icr. 0.5 icr. decr. icr. - icr. Sesor ad coector fixed < icr. decr. icr icr. decr. icr. - - Fig. 4. Calibratio sequece of a torque wrech calibratio device with a target ucertaity of < 0.5%. I the past, there have bee some difficulties with torque wrech calibratio devices whe they were calibrated with torque trasfer wreches of special desig. Due to the greater height of these wreches, the cross force associated with the torque acts at a greater distace from the referece trasducer i axial directio, thus geeratig a larger bedig momet. If the device does ot have a bearig, the the bedig momet actig o the referece trasducer icreases with the height of the wrech. But stadard torque wreches that do ot have this height are the calibrated uder differet coditios, because the bedig momet will be much lower. The guidelie ow requires a iformatio about the axial distace of the cross force. Aother problem is coected with the zero suppressio that is offered by some devices. This feature meas that

4 values that are close to zero are displayed as 0 but i fact they ca deviate from real zero sigificatly. It is required that these devices i case the zero suppressio caot be deactivated for the time of the calibratio shall ot be switched off ad o for self-tarig durig the calibratio. A advatage of the ew revisio of the guidelie is that all mai formulas ad calculatios remai uchaged. I the former versio, there was a classificatio described i the mai text ad a ucertaity calculatio i the aex. Nowadays, ucertaity plays a more importat role ad it is therefore ow part of the mai text, whereas classificatio has bee shifted to a aex. The guidelie ow cotais aother aex with a fully calculated example of a calibratio result, icludig ucertaities, which ca be used as a referece for evaluatio procedures applied by laboratories. 4. CAIBRATION RESUTS 3.. Calibratio result The calibratio result Y is calculated for each torque step as mea value of the icremetal series i differet moutig positios ad at omial legth without takig ito accout repeated series as i () Y j X j I j I j,0 j. () Here, is at least i the case of the fixed coector ad sesor, ad 3 whe both coector ad sesor are turable. There is a umber of parameters that have to be determied (3. through 3.8). All of them are related to correspodig ifluecig quatities. Except for the reversibility (3.8), these parameters are the statistically evaluated ad combied to obtai a resultig measuremet ucertaity or ucertaity iterval (see 5). 3.. Reproducibility The reproducibility b is calculated for each torque step as the stadard deviatio of the values of the icremetal series as i () b j X j Y whe the target ucertaity is less tha 0.5 %. For ucertaities 0.5 % this parameter ca be omitted ad the cotributio of the repeatability (3.3) has to be take istead. Here, is at least but this is ot limited by the guidelie ad additioal series ca be measured Repeatability The repeatability b is calculated for each torque step as the absolute amout of the differece (spa) betwee the values of the iitial ad the repeated measuremets at omial legth as i (3) () b X X. (3) 3.4. Ifluece of the legth The ifluece of the legth b is calculated for each torque step as the differece (spa) betwee the values of the icremetal series with reduced lever legth ad the values of the series with omial lever legth as i (4) b X X. (4),red,om 3.5. Ifluece of the coector The ifluece of the coector b V is calculated for each torque step as the differece (spa) betwee the values of the icremetal series with rotated coector ad the values of the series with the iitial positio of the coector as i (5) b. (5) V X V X Ifluece of the regressio The ifluece of the regressio f a is calculated for each torque step as the differece (spa) betwee the values of the icremetal series ad the values obtaied from the regressio fuctio of third (cubic) or first (liear) order as i (6) f Y. (6) a Y a The regressio fuctio ad the method of its determiatio must be stated. If the idicatio of the calibratio device is fixed ( amed scale ), the the ifluece of the idicatio deviatio (3.7) must be determied istead Ifluece of the idicatio deviatio The ifluece of the idicatio deviatio f q has to be determied oly for devices where the idicatio i torque uits caot be adjusted ( amed scale ) by electroic or other meas. It is calculated for each torque step as the differece (the spa) betwee the idicated values of the icremetal series ad the values of the calibratio torque measured with the trasfer wrech as i (7) f Y. (7) q 3.8. Reversibility The reversibility h is calculated for each torque step as mea value of the differeces (spas) betwee icremetal ad decremetal series i the same moutig positios ad at omial legth as i (8) h m m I j I j j. (8) The reversibility is ot take ito accout for the ucertaity calculatio, therefore there is usually o eed to measure it i differet moutig positios. With m = (8) ca be reduced to a simple differece betwee two values similar to the formulas (4) to (7). The parameter h ca be used for quality assurace ad log-term stability. 5. EASUREENT UNCERTAINTIES The resolutio of the torque wrech calibratio device ad the parameters determied accordig to 3. to 3.6 are

5 statisticaly evaluated usig the distributios give i Table 3. I the case of a cubic regressio, the residual deviatio from the regressio fuctio is cosidered to be a stochastic ifluece. I this case, the resultig stadard ucertaity of measuremet ca be calculated applyig the GU procedure leadig to (9) w δ w( ) w (9) TN i with the ucertaity w( TN ) of the torque trasfer wrech ad the cotributios w(δ i ) of the relevat parameters (see Table 3). I (9), the resolutio must be take twice if the value is calculated from two idicatios: oe at the applied torque ad oe at zero torque. The expaded relative ucertaity is the calculated with the coverage factor k as i (0) i W( ) k w. (0) Table 3. Parameters ad their statistical evaluatio. Parameter Distributio type Stadard deviatio i % r b rectagular ormal b ormal b b V f a rectagular rectagular triagular w w w b w r w b ' V w f r 3 b Y b' b 3 b 3 f 6 V Y Y Y a Y I the case of liear regressio (liear curve fit), the deviatios are cosidered to be systematic ad they should ot be treated like stochastic ucertaity cotributios. The same applies to devices with amed scales. Cosequetly, the expaded relative ucertaity is determied i a differet way ad the ew quatity is called ucertaity iterval. With the coverage factor k, the relative ucertaity iterval is calculated as i () W ' fa, q + k w 6. CONCUSIONS. () The ew draft of the guidelie for the calibratio of torque wrech calibratio devices brigs more clarity to its field of applicatio by restrictig it to torque wreches oly. For other torque tools, like screwdrivers, the effect of differet legths or cross forces ad bedig momets does ot play a role i their calibratio. The draft offers a reduced calibratio effort compared with the old 003 issue, which may be required for a simpler desig of the device ad/or its higher target ucertaity. Although the draft has ot yet bee fially approved (Jue 05), it is expected that it will still be issued i 05. ACNOWEDGENTS The author wats to thak all the cotributors to this ew guidelie draft, especially the members of the DD Torque committee ad the laboratories that participated i the test ru of the ew procedures. REFERENCES [] DD-R Static calibratio of torque tool calibratio devices, Issue 003-0, DD, Web: [accessed ].