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2013 Presentation torque - tension and coefficient of friction of bolts

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Some fundamentals of coefficient of friction of threaded fasteners. Focused on bolts in the automotive industry. ISO 16047 description and requirements in the automotive industry

Some fundamentals of coefficient of friction of threaded fasteners. Focused on bolts in the automotive industry. ISO 16047 description and requirements in the automotive industry

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  • 1. 1PresentationCoefficient of friction onthreaded fasteners in theautomotive industryErik GaldamesBach. of Eng., Chem.Xativa, Spain01
  • 2. 2ContentsIntroductionStandards and specsSymbolsTest of coefficient of frictionFormula of Kellermann-KleinOrigin of formula of coefficient of frictionTotal coefficient of frictionBearing surface under the head of the boltInfluencing factors on coefficient of frictionCoating systemsISO 16047Standards and specs of coefficient of frictionRequirements of coefficient of frictionInternational basic vocabulary
  • 3. 3IntroductionFriccióndebajo decabeza50%Fricción enrosca40%Fuerza deapriete10%10% generatesclamping force40% used inovercomingfriction on thethread50% used inovercomingfriction underthe headTightening process
  • 4. 4IntroductionCoefficient of friction of bolted joints is determined measuring force and torque. It is a standardizedmethod that uses a formula in which F and T mainly and dimensional characteristics of the bolt/nut to bestudiedMating surfaces and reference bolts/nuts are to be the same and agreed so that results are reproducibleand used for comparisonThis test is not suitable to predict behaviour of assembly problems but can give some hints of how thetightening will beVariations of this test under real conditions in the automotive industry can be used in order to predictbehaviour at the assembly (e.g. VDA 235-203)T FFTbTth = T – TbReference nutBoltMeasuringcellsBearing plateNut holder
  • 5. 5IntroductionThe method of calculation is the linear relationship between clamping force and torquebelow the yield point of the boltAn axial force is produced when a bolt is tightened against a bearing surface and a nut bymeans of a pair of forces (torque). This force elongates the bolt and compresses thebearing surfaces in contact, i.e. two opposite forces are producedNot all torque is used to generate clamping; most of the torque applied is used to overcomefriction50% of torque is used in overcoming friction under the head of the bolt, 40% on the threadand only 10% is used to generate clamping forceCompression/Elongation Equilibrium of forces
  • 6. 6Standards and specsDIN 946(withdrawn)Bestimmung der Reibungszahlen von Schrauben und Muttern unter festgelegtenBedindungenISO 16047 Fasteners – Torque/Clamp force testingRenault 01-50-005 Eléments de fixation – Contrôle du coefficient de frottementPSA C10 0054 Vis goujons écrous – Aptitude au frottementFord WZ100 andWZ101Steel Metric Threaded Fasteners Torque/Clamping Force PerformanceVDI 2230 Systematische Berechnung hochbeanspruchter Schraubenverbindungen ZylindrischeEinschraubverbindungenEN 14399-2 Aptitud de uniones atornilladas HV. Ensayo de fuerza y par de aprieteISO 2320 Prevailing torque type steel nuts. Mechanical and performance properties
  • 7. 7SymbolsISO 16047:2005d Nominal diameter Fu Ultimate clamp forced2 Thread flank diameter (basic pitch diameter ofthread)Fy Yield clamp forced4 Hole diamater of equipment T Torquedh Hole diameter of the washer or support plate Tth Thread torqueDo Outer diameter of the bearing surface Tb Bearing surface torque (bearing surface andunder the head of the bolt/nut)Dp Diameter of plain area of bearing plate P PitchDb Diameter of bearing surface under nut or bolthead for friction (theoretical or measured)Rotating angleLC Clamp length th Coefficient of friction on the threadLt Length of complete thread between bearingsurfacesb Coefficient of friction on the bearing surface andunder bolt head/nutF Clamping force tot Total coefficient of frictionFP Proof load acc. to ISO 898-1, ISO 898-2 o ISO898-6
  • 8. 8Test of coefficient of frictionA torque is applied to a bolted joint made of a bolt, a nut and a bearing surface to generate a clampingforce with a rotating unit driven by an encoder motorTorque and clamping force are measured by the measuring headNormally, only clamping force, total torque and torque on the bearing surface under the bolt head can bedirectly measured. Torque on the thread is calculated through a formula. A graph representing force andtorque is represented. The coefficient of friction is represented through the relation between these twovalues0 10 20 30 40 50 60 70 80 90 100Clamping Force/Fv [kN]Torque/Ma[Nm]020406080100120140160Ma/Fv-_001.PRBµges_UL-_001.PRBµges_LL-_001.PRBMa/Fv-_002.PRBµges_UL-_002.PRBµges_LL-_002.PRBMa/Fv-_003.PRBµges_UL-_003.PRBµges_LL-_003.PRBMa/Fv-_004.PRBµges_UL-_004.PRBµges_LL-_004.PRBMa/Fv-_005.PRBµges_UL-_005.PRBµges_LL-_005.PRBMa/Fv-_006.PRBµges_UL-_006.PRBµges_LL-_006.PRBMa/Fv-_007.PRBµges_UL-_007.PRB
  • 9. 9Formula of Kellermann-KleinThe formula for determination ofcoefficient of friction is base on the work ofKellermann-KleinThis formula of Kellermann-Klein waspublished in 1956 by Rudolf Kellermannand Hans Christof Klein in the essay“Berücksichtigung des Reibungszustandesbei der Bemessung hochwertigerSchraubenverbindungen”(10)4154,1154,12122 hobthth dDdPdPFT
  • 10. 10Origin of the formula of coefficient of frictionWhen clamping force is below the yield point of thebolt, coefficient of friction is directly proportional totorque and inversely proportional with clampingforceWhen coefficient of friction is higher, torque ishigher, clamping force is lowerWhen coefficient of friction is lower, torque is lower,clamping force is higherTightening process of a bolt can be decomposed asan object moving upwards through a slope. Formulaof Kellermann-Klein is determined through the studyof this movementTightening of a boltPitch1/2xPitchUnfolded helixPitch anglePitchHelixCifcumference of the circle
  • 11. 11Total coefficient of frictionKellermann-Klein’s formula is too complicated to useit as it is and it is simplified for practical usageIt is assumed that friction under the head and frictionon the bearing surface is the same, making theformula easier to useThe general method for calculation simplifies partialcoefficient of friction under the head and on thethread throughµtot = µth + µb)/2µtot = µth = µbUncertainty of 1% to 2%To determine coefficient of friction it will be necessaryto know:T, Tb, F, measured by equipmentTth is calculated through T = Tb + Tth then, Tth = T -TbP, d2 y Db Dimensional parameters of the bolt/nutTarget values of T and F are obtained through a tablefor the different dimensions of bolts/nuts– It is necessary to know the characteristics of thebolt (diameter, pitch, flank diameter, PC)– Clamping force applied is 75% of proof load acc. toISO 16047F values are determined through ISO 898-1, ISO898-2 (ISO 16047)(5)2577,022btotDdPFT
  • 12. 12Bearing surface under the head of the boltDodhDodhDodhHex bolt Hex bolt with washer Hex flange bolt23 220330hobhhbdDDdDdDD
  • 13. 13Influencing factors on coefficient of frictionLubricants adjust coefficient of friction and reduce variability of friction. Theyadjust coefficient of friction on a certain window so that friction is more regular.Their action relies on the interfering action caused by the molecules of lubricantbetween the mating surfaces and thus, friction is reducedModern coating systems incorporate solid lubricants in their formulation. Thus,not only corrosion protection is obtained; lubrication is additionally among theirpropertiesIn practice, the following factors have an influence on coefficient of friction:– Surface treatment. Type of coating (metallic, zinc flake coatings, lubrication, layerthickness, dirt)– Bearing surface. Hard surface (e.g. roughness, heat treated, non-heat treated steel,aluminium, KTL)– Geometry of the head. Pan head screw, hexagonal bolt, hex flange bolt, diameter ofthe head, washer– Thread of the mating nut. With coating, without coating, with or without oil.Manufacturing process of the nut– Testing conditions. Temperature, humidity, speed of rotationValues of coefficient of friction can be adjusted but these factors may influencetheir predictible behaviour dramatically if out of control or when there is too muchvariation
  • 14. 14Influencing factors on coefficient of frictionLubricants based on emulsions in water can be applied such as waxes, oils and solidlubricants in water mixes (e.g. PE, PTFE, PAK, molybdenum bisulfide). They are dried afterapplication and they provide a stable coefficient of frictionSolid lubricants or sealers with solid integrated lubricants provide less variation ofcoefficient of friction than liquid or lubricants in water emulsions and provide better resultsin automated assemblySolid lubricants also provide better repeated assemblyVariation of coefficient of friction will be higher when working with µ > 0,14. The tend toscatter moreValues under µ < 0,08 are difficult to adjust and are not desirable, since self-looseningeffect may appearValues over 0,25 do not produce sufficient tightening, so there is a high risk of fatiguefractureValues under 0,06 can lead to ultimate clamping load. High risk of fracture.There are some bolted unions that request coefficient of friction of 0,06 to 0,09Uncontrolled lubrication such as oil spraying on the workshop could lead to lowercoefficient of friction and unsafe bolted unions. This may lead to ultimate clamping forceand thus, bolt fracture. This situation must be avoided
  • 15. 15CoatingsCoatings and lubricants help improving friction behaviour and offer less variationof values of coefficient of frictionCoatings for bolted joints in the automotive industry consist mainly on:– Phosphate + post-treatment– Electroplated Zn or Zn alloys (ZnNi, ZnFe) + post-treatment– Zinc flake coatings + post-treatmentAs post-treatments, the following materials are available:– LubricantsWaxesOilsPTFEMoS2– Sealers with integrated lubricantsAnorganic sealersOrganic and anorganic sealers– Organic coatings with integrated lubricantsSealers with integrated lubricants offer corrosion resistance and temperatureresistance besides lubrication properties
  • 16. 16ISO 16047Uncertainty 2%Room temperature, 10ºC to 35ºC, 24 h after coating applicationApplied clamping force, 75% of proof load 0,75·FP (see ISO 898-1, ISO 898-2)Rotation speed = 10 a 40 rpm (M1,6 to M16), 5 to 15 rpm (M16 to M39)Bearing plate or washer type HH or HL– Roughness Ra = 0,5 0,3 µm (Ra < 1,6 µm and Ra < 3,2 µm washer type HL)– Tolerance of flatness acc. to ISO 4759-3, section 3.5.3– Surfacea) Blank and degreasedb) Zinc plating A1J acc. to ISO 4042 and degreased– Minimum thickness according to ISO 7093-1– Hardness 50 to 60 HRc (200 to 300 HV for washer type HL)– Hole diameter dh, acc. to ISO 273, medium series, without chamferingReference nuts for bolt testing– A) ISO 4032 and ISO 8673 class 10 uncoated nuts and degreased.– B) Zinc plated nuts A1J ISO 4042 and degreasedReference bolts for testing nuts– Uncoated and degreased bolts ISO 4014, ISO 4017, ISO 4762, ISO 8765, ISO 15071, ISO 15072– Zinc plated bolts A1J acc. to ISO 4042 and degreased
  • 17. 17Standards and specs of coefficient of frictionISO 16047 Ford WZ100 Ford WZ101 VOLVO STD5511,72BMW GS90003-1GS90003-2µtot -- N/A 0,14 0,03 0,12 – 0,18 0,09 – 0,15F 75% Fp 75% Fp 75% Fp 75% Fp See tables onGS9003-2Temperature 10ºC a 35ºC RT RT 10 – 35ºC 10ºC a 35ºCRpm 10 a 40 rpm M<16 <30 rpm 30 10 rpm < M16 10 – 25 rpm 10 – 25 RPMUncertainty 2% F, T, 3% F, 2% T 2% F, T 2% F, T 2% F, TBearing surface 200 – 300HV (HL)50 – 60 HRC (HH)Steel500 – 600HVSteel200 – 250HV≤ 8.8 200-250 HV10.9 = 300 – 400 HV12.9 = 350 – 450 HVType HH 50 – 60 HRCRoughness Ra 1,6 ≤ 3 mm; Ra3,2 3 < h ≤ 6 mmN4-N5 ISO 1302 Ra 1,2 a 1,6 µm Ra 1,6 max Ra 1,6 max (≤ h 3 mm)Ra 3,2 max (> h 3 mmTolerance flatness See ISO 4759-3class A// 4% // 4% // 4% See ISO 4759-3class ADimensions Acc. to standard Acc. to standard Acc. to standard Acc. to standard Acc. to standardReference nuts ISO 4032, 8673, 4033,8674 6HThread ISO 965/1 6H ISO 4032 6H ISO 4032 6H ISO 4032, 8673, 4033,8674 6HNut surface Uncoated, oil free S309zinc plated passiv. lubr.Uncoated, oil free Uncoated oil free Uncoated, oil freeReference bolt ISO 965/1 6g ISO 4014 6g ISO 965-2 6g ISO 965-2 6GBolt surface Uncoated, oil free S309zinc plated passiv. lubr.Uncoated, oil free Uncoated, oil free Uncoated
  • 18. 18Requirements of coefficient of friction0,07 0,08 0,09 0,10 0,11 0,12 0,13VDA 235-101VW 011 29Ford WZ 101GMW 3359, GMW 3044Renault 01-50-005CPSA C10 00 540,06 0,14 0,15 0,16 0,17 0,18µth, µbLow Friction µtotNormal Friction µtotµth, µbµtot (*) µtot (*)µtotµtotµtotµtot(*)Interval enlarged to include uncertainty of measurement of coefficient of frictionµtotVolvo STD 5511,72 µtotBMW GS 90003-1 µtotµth, µb µth, µb
  • 19. 19International basic vocabularySpanish English French German ItalianCoeficiente de fricción Coefficient of friction Coefficient de frottement Reibungszahl Coefficiente d’attritoPar de apriete, momento deaprieteTightening torque Couple de sérrage Anziehdrehmoment Coppia di serraggio,momento di serraggioCarga, tensión Clamp force Tension Vorspannkraft Precarico, tensioneCarga de prueba Proof load Tension d’épreuve Prüfkraft Carico di provaCarga de rotura Ultimate clamp force Tension de rupture Bruchkraft Carico di rotturaLímite elástico Yield point Limite d’élasticité Streckgrenze Limite d’esnervamentoÁngulo de giro Rotating angle Angle de rotation Drehwinkel Angulo di giroTornillo Bolt, screw Vis, boulon Schraube Vite, bulloneTuerca Nut Écrou Mutter DadoArandela Washer Rondelle Scheibe RosettaEspárrago Stud Goujon Stiftschraube PrigionieroRosca Screw thread Filetage Gewinde Filetto, filettaturaSuperficie de apoyo Bearing surface Surface d’appui Auflagefläche Superficie sottotestaAgujero de paso Clearance hole Taraudage Durchgangsloch Foro de passoPaso de rosca Pitch Pas Steigung Passo
  • 20. 20THE ENDGracias por su atención!Thanks for your attention!Bedankt voor uw aandacht!Danke für Ihre Aufmerksamkeit!

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