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1PresentationCoefficient of friction on threaded fasteners in the automotive industryErik GaldamesBach. of Eng., Chem.Xativa, Spain012ContentsIntroductionStandards 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 vocabulary3Introduction40% used in overcoming friction on the thread50% used in overcoming friction under the headTightening process4IntroductionCoefficient of friction of bolted joints is determined measuring force and torque. It is a standardized method that uses a formula in which F and T mainly and dimensional characteristics of the bolt/nut to be studiedMating surfaces and reference bolts/nuts are to be the same and agreed so that results are reproducible and used for comparisonThis test is not suitable to predict behaviour of assembly problems but can give some hints of how the tightening will beVariations of this test under real conditions in the automotive industry can be used in order to predict behaviour at the assembly (e.g. VDA 235-203)TFFTbTth = T TbReference nutBoltMeasuring cellsBearing plateNut holder5IntroductionThe method of calculation is the linear relationship between clamping force and torque below the yield point of the boltAn axial force is produced when a bolt is tightened against a bearing surface and a nut by means of a pair of forces (torque). This force elongates the bolt and compresses the bearing 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 overcome friction50% of torque is used in overcoming friction under the head of the bolt, 40% on the thread and only 10% is used to generate clamping force

Compression/ElongationEquilibrium of forces6Standards and specsDIN 946 (withdrawn)Bestimmung der Reibungszahlen von Schrauben und Muttern unter festgelegten BedindungenISO 16047Fasteners Torque/Clamp force testingRenault 01-50-005Elments de fixation Contrle du coefficient de frottementPSA C10 0054Vis goujons crous Aptitude au frottementFord WZ100 and WZ101Steel Metric Threaded Fasteners Torque/Clamping Force PerformanceVDI 2230Systematische Berechnung hochbeanspruchter Schraubenverbindungen Zylindrische EinschraubverbindungenEN 14399-2Aptitud de uniones atornilladas HV. Ensayo de fuerza y par de apriete ISO 2320Prevailing torque type steel nuts. Mechanical and performance properties7SymbolsISO 16047:2005dNominal diameterFuUltimate clamp forced2Thread flank diameter (basic pitch diameter of thread)FyYield clamp forced4Hole diamater of equipmentTTorquedhHole diameter of the washer or support plateTthThread torqueDoOuter diameter of the bearing surfaceTbBearing surface torque (bearing surface and under the head of the bolt/nut)DpDiameter of plain area of bearing platePPitchDbDiameter of bearing surface under nut or bolt head for friction (theoretical or measured)qRotating angleLCClamp lengthmthCoefficient of friction on the threadLtLength of complete thread between bearing surfacesmbCoefficient of friction on the bearing surface and under bolt head/nutFClamping forcemtotTotal coefficient of frictionFPProof load acc. to ISO 898-1, ISO 898-2 o ISO 898-68Test of coefficient of frictionA torque is applied to a bolted joint made of a bolt, a nut and a bearing surface to generate a clamping force 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 be directly measured. Torque on the thread is calculated through a formula. A graph representing force and torque is represented. The coefficient of friction is represented through the relation between these two values

9Formula of Kellermann-KleinThe formula for determination of coefficient of friction is base on the work of Kellermann-KleinThis formula of Kellermann-Klein was published in 1956 by Rudolf Kellermann and Hans Christof Klein in the essay Bercksichtigung des Reibungszustandes bei der Bemessung hochwertiger Schraubenverbindungen

10Origin of the formula of coefficient of frictionWhen clamping force is below the yield point of the bolt, coefficient of friction is directly proportional to torque and inversely proportional with clamping forceWhen coefficient of friction is higher, torque is higher, clamping force is lowerWhen coefficient of friction is lower, torque is lower, clamping force is higherTightening process of a bolt can be decomposed as an object moving upwards through a slope. Formula of Kellermann-Klein is determined through the study of this movement

Tightening of a boltPitch1/2xPitchUnfolded helixPitch anglePitchHelixCifcumference of the circle11Total coefficient of frictionKellermann-Kleins formula is too complicated to use it as it is and it is simplified for practical usageIt is assumed that friction under the head and friction on the bearing surface is the same, making the formula easier to useThe general method for calculation simplifies partial coefficient of friction under the head and on the thread throughtot = (th + b)/2tot = th = bUncertainty of 1% to 2%To determine coefficient of friction it will be necessary to 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 table for the different dimensions of bolts/nutsIt is necessary to know the characteristics of the bolt (diameter, pitch, flank diameter, PC)Clamping force applied is 75% of proof load acc. to ISO 16047F values are determined through ISO 898-1, ISO 898-2 (ISO 16047)

12Bearing surface under the head of the boltDodhDodhDodhHex boltHex bolt with washerHex flange bolt

13Influencing factors on coefficient of frictionLubricants adjust coefficient of friction and reduce variability of friction. They adjust 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 lubricant between 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 their propertiesIn practice, the following factors have an influence on coefficient of friction:Surface treatment. Type of coating (metallic, zinc flake coatings, lubrication, layer thickness, 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 of the head, washerThread of the mating nut. With coating, without coating, with or without oil. Manufacturing process of the nutTesting conditions. Temperature, humidity, speed of rotationValues of coefficient of friction can be adjusted but these factors may influence their predictible behaviour dramatically if out of control or when there is too much variation14Influencing factors on coefficient of frictionLubricants based on emulsions in water can be applied such as waxes, oils and solid lubricants in water mixes (e.g. PE, PTFE, PAK, molybdenum bisulfide). They are dried after application and they provide a stable coefficient of frictionSolid lubricants or sealers with solid integrated lubricants provide less variation of coefficient of friction than liquid or lubricants in water emulsions and provide better results in automated assemblySolid lubricants also provide better repeated assemblyVariation of coefficient of friction will be higher when working with > 0,14. The tend to scatter moreValues under < 0,08 are difficult to adjust and are not desirable, since self-loosening effect may appearValues over 0,25 do not produce sufficient tightening, so there is a high risk of fatigue fractureValues 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 lower coefficient of friction and unsafe bolted unions. This may lead to ultimate clamping force and thus, bolt fracture. This situation must be avoided

15CoatingsCoatings and lubricants help improving friction behaviour and offer less variation of values of coefficient of frictionCoatings for bolted joints in the automotive industry consist mainly on:Phosphate + post-treatmentElectroplated Zn or Zn alloys (ZnNi, ZnFe) + post-treatmentZinc flake coatings + post-treatmentAs post-treatments, the following materials are available:LubricantsWaxesOilsPTFEMoS2Sealers with integrated lubricantsAnorganic sealersOrganic and anorganic sealersOrganic coatings with integrated lubricantsSealers with integrated lubricants offer corrosion resistance and temperature resistance besides lubrication properties16ISO 16047Uncertainty 2%Room temperature, 10C to 35C, 24 h after coating applicationApplied clamping force, 75% of proof load 0,75FP (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 HLRoughness Ra = 0,5 0,3 m (Ra < 1,6 m and Ra < 3,2 m washer type HL)Tolerance of flatness acc. to ISO 4759-3, section 3.5.3Surfacea) Blank and degreasedb) Zinc plating A1J acc. to ISO 4042 and degreasedMinimum thickness according to ISO 7093-1Hardness 50 to 60 HRc (200 to 300 HV for washer type HL)Hole diameter dh, acc. to ISO 273, medium series, without chamfering Reference nuts for bolt testingA) ISO 4032 and ISO 8673 class 10 uncoated nuts and degreased. B) Zinc plated nuts A1J ISO 4042 and degreasedReference bolts for testing nutsUncoated and degreased bolts ISO 4014, ISO 4017, ISO 4762, ISO 8765, ISO 15071, ISO 15072Zinc plated bolts A1J acc. to ISO 4042 and degreased17Standards and specs of coefficient of frictionISO 16047Ford WZ100Ford WZ101VOLVO STD 5511,72BMW GS90003-1GS90003-2tot--N/A0,14 0,030,12 0,180,09 0,15F75% Fp75% Fp75% Fp75% Fp

See tables on GS9003-2Temperature10C a 35CRTRT10 35C10C a 35CRpm10 a 40 rpm M