1

Presentation

Coefficient of friction on threaded fasteners in the

automotive industryErik Galdames

Bach. of Eng., Chem.Xativa, Spain

01

2

Contents

IntroductionStandards 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 friction

International basic vocabulary

3

Introduction

Fricción debajo de cabeza

50%

Fricción en rosca40%

Fuerza de apriete10%

10% generates clamping force

40% used in overcoming

friction on the thread

50% used in overcoming

friction under the head

Tightening process

4

Introduction

Coefficient 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)

T F

F

Tb

Tth = T – Tb

Reference nutBolt

Measuring cells

Bearing plate

Nut holder

5

Introduction

The 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/Elongation Equilibrium of forces

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Standards and specs

DIN 946 (withdrawn)

Bestimmung der Reibungszahlen von Schrauben und Muttern unter festgelegten Bedindungen

ISO 16047 Fasteners – Torque/Clamp force testing

Renault 01-50-005 Eléments de fixation – Contrôle du coefficient de frottement

PSA C10 0054 Vis goujons écrous – Aptitude au frottement

Ford WZ100 and WZ101

Steel Metric Threaded Fasteners Torque/Clamping Force Performance

VDI 2230 Systematische Berechnung hochbeanspruchter Schraubenverbindungen Zylindrische Einschraubverbindungen

EN 14399-2 Aptitud de uniones atornilladas HV. Ensayo de fuerza y par de apriete

ISO 2320 Prevailing torque type steel nuts. Mechanical and performance properties

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SymbolsISO 16047:2005

d Nominal diameter Fu Ultimate clamp force

d2 Thread flank diameter (basic pitch diameter of thread)

Fy Yield clamp force

d4 Hole diamater of equipment T Torque

dh Hole diameter of the washer or support plate Tth Thread torque

Do Outer diameter of the bearing surface Tb Bearing surface torque (bearing surface and under the head of the bolt/nut)

Dp Diameter of plain area of bearing plate P Pitch

Db Diameter of bearing surface under nut or bolt head for friction (theoretical or measured)

q Rotating angle

LC Clamp length mth Coefficient of friction on the thread

Lt Length of complete thread between bearing surfaces

mb Coefficient of friction on the bearing surface and under bolt head/nut

F Clamping force mtot Total coefficient of friction

FP Proof load acc. to ISO 898-1, ISO 898-2 o ISO 898-6

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Test of coefficient of friction

A 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 motor

Torque and clamping force are measured by the measuring head

Normally, 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

0 10 20 30 40 50 60 70 80 90 100Clamping Force/Fv [kN]

Torq

ue/M

a [N

m]

0

20

40

60

80

100

120

140

160

Ma/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.PRBµges_LL-_007.PRBMa/Fv-_008.PRBµges_UL-_008.PRBµges_LL-_008.PRBMa/Fv-_009.PRBµges_UL-_009.PRBµges_LL-_009.PRBMa/Fv-_010.PRBµges_UL-_010.PRBµges_LL-_010.PRB

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Formula of Kellermann-Klein

The 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 “Berücksichtigung des Reibungszustandes bei der Bemessung hochwertiger Schraubenverbindungen”

(10) 4154,1

154,1

2

1

2

2

ho

b

th

th dD

dPdP

FT

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Origin of the formula of coefficient of friction

When clamping force is below the yield point of the bolt, coefficient of friction is directly proportional to torque and inversely proportional with clamping force

When coefficient of friction is higher, torque is higher, clamping force is lower

When coefficient of friction is lower, torque is lower, clamping force is higher

Tightening 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 bolt

Pitch

1/2x

Pitc

h

Unfolded helix

Pitch angle

Pitc

h

Helix

Cifcumference of the circle

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Total coefficient of friction

Kellermann-Klein’s 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 throughµtot = (µth + µb)/2µtot = µth = µb

Uncertainty 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 - Tb

P, 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/nuts– It 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)

(5)

2577,0

2

2b

tot Dd

PFT

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Bearing surface under the head of the bolt

Do

dh

Do

dh

Do

dh

Hex bolt Hex bolt with washer Hex flange bolt

2

3 220

330

hob

h

hb

dDD

dD

dDD

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Influencing factors on coefficient of friction

Lubricants 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, 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 rotation

Values 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 variation

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Influencing factors on coefficient of friction

Lubricants 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 µ > 0,14. The tend to scatter moreValues under µ < 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

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Coatings

Coatings 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-treatment– Electroplated Zn or Zn alloys (ZnNi, ZnFe) + post-treatment– Zinc flake coatings + post-treatment

As post-treatments, the following materials are available:– Lubricants

WaxesOilsPTFEMoS2

– Sealers with integrated lubricantsAnorganic sealersOrganic and anorganic sealers

– Organic coatings with integrated lubricantsSealers with integrated lubricants offer corrosion resistance and temperature resistance besides lubrication properties

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ISO 16047

Uncertainty± 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– Surface

a) 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 chamfering

Reference nuts for bolt testing– A) ISO 4032 and ISO 8673 class 10 uncoated nuts and degreased. – B) Zinc plated nuts A1J ISO 4042 and degreased

Reference 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

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Standards and specs of coefficient of friction

ISO 16047 Ford WZ100 Ford WZ101 VOLVO STD 5511,72

BMW GS90003-1GS90003-2

µtot -- N/A 0,14 ± 0,03 0,12 – 0,18 0,09 – 0,15

F 75% Fp 75% Fp 75% Fp 75% Fp See tables on GS9003-2

Temperature 10ºC a 35ºC RT RT 10 – 35ºC 10ºC a 35ºC

Rpm 10 a 40 rpm M<16 <30 rpm 30 ± 10 rpm < M16 10 – 25 rpm 10 – 25 RPM

Uncertainty ± 2% F, T, q ± 3% F, ± 2% T ± 2% F, T ± 2% F, T ± 2% F, T

Bearing surface 200 – 300HV (HL)50 – 60 HRC (HH)

Steel 500 – 600HV

Steel200 – 250HV

≤ 8.8 200-250 HV10.9 = 300 – 400 HV12.9 = 350 – 450 HV

Type HH 50 – 60 HRC

Roughness Ra 1,6 ≤ 3 mm; Ra 3,2 3 < h ≤ 6 mm

N4-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 mm

Tolerance flatness See ISO 4759-3 class A

// 4% // 4% // 4% See ISO 4759-3 class A

Dimensions Acc. to standard Acc. to standard Acc. to standard Acc. to standard Acc. to standard

Reference nuts ISO 4032, 8673, 4033, 8674 6H

Thread ISO 965/1 6H ISO 4032 6H ISO 4032 6H ISO 4032, 8673, 4033, 8674 6H

Nut surface Uncoated, oil free S309zinc plated passiv. lubr.

Uncoated, oil free Uncoated oil free Uncoated, oil free

Reference bolt ISO 965/1 6g ISO 4014 6g ISO 965-2 6g ISO 965-2 6G

Bolt surface Uncoated, oil free S309zinc plated passiv. lubr.

Uncoated, oil free Uncoated, oil free Uncoated

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Requirements of coefficient of friction

0,07 0,08 0,09 0,10 0,11 0,12 0,13

VDA 235-101

VW 011 29

Ford WZ 101

GMW 3359, GMW 3044

Renault 01-50-005C

PSA C10 00 54

0,06 0,14 0,15 0,16 0,17 0,18

µth, µb

Low Friction µtotNormal Friction µtot

µth, µb

µtot (*) µtot (*)µtot

µtot

µtot

µtot

(*)Interval enlarged to include uncertainty of measurement of coefficient of friction

µtot

Volvo STD 5511,72 µtot

BMW GS 90003-1 µtotµth, µb µth, µb

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International basic vocabulary

Spanish English French German Italian

Coeficiente de fricción Coefficient of friction Coefficient de frottement Reibungszahl Coefficiente d’attrito

Par de apriete, momento de apriete

Tightening torque Couple de sérrage Anziehdrehmoment Coppia di serraggio, momento di serraggio

Carga, tensión Clamp force Tension Vorspannkraft Precarico, tensione

Carga de prueba Proof load Tension d’épreuve Prüfkraft Carico di prova

Carga de rotura Ultimate clamp force Tension de rupture Bruchkraft Carico di rottura

Límite elástico Yield point Limite d’élasticité Streckgrenze Limite d’esnervamento

Ángulo de giro Rotating angle Angle de rotation Drehwinkel Angulo di giro

Tornillo Bolt, screw Vis, boulon Schraube Vite, bullone

Tuerca Nut Écrou Mutter Dado

Arandela Washer Rondelle Scheibe Rosetta

Espárrago Stud Goujon Stiftschraube Prigioniero

Rosca Screw thread Filetage Gewinde Filetto, filettatura

Superficie de apoyo Bearing surface Surface d’appui Auflagefläche Superficie sottotesta

Agujero de paso Clearance hole Taraudage Durchgangsloch Foro de passo

Paso de rosca Pitch Pas Steigung Passo

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THE END

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Thanks for your attention!

Bedankt voor uw aandacht!

Danke für Ihre Aufmerksamkeit!