2012 Bolted joints - Bolted joints in real conditions

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A summary of the conditions encountered when tightening bolts in the automotive industry. Experiences when using different materials and different tightening procedures. Based on experience and specifications of the automotive industry

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2012 Bolted joints - Bolted joints in real conditions

  1. 1. BOLTED JOINTSBolt tightening with new materialsby Erik GaldamesTranslation and adaptation of thepresentation “Condiciones reales de montaje”at the Technical Meeting held on 20th July,2012 organized by Galol, L’Olleria (Spain)
  2. 2. Bolted joints – Bolt tightening in real conditionsBasic considerations Tightening of bolts with new materials show achallenge when trying to obtain new results in thedesign of bolted joints Not only considerations of individual components are tobe taken into account in a separate way, it is necessaryto know their interaction and behaviour in their differentcombinations Theoretical data or data from charts are not only thesources to be considered, it is of vital importance toknow the behaviour of the different materials in contact These facts have contributed to seek for practicalsolutions in the different specifications of someindustrial sectors, mainly automotive industry, to studyand determine solutions forr different problems thatmay occur2
  3. 3. Bolted joints – Bolt tightening in real conditionsBasic considerations Types of fasteners Types of coatings Mating materials Thread length Environmental conditions3
  4. 4. Bolted joints – Bolt tightening in real conditionsTypes of fasteners Metric threaded bolts, nuts Self-tapping metric screws Self-tapping screws Studs4
  5. 5. Bolted joints – Bolt tightening in real conditionsDesign considerations Geometry of parts Dimensions and tolerances Internally driven fasteners, externallydriven Clearance hole Thread length Surface pressure. Use of washers, type ofmaterial to fasten5
  6. 6. Bolted joints – Bolt tightening in real conditionsCoatings of fasteners The selection of the adequate coating for a certainapplication plays an important role. The followingfamilies of coatings for fasteners exist in theautomotive industry nowadays, not exclusively: Zinc flake coatings with/without top-coats Electroplated zinc, zinc-nickel with/without top-coats Phosphating + oil6
  7. 7. Bolted joints – Bolt tightening in real conditionsCoatings of the mating surfaces Coatings applied to the mating surfaces tobe fastened with bolts/nuts Zinc flake coatings with/without top-coats Electroplated zinc, zinc-nickel with/without top-coats KTL No coating (e.g. Al-alloys)7
  8. 8. Bolted joints – Bolt tightening in real conditionsCoatings Factors to be taken into account in theirselection: Colour Property class of fasteners. Hydrogen embrittlementavoidance Tightening with hexalobular tool, internal hexagonaltool, etc. Presence of pilot (form of the tip of the bolt) Service temperature Use in combination with locking features ormicroencapsulated coatigns DIN 267-27 or DIN 267-28 Contact with magnesium alloys Ground connections8
  9. 9. Bolted joints – Bolt tightening in real conditionsCoatingsCoating Typical applications Adequate for Less adequate forZinc flake coatings ChassisMotor compartment (areasexposed to high corrosivity)Aluminium jointsBolts ≥ 10.9Microencapsulated coatingallowed (without lubricated top-coat)Ground connectionsExternal thread <M6Internal thread <M10Internal drive features T30ZnNi ChassisMotor compartment (areasexposed to high corrosivity)Aluminium jointsExternal thread < M6Internal thread < M10Bolts ≥ 10.9, ≥ 1000 N/mm2Though less hydrogenembrittlement risk than pure e-plated Zn.Service temperature > 150ºCZnNi black ChassisMotor compartment (areasexposed to high corrosivity)Aluminium jointsExternal thread < M6 withinternal drive featureInternal thread <M10Self-tapping screwsBolts ≥ 10.9, ≥ 1000 N/mm2Though less hydrogenembrittlement risk than pure e-plated Zn.Service temperature > 150ºCPhosphating + oil Multicomponent, oil circuits ortransportationParts in motion withoutcorrosion exposureTransport and limited storagein warehousesParts not multi-componentService temperature > 180ºCZn+passivation+top-coat(silicate)Mg alloys Parts in contact withmagnesium alloysBolts ≥ 10.9Zn+passivation Weld parts Weld parts Bolts ≥ 10.99
  10. 10. Bolted joints – Bolt tightening in real conditionsMaterials Typical materials found in bolted joints Chassis metal sheet Non-structural steel Sintered materials Cast steel Wrought Al alloys Cast Al alloys Mg alloys Ti alloys Zinc cast alloys10
  11. 11. Bolted joints – Bolt tightening in real conditionsMaterialsMaterial Abbreviation Material No. StandardCold formed steel DC01S700MC1.0331.8974EN 10130EN 10149-2Non-allied structural steel S355JR 1.0045 EN 10025-2Steel for quenching andtemperingC45EC34CrNiMo616MnCr51.11921.65821.7131EN 10263-4EN 10083-3EN 10084Sintered materials Sint – D30 - DIN 30910-4Austenitic stainless steel X5CrNi18-12X5CrNiMo17-12-2X6NiCrTiMoVB25-15-21.43031.44011.4980EN 1008-3EN 10269Cast iron EN-GJL-150EN-GJS-40015UEN-GJS-500-7UEN-GJS-600-3UEN-JL1020EN-JS1072EN-JS1082EN-JS1092EN 1561EN 156311
  12. 12. Bolted joints – Bolt tightening in real conditionsMaterialsMaterial Abbreviation Material No. StandardCold formed Al alloys EN-AW-AlSi1MgMn-T6EN AW-AlSi1MgMn-T4EN AW-AlMg4, 5Mn0,7-H111EN AW-6082EN AW-6082EN AW-5083EN 754-2Al alloy castings EN AC-AlSi6Cu4-SFEN AC-AlSi9Cu3(Fe)EN AC-AlSi7Mg0,3-T6EN AC-45000EN AC-46000EN AC-42100EN 1706Mg alloys EN MC-MgAl9Zn1(A)-F-DEN MC-MgAl6Mn-DEN-MC21120EN-MC21230EN 1753Ti alloys TiAl6V4 3.7165.1 DIN 17862Zinc alloy castings ZP3ZP5ZP0400ZP0410EN 1284412
  13. 13. Bolted joints – Bolt tightening in real conditionsMating materials Some softer materials present problems in theassembly due to the increase of friction. E.g. Al and Mgalloys show higher friction compared to steel. Thesame bolt with the same coating may present differentbehaviour when the mating material is an Al-alloy Surface roughness plays an important role, as well asgeometry of the bearing surface of the fastener(concave, convex). This could influence the frictionbehaviour enormously. E.g. Worst case concavesurface. Thread length also has a strong influence. When themating surface between both elements to tighten islarger, unexpected effects may occur, since friction inthe thread is higher13
  14. 14. Bolted joints – Bolt tightening in real conditionsThread lengthMATERIAL Min. RmMin. Brinell HardnessNo. (BHN)Longitud de roscadomín. (tornillos 10.9)Hardened steel,tempered1000 MPa 0,8·dTempered steel,hardened800 Mpa 1,0·dLow-and unalloyedsteel400 Mpa 1,3·dAl and Mg alloycastings80 BHN60 BHN2,0·d (8.8)2,5·d (10.9)2,7·d (≤ 8.8)CuZn alloys 350 MPa 1,3·dSintered parts 510 MPa 1,0·d14
  15. 15. Bolted joints – Bolt tightening in real conditionsOther materials Unions with self-tapping screws on plastics may showunexpected effects Creep of polymers produces drop of preload, thusloosening the union For unions with high preload, use of metal sleeves isrecommended Microencapsulated coatings increase coefficient offriction during assembly. With the introduction of newdeveloped products (µtot 0,12 to 0,16) this effect can beavoided It is not allowed in some automotive specs to usemicroencapsulated coatings over top-coats withintegrated lubricants due to the risk of lack of adhesionof the locking feature over the top-coat15
  16. 16. Bolted joints – Bolt tightening in real conditionsOther factors Use of washers. They change pressure applied on the bolted union Geometry of washers (Aussenträger or concave washer). The mostcomplicated case in bolted unions due to the reduced contact surface Galvanic compatibility (galvanic series). Special attention to materials likeAl, Mg or stainless steel. Cu. Not compatible with most of the coatings that provide cathodic protection Al alloy. Compatible with zinc flake coatings, zinc-nickel Anodized Al. Not recommended for fasteners with zinc flake coatings or electroplatedzinc alloys Mg alloys. Not compatible with zinc flake coatings. Compatible with some electroplatedzinc coatings with special top-coats Specific surface. Avoid relatively small areas of the less noble metal with alarger surface of the most noble metal E.g. A zinc plated bolt fastened to an anodized aluminium sheet or a stainless steelpanel. The surface of the bolt is smaller than the Al sheet (or stainless steel sheet).Finally, the bolt is corroded16
  17. 17. Bolted joints – Bolt tightening in real conditionsOther factorsGalvanic seriesPlatinumGoldGraphiteTitaniumSilverStainless steel (passive)Nickel (passive)BronzeNickel (active)LeadStainless steel (active)IronSteelAluminiumZincMagnesiumCathodicAnodic17
  18. 18. Bolted joints – Bolt tightening in real conditionsTemperature and relative humidity Service temperature. There is a risk of self-loosening athigh temperatures in some coatings with integratedlubricant due to loss of self-retention. For this reason,coating systems must comply with VDA 235-203requirements before approval Relative humidity also has a strong influence. Beforecomparing results it is necessary that parts can beconditioned before testing. Relative humidity has influenced in the assemblyconditions in some cases, so this has led tomisinterpretation over the quality of the parts receivedby the customer Too high humidity may produce a higher lubrication anda too dry environment may increase coefficient offriction18
  19. 19. Bolted joints – Bolt tightening in real conditionsOther factors Type of tightening procedure Tightening below yield point. If torque controlled tightening isperformed, friction has a strong influence and a short variation intorque may influence in a greater variation of preload. E.g. In torquecontrolled tightening, a torque variation of ± 15% ∆T can produce apreload variation of ∆F ± 35%. With angle controlled tightening ∆F ±13% Tightening beyond yield point. The maximum design of the bolt isused when bolt is assembled beyond the yield point. When bolt istightened within the plastic range, friction under the head of the boltplays a less important role with a torque/angle controlled tighteningprocedure High speed tightening. Tightening in two steps (e.g. 200 rpm,stop, final tightening at 20 rpm). Speed of rotation can behigher in some cases, thus friction effects may vary Use of spindle19
  20. 20. Bolted joints – Bolt tightening in real conditionsSummary In the design of bolted joints not only the mechanicalcharacteristics of the bolt have to be consideredindividually. The other materials used in the union playa vital role Furthermore, not only the coating systems appliedinfluence exclusively in the tightening process, otherfactors must be considered. A different coating systemmay influence tightening process dramatically It is of vital importance to know the conditions of thebolted union of the different materials, their interactionand how they behave in the different situationsencountered Temperature and humidity conditions are alsoimportant, mainly when comparing different unions20
  21. 21. Bolted joints – Bolt tightening in real conditionsFurther reading VDI 2230-1 VDA 235-203 KAMAX Schraubenbrevier Pierre R. Roberge. Handbook of corrosion engineering K. Kayser. High-tensile bolted joints. Design Parameters, Assembly,Locking features K. H. Kübler, W. J. Mages. Handbuch der hochfesten Schrauben Erik Galdames, 2006 Guía de las uniones atornilladas21

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