BOLTED JOINTSBolt tightening with new materials

by Erik GaldamesTranslation and adaptation of the presentation “Condiciones reales de montaje” at the Technical Meeting held on 20th July, 2012 organized by Galol, L’Olleria (Spain)

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Bolted joints – Bolt tightening in real conditionsBasic considerations

Tightening of bolts with new materials show a challenge when trying to obtain new results in the design of bolted joints

Not only considerations of individual components are to be taken into account in a separate way, it is necessary to know their interaction and behaviour in their different combinations

Theoretical data or data from charts are not only the sources to be considered, it is of vital importance to know the behaviour of the different materials in contact

These facts have contributed to seek for practical solutions in the different specifications of some industrial sectors, mainly automotive industry, to study and determine solutions forr different problems that may occur

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Bolted joints – Bolt tightening in real conditionsBasic considerations

Types of fasteners Types of coatings Mating materials Thread length Environmental conditions

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Bolted joints – Bolt tightening in real conditionsTypes of fasteners

Metric threaded bolts, nuts Self-tapping metric screws Self-tapping screws Studs

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Bolted joints – Bolt tightening in real conditionsDesign considerations

Geometry of parts Dimensions and tolerances Internally driven fasteners, externally

driven Clearance hole Thread length Surface pressure. Use of washers, type of

material to fasten

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Bolted joints – Bolt tightening in real conditionsCoatings of fasteners

The selection of the adequate coating for a certain application plays an important role. The following families of coatings for fasteners exist in the automotive industry nowadays, not exclusively: Zinc flake coatings with/without top-coats Electroplated zinc, zinc-nickel with/without top-coats Phosphating + oil

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Bolted joints – Bolt tightening in real conditionsCoatings of the mating surfaces

Coatings applied to the mating surfaces to be 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)

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Bolted joints – Bolt tightening in real conditionsCoatings

Factors to be taken into account in their selection: Colour Property class of fasteners. Hydrogen embrittlement

avoidance Tightening with hexalobular tool, internal hexagonal

tool, etc. Presence of pilot (form of the tip of the bolt) Service temperature Use in combination with locking features or

microencapsulated coatigns DIN 267-27 or DIN 267-28 Contact with magnesium alloys Ground connections

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Bolted joints – Bolt tightening in real conditionsCoatings

Coating Typical applications Adequate for Less adequate for

Zinc flake coatings ChassisMotor compartment (areas exposed to high corrosivity)Aluminium joints

Bolts ≥ 10.9Microencapsulated coating allowed (without lubricated top-coat)

Ground connectionsExternal thread <M6Internal thread <M10Internal drive features T30

ZnNi ChassisMotor compartment (areas exposed to high corrosivity)Aluminium joints

External thread < M6Internal thread < M10

Bolts ≥ 10.9, ≥ 1000 N/mm2

Though less hydrogen embrittlement risk than pure e-plated Zn. Service temperature > 150ºC

ZnNi black ChassisMotor compartment (areas exposed to high corrosivity)Aluminium joints

External thread < M6 with internal drive featureInternal thread <M10Self-tapping screws

Bolts ≥ 10.9, ≥ 1000 N/mm2

Though less hydrogen embrittlement risk than pure e-plated Zn. Service temperature > 150ºC

Phosphating + oil Multicomponent, oil circuits or transportation

Parts in motion without corrosion exposureTransport and limited storage in warehouses

Parts not multi-componentService temperature > 180ºC

Zn+passivation+top-coat (silicate)

Mg alloys Parts in contact with magnesium alloys

Bolts ≥ 10.9

Zn+passivation Weld parts Weld parts Bolts ≥ 10.9

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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 alloys

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Bolted joints – Bolt tightening in real conditionsMaterials

Material Abbreviation Material No. StandardCold formed steel DC01

S700MC1.0331.8974

EN 10130EN 10149-2

Non-allied structural steel S355JR 1.0045 EN 10025-2

Steel for quenching and tempering

C45EC34CrNiMo616MnCr5

1.11921.65821.7131

EN 10263-4EN 10083-3EN 10084

Sintered materials Sint – D30 - DIN 30910-4

Austenitic stainless steel X5CrNi18-12X5CrNiMo17-12-2X6NiCrTiMoVB25-15-2

1.43031.44011.4980

EN 1008-3

EN 10269

Cast iron EN-GJL-150EN-GJS-40015UEN-GJS-500-7UEN-GJS-600-3U

EN-JL1020EN-JS1072EN-JS1082EN-JS1092

EN 1561

EN 1563

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Bolted joints – Bolt tightening in real conditionsMaterials

Material Abbreviation Material No. StandardCold formed Al alloys EN-AW-AlSi1MgMn-T6

EN AW-AlSi1MgMn-T4EN AW-AlMg4, 5Mn0,7-H111

EN AW-6082EN AW-6082EN AW-5083

EN 754-2

Al alloy castings EN AC-AlSi6Cu4-SFEN AC-AlSi9Cu3(Fe)EN AC-AlSi7Mg0,3-T6

EN AC-45000EN AC-46000EN AC-42100

EN 1706

Mg alloys EN MC-MgAl9Zn1(A)-F-DEN MC-MgAl6Mn-D

EN-MC21120EN-MC21230

EN 1753

Ti alloys TiAl6V4 3.7165.1 DIN 17862

Zinc alloy castings ZP3ZP5

ZP0400ZP0410

EN 12844

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Bolted joints – Bolt tightening in real conditionsMating materials

Some softer materials present problems in the assembly due to the increase of friction. E.g. Al and Mg alloys show higher friction compared to steel. The same bolt with the same coating may present different behaviour when the mating material is an Al-alloy

Surface roughness plays an important role, as well as geometry of the bearing surface of the fastener (concave, convex). This could influence the friction behaviour enormously. E.g. Worst case concave surface.

Thread length also has a strong influence. When the mating surface between both elements to tighten is larger, unexpected effects may occur, since friction in the thread is higher

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Bolted joints – Bolt tightening in real conditionsThread length

MATERIAL Min. RmMin. Brinell Hardness No. (BHN)

Longitud de roscado mín. (tornillos 10.9)

Hardened steel, tempered

1000 MPa 0,8·d

Tempered steel, hardened

800 Mpa 1,0·d

Low-and unalloyed steel

400 Mpa 1,3·d

Al and Mg alloy castings

80 BHN

60 BHN

2,0·d (8.8)2,5·d (10.9)2,7·d (≤ 8.8)

CuZn alloys 350 MPa 1,3·d

Sintered parts 510 MPa 1,0·d

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Bolted joints – Bolt tightening in real conditionsOther materials

Unions with self-tapping screws on plastics may show unexpected effects

Creep of polymers produces drop of preload, thus loosening the union

For unions with high preload, use of metal sleeves is recommended

Microencapsulated coatings increase coefficient of friction during assembly. With the introduction of new developed products (µtot 0,12 to 0,16) this effect can be avoided

It is not allowed in some automotive specs to use microencapsulated coatings over top-coats with integrated lubricants due to the risk of lack of adhesion of the locking feature over the top-coat

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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 most

complicated case in bolted unions due to the reduced contact surface Galvanic compatibility (galvanic series). Special attention to materials

like Al, 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

electroplated zinc alloys Mg alloys. Not compatible with zinc flake coatings. Compatible with some

electroplated zinc coatings with special top-coats

Specific surface. Avoid relatively small areas of the less noble metal with a larger surface of the most noble metal E.g. A zinc plated bolt fastened to an anodized aluminium sheet or a stainless

steel panel. The surface of the bolt is smaller than the Al sheet (or stainless steel sheet). Finally, the bolt is corroded

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Bolted joints – Bolt tightening in real conditionsOther factors

Galvanic series

Platinum

Gold

Graphite

Titanium

Silver

Stainless steel (passive)

Nickel (passive)

Bronze

Nickel (active)

Lead

Stainless steel (active)

Iron

Steel

Aluminium

Zinc

Magnesium

Cathodic

Anodic

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Bolted joints – Bolt tightening in real conditionsTemperature and relative humidity

Service temperature. There is a risk of self-loosening at high temperatures in some coatings with integrated lubricant due to loss of self-retention. For this reason, coating systems must comply with VDA 235-203 requirements before approval

Relative humidity also has a strong influence. Before comparing results it is necessary that parts can be conditioned before testing.

Relative humidity has influenced in the assembly conditions in some cases, so this has led to misinterpretation over the quality of the parts received by the customer

Too high humidity may produce a higher lubrication and a too dry environment may increase coefficient of friction

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Bolted joints – Bolt tightening in real conditionsOther factors

Type of tightening procedure Tightening below yield point. If torque controlled tightening is

performed, friction has a strong influence and a short variation in torque may influence in a greater variation of preload. E.g. In torque controlled tightening, a torque variation of ± 15% ∆T can produce a preload variation of ∆F ± 35%. With angle controlled tightening ∆F ± 13%

Tightening beyond yield point. The maximum design of the bolt is used when bolt is assembled beyond the yield point. When bolt is tightened within the plastic range, friction under the head of the bolt plays a less important role with a torque/angle controlled tightening procedure

High speed tightening. Tightening in two steps (e.g. 200 rpm, stop, final tightening at 20 rpm). Speed of rotation can be higher in some cases, thus friction effects may vary

Use of spindle

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Bolted joints – Bolt tightening in real conditionsSummary

In the design of bolted joints not only the mechanical characteristics of the bolt have to be considered individually. The other materials used in the union play a vital role

Furthermore, not only the coating systems applied influence exclusively in the tightening process, other factors must be considered. A different coating system may influence tightening process dramatically

It is of vital importance to know the conditions of the bolted union of the different materials, their interaction and how they behave in the different situations encountered

Temperature and humidity conditions are also important, mainly when comparing different unions

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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 atornilladas