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

1. BOLTED JOINTS
Bolt tightening with new materials
by Erik Galdames
Translation and adaptation of the
presentation “Condiciones reales de montaje”
at the Technical Meeting held on 20th July,
2012 organized by Galol, L’Olleria (Spain)

2. Bolted joints – Bolt tightening in real conditions
Basic 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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3. Bolted joints – Bolt tightening in real conditions
Basic considerations
 Types of fasteners
 Types of coatings
 Mating materials
 Thread length
 Environmental conditions
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4. Bolted joints – Bolt tightening in real conditions
Types of fasteners
 Metric threaded bolts, nuts
 Self-tapping metric screws
 Self-tapping screws
 Studs
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5. Bolted joints – Bolt tightening in real conditions
Design 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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6. Bolted joints – Bolt tightening in real conditions
Coatings 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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7. Bolted joints – Bolt tightening in real conditions
Coatings 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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8. Bolted joints – Bolt tightening in real conditions
Coatings
 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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9. Bolted joints – Bolt tightening in real conditions
Coatings
Coating Typical applications Adequate for Less adequate for
Zinc flake coatings Chassis
Motor compartment (areas
exposed to high corrosivity)
Aluminium joints
Bolts ≥ 10.9
Microencapsulated coating
allowed (without lubricated top-
coat)
Ground connections
External thread <M6
Internal thread <M10
Internal drive features T30
ZnNi Chassis
Motor compartment (areas
exposed to high corrosivity)
Aluminium joints
External thread < M6
Internal thread < M10
Bolts ≥ 10.9, ≥ 1000 N/mm2
Though less hydrogen
embrittlement risk than pure e-
plated Zn.
Service temperature > 150ºC
ZnNi black Chassis
Motor compartment (areas
exposed to high corrosivity)
Aluminium joints
External thread < M6 with
internal drive feature
Internal thread <M10
Self-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 exposure
Transport and limited storage
in warehouses
Parts not multi-component
Service 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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10. Bolted joints – Bolt tightening in real conditions
Materials
 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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11. Bolted joints – Bolt tightening in real conditions
Materials
Material Abbreviation Material No. Standard
Cold formed steel DC01
S700MC
1.033
1.8974
EN 10130
EN 10149-2
Non-allied structural steel S355JR 1.0045 EN 10025-2
Steel for quenching and
tempering
C45EC
34CrNiMo6
16MnCr5
1.1192
1.6582
1.7131
EN 10263-4
EN 10083-3
EN 10084
Sintered materials Sint – D30 - DIN 30910-4
Austenitic stainless steel X5CrNi18-12
X5CrNiMo17-12-2
X6NiCrTiMoVB25-15-2
1.4303
1.4401
1.4980
EN 1008-3
EN 10269
Cast iron EN-GJL-150
EN-GJS-40015U
EN-GJS-500-7U
EN-GJS-600-3U
EN-JL1020
EN-JS1072
EN-JS1082
EN-JS1092
EN 1561
EN 1563
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12. Bolted joints – Bolt tightening in real conditions
Materials
Material Abbreviation Material No. Standard
Cold formed Al alloys EN-AW-AlSi1MgMn-T6
EN AW-AlSi1MgMn-T4
EN AW-AlMg4, 5Mn0,7-H111
EN AW-6082
EN AW-6082
EN AW-5083
EN 754-2
Al alloy castings EN AC-AlSi6Cu4-SF
EN AC-AlSi9Cu3(Fe)
EN AC-AlSi7Mg0,3-T6
EN AC-45000
EN AC-46000
EN AC-42100
EN 1706
Mg alloys EN MC-MgAl9Zn1(A)-F-D
EN MC-MgAl6Mn-D
EN-MC21120
EN-MC21230
EN 1753
Ti alloys TiAl6V4 3.7165.1 DIN 17862
Zinc alloy castings ZP3
ZP5
ZP0400
ZP0410
EN 12844
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13. Bolted joints – Bolt tightening in real conditions
Mating 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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14. Bolted joints – Bolt tightening in real conditions
Thread length
MATERIAL Min. Rm
Min. 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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15. Bolted joints – Bolt tightening in real conditions
Other 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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16. Bolted joints – Bolt tightening in real conditions
Other 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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17. Bolted joints – Bolt tightening in real conditions
Other 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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18. Bolted joints – Bolt tightening in real conditions
Temperature 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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19. Bolted joints – Bolt tightening in real conditions
Other 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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20. Bolted joints – Bolt tightening in real conditions
Summary
 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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21. Bolted joints – Bolt tightening in real conditions
Further 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
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