General Do & Don't for bolted joint design. Common mistakes, fastener coatings and effect

1. Overview of Bolted connections
General information, Common mistakes and care points
A T Ansari
28 August 2014

2. Types of Joints
•There are various types of structural engineering joints
•Major joints used in the field of engineering are
–Weld joint
–Bolted Connection
•Bolted Connection can be further sub-grouped as
–Bolt with Nut
–Bolt with threads machined in other components
•Bolted connection will be discussed further…
–Fasteners

3. Benefits of Bolted Connection
•Bolted connections are preferable because…
–Relatively cheaper
–Connect two different materials
–Easy serviceability
–Avoids large structures
–Enables easy logistics of heavy structures
–Avoid special processes on structure itself
–Elimination of NDT inspection and resulting delay of welding

4. Key Quality Controls for Fasteners
•There are various process parameters and product characteristics to be controlled during manufacturing of fasteners as per respective international standards and codes.
•Few are list below to be focused upon while fastener selection
–Grade
–Tensile strength
–Elongation
–Hardness
–Surface coating
–Decarburization

5. Key considerationsFor Bolted connection Design
•Key considerations
–Is it under load condition
–Is it just to connect one part to another
–Bolt with Nut
–Bolt with component having machined threads
–Accessibility
–Surface coating
–Co-efficient of friction
–Torque tightening
–Tension tightening
–Standard fastener
–Special fastener

6. Importance of Correct Parts
•All pre-requisites for joint design must be fulfilled according to global standards and codes
•If Bolt is assembled with Nut
–Bolt and Nut pair to be as per standard norms
–It is not advisable to use mismatching grade of bolt & nut. e.g…
•Grade 10.9 bolt with Grade 8 Nut, this will cause thread rip off in nut at rated torque of 10.9 bolt
•Grade 8.8 bolt with Grade 10 nut, this will cause thread rip off in bolt at rated torque of grade 10 nut
•If lower torque joint will not be preloaded hence chances of loosening the connection in operation.

7. Importance of thread engagement
•If Bolt is tightened with internal threads machined in component
–Design of correct thread engagement is of utmost important
–Thread engagement must be calculated considering actual tensile strength/shear strengths & key thread dimensions of both materials
–It is also advisable to consider incomplete threads of bolt in addition to calculated engagement length as effective thread engagement
–VDI2230, FED-STD-H28/2B & Machinery's Handbook gives more precise calculations hence such scientific calculations to be used
–Generally designers use formula of 1.5d for thread engagement when component strength is lower than fastener, this is always not correct
–Sometimes designer use thread engagement resulting from multiplication of tensile ratio of both material x 0.8d, this is also not correct

8. Common mistakes
•Common mistakes while connection design
–Calculating thread engagement with 1.5d
–Calculating thread engagement with Tensile ratiox0.8d
–Considering bolt length going in machined hole as engagement where incomplete threads of bolts are not engaged, it is generally 2P
–Focusing more on torque irrespective of coating CoF
–Not considering CoF, lubrication etcwhich many times results in thread failures or bolt elongation as bolt reach to preload before the rated torque
–Not considering head friction, this aspect gives some times false torque achievement with preloading the bolt

9. •Consider a component to be assembled by using M36 grade 12.9 bolt with a Ductile Cast Iron housing of EN-GJS-350-22-LT
–As per 1.5d
•54mm
–As per Tensile ratio x 0.8d
•1220/320 x 28.8 = 109.8mm
–As per FED-STD-H28/2B & Machinery's Handbook
•70mm
•This equation uses ratio of shear area of internal/external threads in combination with tensile strengths of both material
–As per VDI2230
•74mm
•This formula uses multiple parameter such as tensile, tensile stress area and various calculated constants but no ratio tensile or shear strengths
•Now this example gives fair idea that how the thumb rules affects!
–1.5d will result in thread rip off of casting part
–Tensile ratio x 0.8d will give better engagement but will cause for undesired machining in casting , extra length of bolt hence extra cost
Thread engagement calculation example
In correct thread engagement calculation methods

10. Thread Engagement Calculation Sample

11. Hot Dip Galvanized Fasteners -Care points
•Hot Dip Galvanized
–For assembly with Bolt & Nut
•Bolt will be 6g + HDG
•Nut will be oversize that is 6AZ (6Z)
•Nut pre-lubricated with MoS2 for constant CoF
–For assembly with machined threads in component
•If it is stud end going in machined internal threads not to be applied with MoS2
•If it is bolt MoS2 to be applied on bolt threads as well as below the head
•Internal threads will be machined to 6H
•Bolt /stud going in internal threads will be of 6az i.e. ~ 6g after HDG
–In any case HDG bolt meant to be used with nut, must not be used with parts having threads machined.
•This will cause torque attainment with bolt preload and thread washed in mating parts
–In any case HDG bolt meant to be used in machined threads, must not be used with nut
•This will cause loosening of joint in long term and unreliable connection because of more clearance between internal and external threads

12. Zinc Flake Coated Fasteners –Care points
•Zinc Flake Coated (flZnnc)
–Bolts will be 6g after coating
–Nuts will be 6H after coating, it is to be noted that even nut threads are coated
–Validated MoS2 to be used to attain required CoF
–Interaction of bolts/stud & nut with oil/grease to be avoided to eliminate uncontrolled CoFand resulting overloading at lesser torque
•Mismatch assemblies –To be avoided
–Sometimes it is seen that HDG bolt/Studs used with flZnncnuts, this must be avoided as it affects fitment and torque attainment

13. References
•ISO 898-1
•ISO 898-2
•ISO 10684
•ISO 10683
•FED-STD-H28/2B
•Machinery's Handbook 18thEdition
•VDI 2230

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