1. BOLTED JOINT ASSEMBLY WIND ENERGY FIELD SERVICE TECHNICIANS
4 BOLTING ASSEMBLY TRAINING - FASTENERSNotes from the Training Course on Bolting Assembly
BOLTED JOINT ASSEMBLY WIND ENERGY FIELD SERVICE TECHNICIANS
THREADED FASTENERS
“THE NUTS AND BOLTS OF NUTS AND BOLTS”
This presentation gives an overview of how the strength of bolts and nuts are defined and specified. The key features of this presentation are:
Background to bolt strength grades
Load-Extension diagrams and the yield and tensile strength.
The 0.2% permanent set limit
Metric bolts
Inch bolts
Strength comparison between inch and metric bolt strengths
Bolt head markings
Nuts, nut strength and nut markings
Washers
© HYTORC / Bolt Science 2006

2. 4 BOLTING ASSEMBLY TRAINING - FASTENERSNotes from the Training Course on Bolting Assembly
The following material is intended to make you aware of the commonly accepted and recognized principles and methods used in assembling bolted joints. However, every possible application of these methods cannot be foreseen and these general guidelines do not supersede any other applicable rules or regulations.
ALWAYS FOLLOW CURRENT COMPANY AND SITE SAFETY RULES AND PROCEDURES WHEN PERFORMING ANY OF THE WORK DESCRIBED IN THIS PRESENTATION.
The authors and sponsors of this course have no control over, and assume no responsibility for any damage, injury or work interruptions due to any person’s use of, or reliance upon these generally accepted methods and principles.
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No course can cover all possibilities and address all problems. The key is to know what to look for, and where to go to get the answers.
Hopefully that is what you will take away from this course.
© HYTORC / Bolt Science 2006

3. As The Threads Turn They Generate A Tremendous Mechanical Advantage
4 BOLTING ASSEMBLY TRAINING - FASTENERSNotes from the Training Course on Bolting Assembly
Threaded fasteners create tension because of their unique shape:
The thread of a screw or bolt acts like a
continuous inclined plane =
A helix can be formed by wrapping a wedge shaped (triangular) sheet of paper around a pencil. Just as a wedge can be used to generate large forces, a threaded fastener can generate very large forces. For example a standard 1 inch diameter bolt will generate a clamp force typically approaching lbs.
As The Threads Turn They Generate A Tremendous Mechanical Advantage
© HYTORC / Bolt Science 2006

4. BUT ONLY SINCE THE 1500’S WERE SCREWS AND BOLTS USED AS FASTENERS
4 BOLTING ASSEMBLY TRAINING - FASTENERSNotes from the Training Course on Bolting Assembly
THE HELICAL THREAD FORM HAS BEEN DOING MAN’S WORK FOR THOUSANDS OF YEARS
Archimedes Screw
(lifting water)
BUT ONLY SINCE THE 1500’S WERE SCREWS AND BOLTS USED AS FASTENERS
PRE-1840’S BOLT
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5. THERE NEEDED TO BE STANDARD FORMS AND SIZES
4 BOLTING ASSEMBLY TRAINING - FASTENERSNotes from the Training Course on Bolting Assembly
THERE NEEDED TO BE STANDARD FORMS AND SIZES
THE FIRST
STANDARD THREADS
WHITWORTH THREAD 1840
SELLERS THREAD 1864
BOTH INCH AND METRIC SHARE THIS PROFILE
MODERN THREAD PROFILE
© HYTORC / Bolt Science 2006

6. 4 BOLTING ASSEMBLY TRAINING - FASTENERSNotes from the Training Course on Bolting Assembly
Thread Terminology
A thread is a groove that in the case of fasteners is typically of a vee form. Enlarging a section of a thread it can be observed as a series of ridges and hollows. Specific names are given to parts of the thread.
The crest is the top most section of the thread. The root is the bottom of the thread. The thread flanks lie between the roots and crest of the threads.
The major diameter of a thread is, for an external thread, the diameter over the crests of the thread. For an internal thread it is the diameter over the thread roots. The pitch diameter cannot be directly measured since there is no marking or indication on the thread to measure. To explain the pitch diameter, it is first necessary to understand what the pitch is. The pitch of the thread is the distance between successive points on the thread form as illustrated in the diagram. For inch threads, the pitch is defined as the number of threads per inch - or tpi. With metric threads the pitch is directly specified.
The pitch diameter is the diameter on the thread that the space between the threads is equal to the space across the thread. The pitch diameter is a critical dimension in determining the acceptability of threaded connections. It is sometimes referred to as the effective diameter. The minor diameter of an external thread is the diameter of the roots of the thread. For an internal thread it is the diameter over the thread crests. Most thread forms have a radius at the root of an external thread. The radius improves the fatigue resistance of the thread.
Pitch for inch (UN or Unified National) is “threads-per-inch” (tpi)
Pitch for metric (ISO) threads is “mm from crest to crest”
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7. Coarse and Fine Threads
4 BOLTING ASSEMBLY TRAINING - FASTENERSNotes from the Training Course on Bolting Assembly
Coarse and Fine Threads
Fine Thread (UNF)
Coarse Thread (UNC)
Threads are often referred to as coarse or fine. This has nothing to do with the thread quality. A coarse or fine thread refers to the pitch size, a coarse thread having a larger pitch than a fine thread for the same thread diameter. There are thread series for both coarse and fine threads. For example the UNF thread refers to a Unified National Fine thread, UNC to a Unified National Coarse. For a given thread diameter, a UNF thread has a smaller pitch - that is, a larger number of threads per inch - than a UNC thread. A standard metric thread is a coarse thread. There is a metric fine thread series but it is normally only specified for specialized applications.
In flange bolting applications, for thread diameters greater than 1 inch, a pitch of 8 threads per inch (8 tpi referred to as 8UN) is normally used. For metric threads above M24, a pitch of 3 mm is usually used. Note that the pitch on these threads is not necessarily the same as a UNC or standard metric thread.
Standard metric threads are a coarse pitch. Fine threads can be obtained but usually they are reserved for special applications
The basic size of a thread is given by its diameter and pitch.
Inch example: 1- 8 UNC
Metric example: M24 x 3
© HYTORC / Bolt Science 2006

8. Stress Area of a Thread Thread Section
4 BOLTING ASSEMBLY TRAINING - FASTENERSNotes from the Training Course on Bolting Assembly
Thread Section
The thread is normally the weakest link of a threaded fastener. That is, tensile failure normally occurs in the threaded region. The thread itself has a complex form and there are obvious difficulties in deciding what area of the thread to use for strength calculations.
Detailed tests were completed in the 1930's to establish the equivalent diameter of bar that could be substituted for the thread. The stress area is the term used for the area of this equivalent section. Carefully conducted tensile tests were performed using plain bars and threaded bars that had been machined from the same bar of steel. Using a diameter that was halfway between the pitch and minor diameters as the bar diameter gave results that were within 3%. The area of bar based upon this mean diameter is known as the stress area. Tables giving the stress area of a particular thread are available. The stress area of a particular sized thread can be determined by calculating the area of section that is the mean of the pitch and minor thread diameters. For inch based threads, the stress area can be determined using the formula below:
© HYTORC / Bolt Science 2006

9. Bolt Strength Grades
4 BOLTING ASSEMBLY TRAINING - FASTENERSNotes from the Training Course on Bolting Assembly
Billions of bolts are made each year, to allow standardization, bolts are usually made to meet a particular strength grade (inch fasteners) or property class (metric fasteners).
Over six billion bolts are made each year. You could specify a bolt material by defining all the properties that you wish the bolt to have, however this would lead to millions of 'specials' that would be very expensive. Bolt strength for steel bolts is usually specified by means of a property class (metric) or grade (inch threads). By specifying a property class, all the key mechanical and physical properties are defined together with the appropriate test methods.
© HYTORC / Bolt Science 2006

10. Metric Bolts
4 BOLTING ASSEMBLY TRAINING - FASTENERSNotes from the Training Course on Bolting Assembly
Metric bolts have their strength specified by a property class. The property class, such as 8.8, defines all the key physical properties required by the bolt.
The two typical property classes used in mechanical engineering are 8.8 and 10.9, the10.9 being stronger than the 8.8 by about 30%.
For metric fasteners, ISO 898 is the key standard that defines property classes for bolt, screws, studs and nuts made of carbon and alloy steel. For bolts there are defined 10 property classes, 3.6 to 12.9, for nuts there are 7 property classes, 4 to 12. Typically a bolt of property class 8.8 is used with a nut of property class 8.
The ISO 898 standard applies to bolts between M1.6 to M39, made from carbon or alloy steel to be used within a temperature range of -50C to +300C (250C for 10.9). The national standards, such as DIN, ASTM, SAE and BS, for metric fastener materials are based upon the ISO standard. A property class for a bolt consists of two figures:The first figure indicates 1/100 of the nominal tensile strength in N/mm2. The second figure indicates 10 times the ratio between lower yield strength (or the 0.2% limit) and the nominal tensile strength. For example the bolt shown has a property class of 8.8. The first figure 8 indicates that the nominal tensile strength is 800 in N/mm2 i.e. 8 x 100. The second figure indicates that the lower yield strength is 640 in N/mm2 i.e. 8/10 x 800 .
© HYTORC / Bolt Science 2006

11. Inch Bolts Grade 8 Grade 5 Used typically in mechanical engineering
4 BOLTING ASSEMBLY TRAINING - FASTENERSNotes from the Training Course on Bolting Assembly
Grade 8
Grade 5
Used typically in mechanical engineering
SAE
Used typically in the oil, gas, power industries
Inch bolts are available to either SAE or ASTM standards. SAE bolts have their strength specified by a grade. The higher the grade number, the stronger the bolt.
The two typical SAE grades used in mechanical engineering are 5 and 8, the 8 being stronger than the 5 by about 30%.
The most common ASTM grade used is B7 which tends to be the standard for the oil/chemical/power industries.
ASTM B7
© HYTORC / Bolt Science 2006

12. 4 BOLTING ASSEMBLY TRAINING - FASTENERSNotes from the Training Course on Bolting Assembly
The above chart illustrates the minimum specified yield strength of the popular bolting materials. (Note that 1 ksi is 1000 psi.) In general the SAE J429 standard applies to inch based fasteners and the ISO 898 standard to metric fasteners. The ASTM A193 standard applies both to metric and inch based fasteners.
For some of the materials, such as B7, the strength decreases as the bolt diameter increases: the specific minimum yield strengths are:
SAE J429 Grade 5 1/4 thru’ 1 inch 92 ksi (634 N/mm²)
Over 1thru 1-1/2 81 ksi (558 N/mm²)
SAE J429 Grade ksi (896 N/mm²)
ASTM A193 B7 < 2.5 inch 105 ksi (724 N/mm²)
2.5 < 4 inch 95 ksi (655 N/mm²)
4 < 7 inch 75 ksi (517 N/mm²)
ASTM A193 B7M <= 2.5 inch 80 ksi (552 N/mm²)
ISO 898 Property Class 8.8 <= M N/mm² (93 ksi)
>M N/mm² (96 ksi)
ISO 898 Property Class N/mm² (136 ksi)
B7M
© HYTORC / Bolt Science 2006

13. Bolt Head Markings ISO Property Class 8.8 SAE Grade 5 ASTM B7
4 BOLTING ASSEMBLY TRAINING - FASTENERSNotes from the Training Course on Bolting Assembly
ISO Property Class 8.8
SAE Grade 5
For both SAE and ISO bolts, the bolt head is marked with two sets of symbols. The first set of symbols is used to indicate the strength of the bolt. For example, three radial marks indicates that the bolt is SAE Grade 5, six, that it is a SAE Grade 8 bolt.
The strength of metric bolts are indicated by the property class, such as 8.8. On studs, the strength is indicated usually by a stamping on the end, such as the B7 marked stud shown above.
The second set of marks is used to indicate who the manufacturer of the bolt was. For example, the JH mark indicates that the bolt was made by the Jinn Her Enterprise Co. of Taiwan in the Republic of China. The triangle on the SAE grade 8 bolt is the mark for Infasco fasteners of Canada.
Markings are important in quality assurance. You can see the grade of an installed bolt and who manufactured it if there are subsequent concerns or problems. Most bolts have the marks raised so that they can only be produced during the primary manufacturing process.
A source for locating manufacturer to the symbol is the publication from Fastener Technology International titled ‘Manufacturer Identification Symbols - International Guide’.
ASTM B7
SAE Grade 8
© HYTORC / Bolt Science 2006

14. Nuts
4 BOLTING ASSEMBLY TRAINING - FASTENERSNotes from the Training Course on Bolting Assembly
Nuts are under compression and generally fail by the thread stripping.
Nuts and bolts are designed so that the bolt will break first by tensile fracture, which is sudden, rather than by thread stripping.
Thread stripping is gradual and can lead to a defective product entering service.
The intention of matching the strength of the nut to that of the bolt is to provide an assembly that is capable of being tightened to the bolt proof load without thread stripping occurring.
A nut is largely subjected to compressive stresses. The mode of failure of nuts is typically by the thread stripping. This is an undesirable mode of failure in that it is gradual and can result in partially failed fasteners being left in assemblies. To prevent this, the nut height is selected so that thread stripping will not occur when a 'proof load' is applied to the nut.
Ensuring that a bolt or nut meets all the requirements of the specification is a time consuming and difficult operation. By specifying a proof stress and the corresponding proof load - a quick and simple check can be made to assess that a batch of fasteners is likely to be satisfactory. The proof load of a nut is the axially applied load the nut must withstand without thread stripping or rupture. The proof load of a bolt, screw or stud is the specified load the product must withstand without permanent set.
The proof load typically is between 88% to 94% of the yield strength for the strength grade or property class. After applying the proof load and then releasing, there should be no permanent extension of the bolt A male mandrel is used for the proof load on nuts. Following the test the nut should be capable of being removed by hand.
Nut thread stripped onto the bolt thread.
© HYTORC / Bolt Science 2006

15. Nut Strength
4 BOLTING ASSEMBLY TRAINING - FASTENERSNotes from the Training Course on Bolting Assembly
To ensure that thread stripping is prevented, the appropriate strength of nut must be matched with the bolt strength.
In general terms, a SAE grade 5 nut is used with a SAE grade 5 bolt, a grade 8 nut with a grade 8 bolt etc.
With metric bolts, a property class 8 nut is used with a property class 8.8 bolt.
It is important to ensure that the appropriate strength of nut is matched to the property class of the bolt. Normally a property class 8 nut is matched with a 8.8 bolt, a 10 nut with a 10.9 bolt etc. If a tightening method is used that results in the bolt reaching yield, then there is the possibility of thread stripping occurring. In such cases, nuts of a higher property class can be used to eliminate the problem, for example a nut of property class 12 used with 10.9 bolts. Thread stripping is when the internal thread is sheared off ending up embedded into the bolt thread.
© HYTORC / Bolt Science 2006

16. Washers
4 BOLTING ASSEMBLY TRAINING - FASTENERSNotes from the Training Course on Bolting Assembly
Washers are often used under bolt heads and nut faces, their purpose is to:
Protect the joint face from damage e.g. embedding
Provide a hard surface with consistent coefficient of friction
Spread the load over a larger area (increases the stiffness of the joint)
To serve their purpose, washers should be THROUGH HARDENED (not just surface hardened) so that they are at least equal to that of the nut/bolt. (38 HRC)
To eliminate the need for washers, many applications use flange headed bolts and nuts. Such fasteners distribute the load over the joint surface and have the advantage that they can’t be missed out.
Indentation in a ‘soft’ washer
© HYTORC / Bolt Science 2006

17. Let’s Try A Few Questions To Ensure
4 BOLTING ASSEMBLY TRAINING - FASTENERSNotes from the Training Course on Bolting Assembly
Let’s Try A Few Questions To Ensure
We Understand The Basics About Fasteners
Please take a few minutes to discuss what we have learned about flanges.
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© HYTORC / Bolt Science 2006

18. 2. Could you safely use a grade 5 nut with a grade 8 bolt? Why?
4 BOLTING ASSEMBLY TRAINING - FASTENERSNotes from the Training Course on Bolting Assembly
QUIZ
1. T/F A torque chart is only accurate for a single grade of bolt and should indicate what that grade is?
2. Could you safely use a grade 5 nut with a grade 8 bolt? Why?
3. Name 3 advantages to using a hardened washer under a nut:
_________________ __________________ ______________
4. Metric threads are now used widely in the world. Is the most common metric thread a coarse or fine thread?
5. Name the parts of the thread:
© HYTORC / Bolt Science 2006

19. No. The nut should always be as strong or stronger than the bolt.
4 BOLTING ASSEMBLY TRAINING - FASTENERSNotes from the Training Course on Bolting Assembly
Quiz answers:
True. Torque is specific to bolt grade. Read the fine print to make sure you have the torque chart that matches your bolt, or vice-versa.
No. The nut should always be as strong or stronger than the bolt.
Protects the flange face. Gives even turning surface under all nuts. Spreads the load making the joint more stiff and the bolt relatively more flexible.
Coarse
Parts are:
pitch
crest
flank
pitch
root
© HYTORC / Bolt Science 2006