1. BOLTED JOINT ASSEMBLY WIND ENERGY FIELD SERVICE TECHNICIANS
2 BOLTING ASSEMBLY TRAINING - PRINCIPLESNotes from the Training Course on Bolting Assembly
BOLTED JOINT ASSEMBLY WIND ENERGY FIELD SERVICE TECHNICIANS
PRINCIPLES OF BOLTED JOINTS
“Why Use Bolts?”
This presentation covers:
The purposes and advantages of bolted joints
Historical background to threads including
Early threads and the work of Joseph Whitworth
William Sellers and the 60 degree thread
The creation of international thread forms
Thread terminology
Coarse and fine threads
The stress area of a thread
The three key components of a joint
The bolt as a tension spring
The joint as a compression spring
The tension spring within a compression spring
How a bolted joint carries load
Load carrying capacity of a joint
The key importance of ensuring that bolts are tight

2. 2 BOLTING ASSEMBLY TRAINING - PRINCIPLESNotes 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.
>
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.

3. IN THIS COURSE WE WILL USE THE GENERAL TERM “BOLTS”
There are many types of bolts, nuts, studs and screws ...
2 BOLTING ASSEMBLY TRAINING - PRINCIPLESNotes from the Training Course on Bolting Assembly
There are literally thousands of different types of threaded fasteners, many having their own particular name. Examples include bolts, studs, screws, u-bolts, and plough bolts.
In this course we will refer to both studs and bolts, as bolts. A stud is a threaded rod that does not have a fixed head or cap but is designed to be used with two nuts.
We call them all “THREADED FASTENERS”
IN THIS COURSE WE WILL USE THE GENERAL TERM “BOLTS”

4. 1. Bolts are removable and reusable
2 BOLTING ASSEMBLY TRAINING - PRINCIPLESNotes from the Training Course on Bolting Assembly
Threaded Fasteners or Bolts Have
Two Great Advantages…
1. Bolts are removable and reusable
You could weld the wheels onto the axles of your car but it wouldn’t make changing a tire very convenient!
Attaching a wheel to a car using 4 or 5 relatively small fasteners illustrates another big advantage, they have a very high load carrying capacity for their size.

5. (we will talk more about this later on)
2 BOLTING ASSEMBLY TRAINING - PRINCIPLESNotes from the Training Course on Bolting Assembly
2. Bolts hold a joint together
<<under tension>>
There are other reasons why bolting is used which include:
They simplify the production of manufactured products. (Try making an engine from a single piece of material.)
Threaded fasteners allow several products to be built around a set of core components facilitating easy and quick assembly.
Threaded fasteners allow parts that are damaged to be easily replaced.
Joints comprising threaded fasteners can have a high load capacity and are fatigue resistant.
(we will talk more about this later on)

6. A BOLT IS A SPRING THE SECRET IS...
2 BOLTING ASSEMBLY TRAINING - PRINCIPLESNotes from the Training Course on Bolting Assembly
THE SECRET IS...
The secret is, a bolt is a spring!
A BOLT IS A SPRING

7. 2 BOLTING ASSEMBLY TRAINING - PRINCIPLESNotes from the Training Course on Bolting Assembly
STRETCH THE BOLT, AND AS IT SPRINGS BACK IT SQUEEZES THE JOINT TOGETHER
Stretch the bolt and as it springs back it squeezes the joint together.

8. WE WANT TO HIT THE SWEET SPOT!
2 BOLTING ASSEMBLY TRAINING - PRINCIPLESNotes from the Training Course on Bolting Assembly
The Key Is: - Enough Spring Action To Hold The Joint Together - But Not Too Much Stretch So That That The Bolt Yields And Longer Springs Back
What happens if you stretch a spring too far? It won’t spring back. The point at which the spring ceases to act like a spring we call its YEILD POINT.
(not where it breaks but where it looses elasticity)
Bolts also have a yield point and if you go beyond it you might as well have a noodle in the joint. The flange will not seal, the joint will not hold and you may not discover that until too late to prevent a disaster.
TIGHTER IS NOT ALWAYS BETTER!
WE WANT TO HIT THE SWEET SPOT!

9. LOAD or Clamping Force WE CALL THIS SPRING ACTION
2 BOLTING ASSEMBLY TRAINING - PRINCIPLESNotes from the Training Course on Bolting Assembly
WE CALL THIS
SPRING ACTION
LOAD or
Clamping Force
We call this spring action LOAD or PRE-LOAD or CLAMPING FORCE.

10. THAN THE PRESSURES TRYING TO ESCAPE…
2 BOLTING ASSEMBLY TRAINING - PRINCIPLESNotes from the Training Course on Bolting Assembly
In the picture is a pipeline flange with 24 bolts each of which has been stretched so that it springs back with a force of 55,000 pounds.
Multiplying the pre-load on each bolt times the number of bolts we get a total force of ALMOST 1.4 MILLION lbs. holding the flange together. As long as the total pressure in the pipe is less than that number, the flange will stay together and won’t leak.
Suppose that the diameter of this pipe was 24” and the gas inside was at 900 psi.
With a little high school geometry I can calculate the area inside the pipe and then multiply the pressure …..
(Pi x radius squared) = ( x 144)900 = 407,000 lbs of force trying to escape.
Therefore the flange will hold together, as a matter of fact there is 3 times as much force holding it together as there is pushing it apart.
THE TOTAL BOLT LOAD
MUST BE GREATER
THAN THE PRESSURES TRYING TO ESCAPE…

11. …OR GREATER THAN THE FORCES TRYING TO SEPARATE THE JOINT…
2 BOLTING ASSEMBLY TRAINING - PRINCIPLESNotes from the Training Course on Bolting Assembly
…OR GREATER THAN THE FORCES TRYING TO SEPARATE THE JOINT…
FOR MACHINERY TO HOLD TOGETHER AND DO ITS JOB

12. 2 BOLTING ASSEMBLY TRAINING - PRINCIPLESNotes from the Training Course on Bolting Assembly
Most springs (including bolts and flanges) behave in predictable ways according to Hooke’s law which states that strain (ΔL/L) is proportional to stretching force (F) and by extension therefore also to stress (F/A)
This can be illustrated graphically in a “stress/strain” diagram
STRESS/STRAIN CURVE .
PSI
ULTIMATE TENSILE STR.
STRESS .
(LOAD)
YIELD
PLASTIC RANGE
ELASTIC RANGE
>
Engineers describe these properties by using a “stress/strain curve or graph”
What this graph shows is that as we begin to add load to a bolt (picture a bolt hanging from the ceiling to which we attach ever increasing weights) it physically gets longer. Notice that the red line is straight meaning that for every pound of additional load we get a predictable amount of stretch in the bolt and the stretch is the same from 1000 psi to 1001 psi as it is from 2000 psi to 2001 psi.
Notice also that the graph is rather steep meaning that steel bolts can take a lot of load without stretching very far. We call this straight portion of the graph the “elastic range” because if we reduce or eliminate the load the bolt will snap back to its original shape without any damage, just like a spring. We count on that happening so that we can re-use the bolts and studs in most applications. Elasticity refers to a material’s ability to revert to its original shape after an applied load has been removed.
A rubber band is often viewed as a very elastic material but “stretchy” is not the same thing as “elastic”. A piano string is more elastic because it can be struck and therefore stretched thousands of times without getting out of tune. Elasticity does not refer to how easy it is to stretch, but how completely it returns to its original shape.
If we continue to add load the bolt will get into what we call the “plastic range” meaning that it will no longer spring back into its original shape.
The point where this change takes place is called the yield point. Notice the line now curves and is more flat meaning that the bolt begins to stretch a lot more with every additional pound added. > If we continue to load it we will reach a point called the “ultimate tensile strength”. After that point the bolt rapidly weakens, loses all its tension and will very quickly break in two. > . ΔL
STRAIN (STRETCH)

13. The Bolt as a Tension Spring
2 BOLTING ASSEMBLY TRAINING - PRINCIPLESNotes from the Training Course on Bolting Assembly
As we just said, when a tensile force is applied to a spring - it extends. In a similar manner when a bolt is tightened a tensile force is induced into the fastener and an extension occurs. As such, it acts exactly like a spring.

14. The Joint as a Compression Spring
2 BOLTING ASSEMBLY TRAINING - PRINCIPLESNotes from the Training Course on Bolting Assembly
But, In a similar manner, when a compressive force acts upon a compression spring it will deflect. Greater the applied force, the greater will be the compression. This is exactly like the joint with a compressive force being applied by the bolt except the deflections (how far the spring moves) are very small.

15. The Tension Spring with a Compression Spring
2 BOLTING ASSEMBLY TRAINING - PRINCIPLESNotes from the Training Course on Bolting Assembly
The bolt and joint combined can be considered as a tension spring compressing a compression spring. The tension in the bolt is balanced by the compression in the joint. Having an extension in the bolt and compression in the spring is important in order that losses from creep and relaxation can be accommodated so that there is sufficient clamp load retained to ensure that the gasket will continue to seal.

16. 2 BOLTING ASSEMBLY TRAINING - PRINCIPLESNotes from the Training Course on Bolting Assembly
A couple of important concepts:
“flexibility” = a material’s ability to stretch, bend or twist without permanent damage, and return to its original shape
a piano spring is very flexible
“stiffness”= a material’s resistance to deflection when subjected to external forces.
Opposite of flexibility. Stiffness not the same as strength (ceramic is very stiff but may not be strong)
BOTH A STEEL JOINT AND A STEEL BOLT ARE OF THE SAME MATERIAL
BUT, WE WANT THE BOLT TO BE FLEXIBLE AND THE JOINT TO BE STIFF
The complex balance between these two opposing springs defines the way in which a bolted joint carries and absorbs load
To understand this relationship we need to discuss the concepts of “elasticity” and “stiffness”
Elasticity refers to a material’s ability to revert to its original shape after an applied load has been removed.
A rubber band is often viewed as a very elastic material but “stretchy” is not the same thing as “elastic”. A piano string is more elastic because it can be struck and therefore stretched thousands of times without getting out of tune. Elasticity does not refer to how easy it is to stretch, but how completely it returns to its original shape.
Stiffness refers to a material’s resistance to deflection when it is subjected to external forces
In extension or compression stiffness is measured as the change in length of the material (ΔL) divided by the applied force (F)
Note that this is not the same thing as “strength” (ability to carry or withstand a load without failure) Ceramic may deflect very little under a load – while mild steel may be more easily compressed, but which is “stronger”?

17. 2 BOLTING ASSEMBLY TRAINING - PRINCIPLESNotes from the Training Course on Bolting Assembly
HOW LONG WOULD YOU EXPECT THE LUG BOLTS ON YOUR CAR WHEELS TO LAST IF YOU PUT THEM ON ONLY FINGER-TIGHT?
HOW LONG DO YOU EXPECT THEM TO LAST WHEN THEY ARE PROPERLY TIGHTENED?
WHAT MAKES THE DIFFERENCE?
WHEN A BOLT IS PROPERLY TIGHTENED EXTERNAL FORCES GET TRANSFERRED TO THE JOINT AND NOT TO THE BOLT
THE “STIFF” JOINT ABSORBS EXTERNAL SHOCKS AND LOADS, AND THE “FLEXIBLE” BOLT DOES NOT FEEL THEM IF IT IS PROPERLY TIGHTENED!

18. THE CONCEPT OF “PRE-LOAD”
2 BOLTING ASSEMBLY TRAINING - PRINCIPLESNotes from the Training Course on Bolting Assembly
THE CONCEPT OF “PRE-LOAD”
HOW MUCH WEIGHT WOULD WE HAVE TO STACK ON THE END OF THE BOARD TO TAKE THE TENSION OFF THE ROPE? 1
SPRING BAR OR
“DIVING BOARD” 10 10 10 10 10 10 10 10
BOARD PULLS
100 lbs. UP
ROPE PULLS
100 lbs. DOWN 10 10
100
WE WILL NOT RELIEVE THE TENSION ON THE ROPE UNTIL EXTERNAL LOAD EXCEEDS THE “PRELOAD” IN THE BOARD
"PRE-LOAD"
SUPPOSE WE HANG 100 LB. WEIGHT
Let’s explore the concept of preload which sometimes is difficult to grasp, but if you think about, it makes perfect sense.
Suppose we have a horizontal spring bar – a diving board would be a good example. Then let’s hang a 100 pound weight on the end, bending the board down. Now let’s tie the board in that position and take the weight away.
What we have done is to “pre-load the spring or board with 100 lbs of tension which can be felt on the rope that is holding it down. The board is pulling up with 100 lbs. and the rope is pulling down with that same 100 lbs. - that’s pre-load.
How many pounds of external or “working load” would we have to stack on the end of the board to take the tension off of the rope?
Would 10 lbs do it? 20? 30? …80? 90? 100?
The answer is that we would not see the tension in the rope relax until the amount of external load exceeds the pre-load.
100
AND THEN TIE THE BOARD DOWN

19. THE SAME THING HAPPENS IN THE BOLTED JOINT
2 BOLTING ASSEMBLY TRAINING - PRINCIPLESNotes from the Training Course on Bolting Assembly
THE SAME THING HAPPENS IN THE BOLTED JOINT
10,000 LBS. PRE-LOAD - JOINT WON’T SEPARATE UNTIL EXTERNAL LOAD EXCEEDS 10,000 LBS.
ZERO PRE-LOAD –
THE JOINT SEPARATES
THEREFORE:
THE GREATER THE PRE- LOAD (up to the yield of the bolt)
THE MORE RESISTANCE TO JOINT SEPARATION AND MOVEMENT

20. Let’s see if we got these concepts straight…
2 BOLTING ASSEMBLY TRAINING - PRINCIPLESNotes from the Training Course on Bolting Assembly
Let’s see if we got these concepts straight…

21. 1.. The function of a bolt or stud is to act like:
QUIZ
1.. The function of a bolt or stud is to act like:
A. A retaining pin B. A screw
C. A spring D. A gasket
2. T / F Bolts do not stretch because they are too hard.
3. At its yield point a bolt will:
A. Break in two B. Be tightened just right
C. Not return to shape D. Heat up under pressure
2 BOLTING ASSEMBLY TRAINING - PRINCIPLESNotes from the Training Course on Bolting Assembly

22. 4. T / F Pre-Load refers to the clamping force or tension exerted by a tightened fastener on a joint “before” it is subjected to outside forces or work loads.
5. Although you won’t design many joints what is one thing you can do to increase the “stiffness” of a joint ___________________?
6. T/F A 5” long 1-1/2” diameter bolt is going to stretch farther (be more flexible) than a 3” long 1-1/2” diameter bolt of the same material, when subjected to the same load?
7. T/F Tight bolts “feel” very little external load, unless and until that external load exceeds the pre-load in the bolt.
2 BOLTING ASSEMBLY TRAINING - PRINCIPLESNotes from the Training Course on Bolting Assembly

23. False. If they do not stretch they do not work.
2 BOLTING ASSEMBLY TRAINING - PRINCIPLESNotes from the Training Course on Bolting Assembly
Quiz answers:
A. Spring
False. If they do not stretch they do not work.
C. Not return to its original shape
True. “pre” means before the joint is subjected to outside forces
Put a washer under the nut or bolt head
True. Although each inch of the similar bolts will stretch the same, one has more inches. (this is one reason engineers design joints with long bolts and not short ones)
True. The joint absorbs most of the external load which will not fall to the bolt until it exceeds the bolt pre-load that was put in by tightening