1. BOLTED JOINT ASSEMBLY WHY BOLTS COME LOOSE and WHAT TO DO ABOUT IT
1 BOLTING ASSEMBLY TRAINING - INTRODUCTIONNotes from the Training Course on Bolting Assembly
BOLTED JOINT ASSEMBLY WHY BOLTS COME LOOSE and WHAT TO DO ABOUT IT
WIND ENERGY FIELD SERVICE TECHNICIANS

2. 1 BOLTING ASSEMBLY TRAINING - INTRODUCTIONNotes 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 LOCAL 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.

3. PROPER BOLTING IS CRITICAL TO WIND TOWER PRODUCTIVITY…
1 BOLTING ASSEMBLY TRAINING - INTRODUCTIONNotes from the Training Course on Bolting Assembly
PROPER BOLTING IS CRITICAL TO WIND TOWER PRODUCTIVITY…
One of the great things about using bolts to put things together is that they can easily be taken apart when we want to. The problem is that sometimes bolts come loose when we don’t want them to, often at exactly the wrong time.
So why do bolts come loose and what can we do to keep them tight, until we want to remove them?
WHY DO BOLTS COME LOOSE?

4. MOST COMMON CAUSES FOR LOOSENING:
1 BOLTING ASSEMBLY TRAINING - INTRODUCTIONNotes from the Training Course on Bolting Assembly
MOST COMMON CAUSES FOR LOOSENING:
INITIAL UNDER-TIGHTENING
INITIAL OVERTIGHTENING
REPEATED RE-TIGHTENING
ELASTIC INTERACTION (UNEVEN TIGHTENING)
EMBEDMENT
VIBRATION AND SHEAR MOVEMENT
Bolts in mechanical assemblies (that is in metal to metal joints without gaskets) generally come loose over time for a few simple reasons. Let’s look at some of the most common causes including:
Initial under tightening
Initial over tightening or repeated re-tightening
Elastic interaction (uneven tightening)
Embedment
Vibration and Shear Movement

5. REMEMBER THE CONCEPT OF “PRE-LOAD”
1 BOLTING ASSEMBLY TRAINING - INTRODUCTIONNotes from the Training Course on Bolting Assembly
REMEMBER THE CONCEPT OF “PRE-LOAD”
SPRING BAR OR
“DIVING BOARD” 1 10
BOARD PULLS
100 UP
ROPE PULLS
100 DOWN
EXTERNAL LOADS DON’T SEPARATE THE JOINT UNTIL THEY EXCEED PRE-LOAD
THE GREATER THE PRE-LOAD (UP TO THE YIELD POINT OF THE BOLT) THE MORE STABLE THE JOINT
100
"PRE-LOAD"
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.
INITIAL UNDER-TIGHTENING

6. TOO LITTLE PRE-LOAD MEANS THE JOINT CAN MOVE
1 BOLTING ASSEMBLY TRAINING - INTRODUCTIONNotes from the Training Course on Bolting Assembly
10,000 LBS. PRE-LOAD - JOINT WON’T MOVE UNTIL EXTERNAL LOAD EXCEEDS 10,000 LBS.
ZERO PRE-LOAD –
THE JOINT MOVES
ONCE THE JOINT MOVES:
THE NUTS WILL LOOSEN
LOAD WILL BE QUICKLY LOST IN THE JOINT
UNPROTECTED BOLT SUBJECTED TO OUTSIDE SHOCKS MAY FAIL
INITIAL UNDER-TIGHTENING

7. HOW DO WE SOLVE THE UNDERTIGHTENING PROBLEM?
1 BOLTING ASSEMBLY TRAINING - INTRODUCTIONNotes from the Training Course on Bolting Assembly
HOW DO WE SOLVE THE UNDERTIGHTENING PROBLEM?
ENSURE ADEQUATE PRE-LOAD ON THE BOLT
KNOW YOUR SPECIFIED TARGET TORQUE OR LOAD
MATCH TARGET TORQUE WITH LUBRICATION USED
CHECK TORQUE, TENSION OR ELONGATION
CRITICAL APPLICATIONS MEASURE AND AUDIT
CALIBRATE YOUR EQUIPMENT REGULARLY
TIGHT BOLTS JUST DON’T OFTEN COME LOOSE
(EXAMPLE OF A 1-8 UN BOLT) 1 25
So how do we solve the under tightening problem?
Make sure that the bolt is properly tightened in the first place by:
Knowing what the proper torque or load is before you start
Making sure that the lubrication or lack of it matches with that assumed by your torque chart
Check your torque or tension or load, don’t just assume
On critical applications measure and audit your results with a secondary measurement
Calibrate your measuring equipment regularly to make sure it is reading correctly
INSTRUTOR’S EXAMPLE OF WHY A PROPERLY TIGHTENED BOLT GENERALLY WON’T COME LOOSE:
A properly tightened fastener is very unlikely to come loose. Here is an example of why…
Assume a 1” diameter bolt with a thread pitch of 8 threads per inch
In order to advance the bolt 1 inch you would have to turn it 8 revolutions
Since the circumference of a bolt is equal to pi x diameter, the length of the thread eight times around the bolt is 8 x 3.14 x 1 = inches (let’s round that to 25 inches)
Therefore we have a triangular wedge or inclined plane that has a rise of 1 and a run of 25 Is anyone concerned that given a coefficient of friction even with the best lubricant of .10 or ten percent, that our bolt is likely to slip down a slope of 25:1 ???
(by the way, the smaller the fastener the more threads per inch, especially on machine bolts and the flatter the angle, while on larger bolts with 8 tpi, the slope gets flatter as the diameter increases.) It is all about keeping the angle of the inclined plane flat enough to resist slippage of the threads “downhill” which just is not very likely GIVEN ADEQUATE PRE-LOAD.
INITIAL UNDER-TIGHTENING 1 25 ?

8. INITIAL OVER-TIGHTENING
1 BOLTING ASSEMBLY TRAINING - INTRODUCTIONNotes from the Training Course on Bolting Assembly
STRESS
STRAIN (STRETCH)
TENSION
STRESS/STRAIN CURVE .
PLASTIC RANGE
ELASTIC RANGE
YIELD
ULTIMATE TENSILE STR.
PSI ΔL
As bad as it is to under tighten, it can be just as problematic to over-tighten.
Remember our discussion about Hooke’s law and the stress strain curve. If we stretch a spring (bolt) beyond its yield point we take it from the elastic range where it springs back into shape into the plastic range where it will not. Extreme over tightening can result in a fastener “necking down” or becoming narrower as it is stretched, just like a piece of taffy.
UNCONTROLLED TIGHTENING CAN TAKE THE BOLTS BEYOND YIELD AND INTO THE “PLASTIC” RANGE
INITIAL OVER-TIGHTENING

9. INITIAL OVER-TIGHTENING
1 BOLTING ASSEMBLY TRAINING - INTRODUCTIONNotes from the Training Course on Bolting Assembly *1
BOLTS IN THIS CONDITION CANNNOT BE RELIED ON TO MAINTAIN SUFFICIENT LOAD UNDER VARYING OPERATING CONDITIONS
Bolts in this condition cannot be relied on to maintain sufficient load under varying operating conditions.
INITIAL OVER-TIGHTENING

10. REPEATED RE-TIGHTENING
1 BOLTING ASSEMBLY TRAINING - INTRODUCTIONNotes from the Training Course on Bolting Assembly
LOOSE BOLTS ARE A “SYMPTOM” OF A PROBLEM, NOT THE CAUSE
THE BOLTED JOINT IS A “SYSTEM” WHERE MISALIGNMENT CAN CAUSE PROBLEMS THAT BOLTS CANNOT SOLVE
BOLTS ARE NOT JOINT ALIGNMENT TOOLS!
REPEATED HAMMERING ON A LOOSE BOLT JUST MAKES THE PROBLEM WORSE
AXIAL MISALIGNMENT
RADIAL MISALIGNMENT
Often bolts in dynamic systems seem to be habitually loose so the preferred solution is to send “Bubba” out with a larger hammer to retighten the bolts.
But loose bolts are most often a symptom of the problem and not the root cause.
For example a joint may be misaligned along its axis where the bolt holes do not line up. The problem here is that on a bolt inserted with the holes misaligned the distance between the nuts on either side will be longer than if the joint were in line. Bolting in this condition means that if the joint shifts in use later on, the distance will shorten. What do you think will happen to the tension in the bolts? Not only that, but since the nuts will not be parallel with the face of the joint the bolts will be bent and the load may be lost or the bolt may be damaged.
Another type of joint problem is what we call “radial” misalignment. Here the two face of the joint are not parallel. The common “solution” is to tighten the bolts on the more open side to bring the two sides together. But think about what is happening. Suppose the desired pre-load load on the bolts is 40,000 pounds but the yield is 60,000. If it takes 40,000 pounds to bring the joint into alignment we still have no effective preload on the joint. And if we continue to tighten we will take the bolt into yield at 60,000 having only provided 20,000 pounds of pre-load. By the way, what do you think is happening to the load in the bolts on the on the opposite side of the flange while we are levering the flange closed?
BOLTS ARE NOT JOINT ALIGNMENT TOOLS! FIX THE PROBLEM OF JOINT ALIGNMENT BEFORE YOU START BOLTING!
Simply sending “Bubba” out with a bigger hammer will probably just make your problem worse.
REPEATED RE-TIGHTENING

11. PAINT OR DIRT UNDER NUTS NON-PARALLEL SURFACES
1 BOLTING ASSEMBLY TRAINING - INTRODUCTIONNotes from the Training Course on Bolting Assembly
EMBEDMENT = SMOOTHING & SETTLING OF MATING SURFACES UNDER LOAD AND ABRASION
PAINT OR DIRT UNDER NUTS
NON-PARALLEL SURFACES
RE-TIGHTENING CAN AFFECT THE TORQUE / LOAD RATIO
FLANGES CAN AND DO DEFORM OVER TIME
EMBEMENT refers to the smoothing and settling of mating surfaces under load and abrasion.
Some of the contributing causes are:
Paint or dirt under the nuts or bolt head. Joint faces are often painted to protect them while they are waiting to be installed. Excessive paint (more than about 5000ths thick) or any grit or dirt should be removed or it could later flake out from under the nut resulting in a lack of tension. Keep in mind that the entire bolt stretch on a bolt is may only be 10,000ths of an inch which could represent at load of 20,000 pounds. Any slight relaxation could lose you all or a significant part of your load, causing the bolt to loosen.
Instructor example: Personal experience by the author involved cutting teeth on the edge of a Caterpillar loader bucket. Operator could not keep the bolts tight. They would loosen after only a few hours. Investigation revealed proper tightening load, but the problem was paint under the nuts.
Non parallel surfaces have already been mentioned. If cocked, the nuts and bolts will have a tendency to settle over time giving up their load
Retightening the same fastener a second time will polish the surfaces of the threads and change the friction coefficient from what existed at the first tightening. This doesn’t mean that you should not re-use fasteners but it does mean that you should not mix used and new bolts on the same joint if you expect to get the same pre-load with the same torque.
Like any other machinery joints can and do relax over time. If sufficient load was applied initially there should always be enough keep the joint in place and functioning properly. Be especially careful with short bolts (length to diameter ratio less than 4:1) as they can loose load much faster than longer bolts.
EMBEDMENT

12. ELASTIC INTERACTION (UNEVEN TIGHTENING)
1 BOLTING ASSEMBLY TRAINING - INTRODUCTIONNotes from the Training Course on Bolting Assembly
MOST BOLTS ARE INTERDEPENDENT (WHAT YOU DO TO ONE AFFECTS THE OTHERS)
PARTICULARLY EVIDENT WITH TENSIONING (A/B PRESSURES)
REASON FOR GRADUAL STEPS AND CROSS-PATTERN TIGHTENING (SEE PATTERN EXAMPLES)
MULTIPLE TOOLS (SIMULTORC)
Most bolts, especially where they are close togeter and in the same joint will be INTERDEPENDENT, that is what you do to one bolt will have an effect on the tension in the others around it or opposite it in the joint.
This is particularly evident when tensioners are involved because we are already dealing with an initial load vs. a residual load problem and this gets complex when we have to over tighten a first bolt so that when we tighten the second, the load in the first is not compromised.
INTRUCTOR NOTE: discuss A/B pressure explanation when using tensioners
To compensate for the load loss that occurs in the bolts tightened in the first pass, it is common practice to apply a higher load to the bolts tightened in the first pass when compared to those tightened in the second pass.
To achieve this higher load in the bolts, a higher pressure is applied on the first pass. This pressure is usually referred to as Pressure A. The bolts tightened on the second pass are tensioned using a lower pressure setting - pressure B.
Typically:
Pressure A = Pressure B + 15% to 20%
This interdependence is the reason why we use cross pattern tightening in gradual steps. You would not tighten the first lug nut on your car all the way, but would go in steps and a star pattern.
(INSTRUCTOR SEE CROSS PATTERN TIGHTENING EXAMPLES)
The best way to deal with even tightening issues is to use multiple wrenches simultaneously. We will demonstrate how this is done a little later.
ELASTIC INTERACTION (UNEVEN TIGHTENING)

13. VIBRATION AND SHEAR MOVEMENT
Use of Locking Devices on Bolted Joints
1 BOLTING ASSEMBLY TRAINING - INTRODUCTIONNotes from the Training Course on Bolting Assembly
Bolt locking devices have been used for over a hundred years but most so-called lock nuts are ineffective– why?
Patents started to appear in the middle of the nineteenth century proposing improvements in bolt and nut design to prevent them unintentionally coming loose. David Cumming in his patent of the 16th June 1868 states that his design would improve the bolt-nut assembly by ‘… thereby preventing the nut from being shaken off as is frequently the case where screw-bolts are used in the construction of railroad-cars, carriages, bridges and other structures’.
VIBRATION AND SHEAR MOVEMENT

14. VIBRATION AND SHEAR LOADING VIBRATION AND SHEAR MOVEMENT
1 BOLTING ASSEMBLY TRAINING - INTRODUCTIONNotes from the Training Course on Bolting Assembly
VIBRATION AND SHEAR LOADING
Research as shown that it’s repeated shear loading (surfaces sliding past one another) , not axial loading that causes bolts to vibrate loose.
Tight bolts clamp the plates & friction keeps them from sliding past one another
No sliding, no loosening!
Many joints use guide pins, keyways or press-fit bolts to resist shear movement
The problem with bolts coming loose under vibration really has its roots in the fact that the bolt is not as large around as the hole. Since the bolt can be pushed from side to side as the two faces of the joint mover relative to one another the bolt is forced into an arc which can alternately load and unload the bolt. Note that in some joints where high or shifting shear loads are present, the designers will use guide pins or keyways or bolts with shanks that are press-fit into the joint, to keep that lateral motion from occurring. In all cases, when shear loads can be a factor, extra care must be taken to ensure that sufficient preload is applied or that some other means is provided to make sure that the joint can’t slip. Once movement occurs the bolts will come loose and the joint will separate in short order.
Machine bolts often have fine threads to help resist loosening from this kind of vibration
VIBRATION AND SHEAR MOVEMENT

15. VIBRATION AND SHEAR MOVEMENT
Junker Vibration Test Machines
1 BOLTING ASSEMBLY TRAINING - INTRODUCTIONNotes from the Training Course on Bolting Assembly
The Junker vibration test machine consists of a fixed lower base and an upper moving section separated from the lower base by needle roller bearings. The bearings are used to eliminate friction between the two parts of the machine so that any transverse loading is sustained solely by the test fastener that secures the two sections together. A load cell is fitted in the lower base to allow the fastener preload to be continuously monitored during the test. A displacement transducer is used to allow the relative displacement between the two sections to be measured.
Junker designed a machine that allows the effectiveness of a fastener locking method to be evaluated
Examples of Junker Test Machines
VIBRATION AND SHEAR MOVEMENT

16. VIBRATION AND SHEAR MOVEMENT
1 BOLTING ASSEMBLY TRAINING - INTRODUCTIONNotes from the Training Course on Bolting Assembly
Junker and other researchers have shown that some commonly used locking methods are ineffective.
Helical spring washers:
Ineffective against vibration
Crush and separate under the pressure of industrial joints
For example a commonly used helical spring washer is frequently used to assist in keeping a nut from backing off by keeping a tension in the bolt if it starts to loosen. These have little place in a heavy industrial joint. First, once the bolt begins to loosen or the nut to turn, virtually all the preload is lost in the first few thousandths of an inch of relaxation. The washer can never hope to retain that kind of load. They even make the joint less secure because under the tremendous loads of a large bolt the split washer tends to crush out of the joint leaving it unstable. Use them on your bicycle if you want, but not in an industrial joint you have to count on.
VIBRATION AND SHEAR MOVEMENT

17. Let’s Try A Few Questions To Ensure
1 BOLTING ASSEMBLY TRAINING - INTRODUCTIONNotes from the Training Course on Bolting Assembly
Let’s Try A Few Questions To Ensure
We Understand About Wind Applications And Loosening
Read the slide)
Please take a few minutes to complete exercise 1 and discuss it.
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(Quiz #2)

18. 1 BOLTING ASSEMBLY TRAINING - INTRODUCTIONNotes from the Training Course on Bolting Assembly
QUIZ
1. Proper bolting techniques are important during which phases of wind turbine work? A. Production B. Installation C. Maintenance D. All of these
2. Elastic interaction refers to the loosening of one bolt when others around it are tightened. How should this problem be addressed? ________________________________________
3. T / F There is no need to worry about tightening bolts in gradually increasing steps in wind turbines because what is done to one bolt has very little effect on the others?

19. 1 BOLTING ASSEMBLY TRAINING - INTRODUCTIONNotes from the Training Course on Bolting Assembly
QUIZ
4. T / F Shear loads try to move the two sides of a joint past one another, while axial loads try to separate and compress the joint?
5. It is rapidly vibrating shear loads that are most responsible for bolts coming loose. Wind turbines have many joints that are susceptible to this kind of vibration. The best thing I can do to avoid bolts coming loose is: A. Use loc-tite B. Tighten properly in the first place C. Use lock washers D. Avoid lubrication
6. T / F Each manufacturer has a specification document that tells me the appropriate torque values and bolting procedures for each critical joint in the tower and turbine?

20. Gradual incremental tightening and a check pass on all bolts
1 BOLTING ASSEMBLY TRAINING - INTRODUCTIONNotes from the Training Course on Bolting Assembly
Quiz answers:
D. All of these
Gradual incremental tightening and a check pass on all bolts
False. Most of the bolts are close enough to one another to have great effect on each other.
True
B. Tighten the bolts properly in the first place
True. Each manufacturer will have specific Bolt Torque Specifications and Work Instructions which must be followed)