©HYTORC 2009 HYTORC -WIND APPLICATIONS 1 BOLTED JOINT ASSEMBLY WHY BOLTS COME LOOSE and WHAT TO DO...

1©HYTORC 2009HYTORC -WIND APPLICATIONS

BOLTED JOINT ASSEMBLY

WHY BOLTS COME LOOSEand

WHAT TO DO ABOUT IT

WIND ENERGY FIELD SERVICE TECHNICIANS


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.


PROPER BOLTING IS CRITICAL TO WIND TOWER

PRODUCTIVITY…


MOST COMMON CAUSES FOR LOOSENING:

INITIAL UNDER-TIGHTENING

INITIAL OVERTIGHTENING

REPEATED RE-TIGHTENING

ELASTIC INTERACTION (UNEVEN TIGHTENING)

EMBEDMENT

VIBRATION AND SHEAR MOVEMENT


100

REMEMBER THE CONCEPT OF “PRE-LOAD”

10 10

10 10

10 10

10 10

10 10

1SPRING BAR OR“DIVING BOARD”

BOARD PULLS 100 UP

ROPE PULLS100 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



TOO LITTLE PRE-LOAD MEANS THE JOINT CAN MOVE

ZERO PRE-LOAD – THE JOINT MOVES

10,000 LBS. PRE-LOAD -JOINT WON’T MOVE UNTIL EXTERNAL LOAD EXCEEDS 10,000 LBS.

ONCE THE JOINT MOVES:

THE NUTS WILL LOOSEN

LOAD WILL BE QUICKLY LOST IN THE JOINT

UNPROTECTED BOLT SUBJECTED TO OUTSIDE SHOCKS MAY FAIL



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)

HOW DO WE SOLVE THE UNDERTIGHTENING PROBLEM?

1

25


8©HYTORC 2009HYTORC -WIND APPLICATIONSINITIAL OVER-TIGHTENING

STRESS

STRAIN (STRETCH)

TENSION

STRESS/STRAIN CURVE

.

.

PLASTIC RANGE

ELASTIC RANGE

YIELD

ULTIMATE TENSILE STR.PSI

ΔL

UNCONTROLLED TIGHTENING CAN TAKE THE BOLTS BEYOND YIELD AND INTO THE “PLASTIC” RANGE


BOLTS IN THIS CONDITION CANNNOT BE RELIED ON TO MAINTAIN SUFFICIENT LOAD UNDER VARYING OPERATING CONDITIONS

*1

INITIAL OVER-TIGHTENING


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

REPEATED RE-TIGHTENING

AXIAL MISALIGNMENT RADIAL MISALIGNMENT


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

EMBEDMENT


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)

ELASTIC INTERACTION (UNEVEN TIGHTENING)


Bolt locking devices have been used for over a hundred years but most so-called lock nuts are ineffective– why?

Use of Locking Devices on Bolted Joints



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

Machine bolts often have fine threads to help resist loosening from this kind of vibration

VIBRATION AND SHEAR LOADING



Junker Vibration Test Machines

Junker designed a machine that allows the effectiveness of a fastener locking method to be evaluated

Examples of Junker Test Machines



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



Let’s Try A Few Questions To EnsureWe Understand About Wind Applications And Loosening

(Quiz #2)


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?


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?


Quiz answers:

1. D. All of these

2. Gradual incremental tightening and a check pass on all bolts

3. False. Most of the bolts are close enough to one another to have great effect on each other.

4. True

5. B. Tighten the bolts properly in the first place

6. True. Each manufacturer will have specific Bolt Torque Specifications and Work Instructions which must be followed)

©HYTORC 2009 HYTORC -WIND APPLICATIONS 1 BOLTED JOINT ASSEMBLY WHY BOLTS COME LOOSE and WHAT TO DO...

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