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©HYTORC 2009 HYTORC -WIND APPLICATIONS 1 BOLTED JOINT ASSEMBLY WHY BOLTS COME LOOSE and WHAT TO DO ABOUT IT WIND ENERGY FIELD SERVICE TECHNICIANS
©HYTORC 2009 HYTORC -WIND APPLICATIONS 2 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.
©HYTORC 2009 HYTORC -WIND APPLICATIONS 3 PROPER BOLTING IS CRITICAL TO WIND TOWER PRODUCTIVITY…
©HYTORC 2009 HYTORC -WIND APPLICATIONS 4 MOST COMMON CAUSES FOR LOOSENING: INITIAL UNDER-TIGHTENING INITIAL OVERTIGHTENING REPEATED RE-TIGHTENING ELASTIC INTERACTION (UNEVEN TIGHTENING) EMBEDMENT VIBRATION AND SHEAR MOVEMENT
©HYTORC 2009 HYTORC -WIND APPLICATIONS 5 100 REMEMBER THE CONCEPT OF “PRE-LOAD” 10 1 SPRING BAR OR “DIVING BOARD” 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 INITIAL UNDER-TIGHTENING
©HYTORC 2009 HYTORC -WIND APPLICATIONS 6 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 INITIAL UNDER-TIGHTENING
©HYTORC 2009 HYTORC -WIND APPLICATIONS 7 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 INITIAL UNDER-TIGHTENING
©HYTORC 2009 HYTORC -WIND APPLICATIONS 8 INITIAL OVER-TIGHTENING STRESSSTRESS STRAIN (STRETCH) TENSIONTENSION STRESS/STRAIN CURVE.. PLASTIC RANGE ELASTIC RANGE YIELD ULTIMATE TENSILE STR. PSI ΔLΔL UNCONTROLLED TIGHTENING CAN TAKE THE BOLTS BEYOND YIELD AND INTO THE “PLASTIC” RANGE
©HYTORC 2009 HYTORC -WIND APPLICATIONS 9 BOLTS IN THIS CONDITION CANNNOT BE RELIED ON TO MAINTAIN SUFFICIENT LOAD UNDER VARYING OPERATING CONDITIONS *1 INITIAL OVER-TIGHTENING
©HYTORC 2009 HYTORC -WIND APPLICATIONS 10 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 MISALIGNMENTRADIAL MISALIGNMENT
©HYTORC 2009 HYTORC -WIND APPLICATIONS 11 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
©HYTORC 2009 HYTORC -WIND APPLICATIONS 12 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)
©HYTORC 2009 HYTORC -WIND APPLICATIONS 13 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 VIBRATION AND SHEAR MOVEMENT
©HYTORC 2009 HYTORC -WIND APPLICATIONS 14 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 VIBRATION AND SHEAR MOVEMENT
©HYTORC 2009 HYTORC -WIND APPLICATIONS 15 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 VIBRATION AND SHEAR MOVEMENT
©HYTORC 2009 HYTORC -WIND APPLICATIONS 16 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 VIBRATION AND SHEAR MOVEMENT
©HYTORC 2009 HYTORC -WIND APPLICATIONS 17 Let’s Try A Few Questions To Ensure We Understand About Wind Applications And Loosening (Quiz #2)
©HYTORC 2009 HYTORC -WIND APPLICATIONS 18 QUIZ 1.Proper bolting techniques are important during which phases of wind turbine work? A. ProductionB. Installation C. MaintenanceD. 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?
©HYTORC 2009 HYTORC -WIND APPLICATIONS 19 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-titeB. 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?
©HYTORC 2009 HYTORC -WIND APPLICATIONS 20 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)
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