Bolt Preload

8/9/2019 Bolt Preload

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Threaded Fasteners Seminar:

Preload Loss and Vibration Loosening

Jeff Jungmann,

Spiralock Corporation


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Fastener Selectionh Underdesigned

or overdesigned?

h Too small can be disastroush

Too large adds weight, costh Not just size, but strengthh Material, property class, plating, friction

h

Correct selection requires understanding of jointbehavior

h Interaction of loads and deflectionsh Joint response to mechanical & thermal loads, vibration

h How to optimize joint for application?

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Joint Loading Axialh Concentrich Stress = Force/Tensile Areah How much load carried by bolt?h Static or dynamic force?h Bolt geometry has stress concentrations

h High stress areas more prone to failuresh

Fatigueh Hydrogen embrittlementh Stress corrosion cracking

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Joint Loading Axialh Eccentric: Bending Momenth Non-linear behaviorh Mating surfaces can separate (gapping)

h Critical! Bolt carries entire loadh Severity influenced by several factors:

h Stiffness of fastened memberh

Bolt hole clearanceh Bolt head diameterh Grip length

very critical, longer is better

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Joint Loading Shearh Bolts typically not intended to

handle lateral loadsh

Friction typeh Design for clamp load to exceed slip loadh Often designed with multiple fasteners

h Bearing typeh Direct load transfer between fastener and holeh

Shoulder boltsh Load limit is shear strength of bolt threads

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Joint Loading Torsionalh Solely a condition during tighteningh Combination of torsion and axial loadh

Torsional preload is minimal, but relaxation occursdirectly after release of toolh Very little torque energy goes to stretch the bolt

h 50% lost to nut faceh 40% lost to friction in threadsh 10% creates load in pitch causing bolt to stretch

h In many cases, fasteners see more strenuousloading during installation than in service

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Tt

i

Tb


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Joint Loading Thermalh Fastener loads change as operating

temperature changesh

Regardless of external load!h Dissimilar metals = different rates of thermalexpansion

h When joint materials move more rapidly thanthe fastener:

h

Cooling contraction causes loss of preloadh Heating

expansion causes embedment, greatertensile stress

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Joint Loading Combined Effectsh Key considerations:

h Fatigue limit -

Goodman Diagramh

Limit alternating loadsh Axial load decreases force holding members

togetherh Need to determine neutral axis of joint to

analyze combined loadsh

Decouple loads and resolve into x and ycomponents

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Estimating Fastener Diameter

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Joint Diagram

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Force

Extension

Boltextensionline

Compression

Joint

compressionline

Joint thickness @zero stress

Bolt length @zero stress

h Tightening sets up stress and strain in bolt & joint membersh Bolt length increases more than joint compresses


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Selecting Preload as % of Yield

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% Applications, important factors

100 Must know in-service loads; critical joints85-95 Needs highly accurate installation method

65-75 Majority of fastener applications50-60 Pressure vessels

35-40 Gasketed

joints

10-20 Fastener serves as shear pin or positive stop


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Hard Joint vs. Soft Jointh Hard Joint

h High preloadh Fatigue resistanth Assembly more rigid than fastenerh

Can see large swings in joint tension

Paper #Bolt Stretch Joint

CompressionExtension

Boltpreload

Force


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How Much Load Carried by Bolt?h Adding load to bolt decreases clamping

force on jointh Bolt in a hard joint carries only a small

fraction of additional load


Compression

Force

Extension

AppliedForce, F


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Hard Joint vs. Soft Jointh Soft Joint

h Gasketed

and noncritical jointsh Fastener is more rigid than assemblyh Bolt takes majority of applied loadh

Joint member compression may be non-linear


CompressionExtension

Non-linear

behavior

Force


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How Much Load Carried by Bolt?h Bolt in a soft joint carries a larger

percentage of additional forceh Risk of fastener moving from elastic

region into yield


Compression

Force

Extension

AppliedForce, F


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Separation Loadh Point where external load reduces clamp

force to zeroh Any additional increase in load is carried

100% by the bolth

Dangerous!


Compression

Force

Extension


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Preload Relaxationh Several possible sources of deformation

h Exceed pressure limits of contact areash

Gasketsh Approach bolt yield

h

Embedmenth Plastic deformation occurs primarily at installationh Rough surfaces, threads, creep in soft materials

h Relaxation dependent on time, temperature,and vibration

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Preload Relaxationh Effect of Embeddingh 64% in 1 st

thread; 90% in first three threads

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Bolt Stretch Joint CompressionExtension

Remainingpreload after

embedment

Plastic

deformation


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Environmental Factorsh Change in temperature causes change in lengthh Coefficient of thermal expansion for steel is ~1/2

that of aluminumh Temperature limits of materials:h Carbon steel starts to anneal at 230

C (450

F)h Stainless steels begin to anneal at 480

C (900

F)

h Heat resistant alloys used in extreme environmentsh Corrosion often begins at areas with high stress

concentration

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Effect of Thread Pitchh More force needed to pull out fine pitch wedgeh Smaller helix angle improves vibration resistance

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FcFf

c f


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Mechanism of Self Looseningh Several mechanisms used to explain self-looseningh Impact load causes assembly to resonateh

Localized cyclic plastic deformation, esp. at 1 st engaged threadh Torsional

energy released

h Bearing surface slipsh Nut rotates in loosening direction

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ContactPressure

Variation

Rotationof the Nut

BendingMoment

Micro-SlipBetweenThreads

ExternalLoad


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Thread Tolerancesh Inherent radial clearance to allow free-spinning

assemblyh Contributes to thread fit variationsh Motion (slip) permitted when preload is low or

external forces are very highh

Amount of motion limited by thread clearance

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Locking Mechanismsh All-metal locknutsh Nylon insertsh Tri-lobular boltsh Split washersh Toothed lockwashersh Serrated nutsh Jam nuts

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h Cotter pinsh Lock wireh Tab lock h Belleville washersh Adhesivesh Opposing rampsh Tapered threadsh Self-locking threads

(Spiralock)


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Vibration Loosening Testh Junkers Test

h Cam driven transverse movementh

Most severe condition to accelerate self-looseningh Treats fastener like it is undersized for application

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Vibration Test Variablesh Comparison studies require apples-to-apples

test parametersh

Amplitude, Frequency, Preload, Hole clearance,Bearing surface

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Vibration Performance

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Preload Locking Thread Formh Axial Load vs. Radial Load

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Self-locking thread has3 times the radial locking load


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Preload Loss Prevention Strategiesh Determine needed clamp load firsth Achieve proper clamp loadh Avoid rough, irregular contact surfacesh Minimize number of joint elements and materialsh Use longer bolts

increase grip length

h Minimize hole clearancesh Fine threaded fasteners are more resistant to

vibration than coarse threadsh Reduce thread clearance (tighter tolerance fit) to

provide better loosening resistanceh Change mechanical behavior of joint

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Referencesh Bickford, John H. Introduction to the Design andBehavior of Bolted Joints

1995, Marcel Dekker, Inc.

h Blendulf, Bengt. Fastening Technology &

Bolted/Screwed Joint Design. 2000.h Bolt Science. Vibration Loosening of Bolts andThreaded Fasteners, Tutorial on Basics of Bolted

Joints. 2008.h Comer, Jess. Threaded Fasteners and the BoltedJoint. 2004.

h

Junker, G.H. New criteria for self-loosening offasteners under vibration, 1969, SAE Trans 78:314-335.

Paper #

Documents

Bolt Preload