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Bolted Joints and Bolt Preload Overview

Dan Griffin - Director of Engineering Design Automation Associates, Inc.www.DAASolutions.com

April 14th, 2011

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Subjects for Discussion

• Why use bolted joints

• Bolt load vs. applied load

• Fatigue

• Prying

• How to choose Bolts, Preload

• Example preload spreadsheet

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Why Use Bolted Joints

•Easy to assemble•Usually Inexpensive •Allows preloading of joint so it remains rigid under various loads•Preload takes advantage of stack spring rate so bolt is little affected by cyclic load•Friction resulting from preload multiplies shear bearing capability

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Bolt Load vs. Applied Load

Loose Bolt Tight Bolt

PA = Load Applied to JointP0 = Bolt PreloadPb = Actual Load in Bolt

The Effectiveness of the Bolted Joint is based upon its Preload!

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Bolt Load vs. Applied LoadFlange “Stack”

“Bolt”

K1A – Bolt Shoulder as ApplicableK1B – Bolt Shank or UndercutK1C – Portion of Threaded Section in TensionK2 – Portion of Bolt Head in CompressionK3 – Portion of Bolt Head in ShearK4 – Portion of nut in CompressionK5 – Thread slip due to Nut Radial GrowthK6A – Flange Portion within Cone of CompressionK6B – 2nd Flange Portion Within Cone of Compression 5

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Bolt Load vs. Applied Load

As

s

bAb

sb

Ass

sb

Abb

ss

bb

sb

A

BA

s

CBA

b

sbTOT

TOTA

PPP

K

KPPP

KKP

KPP

KKP

KPP

KPP

KPP

KKP

KK

K

KKKKKKK

K

KKK

KP

0

0

0

0

0

0

66

5432111

111

11111111

Flange “Stack”

“Bolt”

“Bolt” Group and “Stack” Group are springs in parallel

Each Group is made up of springs in series

Ks is usually 5-10X Kb

So – Pb is nearly constant even though PA is cyclic!

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FatigueHere’s how the preload and stiffness ratio render the properly designed bolted joint immune to fatigue…..

Loose Bolt Tight Bolt

Max Pb=PA

Min Pb=0Mean Load=PA/2Cyclic Load=PA/2

Max Pb = P0+PA Kb/Ks = P0+0.1PA = 2.1PA

Min Pb = P0 = 2PAMean Load = 2.05PA

Cyclic Load = 0.05PA

Assume Kb/Ks=0.1; P0=2PA

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Fatigue

•Loose Bolt will fail in fatigue in almost any cyclic loading situation given bolt stress concentrations•Tight bolt will not experience fatigue or even significant cyclic stress as long as separation is avoided

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Prying

•For thin flanges like the one shown (Typical of aerospace applications), prying due to offsets between bolt, applied load , and flange toe can be significant•Applied load must be scaled up to account for prying•Simple rule of thumb for bolt pattern sizing:

PAPA

Heel

Toe

b

baPP AEffectiveA

,

Prying Factor

•Larger Industrial flanges with long bolts are less susceptible to this •Need FEA to determine with accuracy

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How to Choose Bolts, Preload

This is generally iterative:1. Determine applied loading2. Choose a size and pattern3. Rough estimate prying factor, effective applied load per bolt4. Determine bolt torque, minimum preload based on strength, thread, and

maximum friction coefficient based on lubricant5. Compare effective applied load to minimum preload – PA,Effective/P0

should be less than 0.3-0.5 for Aerospace flanges, can be larger for industrial flanges

6. Adjust bolt size and count to converge on above goal7. Perform FEA to confirm no separation at bolt hole. If you do everything

correctly, you will only need one FEA!8. Given no separation – fatigue analysis should not be necessary!

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Example Preload Spreadsheet

Criteria for Sizing Preload for a given Bolt Geometry, Thread, Lubricant

•Tighten bolts to full capability:90% Sy for Maximum Principal Nominal Stress including tensile and torsion due to wrenchingSome folks use 60% Sy when just considering tensile stress alone which is roughly equivalentAssume minimum friction coeficient (0.10 for oil, 0.02 for some wet anti-seize compounds)

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Example Preload Spreadsheet

KFT

FKT 11

FKT 22

21 KKK

sectan1

sectan

2 1

11

f

fPK

22

33

22

3hH

hH

DD

DDfK

Where:

Bolt Torque

Threaded Section Torque

Head Thrust Face Torque

Threaded Section Friction Factor

Head Thrust Face Friction Factor

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Example Preload Spreadsheet

A

Ft

p

sZ

T1 3

min16

dZ p

2

2

'2

s

t

s

2''

t

st

Tensile Stress

Torsion Shear Stress, where:

Max Shear Stress

Max Principal Stress

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Example Preload Spreadsheet

Type I Type II 0.10 0.15 0.20 0.10 0.15 0.20

.190-24 UNJC-3A 0.1629 0.1368 34 30 28 20 24 0.0162 0.0210 0.0259 0.0152 0.0228 0.0305

.250-20 UNJC-3A 0.2175 0.1864 75 65 65 45 55 0.0207 0.0271 0.0336 0.0182 0.0274 0.0365

.3125-18 UNJC-3A 0.2764 0.2420 160 140 140 105 125 0.0250 0.0331 0.0414 0.0213 0.0320 0.0427

.375-16 UNJC-3A 0.3344 0.2957 280 250 250 180 220 0.0295 0.0393 0.0492 0.0244 0.0366 0.0488

.4375-14 UNJC-3A 0.3911 0.3472 460 410 420 310 370 0.0342 0.0457 0.0573 0.0292 0.0439 0.0585

.500-13 UNJC-3A 0.4500 0.4028 690 620 630 470 560 0.0385 0.0517 0.0650 0.0323 0.0485 0.0647

.5625-12 UNJC-3A 0.5084 0.4574 1000 900 925 675 825 0.0429 0.0578 0.0728 0.0372 0.0557 0.0743

.625-11 UNJC-3A 0.5660 0.5105 1350 1200 1250 900 1100 0.0474 0.0641 0.0808 0.0403 0.0604 0.0805

.750-10 UNJC-3A 0.6850 0.6240 2400 2100 2300 1700 2000 0.0558 0.0758 0.0960 0.0482 0.0723 0.0963

.875-9 UNJC-3A 0.8028 0.7352 3900 3500 3700 2700 3300 0.0644 0.0879 0.1115 0.0561 0.0841 0.1121

1.000-8 UNJC-3A 0.9188 0.8430 5900 5300 5600 4200 5000 0.0733 0.1002 0.1273 0.0640 0.0960 0.1280

Type I Type II Type I Type II Type I Type II 0.10 0.15 0.20

.190-24 UNJC-3A 637 764 456 547 355 426 891 638 497 65.2 45.3 55.8 88.4

.250-20 UNJC-3A 1156 1413 826 1009 642 784 1669 1192 927 65.8 40.8 52.4 85.3

.3125-18 UNJC-3A 2267 2699 1612 1919 1250 1488 3023 2149 1666 70.7 39.9 53.3 88.6

.375-16 UNJC-3A 3341 4083 2370 2897 1835 2243 4640 3292 2549 72.7 37.0 51.9 88.2

.4375-14 UNJC-3A 4888 5834 3461 4131 2677 3195 6622 4689 3627 75.2 36.1 52.2 89.8

.500-13 UNJC-3A 6638 7909 4690 5588 3624 4318 8898 6287 4858 75.1 35.0 51.3 88.8

.5625-12 UNJC-3A 8434 10308 5944 7265 4587 5606 11557 8146 6285 75.6 34.2 51.0 88.8

.625-11 UNJC-3A 10265 12546 7233 8840 5580 6820 14257 10046 7750 74.9 34.0 50.6 88.0

.750-10 UNJC-3A 16357 19244 11479 13504 8837 10396 22130 15530 11956 77.8 32.6 50.7 89.7

.875-9 UNJC-3A 22420 27402 15699 19188 12072 14755 30723 21514 16543 77.8 31.7 50.2 89.1

1.000-8 UNJC-3A 30588 36415 21405 25482 16454 19588 40784 28540 21938 78.6 31.4 50.3 89.6

Pitch

Diameter

(in.)

Recommended

Torque - self

locking, f =.10,

(lb.-in.)

Coarse Threads

Oil Lubricated

Low Strength Material (100-149 KSI)

TABLE 2 - Torque, Stress and Load Data for Bolts with:

Min

Minor

Diameter

(in.) Min. Friction Factor Friction Factor

σt τs

Thread Size

Recommended Torque

- free running, f

=.10, (lb.-in.)

K1

Fmin = Bolt Loads With Min. Torque, lbsFmax = Bolt Loads

with Max. Torque, lbs

Stresses Based on Max. Torque

and f =.10 (KSI)

K2

Max.Min.Max.

τs' σt'

TABLE 2 Continued

Thread Sizef = .10 f = .15 f = .20 Friction Factor

Size/Thread

Thread TypeLubricantStrength Class

Friction Coefficients

Min Bolt Loads

Torques

Stresses(<90% Sy)

Max Bolt Loads

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