LECTURE # 38LECTURE # 38

BOLT STRENGTHBOLT STRENGTH

The strength is specified by stating the The strength is specified by stating the minimum proof strength, or minimum minimum proof strength, or minimum proof load, and the minimum tensile proof load, and the minimum tensile strengthstrength

The proof load is the maximum load The proof load is the maximum load (force) that a bolt can withstand (force) that a bolt can withstand without acquiring a permanent setwithout acquiring a permanent set

The proof strength is the quotient of The proof strength is the quotient of the proof load and the tensile-stress the proof load and the tensile-stress areaarea

The proof strength is about 90 The proof strength is about 90 percent of the 0.2 percent offset percent of the 0.2 percent offset yield strengthyield strength

ASTM threads are shorter (deals ASTM threads are shorter (deals mostly with structures) and generally mostly with structures) and generally loaded in shearloaded in shear

TENSION CONNECTIONS-THE TENSION CONNECTIONS-THE EXTERNAL LOADEXTERNAL LOAD

A clamping force,(preload) FA clamping force,(preload) Fii is applied is applied by tightening the nut before external by tightening the nut before external force, P is appliedforce, P is applied

FFii = preload = preload P = external tensile loadP = external tensile load PPbb = portion of P taken by bolt = portion of P taken by bolt PPmm = portion of P taken by members = portion of P taken by members

FFbb = P = Pbb + F + Fii = resultant bolt load = resultant bolt load FFmm = P = Pmm – F – Fii = resultant load on the = resultant load on the

membersmembers

The load P is tension, and it causes the The load P is tension, and it causes the connection to stretch, or elongate, connection to stretch, or elongate, through some distance through some distance

andandb

b

k

P

m

m

k

P

m

bmb k

kPP

Since, P = PSince, P = Pbb + P + Pmm

The resultant bolt load isThe resultant bolt load is

FFmm < 0< 0

mb

bb kk

PkP

imb

bibb F

kk

PkFPF

Fig. 15.9Fig. 15.9

Fig. 15.9Fig. 15.9

Fig. 15.10Fig. 15.10

Fig. 15.9Fig. 15.9

Fig. 15.10Fig. 15.10

Fig. 15.11Fig. 15.11

FFmm < 0< 0

These results are valid only as long as some These results are valid only as long as some clamping load remains in the membersclamping load remains in the members

i

mb

mibm F

kk

PkFPF

In all cases, the members take over In all cases, the members take over 80 percent of the external load 80 percent of the external load

Making the grip longer causes the Making the grip longer causes the members to take an even greater members to take an even greater percentage of the external load percentage of the external load

TORQUE REQUIREMENTSTORQUE REQUIREMENTS A high preload is very desirable in A high preload is very desirable in

important bolted connectionsimportant bolted connections

If the overall length of the bolt can If the overall length of the bolt can actually be measured with a actually be measured with a micrometer when it is assembled, the micrometer when it is assembled, the bolt elongation due to preload Fbolt elongation due to preload F ii can can be computed using formulabe computed using formula

EAlFi

The nut is simply tightened until the The nut is simply tightened until the bolt elongates through the distance bolt elongates through the distance

This ensures that the desired preload This ensures that the desired preload has been attainedhas been attained

The elongation of a screw cannot be The elongation of a screw cannot be measured, because the threaded end measured, because the threaded end is often a blind holeis often a blind hole

The torque wrench has a built-in dial The torque wrench has a built-in dial which indicates the proper torquewhich indicates the proper torque

With impact wrenching, the air With impact wrenching, the air pressure is adjusted so that the pressure is adjusted so that the wrench stalls when the proper torque wrench stalls when the proper torque is obtainedis obtained

Or in some wrenches, the air Or in some wrenches, the air automatically shuts off at the desired automatically shuts off at the desired torquetorque

The snug-tight condition is the The snug-tight condition is the tightness attained by a few impacts tightness attained by a few impacts of an impact wrenchof an impact wrench

Or the full effort of a person using an Or the full effort of a person using an ordinary wrenchordinary wrench

When the snug-tight condition is When the snug-tight condition is attained, all additional turning attained, all additional turning develops useful tension in the boltdevelops useful tension in the bolt

Turn-of-the-Nut MethodTurn-of-the-Nut Method

The turn-of-the-nut method is the easiest The turn-of-the-nut method is the easiest and least expensive method for installing and least expensive method for installing fasteners with the proper bolt tension. fasteners with the proper bolt tension.

The procedure generally works as The procedure generally works as follows. An iron worker tightens the bolt follows. An iron worker tightens the bolt and nut as tight as possible using a spud and nut as tight as possible using a spud wrench or a pneumatic impact wrenchwrench or a pneumatic impact wrench

A chalk mark or paint is then made on the A chalk mark or paint is then made on the bolt and nutbolt and nut

The bolt is tightened further by either The bolt is tightened further by either hammering on the spud wrench or using a hammering on the spud wrench or using a pneumatic impact wrench until the rotating pneumatic impact wrench until the rotating part has rotated the required amountpart has rotated the required amount

The paint or chalk mark shows how far the The paint or chalk mark shows how far the part has rotated and the rotation is always part has rotated and the rotation is always measured relative to the rotation of the measured relative to the rotation of the bolt. bolt.

The turn-of-the-nut method requires The turn-of-the-nut method requires that you compute the fractional that you compute the fractional number of turns necessary to develop number of turns necessary to develop the required preload from the snug-the required preload from the snug-tight conditiontight condition

For heavy hexagon structural bolts, For heavy hexagon structural bolts, the turn-of-th-nut specification states the turn-of-th-nut specification states that the nut should be turned a that the nut should be turned a minimum of 180minimum of 180oo

from the snug-tight from the snug-tight condition under optimum conditionscondition under optimum conditions

A good estimation of the torque A good estimation of the torque required to produce a given preload required to produce a given preload isis

2sec

sec1

2cci

m

mmi dF

ld

ddFT

2sectan1

sectan

2ccimi dFdF

T

mdl tan

Th diameter of the washer face of a Th diameter of the washer face of a hexagonal nut is the same as the hexagonal nut is the same as the width across flats and equal to 1.5 width across flats and equal to 1.5 times the nominal sizetimes the nominal size

ddcc = (1+1.5d)/2 = 1.25d = (1+1.5d)/2 = 1.25d

dFd

dT ic

m

625.0sectan1

sectan

2

Define a torque co-efficient KDefine a torque co-efficient K

T = K FT = K Fi i dd

The coefficient of friction depends The coefficient of friction depends upon the surface smoothness, upon the surface smoothness, accuracy, and degree of lubricationaccuracy, and degree of lubrication

cm

d

dK

625.0sectan1

sectan

2

On the average, both f and fOn the average, both f and fcc are are about 0.15about 0.15

K = 0.26 for f = fK = 0.26 for f = fcc =0.15 no matter =0.15 no matter what size bolts are employed and no what size bolts are employed and no matter whether the threads are matter whether the threads are coarse or finecoarse or fine

BOLT PRELOAD-STATIC BOLT PRELOAD-STATIC LOADINGLOADING

iimb

bb FCPF

kk

PkF

mb

b

kk

kC

im FPCF 1

The tensile stress in the bolt can be The tensile stress in the bolt can be found by dividing both terms of first found by dividing both terms of first equation by the tensile-stress area, Aequation by the tensile-stress area, Att

The limiting value of The limiting value of bb is the proof is the proof strength, Sstrength, Sbb

t

i

tb A

F

A

CP

With the introduction of a load factor With the introduction of a load factor nn

Any value of n > 1 ensures that the Any value of n > 1 ensures that the bolt stress is less than the proof bolt stress is less than the proof strengthstrength

Pt

i

t

SA

F

A

PnC

PC

FASn itP

JOINT SEPARATIONJOINT SEPARATIONFor safe joint, external load be smaller thanFor safe joint, external load be smaller than

that needed to cause the joint to separatethat needed to cause the joint to separate

If separation does not occur, then the entireIf separation does not occur, then the entire

external load will be imposed on the boltexternal load will be imposed on the bolt

Let PLet Poo be the value of the external load that would cause joint separation be the value of the external load that would cause joint separation

LlllllllllllllLlllllllllllll

Fig. 15.15Fig. 15.15

At separation, FAt separation, Fmm = 0, = 0,

(1 – C)P(1 – C)Poo – F – Fii =0 =0

Let the factor of saftey against joint Let the factor of saftey against joint separation beseparation be

P

Pn oo

)1( CP

Fn io

Fig. 8.18Fig. 8.18

Fig. shows stress-strain diagram of a Fig. shows stress-strain diagram of a good quality bolt materialgood quality bolt material

No clearly defined yield pointNo clearly defined yield point

Fracture at the tensile strengthFracture at the tensile strength

No matter how much preload is given No matter how much preload is given to bolt, it will retain its load-carrying to bolt, it will retain its load-carrying capacitycapacity

The pr-tension is the muscle of the The pr-tension is the muscle of the joint and its magnitude is determined joint and its magnitude is determined by the strengthby the strength

If the full bolt strength is not used, If the full bolt strength is not used, then money is wastedthen money is wasted

Good quality bolts can be preloaded Good quality bolts can be preloaded into the plastic range to develop more into the plastic range to develop more strengthstrength

A bolt will either fracture during A bolt will either fracture during tightening or not at alltightening or not at all

It is recommended for both static and It is recommended for both static and fatigue loading that the following be fatigue loading that the following be used for preload :used for preload :

Where FWhere FPP is the proof load, obtained is the proof load, obtained from the equation Ffrom the equation FPP = A = Att S SPP

SSPP is the proof strength obtained from is the proof strength obtained from tables 8-4 to 8-6tables 8-4 to 8-6

sconnectionpermanentforF

sconnectionreusedforFF

P

Pi

90.0

75.0

For other materials, an approximate For other materials, an approximate value isvalue is

SSPP = 0.85 S = 0.85 SYY

Fig. Fig. 8.178.17

Fig. Fig. 8.188.18

Fig. 8.19Fig. 8.19