DesignofBoltedJoints_16-20

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F A S T E N E R SDesign of Bolted Joints forGeneral Engineering

Selection of TensileStrength of Bolts

Bolted joints in which strength isthe main design consideration, can,in most cases, be moreeconomically designed when ahigh tensile bolt is used ratherthan a mild steel bolt. Fewer boltscan be used to carry the same totalload, giving rise to savings notonly from the cost of a smallernumber of bolts, but alsomachining where less holes aredrilled and tapped, and assemblywhere less time is taken.

Selection of Coarseand Fine Threads

In practically all cases the coarsethread is a better choice. Thecoarse threads provide adequatestrength and great advantages inassembly over fine threads. Theformer are less liable to becomecross threaded, start more easily,particularly in awkward positions,and require less time to tighten.

In cases where fine adjustment isneeded, the fine thread should beused. Providing bolts are tightenedto the torque specified in Tables18-23 there should be no tendencyto loosen under conditions ofvibration with either coarse or finethreads.

Types of Loading onJoints

Examine the forces being appliedto the joint to decide which of thefollowing types fits the conditions.

a) Joints carrying direct tensileloads (See Fig. 9).

b) Joints carrying loads in shear(See Fig. 10-11). Types 1 and 2.

c) Flexible gasket joints forsealing liquids or gases underpressure (See Fig. 12).

Joints Carrying DirectTensile Loads

(1) Safety Factor. Apply a safetyfactor according to the nature ofthe loading. Except in the case ofthe flexible gasket joint, the safetyfactor on a bolt differs from mostother applications in that it doesnot affect the stress of the bolt, butrefers to the factor by which thesum of the preload on all the boltscomprising the joint exceeds thedesign load applied. Regardless ofthe nature of the load, the boltsshould still be preloaded to 65% oftheir yield stress using therecommended torque values as setout in Table 18-23.

Safety Factor =

Sum of preload on all the boltscomprising the joint

Design applied load

For design purposes, the preloadon each bolt should be takenaccording to the bolt size and boltmaterial as shown in Tables 18 to

45845 Blacks Bolt body 3/9/06 2:10 PM Page 16

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F A S T E N E R S


Design of Bolted Joints forGeneral Engineering

23 and the safety factor selectedfrom the following table:-

Table 14

Nature of Loading Safety Factors*

Steady Stress 1.5 – 2

Repeated Stressgradually applied 2 – 3.5shock 4.5 – 6

* Applies to joints with direct tensileloads only and assumes all bolts aretightened to 65% of the yield stress.

(2) Total Required Preload†.Determine this from safety factor(S) and applied load (L).

Total required preload F = S x L

(3) Selection of BoltMaterial, Bolt Size, Numberof Bolts. By selecting a suitablebolt size and bolt material, therequired number of bolts can bedetermined from –

N = Ff

Where N is the number of bolts, Fis the total required preload and fis the recommended preload (seeTables 18-23) on the bolt for theparticular size and materialselected.

(4) Specify TighteningTorque. Ensure that the bolts arefully tightened to the torquerecommended in Tables 18-23 forthe particular bolt size andmaterial.

(5) Positioning of the Bolts.The bolts should be placed as nearas possible to the line of directtensile load. By doing this,secondary bending stresses in thebolts and bolted members arereduced to a minimum.

Joints Carrying Loadsin Shear

The design procedure formechanical joints carrying thistype of loading can be based onwell established practice laid downfor structural joints carrying staticloads, provided the design loadsare increased by adequate factorsto allow for cyclic loads, shock, etc.These factors will varyconsiderably according to theapplication, and must be based onthe designer’s experience. Boltedjoints carrying loads in shear fallinto two types:-

1. Joints in which the load istransferred through the boltedmembers by bearing of themember on the shank of thebolt and shear in the bolt.

2. Friction type joints, where theload is transferred by thefriction developed between themembers by the clampingaction of the bolts.

† Note: At time of publication there are no “Allowable Stress” code provisions forgeneral mechanical engineering design of bolted joints. This information isprovided for guidance only.


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Load Transfer byBearing and Shear

Such joints may be designed usingallowable values for shear in thebolts and bearing on the jointmembers such as those given inAS 1250 or under the limit statesprovisions of AS 4100.

Guidance on bolt shear capacity isgiven on page 14. The loweststrength, whether it be in shear orbearing, is used to compute therequired number of bolts to carrythe design load.

The allowable values for shear andbearing depend not only on boltsize, but also on the tensilestrength of the bolt, and whetherthe bolt is in a close fittingmachined hole (not greater than0.25mm clearance) or is fitted in aclearance hole (up to 2-3mmclearance).

Careful consideration should begiven to the properties of thematerial in the bolted members toensure they are capable ofwithstanding bearing loads. Tensilestrength and yield stress of Blacksbolts can be obtained from Tables5-11. Care must be taken that thepitch of the bolt spacing issufficient to ensure that the boltedmembers are not weakened by the

bolt holes to the extent that theycannot safely carry the load. Toachieve this it may be necessary touse more than one row of bolts.Staggering of bolt holes canminimise reduction of membercapacity. If more than twomembers are bolted togetherslightly higher values are permittedin bearing on the central member,and the area considered forcalculating strength in shear isincreased by two or four times forbolts in double or quadruple shear.

Friction Type Joints

These joints are made up usinghigh strength bolts fitted inclearance holes and tightenedunder careful control to develop apreload equivalent or greater thanthe bolt yield load.

The mechanism of carrying load isby friction developed between themating faces, and it is wellestablished that this type of joint isconsiderably stronger than ariveted joint. Refer to AustralianStandard 4100.

General Rules toReduce Possibility ofBolt Failure Due toFatigue

The following general rules shouldbe observed to minimisepossibility of fatigue of bolts underhigh alternating or fluctuatingstresses.


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F A S T E N E R S


Design of Bolted Joints forGeneral Engineering

(1) Most Important. Tightenbolt effectively to ensure aninduced tension or preload inexcess of the maximum externalload.

(2) Bolt extension in tighteningshould be high. This can beachieved by:-

a) At least 1 x bolt diameter of“free” thread length under thenut.

b) Use of small high strengthbolts in preference to largerlow strength bolts.

c) In extreme cases a “waistedshank” bolt can be considered.

(3) Rolled threads are preferable tomachined threads.

(4) Under conditions of extremevibration the use of locknuts suchas the Conelock or Nyloc nutshould be considered to avoidpossibility of a loosened nutvibrating right off the bolt beforedetection.

(5) Bolt head and nut should be onparallel surfaces to avoid bending.

(6) Non axial bolt loadingproducing a “prising” actionshould be avoided where possible.

Flexible Gasket Jointsfor Sealing Liquids orGases Under Pressure

This type of joint differs from thetwo preceding types in that thestress in the bolt varies with theworking load.

This is because the flexible gasketmaterial has a much lower elasticmodulus than the bolt, andcontinues to exert virtually thesame force on the bolts whenadditional load is applied to thejoint. The resulting effect is that theworking load is added to the boltpreload in this case, so the designprocedure must be modifiedaccordingly.

(1) Design Pressure Load.Determine the design load Q onthe joint by multiplying theeffective area A on which thepressure is acting by the liquid orgas pressure P by S where S is thesafety factor selected from Table14.

Q = APS

(2) Total Preload Required.To the design pressure load, Q add10%, and this is the sum of thepreload F that should be applied tothe bolts comprising the joint.

F = Q + 10Q100

i.e. F = 1.1Q

(3) Total Design Load onBolts. In the case of a flexiblegasket type of joint the designpressure load Q on the joint isadded to the preload F on thebolts, giving the total design loadW on the bolts.

W = Q + F

(4) Select Bolt Material. Fromthe following table of yield stressesselect the bolt material.


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Table 15

Proof Load StressBolt Type lbf/in2 MPa

Blacks AS 2451 Bolts –1/4” – 3/4” 35,900 248Over 3/4” 33,600 232

Blacks MetricCommerical Bolts 32,630 225

Blacks Metric PrecisionPC 8.8 Bolts 95,725 660

Blacks SAE Grade 5 High Tensile Bolts –

1/4” – 1” 85,000 586

Blacks SAE Grade 8High Tensile Bolts 120,000 827

(5) Select Bolt Size andDetermine Number of Bolts.From the desired bolt size andcorresponding Stress Area “As”, (seeTables 5-11) determine the numberof bolts N from the yield stress Yand the total design load W on thebolts.

Number of bolts required N = WYAs

(6) Setting of TighteningTorque. In this case the bolts canonly be tightened to a preload wellbelow the yield stress so the torque

figure T for the bolt size andmaterial selected listed in Tables18-23 must be reduced bymultiplying by a factor of 0.806.

Tightening torque to be applied tobolts of a flexible gasket type ofjoint.

t = 0.8 T

Metal to MetalPressure Tight Joints

The stress in the bolts in a flexiblegasket type joint varies with load,and under rapidly fluctuatingloads they can be subject tofatigue. It is therefore desirable touse wherever possible, metal tometal pressure tight joints, as theseare not subject to fatigue.

The design procedure for a metalto metal pressure tight joint isexactly the same as for jointscarrying direct tensile loads oncethe pressure load is determined.


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DesignofBoltedJoints_16-20