Introduction to Bolts - The Importance of Preloading

7/28/2019 Introduction to Bolts - The Importance of Preloading

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Introduction

The complexity of the simple nut and bolt isfrequently underestimated. A fully tightened bolt doesnot perform like a loose bolt. A fully tightened bolted

joint can sustain millions of load cycles withoutproblems, a joint consisting of untightened bolts willfrequently fail within a few cycles. The reason for thisis the way a bolted joint carries anexternal load - a fully tightened bolt

sustains only a small proportion of any externally applied load.This tutorial seeks to explain why this occurs.

Presented in this tutorial are details about the basics of boltedjoint technology and in particular on the mechanics of the load

transfer mechanism involved in such joints. There are a number of pages tothis tutorial covering the topic from the basics. Additional pages are beingadded as and when time permits. Bolt Science is committed to providingassistance on bolted joint technologies to individuals, companies and otherorganisations. If you have a question on any of the topics - why not email usand we will try to answer your query.

Why Bolt Preload isImportant

Over the last fifty years great improvementshave been made by the fastener industry inimproving the design and reliability of theirproducts. However, no matter how welldesigned and made the fastener itself is, itcannot alone make the joint more reliable.Fastener selection based upon anunderstanding of the mechanics of how athreaded fastener sustains loading and theinfluence that tightening procedures can playis also needed. This article provides anintroduction to the basics of bolted joints andthe major factors involved in the design ofsuch joints.


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It is not widely understood how a bolted joint carries a direct load. A fullytightened bolt can survive in an application that an untightened, or loose bolt,would fail in a matter of seconds. When a load is applied to a joint containing atightened bolt it does not sustain the full effect of the load but usually only asmall part of it. This seems, at first sight, to be

somewhat contrary to common sense. Figure1A shows a bolt and nut securing a bracket toa support plate.With the nut loose on the bolt, if a weight of 1unit is added to the bracket, as shown in figure1B, then the force in the bolt shank willincrease by 1. However, if the nut is nowtightened and the weight applied, the force inthe bolt shank will not increase by 1 butusually by only a small fraction of this amount.An understanding of why the bolt does notsustain the full effect of the applied load isfundamental to the subject.

A model can often be of help in understanding why the bolt does not sustainthe full effect of the applied load. Figure 2 is an attempt to illustrate the loadtransfer mechanism involved in a bolted joint by the use of a special fastener.In the case of this fastener no significant load increase would be sustained bythe fastener until the applied load exceeded the fastener's preload. (Preload isthe term used for a bolt's clamp force.)

Appying an External Force to a BoltedJoint

A model can often be of help inunderstanding why the bolt does notsustain the full effect of the applied load.Figure 2 is an attempt to illustrate the

load transfer mechanism involved in abolted joint by the use of a specialfastener. In the case of this fastener nosignificant load increase would besustained by the fastener until theapplied load exceeded the fastener'spreload. (Preload is the term used for abolt's clamp force.)


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With the special fastener shown, the bolt is free to move within its casing, a compression spring isincluded within the casing so that if the bolt is pulled down the spring will compress. A scale on the sideof the casing indicates the force present in thespring and hence the force present in the shankof the bolt. Figure 2A illustrates this special

fastener in its untightened condition.The bolt is now inserted through a hole in asupport plate and a bracket attached to thespecial fastener by securing a nut to the threaded

shank. If the nut is now rotated so that the headof the bolt is pulled down, the spring will becompressed. If the nut is rotated so that 2 forceunits are indicated on the casing, the compressiveforce acting on the spring will be 2 and the tensileforce in the bolt shank will also be 2. This isillustrated in figure 2b; this is like a tightened boltwithout any working load applied.

If a weight is now added to the bracket (figure 2c)

of value 1, then the initial reaction is to think thatthe load in the bolt must increase, otherwise whathappens to the additional force? Surprisingly itwill keep at its existing value of 2 - it will not 'feel'any of the additional force. To visualise why thisis so - imagine what would happen if the load inthe bolt did increase. To do this it would compress

the spring more and a gap would be madebetween the bracket and the plate. If such a gapwas to form then it would mean that there would be 2 units of force acting upwards - due to the spring,and 1 unit of force acting downwards from the applied weight. Clearly this force imbalance would notoccur. What does happen is that the effect of the applied load is to decrease the clamp force that existsbetween the plate and the bracket. With no load applied the clamp force is 2 units, with the load appliedthis decreases to 1 unit of force. The bolt would not actually 'feel' any of the applied force until it

exceeded the bolts clamp force.Older design procedures proposed calculation methodsbased upon the idea that the bolt will not 'feel' any ofthe applied load until it exceeds the bolts clamp force.That is, the bolt should be sized so that its clampforce is equal to the external load after a factor ofsafety has been included. With the special fastenerused in this example the stiffness of the fastener is farsmaller than the stiffness of the plate and bracket itclamps. Practical fasteners differ from that shown infigure 2 in that elongation of the fastener andcompression of the clamped parts occurs upontightening. This compression results in the boltsustaining a proportion of the applied load. As theapplied force reduces the clamp force existing withinthe joint an additional strain is felt by the bolt which

increases the force it sustains. The amount of theadditional force the bolt sustains is smaller than theapplied force to the joint. The actual amount of forcethe bolt sustains depends upon the ratio of stiffnessesof the bolt to the joint material.

The best way to understand and visualise how theforce sustained by the bolt depends upon the jointstiffness is by the use of joint diagrams. These are the

subject of the next page in this basics of bolted joints tutorial.


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What is a Joint Diagram?

To help visualise the loading within bolted connections, joint diagrams havebeen developed. A joint diagram is a means of displaying the load deflectioncharacteristics of the bolt and the material that it clamps. Joint diagrams can

be used to assist in visualising how a bolted joint sustains an external forceand why the bolt does not sustain the whole of this force.

The diagram shown above presents the way that the basic joint diagram is constructed. As a nut is

rotated on a bolt's screw thread against a joint, the bolt is extended. Because internal forces within thebolt resists this extension, a tension force or bolt preload is generated. The reaction to this force is aclamp force that is the cause of the joint being compressed. The force-extension diagram presentedabove shows the bolt extension and the joint compression. The slope of the lines represents the stiffnessof each part. The clamped joint usually being stiffer than the bolt.

The basic joint diagram is formed by moving the compression line of the joint to the right. A triangle is

formed because the clamped force tending to compress the joint is equal to the bolt preload. Positiveextension is to the right such as that sustained by the bolt, negative extension (compression) is to theleft and is sustained by the joint material.

Joint Diagrams with External ForcesApplied


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When an external tensile force is applied to the joint it has the effect of reducing some of the clamp forcecaused by the bolt's preload and applying an additional force to the bolt itself. This is illustrated in the

joint diagram shown above. The external force acts through the joint material and then subsequently intothe bolt. At first sight it may seem a bit strange to place the applied force in the position shown in thediagram. However, it should be realised that the load on the bolt cannot be added without decreasing theclamp force acting on the joint. As can be observed from a study of the diagram, the actual amount ofincrease in the bolt force is dependent upon the relative stiffness of the bolt to the joint.

As an illustration of the importance of the relative stiffness of the bolt to the joint, presented above is ajoint diagram for a 'hard' joint (a low stiffness bolt with a high stiffness joint). In this case, because of thesteep stiffness slope of the joint, the bolt will only sustain a small proportion of the applied force.

With a 'soft' joint (a high stiffness bolt with a low stiffness joint), because the stiffness slope of the bolt is

greater than that of the joint, the bolt would sustain the majority of the applied force. Study of thesediagrams provides understanding of why high performance bolts have shanks that have been reduced toa diameter below that of the outside diameter of a thread. By reducing the shank diameter in this mannerthe stiffness of the fastener is reduced so that it will not sustain as much of any applied force that it

would otherwise do. If the shank diameter is not reduced to a diameter below that of the stressdiameter(see stress area in the glossary)then the strength of the fastener will not normally beimpaired.

The Effect of a Large Applied External Force
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As the external force is increased the force acting on the bolt is proportionallyincreased. At the same time the clamp force acting on the joint is decreased. Ifthe external force continues to increase then either:

1) The proportion of the external force acting on the bolt together with the bolt's preload, results in theyield of the bolt material being exceeded with the imminent likelyhood of bolt failure. Even if failure does

not immediately occur when the external force is removed, the preload will be reduced. The joint diagramshowing an external force causing the bolt to yield is illustrated below.

2) The clamp force acting on the joint will continue to decrease until it becomes zero. Any furtherincrease in the applied force will result in a gap forming between the plates comprising the joint and thebolt sustaining all of the additional force. This is illustrated in the joint diagram below.

If a gap does form between the plates comprising the joint then the bolt or bolts are almost always

subjected to non-linear loadings from bending and shear forces acting. This usually quickly leads to boltfailure. Hence it is normal to set a design criteria that the applied forces must not under anycircumstances result in a gap forming within the joint.

The Effect of a Compressive External Force

If the joint experinces a compressive external force this has the effect ofincreasing the clamp force acting on the joint and decreasing the tension in thebolt. This is illustrated with the joint diagram shown below. If the compressiveexternal force is great enough then either:

1. The tension in the bolt can be reduced to a low value - if the external load is cyclic then the bolt couldfail due to fatgue (since it is experiencing tension variations under a compressive external force). Also thebolt is more susceptable to vibrational loosening.


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2. The yield limitations of the clamped material may be exceeded since the joint is sustaining acompressive force in addition to that provided by the bolt's preload. This will result in some permanentdeformation that upon the release of the external force a loss of bolt preload would result.

The Effect of Joint Deformation lossdue to Embedding

A joint diagram showing the effect of embedding is presented below.

When a bolt is tightened, very high local pressures can exist in the contact areas on the threads and

under the nut/bolt. Local plastic deformation can occur at these interfaces by flattening of surfaceroughness. This plastic deformation has the effect of reducing a bolt's preload. Research has beencompleted to establish guide values for the amount of embedding that typcially occurs within joints. Theamount of embedding determined is a loss of joint deformation. This can be converted into force by

calculation or by the aid of a joint diagram.

Bolt Preload Variation due to theTightening Method

The effect that the method of tightening has on determining what size of bolt isrequired to fulfil a specific function is largely underestimated. If several bolts ofthe same size are tightened by the same method then there will be variation inthe bolt's preload - they won't have all the same value. This variation is

influenced by such factors as variation in friction characteristics in the threadand under the nut face, thread form and pitch variations, variations in the


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surface flatness etc. Hence for any particular tightening method there will be amaximum anticipated preload and a minimum given a set of conditions.

The tightening factor is a measure of the scatter in a bolt's clamp force as a result of the tighteningmethod used to tighten the fastener. It is defined as the maximum bolt clamp force divided by theminimum value anticipated for that tightening method. For tightening with a torque wrench the

tightening factor is usually taken as 1.6; i.e. the maximum preload value is 1.6 times the minimum.

A joint diagram showing the effect of preload variation and embedding is presented below.

Since the bolt is not to be broken by overtightening on assembly, it must be selected for the maximuminitial preload. Hence for a given bolt size, the smaller the tightening factor, the larger the residualpreload is remaining to sustain the applied forces to the joint.

Joint diagrams can display a significant amount of information about the jointbut in our experience many people find them difficult to interpret andunderstand. Preload Requirement Charts are a way to graphically display theresults of a joint analysis in a clear and understandable manner.

By way of example, consider the joint shown below that is subject to combinedaxial and shear loading. For information, the bolt is M12 property class 10.9,the joint thickness is 20 mm with an axial load of 15 kN and a shear force of 4kN being applied. (If the joint consists of several bolts, it is first necessary todetermine the loading on an individual bolt.)


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One key aspect to appreciate is that the root cause of the majority of bolt/jointfailures is due to insufficient preload. It is unusual for the bolt to be overloaded.If the preload provided by the bolt is insufficient, joint separation andmovement can occur resulting in possible bolt fatigue and self-loosening issues.In order that such problems do not occur it is vital that there is sufficientresidual clamp force acting on the joint interface after accounting for theeffects of the applied forces and embedding losses. A Preload RequirementChart graphically illustrates this point as it looks at the forces acting on thejoint interface. Such a chart is shown below for the above joint.


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The above chart was produced by theBOLTCALCprogram, but such charts canbe produced manually. Explaining each of the parts of the chart in turn:Embedding Loss: Embedding is localised plastic deformationthat occurs under the nut face, in the joint faces and in the threads as a resultof flattening of the surface roughness. Embedding results in a loss of clamp

force acting on the joint. If the joint and bolt stiffness can be established, theamount of this force loss can be quantified if the surface roughness of thecontact surfaces is defined. In the above chart, a loss of 10 kN is anticipated.Large amounts of embedding loss can occur in joints with a short grip lengthconsisting of many interfaces.

Axial Force Requirement: In a preloaded joint, themajority of the applied axial load reduces the clamp force on the joint interfacerather than increasing the load in the bolt (see an earliertutorialfor anexplanation). The amount of the axial load that unloads the joint interface canbe determined from the joint/bolt stiffness calculations. In this example, of the15 kN applied force, 13.8 kN reduces the clamp force on the interface (theremaining 1.2 kN increases the load in the bolt). To simplify, when handcalculations are being completed, the conservative assumption is often appliedthat all the applied axial load reduces the clamp force on the joint interface.

Shear Force Requirement: The majority of joints inmechanical engineering use clearance holes and any shear load is transmittedby friction grip. That is, the clamp force on the joint interface generates afriction force that resists any applied shear loading. On such joints, if slippageis prevented, the bolts do not directly sustain any shear loading, however they

have to provide sufficient clamp force to prevent joint movement. To achievethis, the clamp force required is the shear force divided by the coefficient offriction present between the joint surfaces (for the single shear plane presentin the joint shown above). Since the coefficient of friction is usuallysignificantly less than 1, this requirement results in a significantly larger clampforce being required than the magnetude of the shear force. In this example,the applied shear is 4 kN which, if a coefficient of friction of 0.2 is assumedbetween the joint plates, results in a minimum clamp force of 20 kN (i.e.4/0.2).

Total Preload Requirement: This represents theminimum preload required to be provided by the bolt. It is the sum of theembedding loss, the amount of the applied axial force that reduces the clampforce on the joint and the clamp force needed to prevent slippage of the jointdue to a shear loading.

Preload Variation: In an ideal world the preload provided bythe bolt would be known to an exact value and would be the same for everybolt tightened. Unfortunately there is no low cost means of tightening a boltand knowing, precisely, the preload value. Techniques such as tightening thebolt to a specific torque value results in variation in the preload between,apparently, identical bolts. This is as a result of not being able to apply thetorque to the same exact value each time, variation in the hole and bolt
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tolerances but more importantly, variation in the coefficient of friction presentin the threads and under the nut/bolt face. To design a joint successfully thisscatter in the preload must be taken into account. This can be done in anumber of ways but usually either by determing the minimum/maximumpreloads from knowledge of the friction variation or by the use of a tightening

factor.

The problem: In the above chart the total preload requirementexceeds the minimum preload. What this means is that on some, but not alljoints, the preload will be insufficient to resist the applied forces. In such cases,joint failure can be anticipated. The failure islikely to be by either bolt fatigue(due to bending due to the joint slipping and separating) or by self-loosening(due to joint movement).The solution: In general, changes can be made to increase theminimum preload value (by using a stronger or larger bolt or changing thetightening method) or by reducing the applied forces (by using more bolts inthe joint, or by increasing the friction between the joint interface and soreducing the shear force requirement etc.) Shown below is the chart forchanging the tightening method to torque and angle. If applied correctly. thismethod will consistently provide a high preload value.


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Factor of Safety: A question which often arises is how muchof a gap there should be between the total preload requirement and theminimum preload value. This depends essentially upon engineering judgement.If the applied forces are accurately know, if product testing is going to becompleted, then the gap can be small. If the forces are not known accurately,and the consequences of failure disastrous, then a larger gap would be sensible.The consequence of having a generous factor of safety is that a larger bolt size(or higher strength bolt or better tightening method etc.) would be neededthen which would otherwise be the case. This can result in a more expensiveand less competitive product.Preload Requirement Charts can be developed to include other effects such asthe effect on bolt loading of differential thermal expansion. They are a usefulmethod for joint analysis and solving bolting issues.

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