Bolted joint - WikiHQ

A bolted joint is one of the most common elements in construction and machine design. It consists of a male threaded fastener (e. g., a bolt) that captures and joins other parts, secured with a matching female screw thread. There are two main types of bolted joint designs: tension joints and shear joints.

The selection of the components in a threaded joint is a complex process. Careful consideration is given to many factors such as temperature, corrosion, vibration, fatigue, and initial preload.

Joint types

Tension joint

There are two types of tension joint: non-preloaded and preloaded.

Non-preloaded tension joint

These joints are not tightened to a precise preload, and the tension is mainly used to keep parts together without generating a high clamping force. An applied tensile load may cause separation of the joint. This type of joint should not be used where it is frequently subjected to variations of tensile loading.

The bolt remains secure without the relative movement that could cause loosening, potentially eliminating the need for additional locking mechanisms.

The joint should be designed so that the preload always exceeds the external tensile load to prevent separation. If the external tensile load exceeds the preload, the joint will separate, allowing relative motion between the components, potential bolt loosening, and increased load on it.

In both the preloaded tension and slip-resistant shear joints, some level of preload in the bolt and resulting compression in the clamped components is essential to the joint integrity. The preload target can be achieved by a variety of methods: applying a measured torque to the bolt, measuring bolt extension, heating to expand the bolt then turning the nut down, torquing the bolt to the yield point, testing ultrasonically, or by applying a certain number of degrees of relative rotation of the threaded components. Each method has a range of uncertainties associated with it, some of which are very substantial.

Shear joint

There are two types of shear joint: slip-resistant and the bearing type.

In some applications, joints are designed so that the fastener eventually fails before more expensive components. In this case, replacing an existing fastener with a higher strength fastener can result in equipment damage. Thus, it is generally good practice to replace old fasteners with new fasteners of the same grade.

Formulas

The force in the bolt of a joint, which has not separated, is<math display="block">F_b=F_p+C\cdot F_e</math>and in the clamped parts<math display="block">F_c=-F_p+(1-C)\cdot F_e</math>where

<math>F_e</math> is the external applied force, and

<math display="inline">F_p</math> is the bolt preload.

The portion of an external load carried by the bolt is the joint stiffness ratio<math display="block">C=k_b/(k_b+k_c)</math>where

<math display="inline">k_b</math> is the stiffness of the bolt,

<math>k_c</math> is the stiffness of the clamped parts

Separation of the clamped parts occurs when the force at the clamped surfaces is zero (<math display="inline">F_c=0</math>) thus the separation force is<math display="block">F_{ec} =F_p /(1-C)</math>Separation under the bolt head occurs when the force in the bolt is zero <math>(F_b=0)</math> thus the separation force is,<math display="block">F_{eb}=-F_p/C</math>

thumb|Bolt Joint Stiffness Ratio and Load Sharing

The accompanying graph and table illustrate how the relative stiffness of the clamped parts and the bolt affects the portion of applied load carried by it. For example, when the stiffness of the clamped parts equals that of the bolt (the blue curve), an external load in the range from minus to plus twice the preload results in only 50% of the applied load being transferred to the bolt, as the total load in the bolt only varies by twice the preload. If the tensile applied load exceeds twice the preload, the clamped parts separate, and the bolt carries the entire load. Conversely, if the compressive load is lower than twice the preload, separation at the bolt head occurs, and the force in the bolt is zero. The curve representing a clamped parts-to-bolt stiffness ratio of 0.01 shows that when the relative stiffness of the clamped parts is very low, almost all of the load is transferred to the bolt, down to the point where a compressive load equals the preload, and separation at the bolt head occurs, reducing the force in the bolt to zero.

{| class="wikitable defaultcenter" style="white-space:nowrap;"

|+Effect of Bolt to Clamped Parts Stiffness Ratio

! rowspan="2" |Clamped Parts Stiffness Bolt Stiffness

! rowspan="2" |Joint

Stiffness

Ratio

! rowspan="2" |Portion of Applied Load Transferred to Bolt

! colspan="2" |Range of Joint Integrity Applied Load Bolt Preload

!Separation at Bolt Head

!Separation of Clamped Parts

|0.01

|0.99

|99 %

| -1.0

|100

|0.5

|0.67

|66 %

| -1.5

|3.0

|1.0

|0.50

|50 %

| -2.0

|2.0

|3.0

|0.25

|25 %

| -4.0

|1.3

Calculating the torque

Engineered joints require the torque to be chosen to provide the correct tension preload. Applying the torque to fasteners is commonly achieved using a torque wrench. The required torque value for a particular fastener application may be quoted in the published standard document, defined by the manufacturer or calculated. The side of the threaded fastening having the least friction should receive torque while the other side is counter-held or otherwise prevented from turning.

A common relationship used to calculate the torque for a desired preload takes into account the thread geometry and friction in the threads and under the bolt head or nut. The following assumes standard ISO or National Standard bolts and threads are used:

:<math>T = K P_{pre} d </math>

where

:<math>T </math> is the required torque

:<math>K </math> is the nut factor

:<math>P_{pre} </math> is the desired preload

:<math> d </math> is the bolt diameter

The nut factor K accounts for the thread geometry, friction, pitch. When ISO and Unified National Standard threads are used the nut factor is:

{| class="wikitable floatleft"

|+ Accuracy of bolt preload based on bolt preload method

|Method

|| Accuracy

|Torque wrench on unlubricated bolts

|| ± 35%

|Torque wrench on cad plated bolts || ± 30%

|Torque wrench on lubricated bolts

|| ± 25%

|Preload indicating washer

|| ± 10%

|Computer-controlled wrench (below yield) || ± 15%

|Computer-controlled wrench (yield sensing)|| ± 8%

|Bolt elongation

|| ± 5%

|Strain gauges

|| ± 1%

|Ultrasonic monitoring

|| ± 1%

The preferred bolt preload for structural applications should be at least 75% of the fastener's proof load Fasteners should only be torqued if they are fitted in clearance holes.

Torque wrenches do not give a direct measurement of the preload in the bolt.

More accurate methods for determining the preload rely on defining or measuring the screw extension from the nut. Alternatively, measurement of the angular rotation of the nut can serve as the basis for defining screw extension based on the fastener's thread pitch. Measuring the screw extension directly allows the clamping force to be very accurately calculated. This can be achieved using a dial test indicator, reading deflection at the fastener tail, using a strain gauge, or ultrasonic length measurement.

Bolt preload can also be controlled by torquing the bolt to the point of yielding. Under some circumstances, a skilled operator can feel the drop off of the work required to turn the torque wrench as the material of the bolt begins to yield. At that point the bolt has a preload determined by the bolt area and the yield strength of the bolt material. This technique can be more accurately executed by specially built machines. Because this method only works for very high preloads and requires comparatively expensive tooling, it is only commonly used for specific applications, primarily in high performance engines.

There is no (as yet) simple method to measure the tension of a fastener in situ. All methods, from the least to most accurate, involve first relaxing the fastener, then applying force to it and quantifying the resultant amount of elongation achieved. This is known as 're-torqueing' or 're-tensioning' depending on which technology is employed.

Technologies employed in this process can be:

An electronic torque wrench is used on the fastener in question, so that the torque applied can be measured as it is increased in magnitude.

Recent technological developments have enabled tensions to be established (± 1%) by using ultrasonic testing. This provides the same accuracy to that of strain gauging without having to set strain gauges on each fastener.

Another method that indicates tension (mainly in erecting steel) involves the use of crush-washers. These are washers that have been drilled and filled with orange RTV. When a given force has been applied (± 10%), orange rubber strands appear.

Large-volume users (such as auto makers) frequently use computer-controlled nut drivers. With such machines, the computer is in control of shutting off the torque mechanism when a predetermined value has been reached. Such machines are often used to fit and tighten wheel nuts on an assembly line, and have also been developed for use in mobile plant tire fitting bays on mine sites.

Thread engagement

Thread engagement is the length or number of threads that are engaged between the screw and the female threads. Bolted joints are designed so that the bolt shank fails in tension before the threads fail in shear, but for this to hold true, a minimum thread engagement must be achieved. The following equation defines this minimum thread engagement:

:<math>L_e = \frac{2 \times A_t}{0.5 \pi \left( D - 0.64952 p \right)}</math>

Where Le is the thread engagement length, At is the tensile stress area, D is the major diameter of the screw, and p is the pitch. This equation only holds true if the screw and female thread materials are the same. If they are not the same, then the following equations can be used to determine the additional thread length that is required:

:<math>J = \frac{\text{tensile strength of external thread material{\text{tensile strength of internal thread material</math>

:<math>L_{e2} = J \times L_e</math>

Where Le2 is the new required thread engagement.

While these formulas give absolute minimum thread engagement, many industries specify that bolted connections be at least fully engaged. For instance, the FAA has determined that in general cases, at least one thread must be protruding from any bolted connection. [http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/99c827db9baac81b86256b4500596c4e/$FILE/Chapter%2007.pdf]

Failure modes

When doing a failure mode analysis for bolts that have broken, come loose or corroded, careful consideration must be given to the below failure modes:

;Overloading

:Overloading occurs when operating forces of the application produce loads that exceed the clamp load, causing the joint to loosen over time or fail catastrophically.

;Over-torquing

:Over-torquing might cause failure by damaging the threads and deforming the fastener, though this can happen over a very long time. Under-torquing can cause failures by allowing a joint to come loose, and it may also allow the joint to flex and thus fail under fatigue.

;Fatigue

:When axial or transverse loading overcomes the bolts preload or causes the bolt to slip transversely, movement in the bolt can cause small cracks to build up in the material eventually leading to fatigue failure of the bolt or male threaded component. According to Bill Eccles from boltscience, [In the vast majority of applications, the most effective way to ensure that the bolt is fatigue resistant is to ensure that it is tightened sufficiently...]

;Brinelling

:Brinelling may occur with poor quality washers, leading to a loss of clamp load and subsequent fatigue failure of the joint.

;Other failure modes

:Other modes of failure include corrosion, embedment, and exceeding the shear stress limit.

Bolted joints may be used intentionally as sacrificial parts, which are intended to fail before other parts, as in a shear pin.

Locking mechanisms

thumb|right|Bolted joints in an automobile wheel. Here the outer fasteners are four studs with three of the four nuts that secure the wheel. The central nut (with locking cover and [[Split pin|cotter pin) secures the wheel bearing to the spindle.]]

Locking mechanisms keep bolted joints from coming loose. They are required when vibration or joint movement will cause loss of clamp load and joint failure, and in equipment where the security of bolted joints is essential. A prevalent test for the self-loosening behaviour is the Junker test.

;Jam nuts

:Two nuts, tightened on each other. In this application a thinner nut should be placed adjacent to the joint, and a thicker nut tightened onto it. The thicker nut applies more force to the joint, first relieving the force on the threads of the thinner nut and then applying a force in the opposite direction. In this way the thicker nut presses tightly on the side of the threads away from the joint, while the thinner nut presses on the side of the threads nearest the joint, tightly locking the two nuts against the threads in both directions.

;Prevailing torque nuts (locknuts)

:An insert on the internal threads (either metallic or non-metallic, e.g. Nyloc nut) or a plug/patch of non-metallic material on the external threads is installed. This material binds against the threads of the opposing fastener with a friction force and creates a prevailing torque, which resists the backing-out or loosening of the fastener.

;Chemical locking compounds (thread-locking compound)

:The use of a chemical locking compound binds the threads together when the compound cures. Examples of such a compound includes anaerobic compounds such as Loctite, which cures in the absence of oxygen and acts as an adhesive to lock the threads of the joint together.

In the bearing joint, the bolt itself limits lateral movement of the elements by the shank of the bolt (the unthreaded portion of a bolt near its head) bearing upon the sides of the holes in the clamped elements. Such joints require less clamping force, because a high level of friction between the clamped surfaces is not required. The clearance between the bolt and the holes means that some lateral movement may occur before the bolt bears against the sides of the holes to allow for thermal expansion and other slight motions.

Even when designed as a bearing joint, the surface friction between the clamped elements may be sufficient to resist movement for some time, especially when the building may not yet be fully loaded – thus it operates initially as a friction joint. When the lateral force becomes sufficient to overcome this friction, the clamped elements move until the sides of the holes bear against the shank of the bolt. This movement – "slip into bearing" – usually starts and stops very suddenly, often releasing elastic energy in the associated elements through sound, resulting in a loud but harmless bang.

International standards

SA-193/SA-193M: "Specification for alloy-steel and stainless steel bolting materials for high-temperature service"
SA-194/SA-194M: "Specification for carbon and alloy steel nuts for bolts for high-temperature service"
SA-320/SA-320M: "Specification for alloy steel bolting materials for low-temperature service"
EN 1515: "Flanges and their joints - Bolting"
EN 1515-1: "Flanges and their joints - Bolting - Part 1: Selection of bolting"
EN 1515-2: "Flanges and their joints — Bolting — Part 2: Classification of bolt materials for steel flanges, PN designated"
EN 1515-2: "Flanges and their joints — Bolting — Part 3: Classification of bolt materials for steel flanges, class designated"
ISO 4014: "Hexagon head bolts - Product grades A and B"
ISO 4017: "Hexagon head screws - Product grades A and B"
ISO 4032: "Hexagon nuts, style 1 - Product grades A and B"
ISO 4033: "Hexagon nuts, style 2 - Product grades A and B"

References

;Notes

;Bibliography

External links

Bolted Joint Calculator
Bolt Formulas and Calculators
The banging bolt syndrome
Bolted Joints, Formulas and Calculators

AISC

Banging bolts — another perspective AISC
Bolt Science - The Jost Effect
Threaded Fasteners - Tightening to Proper Tension US Department of Defense document MIL-HDBK-60, 2.6MB pdf.
Fastener Design Manual, NASA-RP-1228, 100pp, 1990 NASA handbook, 5.1 Mb, pdf.
Mechanics of screws
FAA Advisory Circular 43.13-1B, Paragraph 7-37 "Grip Length"
Bolted Joint Analysis
Bolted Joint Design, Fastenal Engineering & Design Support

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