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.Barrett, R. T. (1990). Fastener design manual (No. NAS 1.61: 1228).
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.
Preloaded tension joint
In these joints, the bolt is tightened to apply a specific preload, generating a tensile force in the bolt and an equal compressive force in the clamped parts. This ensures that any applied tensile loads are distributed between the bolt and the clamped parts (see theory section), which has some advantages:
For cyclical tension loads, the bolt does not experience the full amplitude of the load, which increases its fatigue life. If the material has an endurance limit, the fastener's life may extend indefinitely.Collins, p. 481. 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.
Slip-resistant shear joint The bolt is tightened to a specified preload in these joints. This preload generates friction between the clamped parts, allowing shear loads to be transferred through the joint faces via friction, rather than being carried directly by the bolt. Relative motion between the clamped parts is prevented and thus any fretting wear that could result in the development of fatigue cracks. Any tensile loads applied to these joints, usually have a secondary effect, but they do reduce the effective preload.
The relative stiffness of the bolt and the connected parts is less critical in calculating preload for a slip-critical joint compared to a preloaded tension joint (see theory section). Methods of applying preload are described in the preloaded tension joint section.
Bearing type shear 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. In this type of joint, the shear load is transmitted through bearing contact between the bolt and the walls of the bolt holes in the connected parts. When a shear load is applied, the connected parts move and the bolt shank makes contact with the hole walls, which transfers the load from the parts to the bolt. This causes a shear stress in the bolt at the junction of the connected parts, which it resists through its shear strength. As bearing type joints rely on this direct contact, they are more susceptible to wear and deformation of the bolt holes under high or repeated loads, which can lead to bolt fatigue or elongation of the holes over time. Any tensile loads applied to these joints, usually have a secondary effect, but they can reduce available shear strength of the bolt.
A clevis linkage, which relies on a locking mechanism (like lock washers, thread adhesives, and lock nuts), is a bearing type shear joint.
Theory
Key concepts Typically, a bolt is tensioned (preloaded) by the application of a torque to either the bolt head or the nut. The applied torque causes the bolt to "climb" the thread causing a tensioning of the bolt and an equivalent compression in the components being fastened by the bolt. The preload developed in a bolt is due to the applied torque and is a function of the bolt diameter, the geometry of the threads, and the coefficients of friction that exist in the threads and under the torqued bolt head or nut. The stiffness of the components clamped by the bolt has no relation to the preload that is developed by the torque. The relative stiffness of the bolt and the clamped joint components do, however, determine the fraction of the external tension load that the bolt will carry and that in turn determines preload needed to prevent joint separation and by that means to reduce the range of stress the bolt experiences as the tension load is repeatedly applied. This determines the durability of the bolt when subjected to repeated tension loads. Maintaining a sufficient joint preload also prevents relative slippage of the joint components that would produce fretting wear that could result in a fatigue failure of those parts.
The clamp load, also called preload of a fastener, is created when a torque is applied, and so develops a tensile preload that is generally a substantial percentage of the fastener's proof strength. Fasteners are manufactured to various standards that define, among other things, their strength. Torque charts are available to specify the required torque for a given fastener based on its property class (fineness of manufacture and fit) and grade (tensile strength).
When a fastener is tightened, a tension preload is develops in the bolt, while an equal compressive preload forms in the clamped parts. This system can be modeled as a spring-like assembly, where the clamped parts experience compressive strain, and the bolt tensile strain. When an external tensile load is applied, it reduces the compressive strain in the clamped parts and increases the tensile strain in the bolt. The load carried by the bolt and the clamped parts is in proportion to their stiffness because they both experience the same induced strain. As a result, the external load is shared across the joint rather than being solely carried by the bolt. In a well-designed joint, about 10-20% of the applied tensile load is carried by the bolt with the greater part transferred through the clamped parts because they are much stiffer than it. This reduction in the proportion of load transferred to the bolt is important in applications with cyclic loading, as bolts have low fatigue strength due to the stress concentration in their threads.
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, isand in the clamped partswhere
is the external applied force, and
is the bolt preload.
The portion of an external load carried by the bolt is the joint stiffness ratiowhere
is the stiffness of the bolt,
is the stiffness of the clamped parts
Separation of the clamped parts occurs when the force at the clamped surfaces is zero () thus the separation force isSeparation under the bolt head occurs when the force in the bolt is zero thus the separation force is,
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.
+Effect of Bolt to Clamped Parts Stiffness RatioClamped Parts StiffnessBolt StiffnessJoint Stiffness
RatioPortion of Applied Load Transferred to BoltRange of Joint IntegrityApplied LoadBolt PreloadSeparationat Bolt HeadSeparation ofClamped Parts0.010.9999 % -1.01000.50.6766 % -1.53.01.00.5050 % -2.02.03.00.2525 % -4.01.3
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:
where is the required torque is the nut factor is the desired preload 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:
where = the mean thread diameter, close to pitch diameter. = nominal bolt diameter = (thread pitch)/(pi * dm) Thread Pitch = 1/N where N is the number of threads per inch or mm = friction coefficient in the threads = half the thread angle (typically 60°) = 30° = friction coefficient under torqued head or nut
When = = 0.15, the dimensions used correspond to any size coarse or fine bolt, and the nut factor is K ≈ 0.20, the torque/preload relationship becomes:
A study of the effect of torquing two samples, one lubricated and the other unlubricated, 1/2 in.- 20 UNF bolts to 800 lb-in, produced the same mean preload of 7700 lbf. The preloads for the unlubricated bolt sample had a standard deviation from the mean value of 1100 lbf, whereas the lubricated sample had a standard deviation of 680 lbf. If the preload value and torques are used in the above relation to solve for the nut factor it is found to be K = 0.208, which is very close to the recommended value of 0.20
+ Accuracy of bolt preload based on bolt preload method Method AccuracyTorque wrench on unlubricated bolts ± 35%Torque wrench on cad plated bolts ± 30%Torque wrench on lubricated bolts