hjwx hjwx A Tutorial on the Basics of Bolted Joints The complexity of the simple nut and bolt is frequently underestimated. A fully tightened bolt does not perform like a loose bolt. A fully tightened bolted joint can sustain millions of load cycles without problems, a joint consisting of untightened bolts will frequently fail within a few cycles. The reason for this is the way a bolted joint carries an external 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 bolted joint technology and in particular on the mechanics of the load transfer mechanism involved in such joints. There are a number of pages to this tutorial covering the topic from the basics. Additional pages are being added as and when time permits. Bolt Science is committed to providing assistance on bolted joint technologies to individuals, companies and other organisations. If you have a question on any of the topics - why not email us and we will try to answer your query. Why preload is important Bolt Science is committed to providing expertise in bolted joint technology and to this end we present the following information for education purposes. Over the last fifty years great improvements have been made by the fastener industry in improving the design and reliability of their products. However, no matter how well designed and made the fastener itself is, it cannot alone make the joint more reliable. Fastener selection based upon an understanding of the mechanics of how a threaded fastener sustains loading and the influence that tightening procedures can play is also needed. This article provides an introduction to the basics of bolted joints and the major factors involved in the design of such joints. It is not widely understood how a bolted joint carries a direct load. A fully tightened 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 a tightened bolt it does not sustain the full effect of the load but usually only a small part of it. This seems, at first sight, to be somewhat contrary to common sense. Figure 1A shows a bolt and nut securing a bracket to a support plate. With the nut loose on the bolt, if a weight of 1 unit. is added to the bracket, as shown in figure 1B, then the force in the bolt shank will increase by 1. However, if the nut is now tightened and the weight applied, the force in the bolt shank will not increase by 1 but usually by only a small fraction of this amount. An understanding of why the bolt does not sustain the full effect of the applied load is fundamental to the subject. A model can often be of help in understanding why the bolt does not sustain the full effect of the applied load. Figure 2 is an attempt to illustrate the load transfer 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 by the fastener until the applied load exceeded the fastener's preload. (Preload is the term used for a bolt's clamp force.) Appying a force to a bolted joint A model can often be of help in understanding why the bolt does not sustain the full effect of the applied load. Figure 2 is an attempt to illustrate the load transfer 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 by the fastener until the applied load exceeded the fastener's preload. (Preload is the term used for a bolt's clamp force.) With the special fastener shown, the bolt is free to move within its casing, a compression spring is included within the casing so that if the bolt is pulled down the spring will compress. A scale on the side of the casing indicates the force present in the spring and hence the force present in the shank of the bolt. Figure 2A illustrates this special fastener in its untightened condition. The bolt now is inserted through a hole in a support plate and a bracket attached to the special fastener by securing a nut to the threaded shank. If the nut is now rotated so that the head of the bolt is pulled down, the spring will be compressed. If the nut is rotated so that 2 force units are indicated on the casing, the compressive force acting on the spring will be 2 and the tensile force in the bolt shank will also be 2. This is illustrated in figure 2b; this is like a tightened bolt without any working load applied. If now a weight is added to the bracket (figure 2c) of value 1, then the initial reaction is to think that the load in the bolt must increase, otherwise what happens to the additional force? However it will keep at its existing value of 2 - it will not 'feel' any of the additional force. To visualise why this is so - imagine what would happen if the load in the bolt did increase. To do this it would compress the spring more and a gap would be made between the bracket and the plate. If such a gap was 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 not occur. What does happen is that the effect of the applied load is to decrease the clamp force that exists between the plate and the bracket. With no load applied the clamp force is 2 units, with the load applied this 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 methods based upon the idea that the bolt will not 'feel' any of the applied load until it exceeds the bolts clamp force. That is, the bolt should be sized so that its clamp force is equal to the external load after a factor of safety has been included. With the special fastener used in this example the stiffness of the fastener is far smaller than the stiffness of the plate and bracket it clamps. Practical fasteners differ from that shown in figure 2 in that elongation of the fastener and compression of the clamped parts occurs upon tightening. This compression results in the bolt sustaining a proportion of the applied load. As the applied force reduces the clamp force existing within the joint an additional strain is felt by the bolt which increases the force it sustains. The amount of the additional force the bolt sustains is smaller than the applied force to the joint. The actual amount of force the bolt sustains depends upon the ratio of stiffnesses of the bolt to the joint material. The best way to understand and visualise how the force sustained by the bolt depends upon the joint stiffness is by the use of joint diagrams. These are the subject of the next page in this basics of bolted joints tutorial. What is a Joint Diagram? To help visualise the loading within bolted connections, joint diagrams have been developed. A joint diagram is a means of displaying the load deflection characteristics 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 force and 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 the bolt resists this extension, a tension force or bolt preload is generated. The reaction to this force is a clamp force that is the cause of the joint being compressed. The force-extension diagram presented above shows the bolt bolt extension and the joint compression. The slope of the lines represents the stiffness of 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. Positive extension is to the right such as that sustained by the bolt, negative extension (compression) is to the left and is sustained by the joint material. Joint Diagrams with External Forces Applied When an external tensile force is applied to the joint it has the effect of reducing some of the clamp force caused by the bolt's preload and applying an additional force to bolt itself. This is illustrated in the joint diagram shown above. The external force acts through the joint material and then subsequently into the bolt. At first sight it may seem a bit strange to place the applied force in the position shown in the diagram. However, it should be realised that the load on the bolt cannot be added without decreasing the clamp force acting on the joint. As can be observed from a study of the diagram the actual amount of increase 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 a joint diagram for a 'hard' joint (a low stiffness bolt with a high stiffness joint). In this case, because of the steep 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 these diagrams provides understanding of why high performance bolts, such as one shown in the drawing below, have shanks that have been reduced to a diameter below that of the outside diameter of a thread. By reducing the shank diameter in this manner the 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 stress diameter (see stress area in the glossary) then the strength of the fastener will not normally be impaired. The Effect of a Large Applied External Force As the external force is increased the force acting on the bolt is proportionally increased. At the same time the clamp force acting on the joint is decreased. If the 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 the yield 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 diagram showing 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 further increase in the applied force will result in a gap forming between the plates comprising the joint and the bolt 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 bolt failure. Hence it is normal to set a design criteria that the applied forces must not under any circumstances 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 of increasing the clamp force acting on the joint and decreasing the tension in the bolt. This is illustrated with the joint diagram shown below. If the compressive external 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 could fail due to fatgue (since it is experiencing tension variations under a compressive external force). Also the bolt is more susceptable to vibrational loosening. 2. The yield limitations of the clamped material may be exceeded since the joint is sustaining a compressive force in addition to that provided by the bolt's preload. This will result in some permanent deformation that upon the release of the external force a loss of bolt preload would result. The Effect of Joint Deformation loss due 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 surface roughness. This plastic deformation as the effect of reducing a bolt's preload. Research as been completed to establish guide values for the amount of embedding that typcially occurs within joints. The amount 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 the Tightening Method The effect that the method of tightening has on determining what size of bolt is required to fulfil a specific function is largely underestimated. If several bolts of the same size are tightened by the same method then there will be variation in the 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 thread and under the nut face, thread form and pitch variations, variations in the surface flatness etc. Hence for any particular tightening method there will be a maximum 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 tightening method used to tighten the fastener. It is defined as the maximum bolt clamp force divided by the minimum 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 maximum initial preload. Hence for a given bolt size, the smaller the tightening factor, the larger the residual preload is remaining to sustain the applied forces to the joint.