Threaded fasteners are widely used in the industry today, but are also a notable cause of service problems and the like. Besides possible safety implications, fastener failures can represent a significant proportion of a product's warranty costs. In the following paragraphs a brief introduction to bolted joints is to be exposed indicating mostcommon failure causes and advices to avoid them.
Bolted joints are probably the best choice in order to apply a desired clamp load to assemble two pieces, at a low cost, with the option to disassemble whenever it is necessary. Furthermore, the simplicity of their mechanism to develop and maintain the desired clamp force has made them very popular and has become one of the most acceptedengineering products. In a negative sense, this simplicity may have made some users reluctant and therefore disregard some important issues associated with a bolted joint.
Figure 1: Different Bolts and nuts (www.forjasbolivar.com)
The correct design of mechanical joints depends on the correct evaluation of factors such as the intended use of the joint, preferred failure point,surrounding conditions (temperature, corrosion, vibration, etc.), material and geometry, bolt type, resistance grade, desired preload, installation method, etc.
It would be impossible to cover all the assortment of joint combinations in these few pages. Therefore the main purpose is to give a brief explanation of the fundamental principles of bolted joints. The term “bolt” will be used to name allkinds of bolts and screws unless otherwise indicated. The joint will be supposed to be of carbon steel, but the same theories apply to joints made of other metals.
Bolted joints are one of the most widely used mechanical components used in the industry today. It is basically the joint of two parts or components using a bolt. When the bolt is tightened and torque is applied to it, the angularmotion is changed into linear motion creating a pressure force between the two components as if they were placed into a press.
It is popular to state that "a bolt can be tightened until it fails”, but this is not a totally true statement. When a bolt is tension loaded, it will expand elastically until it reaches its elastic limit which is the point where the maximum stress can be appliedwithout obtaining a permanent deformation. From that point on, the material moves to its plastic behaviour and elongates at a higher rate until it reaches the fracture point. Figure 2 shows the typical stress – strain diagram for three bolt resistance grades.
Figure 2: Stress – Strain diagram for different bolt resistance grades.
1. Preloaded bolted joints
When a connection isdesired to resist high external tensile loads and shear loads, the bolt should be pretensioned in order to get a more stable component. In figure 3 the bolt has first been tightened to produce an initial tensile preload P, after which the external tensile load F is applied. The effect of the preload is to place the parts under compression for better resistance to the external tensile load and tocreate friction between the parts to resist possible shear loads.
Figure 3: Externally loaded bolted joint.
Bolts retain considerable tensile strength capacity after tightening, even if preloaded beyond their yield strength. Therefore, tightening to yield optimizes preload without the risk of imminent failure, actually, for stiff metal to metal joints, the minimumpreload on the bolt should be 90% of the yield load. In any rigid joint, the higher the preload, the greater the protection against joint separation and slip. This procedure will also reduce the possibility of a joint loosening because of external vibrations or due to relaxation of the components and will increase the resistance to cycle (fatigue) loads. Also, since bolt materials are relatively...