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Key Factors in Bolted Joint Failure
Published in Randy Riddell, Practical Root Cause Failure Analysis, 2022
Fastener failures are very common in every industrial environment; however, there are many elements of fastener joints that are not fully understood by users. While there are many types of fasteners, we will concentrate on mostly the bolt- or screw-type fastener as this accounts for most of the fastened joints used on machines. Many use the terms “bolt” and “screw” interchangeably but there is a technical difference. A bolt has a nut as its counterpart while a screw is threaded into a blind hole that is tapped into the mating part. The fastener can be a bolt in one assembly and the same fastener can be a screw in another. As typically done in industry, the term bolt may be used in a generic sense for the rest of this discussion. Screw will be used for the specific application as referring to an actual screw, by definition.
Basic Concepts of Fasteners
Published in Harold Josephs, Ronald L. Huston, Blake’s Design of Mechanical Joints, 2018
Harold Josephs, Ronald L. Huston
As with other joining processes, the primary function of most (but certainly not all) threaded fastener joints is to hold two or more things together. Some bolting systems are utilized to pull things together as part of assembly operations, while other bolting systems are utilized to counteract shear forces. As we shall see in the coming chapters a threaded fastener joint can be modeled as a series of springs. However, caution is in order in that the spring energy and therefore the stress distribution in the fastener spring system is non-uniform. This results in the fastener joint being an unstable system wherein multiple long- and short-term effects can (and often do) modify the distribution of stress and stored energy in the system. The terms bolt and screw are often used interchangeably. We, however, will take the term bolt to include those threaded fasteners that are typically associated a nut or tapped hole and the term screw to include those threaded fasteners with a sharp end as in self-drilling screws for metals or as typically found in wood screws. An overview of the advantages and disadvantages of bolted joints is presented in Table 2.5.
Power Screws, Fasteners, and Connections
Published in Ansel C. Ugural, Youngjin Chung, Errol A. Ugural, Mechanical Engineering Design, 2020
Ansel C. Ugural, Youngjin Chung, Errol A. Ugural
The common element among screw fasteners used to connect or join two or more parts is their thread. Screws and bolts are the most familiar threaded fastener types. The only difference between a screw and a bolt is that the bolt needs a nut to be used as a fastener (Figure 15.10(a)). On the other hand, a screw fits into a threaded hole. The same fastener is termed a machine screw or cap screw when it is threaded into a tapped hole rather than used with a nut, as shown in Figure 15.10(b). Stud refers to a headless fastener, threaded on both ends, and screwed into the hole in one of the members being connected (Figure 15.10(c)).
Effects of aging temperature and humidity on the response of medium and high strength threadlockers
Published in The Journal of Adhesion, 2022
D. Croccolo, M. De Agostinis, S. Fini, G. Olmi, L. Paiardini, F. Robusto
Adhesives are nowadays widely used in the field of mechanical joints since they provide a reliable design solution. They generally provide an acceptable strength with respect to specification, along with lightweight properties thanks to the high strength/weight ratio. Moreover, they can provide other features, such as the improvement of corrosion or fretting resistance.[1] Several studies have been carried out with regard to anaerobic adhesives, to investigate the effect of the pressure acting on the bonding on its static strength.[2–6] The same studies propose some design tips regarding the joint most suitable proportioning. In particular,[6] an experimental campaign was carried out to assess the influence of the aspect ratio (coupling length over diameter) on the shear strength of press-fitted and adhesively bonded joints. Focus was placed on the influence on strength of the clearance/interference levels between the shaft-hub coupling. It was pointed out that the joint overall strength takes advantage of the capability of the adhesive of filling the voids among the roughness crests, thus incrementing the actual mating surface. Therefore, in hybrid joints, when adhesive is added, a remarkably high coupling strength can be achieved, also preventing the mating surfaces from fretting wear. As a consequence, anaerobic adhesives can be used not only to replace a conventional coupling but also to integrate, thus improving the mechanical strength. Anaerobic adhesives are indeed widely used in the field of threaded connections, in order to avoid self-loosening. The adhesive, commonly regarded as threadlocker, has the capability of safety locking the threaded fastener against self-loosening that may be induced by vibration or service loads. In addition, threadlockers are able to provide an additional protection against moisture and corrosion by sealing the voids between the screw and the nut threads.[7,8] These adhesives are generally anaerobic acrylic and are able to cure at room temperature, when in contact with a metal in the absence of oxygen. The cure is generally affected by the physical properties of the involved metal and by the oxygen concentration. The most frequently used metals for threaded fasteners (e.g. steel and aluminum alloys) are active enough to cure the adhesive rapidly. Threadlockers are commonly classified as weak, medium, and strong, based on their achievable strength upon polymerization, and regarding their tribological effect upon tightening. Regarding this point, the capability of medium and strong threadlockers of lubricating the mating surfaces upon tightening is investigated in.[8] Some studies dealing with the beneficial properties of threadlockers are available in the technical literature, e.g., in,[9,10] but must be more properly regarded as internal studies by suppliers. A low number of studies on threadlockers are available in the scientific literature, and most of them deal with breakaway untightening torque increment as an effect of threadlocker applications.[11–13]