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Mechanical-fastening:the non welded alternative.SOLUTIONS with Effective, Powerful Advice
So why should it find its place in a welding website?
Because it could be a useful option when welding is simply unsuitable. In those cases it is not only preferable, it is imperative to look for alternative solutions. Fasteners can be described as devices that fulfill the function of transmitting mechanical loads. They keep two or more elements of an assembly in relative position, assuring continuity, stability and mechanical strength as needed. Mechanical-fastening can be a useful alternative way to provide joining of elements. That should be considered whenever there are good reasons to do so in place of more traditional welding or brazing processes.
Which could be such good reasons? Well, the most striking one could be the objection to the use of heat. Mechanical-fastening is generally performed without heating, except maybe in a few particular cases (i.e. interference fit or hot mounted rivets). It is known that all fusion welding processes and also a few other ones, use heat to promote melting and coalescence of the metal elements to be joined. High heat needed for fusion may have adverse effects on the metallurgical structures affected, to the point of impairing their mechanical properties.
Other reasons are the possible need of easy disassembling for maintenance, unavailable with welded elements. Or the ease of joining dissimilar materials, sometimes problematic with welding, but achievable by Mechanical-fastening that circumvent welding difficulties. Speed and economy of accomplishment may be the governing motive for selecting, rather than more traditional methods of joining, self-piercing riveting and clinching. See our page on Clinching. These are Mechanical-fastening methods which enjoy substantial development and progress in recent times. Mechanical-fastening includes different types of joining devices like threaded fasteners, rivets of many different kinds, and press fit joining. Threaded fasteners (bolts and screws, nuts) are applied in assemblies of machine elements for relatively thick sections. Their usage permits disassembling and re-assembling. Holes are needed and bilateral access may be required except if threads are machined in the material, or if a nut is secured to the part before assembling. A specified torque moment may be specified for assembling. Relaxation of bolting elements caused by creep at elevated temperature should be considered. The joining stress is likely to decrease remarkably if heat is applied during operation. If it is imperative that bolting stress be assured at all time during service, (i.e. between a vessel and its bolted cover), then creep resistant bolt materials must be selected. If vibration is a factor special provisions must be put in place to lock the bolted elements to avoid their self unscrewing. Several means (i.e. locking wires and special washers) are available to assure this requirement. Self tapping screws can be used to join thin sheets. In all these cases due attention has to be paid to the possibility of galvanic corrosion if different materials are making contact in presence of humidity or of an electrolyte. The problem should be solved by correct selection of materials, by suitable plating or by insulation of the joint with proper means. Rivets were used for centuries in the Mechanical-fastening construction of structures (bridges, towers, ships) and of fluid retaining vessels, even under pressure. In all cases the joint is between overlapping elements. In this case, to join together two or more plates, holes have first to be drilled in place. Steel rivets, with one head formed at one side of the shank, are heated to red color, then inserted from one side through the plates, the head held against a suitable mass and riveted from the other side to form the second head. Upon cooling and shrinking the rivet shank exerts considerable tensile force, providing powerful joints, whose strength is based on friction between members, rather than on rivet shear strength. Aluminum rivets are formed cold. High strength heat treated aluminum alloy rivets reach their softest condition upon quenching from solutioning temperature. This is the time, while the material is still soft, to form the second head. But this is a metastable condition as the material develops the highest mechanical properties by precipitation or natural aging at room temperature in a matter of a few days. Therefore, to delay aging, solutioned rivets must be kept cold, in a freezer, until riveting time. Otherwise, after hardening, they crack upon riveting. Blind rivets are used for Mechanical-fastening where access is available from one side only. After having inserted the special hollow rivet in a hole for holding together the different elements of the assembly, the rivet bottom is expanded. This is done by the head of the preplaced mandrel, that runs through the hollow rivet, as it is pulled back with special pliers. The mandrel breaks at the notch, after completing its function, and is discarded. Dissimilar materials can be joined this way. Hollow self-piercing rivets pierce and clinch two or more sheets together in one operation using special tools and dies. Rivets are of proprietary design. Operation can be automated for fast performance. With tools of various shapes, ductile metals can be joined at several locations by clinching or press joining. It means indenting and deforming locally both materials to obtain mechanically interlocking joints without any external device. This is done by pressing a punch to indent or pierce sheets against a special die, frequently of split-type, to produce a button on the underside, thereby creating a mechanical interlock between the sheets. Note that this is different from cold welding where a metallurgical joint is produced. See Cold Welding. Self-piercing riveting and clinching are increasingly applied in the high speed assembly of sheet materials in automotive and appliance industries. These methods permit the use of preplated or painted sheet metals. They also allow Mechanical-fastening of materials like aluminum and steel that are incompatible for fusion welding. It should be said for completeness that recent developments achieve this feat that was unthinkable only a short time ago. Structural adhesives can be used alone or combined with joint point capabilities of many of these methods, with curing being performed after Mechanical-fastening.
Joint strength of any single joining element of the above Mechanical-fastening systems is generally lower than that of spot welds. But layouts are designed to meet requirements for the joint as a whole. There are not generally approved Standards relative to Mechanical-fastening, except for machine threaded bolts and nuts. Many of the methods were developed to solve specific production problems in certain industries with the general aim to increase productivity while at the same time reducing workforce requirements and costs. Quality is generally assured by visual inspection, by measurement of the deformed thickness and by occasional sections. Machine settings and process monitoring are performed by recording pressure and displacement as joints are formed and are checked by statistical process control (SPC). Visual display of graphs of force as a function of displacement are checked against reference curves to detect faulty operation. There is a large abundance of designs and proprietary systems, mostly suitable for automated applications. Therefore the selection of suitable equipment and tools for modern Mechanical-fastening, should be performed as a joint project with the suppliers. Reasonable production goals and a firm budgetary frame should be established in advance.
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