Screws, mechanical fasteners

Aerobic adhesives were developed as an advance over anaerobic and cyanoacrylate adhesives. Anaerobic (threadblocking) adhesives are used to augment mechanical fasteners such as screws, bolts, or press-fits. Cyanoacrylates tend to be used for nondurable bonding on rigid surfaces and for durable rubber bonding. 

Combining is a process of joining two or more materials together. Using mechanical fasteners such as nails, staples, screws, bolts, and rivets is a common method of combining materials. One advantage of mechanical fasteners is that they can join together different types of materials. Mechanical fasteners also allow for easy assembly and disassembly of a prototype. 

A large variety of mechanical fasteners can be used for joining plastic parts to themselves and to other materials. These include machine screws, self-tapping screws, rivets, spring clips, and nuts. In general, when repeated disassembly of the product is anticipated, mechanical fasteners are used. 

Machine Screws, Bolts. Parts molded of thermoplastic resin are sometimes assembled with machine screws or with bolts, nuts, and washers (Fig. 7.6), especially if it is a very strong plastic. The threads used to engage the mechanical fastener may either be molded into the part (molded-in threads or molded-in inserts) or attached after the molding operation (post-molded inserts). 

FIGURE 7.6 Mechanical fastening with (a) self-tapping screws and (b) bolts, nuts, and washers. ... 

Because of the lower ductility of the fluorocarhon materials (e.g., TFE, CTFE, FEP), snap-fit and press-fit joints are seldom nsed. Rivets or studs can be used in forming permanent mechanical joints. These can be provided with thermal techniques on the melt-processable grades. Self-tapping screws and threaded inserts are used for many mechanical joining operations. In holted connections, some stress relaxation may occur the first day after installation. In such cases, mechanical fasteners should he tightened thereafter, stress relaxation is negligible. 

Joining Vj6i-nig (14c) n. The process of assembhng plastic parts by means of mechanical fastening devices such as rivets, screws, clamps, etc. 

Polyimide parts can be joined with mechanical fasteners. Self-tapping screws must be strong enough to withstand distortion when they are inserted into the polyimide resin which is very hard. Polyimide parts can be bonded with epo adhesives. Only abrasion and solvent cleaning is necessary to treat the substrate prior to bonding. Tbe plastic part will usually have a hi er thermal rating than the adhesive. Thermosetting polyimides carmot be heat welded or solvent cemented. 

Whatever mechanical fastener is used, particular atteution should be paid to the head of the fastener. Cortical heads, called flat heads, produce undesirable tensile stresses and should not be used. Bolt or screw heads with a flat underside, such as pan heads, round heads, and so forth are preferred because the stress produced is more compressive. Flat washers are also suggested and should be used under both the nut and the fastener head. Sufficient diametrical clearance for the body of the fastener should always be provided in the parts to be joined. This clearance can nominally be 0.25 mm (0.010 in.). 

By definition, a structural bond involves the formation of a load-bearing joint between high-strength materials, typically metal, wood, ceramic, and certain plastics. The first step in the selection of a joining method should be a comparison of the relative merits of the available techniques.In addition to structural adhesives, these include a number of mechanical fastening methods, such as screws, bolts, nails, staples, and rivets, as well as metal fusion processes. Many of these techniques are usually associated with the joining of metal structures. 

Most structures require the joining of one or more similar or dissimilar materials in their fabrication. Conventional joining techniques have employed mechanical fasteners, such as screws, nails, bolts, and rivets. These joining techniques result in large stresses near the fastener, produced by nonuniform load distributions. 

Total replacement of the affected structural timber element addition of mechanically fastened metallic external bracing repair by use of self-tapping screws etc. 

PP is a tough and resilient plastic and many types of mechanical fastening techniques, (Section 7.2), can be easily used. However, PP is prone to creep and screws used under high stress for assembly may slack due to creep. In these cases, metal inserts should be preferred since they increase the load-bearing area in PP. Metal inserts may be moulded in or pressed in after injection moulding. However, the stress concentration effect on the impact strength due to sharp-edged inserts should be carefully considered. 

Mechanically mounted PBA—This is mounted and/or supported in a mechanical assembly or housing with a number of mechanical fasteners around its periphery (and internally to the PBA for additional support, if required). The most common mechanically mounted PBA is the mother board, such as is frequently used in personal computers. Mechanically mounted PBAs are one of the members of an electrical/electronic assembly and are physically mounted in the assembly using one or more mechanical fasteners, such as screws, chps, and standoffs. A common example of a mechanically mounted PBA is a modular power supply. 

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