Mechanical Fastener Connections

Mechanical fastener connections are used when there is the need to disassemble the components. The most common reason for disassembly is for repair of the internal components of a device, which can be electronic, mechanical, or both. There are two main types of mechanical connections for assembling polymer components that will be considered. One type is the threaded inserts and bolts. The other common situation is when screws are threaded directly into the unthreaded polymer for mechanical fastening of another component. Threads can also be molded into the polymer component. Molding in threads increases the cost of the molding operation. 

FIGURE 12.13 Thread forming and thread tapping screws. 

FIGURE 12.14 Three-lobed thread forming screw. 

FIGURE 12.15 Boss design for self-tapping screws. 

FIGURE 12.16 Screw torque when threading into plastic. 

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A potential structural application of epoxy resin adhesives in bridge construction is to use them to form the necessary shear connection between steel girders and the concrete deck slab in composite bridges, in place of conventional welded mechanical fasteners. There are three ways in which bonded shear connections between steel and concrete might be achieved. 

Fastener connection—A mechanical joint held together by fasteners that are additional to the components being joined. The fastener may be used directly in the transfer of load between the components to be joined (e.g. as in a bolted, riveted or screwed connection) or indirectly (e.g. as in a friction gripped, clamped or strapped connection). 

P(l) This section covers the design of mechanical joints where at least one of the primary components to be joined is made of glass FRP. Such joining techniques include fastener connections, friction joints (shear loads), contact joints (direct loads), threaded joints, strap joints and joints incorporating embedded fasteners (see 5.i.4 P(2)). 

Bonded inserts are also commonly used in sandwich structures, most typically in panels. The insert is bonded into the core and to one or both skins of the sandwich. Loads are then directed to the joint through the mechanical fasteners (i.e. the insert) in their axial direction, causing out-of-plane loading of the sandwich component. Sandwich structures typically have thin skins and therefore the loads from the insert are mainly transferred to the core. If the insert is loaded in the in-plane direction of the sandwich member, the connection is categorised as an embedded insert and it should be designed as a mechanical connection. 

Three connections (Table 1) had mechanical fastening by M16 grade 8.8 steel bolts with 30 mm diameter standard size washers. These were tightened to a torque of 23.8 N m, as recommended in reference 1 for FRP bolts. To aid buildability in connections Wmj bt, Wmj bt+bd and Wmj bd, bolt holes had a clearance of 2 mm. After test Wmj bt was conducted, the specimen was modified such that the holes in the members and cleats (Figure 3) were re-drilled to provide a tight fit for M20 steel bolts. This additional test, Wmj bt.2, was conducted for a number of reasons (reference 10). 

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. 

Nylon 6,6. Nylon 6,6 appears in automotive applications mostly in impact-modified, blended, and filled (mostly glass reinforced, with or with out impact modifiers) form. Impact modification of nylon 6,6 is of significant commercial importance, especially for automotive uses. Typical applications include fasteners for interior and exterior components, a host of connectors, holding fixtures, and radiator end caps, and a fairly common application is for air intake manifolds (Fig. 9.4). These applications rely on the ductility and toughness of impact-modified nylon 6,6 as a mechanism to connect assemblies or join components. 

Bonded joints are extensively employed in the construction of composite structures in aerospace applications, maritime structures, lifting equipment, wind mills as well as automotive industries [3, 4, 5], Unlike the bolt hole in mechanical fastening that causes a stress concentration in the composite joint plates, adhesively bonded joints minimize the potential for stress concentration within the joint. Besides, applications where lower structural weight, improved damage tolerance design philosophy are required, adhesively bonded joints provides a potential solution. Bonded joints are an efficent fastening solution also for hybrid structures, i.e., structures where composite parts are connected to metal parts. 

Although a few tests on bonded pultruded GFRP WF prohle beam-to-column joints have been reported (see Sanders et al., 1996), in practice mechanical fastening is the preferred means of connecting beams to columns in frame structures made of these materials. Consequently, only tests on bolted configurations of these joints are considered in this sub-section. 

Total replacement of the affected structural timber element replacement of the affected timber with new timber connected to the remaining sound timber by mechanically fastened metallic plates or profiles (internal or external) external addition of metallic plates or profiles mechanically fastened to the sound timber etc. 

Use of metallic cables, tendons and mechanical anchoring devices addition of new structural members improvement of connections between structural elements with mechanically fastened metallic plates, profiles or other connecting devices etc. 

Structural timber-concrete composites (e.g., timber-concrete composite slabs and timber wall-concrete deck composite in which the connection between the two common constmction materials is made through a bonded joint). The aim is to replace traditional mechanical fasteners by an adhesive connection, which has several advantages in comparison with the former for instance, a bonded joint is able to distribute the applied load over the entire bonded joint area, resulting in a more uniform distribution of stress (compared to mechanical point connections), requires little or no damage to the adherends, adds very little weight to the structure, and has a superior stiffness and fatigue resistance. 

Mechanical Engineer Fastener identified Weight/tolerances Resistance to abrasion Vibration requirements (x, y, z) Form factor/extemal dimensions — tolerance study Terminal type Torque requirements for connections requirements and analysis Leakage Test Case material... 

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