MECHANICAL FASTENING

Mechanical fasteners are screws and bolts, self-tapping screws, or inserts. They may be reusable and can be moulded in metal or plastic. Plastic fasteners are of low weight and are corrosion and impact resistant. The high stress produced near fasteners can make parts more susceptible to chemical and thermal attack. The design stress, residual stress, and assembly stress in a component should not exceed the maximum allowable stress for the plastic. To reduce the stress concentration, inserts with smooth rounded surfaces can be used, which produce less stress than knurled inserts. 

A selection of semi-permanent mechanicai fastening systems 

All mechanical fastening systems can be manually or semi-automatically performed during assembly or installation, however, not all fastening systems readily lend themselves to full automation. 

High production rates possible depending on the fastening system and degree of automation. Also dependent on time to open and close fastening system. 

Identification of possible failure modes (tension, shear, bearing, fatigue) and calculation of stresses in the fastener at the design stage recommended in Joints subjected to high static, impact and/or 

Examination of the stresses in the Joint area under the fastener important to determine the load bearing capability and stiffness of the parts to be Joined. 

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Clothes. Laboratories should be equipped with a sufficient number of fireproof blankets, so that a blanket is available at any point of the laboratory at a few seconds notice. Each blanket should be kept in a clearly labelled box, the lid of which is closed by its own weight and not by any mechanical fastening, which might delay removal of the blanket. The box itself should be kept in some open and unencumbered position in the laboratory. 

Polymers are characteri2ed as thermosetting and thermoplastic with respect to the methods by which they are joined. Thermosetting polymers are permanently hard and do not soften upon the apphcation of heat they are joined by mechanical fasteners and adhesives. Several methods have been devised to join thermoplastic polymers, as weU as thermoplastic composite materials, which soften upon heating. 

Single-ply membranes offer the widest range of systems in the roofing industry. The three basic systems are ballasted, fuUy adhered, and mechanically fastened. From a cost standpoint, the fuUy adhered system is the most expensive to install, the ballasted system the least. The protected-membrane roofing system can be used with any of the basic systems. The specifications for these systems are pubHshed by the various manufacturers. The following gives a brief description of the roof assembhes. 

Fully Adhered. The substrate, ie, insulation, cover board, etc, that the single-ply membrane is to be attached to is either fuUy adhered or mechanically fastened to the deck. However, there are also appHcations where the membrane is adhered directly to the deck. The membrane is then adhered to the substrate. The typical method for adhering the membrane to the substrate is by applying a contact adhesive to the membrane and substrate, rolling the membrane into place, and brooming once the adhesive is ready. There are one-sided appHcations where the membrane is roUed directly into the adhesive that has been appHed to the substrate only. The membrane used in this appHcation method may be fleece-backed. FuUy adhered systems can be installed on any slope. The fuUy adhered appHcation offers a smooth surface that is easy to maintain and inspect, as weU as exceUent wind resistance on account of positive attachment. 

Mechanically Fastened. The substrate, ie, insulation, cover board, etc, which the single-ply membrane must go over, is typically mechanically fastened to the deck using a low density of fasteners. However, there are also appHcations where the membrane is appUed directly to the deck. The typical... 

There are also mechanically fastened systems where large sheets are laid over the substrate and seamed together. The mechanical fastening system to hold the membrane to the deck is placed at the appropriate density either over the membrane to fasten the system to the deck or under the membrane to which it is affixed. 

The prime installation method is mechanically fastened but fully adhered and ballasted appHcations can also be used. CSPE exhibits strong resistance not only to weathering but also to a broad range of chemicals and pollutants it is also inherently ozone-resistant. It can be produced in many colors and the sheet widths are typically 5—6.5 ft (1.5—1.65 m). The physical characteristics of a CSPE sheet have been described (17) (see Elastomers, SYNTHETIC-Cm OROSULFONATED POLYETHYLENE). 

EPDM is by far the most widely used material in the ballasted roofing system constmction. Because of EPDM s flexibiHty, very large sheets of up to 10,000 square feet (929 m ) can be deHvered to the job site in compact roUs that offer reduced labor on the roof in the seaming process. The typical EPDM sheet size used in ballasted systems is 12 by 30 m and 1.1 mm thick. EPDM is also widely used in both the fully adhered and mechanically fastened roofing systems. In these constmctions, both 1.1- and 1.5-mm thick material is used with widths from 2.1 to 15 m. A majority of the installations use nonreinforced sheet, although reinforced membrane can also be used in all of the system types. The majority of the reinforced sheets go into mechanically fastened systems. 

CPA. Copolymer alloy membranes (CPAs) are made by alloying high molecular weight polymeries, plasticizers, special stabilizers, biocides, and antioxidants with poly(vinyl chloride) (PVC). The membrane is typically reinforced with polyester and comes in finished thicknesses of 0.75—1.5 mm and widths of 1.5—1.8 m. The primary installation method is mechanically fastened, but some fully adhered systems are also possible. The CPA membranes can exhibit long-term flexibiHty by alleviating migration of the polymeric plasticizers, and are chemically resistant and compatible with many oils and greases, animal fats, asphalt, and coal-tar pitch. The physical characteristics of a CPA membrane have been described (15). 

BP. These nitrile alloy membranes are compounded from PVC, flexibilized by the addition of butadiene—acrylonitrile copolymers, PVC, and other proprietary ingredients. Typically reinforced with polyester scrim, NBP membranes are 1 mm thick and have a width of 1.5 m. They ate ptedominandy used in mechanically fastened roofing systems. NBP membranes exhibit excellent teat and puncture resistance as well as good weatherabihty, and remain flexible at low temperatures. They ate resistant to most chemicals but ate sensitive to aromatic hydrocarbons. The sheet is usually offered in light colors. The physical characteristics of NBP membranes have been described (15). 

The greater portion of PVC is installed in the mechanically fastened roofing system a lesser portion is installed in fully adhered appHcations. Although PVC was once heavily used in ballasted roofing systems, there are only a small number installed in the 1990s. Fleece-back membrane is popular in the PVC constmction for both fully adhered appHcations as well as in appHcations where a separator sheet is needed. PVCs ate resistant to vafious weather conditions, bactefial growth, and industfial chemicals. These membranes ate chemically incompatible with bituminous materials. PVCs ate offered in a variety of colors. The physical characteristics of a PVC membrane have been described (15). 

Of obvious importance to aircraft is the smoothness of exterior surfaces. Smooth aerodynamic surfaces reduce aerodynamic drag, resulting in higher airspeeds and increased efficiency. Mechanical fasteners, even countersunk flush fasteners, introduce disruptions in the airflow over the exterior surface. Even the slight deformation of thin sheets around fasteners produces drag. Adhesively bonded structure has no fasteners to disrupt airflow and is more capable of producing the smooth continuous contours that are so common on aircraft. 

As mentioned above, aircraft structure is typically quite thin with numerous small fasteners to achieve efficient load transfer through joints. Mechanical fasteners are laborious to install. Adhesive bonding of large area doublers and joints can be accomplished at significant labor hour savings over equivalent mechanically fastened designs. 

Local repair of bonded structure damage from mechanical sources (bird-strike, equipment impact, etc.) can either be bonded or mechanically fastened patches. In general it is more likely that sandwich structure is repaired using bonded patches because of the limited compression capability of sandwich structure and the risk of introducing water into the honeycomb from penetrating fasteners. 

For composite stiffeners, all shapes are builtup from individual layers of material. Of course, some stiffener shapes can be produced by roll forming or pultrusion, for example, and then fastened to panels. Or, the stiffened panel could be made in a single operation involving the placement, usually by hand, of individual laminae of various dimensions in positions such that a builtup structure results. Stiffeners can be fastened to panels by bonding, stitching, or mechanical fastening. 

Simply bonding a stiffener to a panel with adhesive is certainly a very feasible and natural procedure with typical composite structure construction. We have not discussed any procedure for joining parts except co-curing. Alternatively, to use film adhesive for bonding parts together, we simply cut a sheet or film of adhesive to the proper size, place it between the two parts that we wish to bond together, and then go through a cure cycle that causes the adhesive to adhere to both the stiffener and to the panel itself. We can also mechanically fasten any stiffener we like to a panel. 

Mechanical fasteners Soldering and brazing Fusion welding... 

Quadraxially oriented (four directional layer) glass fabric-TS vinyl ester polyester RP sheet panels with a foam core and gel coating are used. Most of the panels are 3 mm thick with molded-in rib structure supports. Body skins are bonded to the chassis with a double-stick acrylic tape developed by 3M Co. as well as mechanical fasteners. Unlike most steel designs, no B-pillar structural component between the front and rear doors is required thus providing more interior space and easy entry since doors open in opposite directions. 

Thermoplastics Mechanical Fasteners Adhesives Spin and Vibration Welding Thermal Welding Ultrasonic Welding Induction Welding Remarks... 

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