Self-tapping screw

For mechanical assemblies using screws can be detached indefinitely, with the exception of self-tapping screws, which can be loosened and retightened only a limited number of times. The best guideline for the designer is to prefer any assembly design that converts... 

Finally, self-tapping screws require additional torque to cut or form their threads. This... 

Mark a center line on the top rear of the 8 tube. Measure 5/8 down each side from the center line and mark the tube. Measure 1 from the rear of the tube and mark again,intersecting the first 2 lines. These two points are the location for the tube support screws They can be drilled and tapped for machine screws or simply drilled for self tapping screws or pop rivets. Be sure that whatever fasteners used do not extend through the collar into the barrel itself. This allows the collar/ tube assembly to be removed from the barrel by loosening the bottom screw only. 

The holding power of screws can be affected by the size of the pilot drill-hole, the depth of screw engagement, the outside diameter of the screw, and quality of the bone [Cochran, 1982 DeCoster et al., 1990], Therefore, the selection of the screw type should be based on the assessment of the quality of the bone at the time of insertion. Under identical conditions, self-tapping screws provide a slightly greater holding power than non-self-tapping screws [Tencer et al., 1993]. 

This has a polypropylene (PP) seat, with welded tubular-steel legs. Use a Phillips screwdriver to remove the four screws that attach seat to the legs. These self-tapping screws (Fig. 1.5) with sharp, widely spaced threads, are much longer than the typical 4 mm thick seat. When screwed into a moulded cylindrical boss on the hidden side of the seat, the threads cut grooves in the initially smooth plastic. It is supported by four or more buttresses, to prevent bending loads causing failure, where the boss joins the seat. Measure the thickness of the buttresses. 

Figure 1.5 Self-tapping screw for attached tubular metal legs, and the boss with buttresses under the seat of a PP stacking chair.

Figure 13.17 shows that, when bosses are provided for self-tapping screws, they should be separated from the main wall of the product, to avoid the creation of thick regions. Note the number of sharp internal corners in the moulding. The bosses may need to be supported by buttresses that connect to other parts of the product. An extreme example occurs in stacking chairs (Fig. 1.5). 

A manufacturer of restaurant appliances fastens a stainless steel top to a combined counter/refrigerator/stove imit. The standard attachment technique included both welding and the use of self-tapping screws. An aerobic acrylic adhesive has replaced these techniques resulting in a lower cost assembly requiring less skilled labor. 

Reduction of the number of product components, usually achieved by combining functions. Typical examples include box and cover assemblies joined with live hinges, the use of self-tapping screws to eliminate nuts, and staking of molded plastic bosses instead of metal rivets or screws. 

This description applies equally to assemblies using threads tapped in the plastic or to self-tapping screws. Screws are generally used for larger assemblies, such as enclosures for electrical apparatus, etc., and for any size assembly that must be taken apart for servicing or into which are fastened other components which may have to be removed fm servicing, etc. 

LS Standard screw length LS should be a standard screw lengthy r ardless of whether the screw requires a tapped hole or is a self-tapping screw. 

LT must be greater than LE plus any pointed portion of the screw otherwise, the screw would bottom in the hole and 1) make it impossible to tighten the joint and 2) could split the plastic. If the hole will be tapped (rare today), LT must be deep enough to permit a run-out for the tap. For self-tapping screws, the difference between LT and LE depends on the selected screw. In either case, the designer must use the recommendations for proper hole dimensions from the screw manufacturer s charts. 

In example 2A, a short, standard screw is easily obtained in the diameter required, regardless of whether the hole in the base is tapped or a self-tapping screw is used. The problems here are ... 

Use a cover meter to locate the steel and determine rebar spacing. Make a connection to the steel either by exposing it or using already exposed steel. The connection must be metal to metal and secure. Self-tapping screws are frequently used. 

Electrical connections may be made by self tapping screws, welding or brazing. Extra bars may need to be welded in if continuity between bars is inadequate. Connections are usually protected with epoxy glue applied over them. Cabling may be run through the concrete overlay, over the anode or from the back of the protected surface. Cables are run into junction boxes for splicing and connections. There must be at least two connections per zones for redundancy. There may be separate connections for the reference electrodes. 

Self-tapping screws are used for fast-assembly work. They also offer good resistance to loosening through vibration. These screws are specially hardened and produce their own threads as they are screwed into a prepared pilot hole, thus eliminating the need for a separate tapping operation. 

Figure 12.4. Self-tapping screws (a) thread-forming (b) threadcutting (c) self-piercing-and-tapping...

FRP cannot be welded or easily shaped after fabrication. Screws are not recommended if the primary role of the joint is structural, because both self tapping screws and tapped hole screws cause extensive damage to the laminates. If screws are to be used for structural applications, it should be possible to cast metal blocks into the composite at the time of manufacture or to mount metal blocks on the composite and then to drill and tap into them in the conventional way. 

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. 

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

Press fitting has been found to provide joints of high strength at minimum cost. Acetal copolymer can be used to provide snap-fit parts. Use of self-tapping screws may provide substantial cost savings by simplifying machined parts and reducing assembly costs. 

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. 

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