Mechanical fasteners allow reassembly, thin-material attachment

By Craig Link, Applications Engineering Manager, Penn Engineering & Mfg. Corp.

Specifying the most effective fastening or joining method is not simply a matter of preference for design engineers striving to adhere to the principles of "Design for Assembly," "Design for Manufacturing," and "Design for Serviceability." The challenge is how to enhance assembly efficiencies, ensure end-product integrity, and promote product-servicing capabilities.

The inherent disadvantage of more "permanent" fastening techniques, such as adhesives and other "material" fastening methods, is their inability to allow for product disassembly when in-field service is required-a demand in the electronics, automotive, communications, and countless other high-tech industries. Welding presents the same problem, and further produces undesirable fumes and burn-outs. With welding, there is the additional need to use complicated electrodes and pilots, which can negatively impact assembly timelines.

However, some mechanical fasteners have inherent drawbacks in meeting DFA, DFM, and DFS requirements. For example, weld nuts require tedious, time-consuming indexing and re-tapping operations to remove weld splatter. Sheet-metal screws can fall short in reusability and eventual holding power. Some screw types require the time-consuming process of tapping and may not be able to provide the necessary strong threads in thin metal sheets.

With most mechanical fasteners, users must keep multiple parts in stock, and handling several parts during the installation procedure can delay production. Once such hardware is installed, there is always the risk that parts will loosen, fall out, and damage components.

Within the general mechanical fastener category, self-clinching fasteners solve these problems, while offering performance and service advantages that alternative fastening methods cannot. They represent a proven and practical method to attach thin metal sheets while meeting DFA, DFM, and DFS objectives.

Upon installation, self-clinching fasteners provide threads in metal sheets as thin as 0.020 inch (0.5 mm) and become a permanent part of the component in which they are installed. They will not become loose or fall out, and can reduce the number of parts to be handled or kept in inventory.

The net result is more streamlined and cost-effective product assembly. Just as importantly, self-clinching fasteners have made a wide variety of thin-metal designs possible for the first time. Today, dozens of self-clinching fastener types in more than 12,000 variations are available for virtually any application.

Generally, self-clinching fasteners take less space and require fewer assembly operations than caged or anchor nuts. They also have greater reusability and more holding power than sheet-metal screws. They are used chiefly where good pull-out and torque loads are required in sheet metal too thin to provide secure fastening by any other method.

These fasteners can be installed during fabrication or during final assembly, and permit high-volume installation that results in accelerated production and lower installed cost. Self-clinching fasteners often support a thinner sheet metal and because of their compact design and low profile, provide a neat appearance.

A self-clinching fastener should be specified whenever a component must be readily replaced and where loose nuts and hardware would not be accessible. If the attaching nuts and screws cannot be reached after a chassis or cabinet is assembled, self-clinching fasteners can be installed during metal fabrication. They simplify and expedite component mounting and assembly operations, including those performed in the field.

Self-clinching nuts, studs, spacers, and standoffs are used worldwide by producers of spacecraft, aircraft, automobiles, electronic and industrial appliances and equipment, business machines, transportation equipment, medical devices, and communications systems, among others. Even "low-tech" products such as lawn power tools, commercial lighting equipment, and bicycles use self-clinching fasteners for secure attachment, reliability, low installed cost, and end-product performance and serviceability.

Adhesives take stress out of permanent fastening

John Cocco P.E. , Director of Application Engineering, Loctite Corp.

There are three major methods of assembly in today's manufacturing environment, offering varying degrees of effectiveness depending on application needs. Mechanical fastening secures substrates with bolts, screws, or rivets. Thermal fastening welds, brazes, or solders two similar metal substrates. Chemical assembly bonds substrates using adhesives.

Adhesives have found widespread design acceptance for structural bonding, cylindrical assembly, mechanical fastener threadlocking, and flange and thread sealing, among other applications. Without using adhesives to bond dissimilar materials, many designs could not be produced. Adhesives are environmentally safe, durable, and less sensitive than other fastening methods to outside factors affecting performance in demanding applications.

Adhesives also offer significant benefits over mechanical fastening methods. Rather than concentrating stress at a single point, adhesives distribute stress load over a broader area, resulting in a more even distribution. An adhesive-bonded joint better resists flex and vibration stresses than, for example, a riveted joint.

Adhesives form a seal as well as a bond, eliminating corrosion that occurs in a mechanically fastened joint. They join irregularly shaped surfaces more easily than mechanical fasteners, allow different substrate materials to be permanently bonded, negligibly increase the weight of an assembly, and create virtually no change in the dimensions or geometry of the designed part.

Mechanical fastening methods and adhesives sometimes work together to form a stronger bond than either method alone. Threadlocking and gasketing are two examples. Design engineers who want to improve the safety and quality of an assembly will use a mechanical fastener in tandem with a threadlocking adhesive. The anaerobic threadlocker guarantees the assembly will not fail or loosen, and that corrosion will not reduce or otherwise have an adverse affect on the fastener's life.

Similarly, liquid form-in-place gasketing materials dress conventional rubber, paper, or cork gaskets, and often completely replace cut gaskets. These anaerobic dressings fill surface imperfections in the mating flanges and extend gasket life.

Most industries use adhesives in some manner. In fact, it would be difficult to name an industry that does not. Look around you in everyday life-most everything used for work and leisure incorporates adhesives. Adhesives are commonly used in manufacturing electrical and electronic devices, consumer products, appliances, automobiles, aircraft, medical devices, and many general industrial assembly applications.

For example, adhesives are widely used in loudspeaker manufacturing, attaching more than 14 different flexible components. Without adhesives, the cost to manufacture speakers would be prohibitive. In the appliance industry, adhesives bond assemblies, and sealants maintain housings and prevent fasteners from loosening. Adhesives are widely used in plastic assembly, bonding a range of products from disposable medical devices to sporting equipment to recreational vehicle sub-assemblies.

Adhesives available to manufacturers range from simple, natural compounds to complex chemical formulations. Loctite Corp. offers a range of high-performance engineering adhesives, including durable epoxies; anaerobic threadlockers and gasketing materials; tough yet flexible urethanes; fast-fixturing acrylics; light-cure acrylics that cure on demand; cyanoacrylates (or instant adhesives); and flexible, weather-resistant silicones.

The majority of design engineers and manufacturers know the benefits of adhesives and understand that by eliminating mechanical components they can reduce associated costs in the assembly process. However, some designers are not aware of the benefits of adhesives; some still do not trust chemical bonding over mechanical fastening. The educational process will continue into the future as companies continue to introduce innovative adhesive technologies to solve complex assembly challenges.