Thread locking applications

Activation by a metal surface also takes place in the commercially important anaerobic adhesives. These one-part adhesives are stable in the package, but cure quickly in an oxygen-free environment such as a tightly controlled bond line. Important applications include thread-locking, sealing, retaining, and some structural bonding [111]. A representative model formulation has recently been described [112] (Fig. 3). 

Thread locking. The first applications for anaerobic adhesives were for locking threaded fasteners. Filling the inner space between a nut and bolt with a hard, dense material prevents self-loosening. 

One of the most important applications for anaerobics is the thread-locking of mechanical fasteners which might loosen with vibration. A range of strengths are available to allow disassembly and use on a wide range of thread diameters. 

Anaerobic and cyanoacrylate adhesives are both monomeric materials which polymerize rapidly within the glue-line once the latter is formed. Setting can be very rapid indeed with cyanoacrylates and this implies that rapid and correct location of the adherends is necessary. Anaerobics also set more rapidly but movement of the surfaces is possible after polymerization has commenced. However, such movement is inimicable to the highest bond strengths but, because of its possibility, jigging is desirable. In the most common use of such adhesives, namely thread locking, the application is, in effect, its own... 

Fig. 6.1. These screws, used in a computer printer, have been retained with a thread locking compound note the excess adhesive which indicates that application has been accomplished.

Anaerobic Anaerobic adhesives cure in the absence of oxygen. One of the most common applications is that of thread-locking compound. Beyond the actual use, there are two interesting things about thread-locking compounds. First, why do not... 

Anaerobic adhesives mainly serve the purpose of protecting threads from undesired loosening as a consequence of vibrations or dynamic stress and represent a preferred alternative to mechanical locking devices. Furthermore, they enable shaft-hub connections, for example, when mounting a gear wheel on a shaft (Figure 11.6). They have also delivered optimal performance as liquidly applied surface seals alternatively to the solid seals required in the respective dimensions. Figure 4.6 shows these application possibilities. 

These screws and bolts eliminate the need for nuts and washers and so provide cost savings. Common types incorporate a nylon insert, either as a small plug or as a strip along the length of the thread. Fig. 12.7(a). A development of this principle is the application of a layer of nylon over a patch of thread. As the screw is engaged in the mating part, the nylon is compressed and completely fills the space between thread forms. This provides an interference which will resist rotation of the screw. Fig. 12.7(b). A more recent development is the application to an area of thread of a chemical adhesive which is completely dry to the touch. Fig. 12.7(c). The liquid adhesive is encapsulated within the film. When the two threaded parts are assembled, the micro-capsules of adhesive are broken, releasing the adhesive, which hardens and provides a reliably sealed and locked thread. 

Applications of lock stitches include seaming operations on all types of garments and run stitching. Lock stitch is extensively used for joining fabrics collar, cuff, pocket, sleeve, facing etc. Lockstitch type 301 is the simplest, which is shaped from the needle thread and the bobbin thread (Figure 10.9). 

Stitches in this class are sometimes referred to as double-locked stitches, because the needle thread is interconnected with two loops of the under-thread. Applications of multi-thread chain stitches are seaming operations on all types of garments. 

Threaded fasteners are occasionally supplied pre-coated with a locking medium in the form of a band of microencapsulated epoxy or anaerobic adhesive around the appropriate section of the screw thread. This ensures that the adhesive application is not overlooked. While microencapsulation is attractive and useful, its chequered history indicates limited potential any one of the following major drawbacks may be encountered in practice ... 

However, it is not generally realised that such adhesives can be used to lock threaded fasteners and yet allow subsequent adjustment. A typical example is an adjuster screw mechanism of a reed switch. Other applications include tuning circuits in electronic components and adjusters in hydraulic brake mechanisms. 

The review of anerobic adhesives presented by M. Hauser depicts a new type of adhesive. The materials described, cure (pol3nnerize) only under anerobic conditions. In presence of air, curing of the adhesive is inhibited. The chemistry involved was first reported in 1940. Over the next twenty years, the state of the art progressed to the point where stable, commercially useful adhesives which cured only in the absence of air could be produced. The application was mainly for locking of threaded fasteners. 

Molded-in inserts represent inserts that are placed in the mold before the plastic resin is injected. The resin provides is then shaped to the part geometry and locks the insert into its body. Molded-in inserts provide very high-strength assemblies and relatively low unit cost. However, molded-in inserts conld increase part cycle time while the inserts are manually placed in the mold. When the application involves infrequent disassembly, molded-in threads can be used successfully. Coarse threads can also be molded into most materials. Threads of 32 or finer pitch shonld be avoided, along with tapered threads, because of excessive stress on the part. If the mating connector is metal, overtorque will result in part failure. 

The first successful application of anaerobic adhesives was to replace lock washers in threaded fastener assemblies (e.g., nuts and bolts), where they fulfil the dual function of locking and sealing the assembly. Frequently referred to as liquid lock washers, anaerobics adhere strongly to the metal surfaces and prevent loosening from vibration. They also completely fill the void space in a threaded assembly and protect it against corrosion. This remains the most... 

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