Co-axial Joints

LOCKING, BONDING SEALING THREADED AND PLANE CO-AXIAL JOINTS. 

Although there is, as might be expected, a grey area between structural and mechanical applications of adhesives, it helps understanding to consider them separately. Generally, structural adhesives are used in some form of overlap joint, as distinct from the co-axial joints formed from turned parts, where anaerobic adhesives (see Chapter 3 and Section 5.1.2) are normally employed. However, structural adhesives may be used in co-axial configurations when maximum performance is required. 

With co-axial joints, an increase in temperature does not necessarily cause the anticipated reduction in performance because the differential expansion of the adhesive and the assembled components may generate very high compression forces on the adhesive which in turn result in an increase in observed shear strength. It is not generally realised that typical adhesives have thermal expansion co-efficients between seven and twenty times those of common engineering metals, as shown in Table 2.2. 

Provided that co-axial joints will be loaded in compression - as they very often are - then cleanliness of the surfaces to be bonded is not too important. 

If one or more of the parts in a bonded assembly is made of a plastics material or soft alloy, then the overall performance will be lower than if the parts had been made of steel. It is important to remember that the shear strength of co-axial joints shown during the displacement of the parts is actually a measure of the bonding and mechanical jamming action of the hardened adhesive. The emphasis on either bonding or jamming depends upon the type of adhesive. 

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We will consider a torsional load on a co-axial joint, with an overlap of 25 nun using a single part hot setting epoxy adhesive. The inner tube external diameter is 25 mm and both tubes are 1.5 mm thick steel. A torsional load of 100 Nm and a glue line of 0.05 mm gives a stress distribution as shown in Fig. 30. 

Co-axial joints are also common in the design of filter or heat exchanger end caps. Here, the bonding process has replaced soldering or brazing. In these cases, because stress levels on the joints are low, an interference fit at one end of the joint can provide the basis of simple low cost assembly. If a cluster of tubes are required to be bonded and sealed, the selection of a free flowing adhesive, with the required performance under service conditions in combination with a simple moulded end cap, is the answer (see Fig. 50). 

Although there is a grey area lying between the structural and the mechanical use of adhesives it is useful to consider the two applications to be quite separate. They may be conveniently, if roughly, defined as the large scale application of adhesives to some form of overlap joint and, in the case of mechanical assembly, the use of small quantities of adhesive on co-axial joints made from turned and fitted parts (Fig. 7.2). 

Fig. 7.2. The major engineering joints, (a) The lap joint this simple form, and its many variants, is the basis of most bonded structures, (b) The co-axial joint this form is particularly suited to the assembly of mechanical components. The anaerobic adhesives employed are normally quite different in nature from those...

Load and mode of loading. Whatever the load, co-axial joints are usually best assembled with anaerobic adhesives unless special circumstances demand otherwise. By contrast, there are many candidates for a lap joint based assembly. A crude, but fairly effective, means of selecting which types may be used is based on the anticipated load. If only nominal loads are to be borne any adhesive may be used, but once the working load exceeds an average level of 3-5 MPa (500 psi) then only structural adhesives of the thermoset type should be used. This figure may appear to be very low but it has been derived from the considerations reviewed below (see Design ). Incidentally, the fact that thermally induced stresses can lift to quite high levels what would otherwise be nominal loads should not be overlooked. 

It follows therefore that anaerobic adhesives are suited to co-axial joints made from thick, rigid materials whose surfaces are not highly polished and where bond lines are thin. Joint loads should preferably be shear, torsional or compressive in nature. 

Hitch, D. and Beevers, A, The analysis of adhesive-bonded co-axial joints. Adhesives, Sealants and Encapsulants Conference, November 1985. Wainwright, P. and Bolt, R., Engineering Materials and Design, October 1977. 

In both cases, a distinction must be made between the many variants of lap joints on the one hand and co-axial assemblies in general on the other. Threaded components must be singled out, for, when bonded, they do not behave as slip-fitted unthreaded parts. They are therefore dealt with separately in Sections 2.3.1 and 2.3.2. 

Finally, care must be taken, either in the choice of adhesive or in the assembly technique, to ensure that the adhesive is not wiped from the surface as the parts are placed together. This is likely where, for example, one part is slid into position over another the sliding action tends to wipe the adhesive from the joint area, forming pockets and reducing overall performance. In this context, co-axial assemblies must of course be slid together because of the very nature of the joint. This can cause severe problems for some adhesives but presents no difficulties for the anaerobic varieties, which are formulated specifically to cope with this type of work. 

Anaerobic adhesives are not generally stringy in nature and contain no solid catalysts, so they are suited for application on many relatively small parts -particularly co-axial assemblies where they can be placed with ease. Their characteristics ensure that they readily fill the minute gaps always found in interference-fitted joints. Thus, they can be used both to seal and supplement the overall performance of such joints. 

Figure 2.16 compares the relative co-axial shear strength (measured on the Collar and Pin test specimens described in MoD DTD 5628, Method H) of a number of adhesives with a resistance to disassembly similar to related press-fitted collar and pin components. The strength of the bonded joints is considerably in excess of the resistance to movement displayed by friction- fitted assemblies. 

Nevertheless, experience shows that, for the vast majority of like-to-like, co-axial assemblies with diametric clearances around 0.05 mm, temperatures between -55°C and +80°C will be readily accommodated. Adhesives with a higher Tg will cope up to 120°C without major strength loss, and those designed for elevated temperatures perform, on collar and pin assemblies, up to 200°C. This is seen clearly in Figure 2.19, which shows the performance of a highly cross-linked anaerobic adhesive specifically formulated for maximum performance on collar and pin assemblies at elevated temperatures. It cannot be emphasised too strongly that the data given relate only to collar and pin assemblies and must not be applied to lap joints. 

Joint sealing and retention of co-axial components - often both in the same application - are prime examples. Components can be threaded (screws or pipes), or splined or smooth (eg bearings). Special versions of these adhesives (often not truly anaerobic - in that primers may have to be used) give much higher levels of effective adhesion than the normal materials and so may be used in lap joints. Other versions are formulated as gasketting media. 

No other adhesive type has the versatility of this family for assembly (with dismantling possible) of co-axially fitting parts. But for lap joints, the ultimate performance of even the best of the truly adhesive anaerobics is readily surpassed by other groups - particularly the toughened acrylics and toughened epoxies. However, both of these may be less convenient to use. 

Essentially limited as a class to co-axial mechanical assembly, retention and sealing, they also make good general purpose gasketting media. The cure rate depends upon surface activity and may require a supplementary catalyst. The family copes with the gaps of normal engineering practice. As clearances increase, the anaerobics capacity to cope well falls rapidly. The majority of materials in the family are only suitable for use in lap joints as gasketting media or to seal a gap. Only the special anaerobic materials can be considered to be true adhesives and suitable for use on unsupported lap joints. 

Some plastics eg polyethylene, polypropylene, PTFE and silicone rubbers are particularly difficult to bond - except when used in co-axial (slip fitted or threaded) joints. 

Is the joint co-axial, ie composed of round, turned, threaded or fitted parts (Note not co-axial butt-jointed parts)... 

Of all adhesives, only anaerobics have been formulated to give reproducible and different levels of strength. They are the only adhesives which can be reliably used for the assembly of fitted, co-axial mechanical components where disassembly may be required for maintenance or other reasons. However, other types can be considered to be temporary and may be used in simple lap joints, e.g. adhesive tapes. A particular problem is presented here by adhesives which, in some applications, may reasonably be considered permanent - elsewhere their role is less readily defined. A particularly good example being the hot melt adhesives. 

From a stress distribution viewpoint, a co-axial tubular joint is much preferred since this type of joint cannot be subjected to the destructive peel stresses. We can consider a tube which slides inside the second tube such that the adhesive fills the annular gap (Fig. 29). 

Figure 29 A section through a co-axial tubular joint.

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