Joints practical assembly

One of the more difficult features to quantify in detail is welded joints and assemblies. A common way to indicate the standard of finish on welded stainless steel components is by the provision of small examples. It is comparatively easy to achieve any required finish on a small component that is fully accessible and manoeuvrable, but quite a different matter to attain the same quality over every minute region of a large and complex fabrication. To remove subjective judgement as far as practical from the inspector s task it may be useful to have examples of a slightly lower grade of finish, and/or of what is not acceptable at any point of the final equipment, for comparison. 

In outdoor type switchgear or controlgear assemblies the normal practice is to provide a double door in the front to house the front panel and protect the door knobs, meters, lights, pushbuttons, reset knobs or other accessories mounted on the door and thus prevent water or dust leaking through joints, knockouts and fitments etc. It is also recommended to have a canopy on the top of the enclosure to protect the panel from direct rain. Figures 13.6 and 13.28 illustrate this type of construction. 

In the cubicle construction of a switchgear assembly the busbar chamber is normally located at the top of the assembly and runs through the length of it. It is usually suitable for extension, through fish joints at either end, if required at a later date. For installations having top cable entry, the busbar chamber may also be located at the bottom of the assembly or the depth of the panel increased, with an additional shroud between the top busbar chamber and cable chamber. From these main busbars are tapped the vertical buses for each vertical panel. Manufacturers may adopt different practices for horizontal and vertical busbar arrangements to economize on their cost of production. We illustrate the most common types of busbar arrangements. 

Problems with service experience include obtaining sufficient information on the actual conditions of service, finding that these conditions are much milder than the design conditions, or that the polymer formulation used has now been superseded. Life prediction cannot consider failures caused by poor workmanship, incorrect use or maintenance, or faulty design, particularly of joints, attachments or in the choice of neighbouring materials. Many failures in practice are due to degradation in processing, faulty assembly, or to abuse. 

An adhesive is a material capable of holding together solid materials by means of surface attachment. Adhesion is the physical attraction of the surface of one material for the surface of another. An adherend is the solid material to which the adhesive adheres and the adhesive bond or adhesive joint is the assembly made by joining adlierends together by means of an adhesive. Practical adhesion is the physical strength of an adhesive bond It primarily depends on the forces of adhesion, but its magnitude is determined by the physical properties of the adhesive and the adherend, as well as the engineering of the adhesive bond. 

One must also bear in mind the effect of differences in the coefficient of linear thermal expansion between the two parts. If the coefficient of linear thermal expansion is greater for the shaft than it is for the hub, the gap will tend to close at elevated temperatures and become greater at low temperatures. Whether or not this is desirable depends on whether the joint is intended to be fixed or sliding at elevated temperatures. In the case of a fixed joint, it may be desirable to elevate the temperature of the hub (only) for assembly in order to assure that the joint remains fixed at elevated temperatures. However, this practice can lead to stress cracking of the hub at low temperatures if it is not strong enough to withstand the added stress under such conditions. 

To make an economical and practical joint, the surface preparation methods must also meet several other requirements. They must be safe to handle and should not be flammable or toxic. They should be inexpensive and provide fast processing times. The prebond processes should be easy to monitor and control in a production situation. In addition, the process should not in itself leave a weak boundary layer. If chemical solutions are used, they should rinse off easily and not continue to react with the surface past the time when the bond is made. The surface preparation process should allow for practical working time between preparation and application of the adhesive or sealant. Finally, the surface provided by the treatment should not change once the assembled joint is made and placed into service. 

Determining the Strength of Adhesively Bonded Plastic Lap-Shear Sandwich Joints in Shear by Tension Loading, Practice for (D 3164) Strength Properties of Adhesives in Shear by Tension Loading of Laminated Assemblies, Test for (D 3165)... 

Loads to be borne by and distributed within the various layers of the joint shown in Figure 2.1 and Practical aspects of assembly. 

These materials are probably best bonded with acrylic-based toughened adhesives. Both the adhesive and its associated initiator, in their liquid forms, are likely to cause stress cracking in prolonged contact with the acrylic face. These adherends should therefore be bonded using pre-mixed adhesive/initiator if at all practical. Joints based on the acrylic face should never be used for structural assembly work. 

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. 

---