Mechanical Joining

7th March 2016

MECHANICAL JOINING

The methods of assembly of parts into a mechanical structure presents a range of options for engineers to consider, all of which have tradeoffs between manufacturing cost, performance and impact on the end product’s weight.  The type of joining technique is crucial to manufacturing decisions and therefore significant in design. The availability of specialized equipment and skilled labor, plus the cost of materials and energy will guide the decision. A number of structural joining technologies and methods are explored below.

RIVETS, BOLTS AND SCREWS

Historically, perhaps the most common structural joining technique is riveting. From buildings, to ships, to automobiles, rivets were the assembly method of choice in the past. While usage has diminished riveting is still used in when lightweight and high strength are at a premium hence the continued application in the construction of aircraft. Clearly a riveted joint is not designed for disassembly in the same way as a screw fastener. However this kind of joint does not cause the material deformation and heat effects possible with welding.

Bolts and screws offer an ease of disassembly that is not possible with a riveted joint, but this feature can be a disadvantage in some environments when a fastener can become loose.

WELDING

Welding is a very flexible joining method that it common across many industries. Welding has the advantage over other joining methods in that it does not always require any additional overlap in the joined materials, therefore lending offering a relatively lightweight fabrication method.

The biggest limitation of welding is the heat-affected zone. The effect of welding can be detrimental to the surrounding material. Depending on the material and the heat input by the welding process, the heat-affected zone can be of varying size and significance. In addition, the high temperatures of many welding techniques, combined with rapid cooling of the joint can lead to deformations within the material and residual stresses. The stresses introduced can have detrimental effects on the strength and fatigue life of the material.

The most common forms of welding include:

• Arc welding
• Gas welding
• Resistance welding
• Energy beam or laser welding
• Solid state welding
• Friction stir welding

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