Mechanical Joints Overview – Part I – Permanent Joints

The history and evidence of joints can be traced back to the Bronze Age when the metals started to gain predominance. At present there are various joints that suit the specific requirement and are related to the metals that require fastening. There are basically two major categories of joints: Permanent joints and Temporary joints. This article is about the former.

Permanent joints are preferred where the load is high and the durability plays an important role. Some advantages are high load bearing capacity, higher yield strength of the combined structure, and better distribution of stresses through the material. There is also a decreased chance that the parts will separate. The main drawback, of course, is that the joint has to be broken if any alterations are needed and some times installation is tricky, requiring complicated instruments, shields, etc. The two main types of permanent joints are welding and riveting.

Metals can be generally joined using the welding technique – although there are some exceptions, but almost any metal material (aluminum, carbon steel, stainless steel, tungsten) has a variation that is suitable for welding. Welding is a type of permanent joint where the metals are melted (using high temperature) and with the aid of a filler material a weld pool is created which is nothing but a puddle of molten metal. The joint is made permanent by reducing the work piece temperature to that of the room temperature. There are different types of welding: Arc welding, Gas welding, Resistance welding etc. Though the principle is similar, the method of achieving the joint is different. The advantage of welding is that it is strong, durable, it can withstand a high stress value and it can also be used to join dissimilar metals. However, it must be noted that the strength of welded region is significantly lower than the metal’s one. The yield point of the melted and cooled material is about 50-60% of the metal “unmelted” yield point.

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A riveted joint is a permanent joint which uses rivets to fasten two materials. A rivet is a structure that has a hemispherical head on one side and a cylindrical shaft on the other. Made from Aluminium alloys, steel, or CRES and other special metals like titanium, nickel, etc. the riveted joints have a wide range of applications ranging from aircraft bodies to high pressure boilers. There many types of rivets, with the simplest process being the following: the rivet is inserted into a pre-drilled hole of the same diameter and the other end is ‘upset’ i.e. deformed to a diameter that is 1.5 times greater than the original diameter. In riveted joints, more than one rivet is always used to fasten the materials, positioned in a specific array that is either parallel to each other or runs in a zigzag manner. Riveted joints are strong and can handle high stress (especially in shear), however they can also fail with a high value of tension force.

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