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Training & Education

Locking joints

By Udo O.J. Huff, M.ED.

Different rivet types and methods are suited for diverse applications

July/August 2012 - Riveting is appropriate when unwanted changes in a metal’s grain structure could occur with a heat-applied method like welding. Cold riveting leads to a positive-locking joint and hot riveting to a force-locking joint. There are different variables to consider in determining which riveting process to implement, depending on the application. FFJournal’s May issue introduced riveting as an ideal process by upsetting, or compressing, an auxiliary joining component.

The riveting joint refers to how the sheet metal parts are placed together. In lap riveting, one plate is laid on top of the other. In butt-joint riveting, the ends of the plates are fitted square against each other. Materials for rivets are low-carbon steels. The rivet and the workpiece should be the same base material so that no electrochemical corrosion occurs.

Rivet seams and rivet pitches

If the riveted structure needs to withstand large forces and stress, several rivets are placed along the rivet joint line. The rivet seam may consist of one or more rivet rows and may have the layout of a chain seam or a zigzag seam.

For the riveting process, holes for the rivets first have to be drilled. Those holes, seen in the cross-section of the metal plates, will weaken the plate. The rivet hole can cause a notch effect. The tensile stresses generated are not distributed uniformly in the cross-section of the workpiece but reach peak values at the walls of the rivet holes. Because of the loss of tensile strength, there must be a minimum distance between the rivet holes and from the edge of the workpiece to the rivet holes. Another rule is to determine the distance from the rivet center to the outside edge of the workpiece.

When structured steels are connected via gusset plates, for example, the rivets must be placed away from the edge in such a way that lets the rivet heads be upset.

Special rivets

In a normal setting, the riveting point is accessible from both sides. Hollow rivets are sleeves with a flat rim on one end. After the rivet has been led into the hole, the other end is folded with a special device. This riveting process has been used for thin sheet metal, cardboard and leather.

Mushroom rivets are used to connect thin sheet metal plates. The snap head rests on a smooth base and the rivet is struck from the other side. In this case, the riveting point is accessible only from one side.

Thermal rivets are drilled into the rivet’s shank end. An explosive charge is pressed into the hole. When heated to 120 degrees to 130 degrees Celsius, this charge explodes, causing the inaccessible snap head side to bulge, locking the rivet.

Blind rivets are hollow rivets with a conical or spherical end inserted with the drift end. If the end of the drift is forced through the rivet, it expands and the drift head even can crack at a certain pressure.

Expanding rivets are slotted at the ends. The end of the rivet shank expands when a grooved pin is driven into it.

No matter which type of rivet or process is used, riveting faults reduce the strength of the joint and the rivet. Displaced rivet holes and rivets that have been struck askew change the uniform fiber flow in the rivet. Such a fault reduces the clamping force. This also occurs because of rivet heads that have not been formed fully. If the rivet hole is too large (or the rivet too small), the strength of the joint will be weakened. FFJ

Udo O.J. Huff is an independent consultant with project experience in machine building, welding engineering, training and development. He holds Master of Education and Bachelor of Science in Technology degrees from Bowling Green State University. Questions or comments? E-mail uhuff@sbcglobal.net.

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