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Joining by deformation

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

Riveting locks joints when heat-affected fusing methods are unsuitable

May 2012 - Reliance on riveting has declined. It generally is being replaced by welding, soldering and brazing. However, it’s still important. Riveting is applicable where unwanted changes in a metal’s grain structure would occur when heat is applied.

Special rivets often are used for special types of joints. Aircraft manufacturers, for example, use the riveting process for joining aluminum alloys in the fuselage, wings and outer structural metals. The aerospace industry also uses special rivets with small explosives for hard-to-reach areas and joining applications.

The rivet joint is created by deforming the structural component or the auxiliary joining component. Some structural elements, such as plating or sheet metal, are joined by folding, flanging or other deformation methods.

Riveting joins by upsetting (compressing) an auxiliary joining component. Cold riveting creates a positive-locking joint while hot riveting yields a force-locking joint. Auxiliary joining components include rivets, tubular rivets and special application rivets.

Basics and tools
An undriven rivet consists of a head and shank. The length is specified without including the head in the case of button head rivets. However, the head is included in the length of flush (flat-head) rivets.

The rivet length must include additional allowance for forming a second head by upsetting. Exact specifications of this allowance can be found in tables. A rule of thumb applies to button-head rivets:

Allowance = (1 mm/0.0393 in. to 2.5 mm/0.0984 in.) x diameter

With this formula, the allowance increases exponentially in relation to the rivet length and diameter of the undriven rivet. A flat head rivet is 1 mm/0.0393 in. to 2 mm/0.07874 in. shorter.

Going back 40 or 50 years in the history of industrial riveting, someone might find teams riveting structures like bridges, containers and vessels. The riveting team started with a person at the fire pit heating rivets and using a tool to throw the hot rivets to a catcher. The catcher then would place the rivet. From the opposite side, a third person with a steady rest would set the head. With the head set, the catcher would strike the rivet head with a large riveting hammer. When the rivet cooled, it would contract and harden, securing the joint.

Now, one person uses an induction heat tool while placing the hot rivet. There is still an opposite person with the steady rest setting the head. But the induction tool works also as a pneumatic hammer to drive and form the head.

The required tools are a plate-closing tool, a steady rest and a header. Pneumatic or electric hammers are used to increase the work speed. The parts to be joined must lie flush against each other and must be drilled together.

The holes must be exactly vertical and the edges spot faced. The rivet hole diameter must be such that the rivet can be inserted easily.

Small steel rivets up to 10 mm (3⁄8 in.) are cold headed. In hot riveting, the end of the shank is made white hot and the set head red hot.

The inserted rivet is pulled tight with a plate-closing tool. The shank is then top headed with the riveting hammer and the driven head struck with the header. If necessary, both heads are caulked.

In cold riveting, the rivet fills the hole. Forces that cause the plates to be pressed together are relatively small. Tensile forces in the plates are transmitted by the rivet shank, which is now subject to shearing stress. This creates a positive locked joint.

Hot-driven rivets shrink in length, tightening the joint cross section during cooling. A clamping force, or frictional connection, is generated, which prevents the plates from shifting force under load. 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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