July 2, 2012
A fluid-carrying line usually consists of a length of tube or pipe that has a fitting that is either attached at the end of the tube by a conventional swaging method or attached to the tube by welding. A little-known process, elastomeric swaging, bulge-forms the tube or pipe ends. Its niche is in small-diameter, heavy-wall applications that carry fluid under substantial pressure.
If you’re familiar with internal swaging, you know you have a couple of choices to perform the operation. Conventional processes use a hard tool that expands or rotates inside the tube to make a predetermined form. A little-known process, elastomeric swaging, is another choice. It uses a flexible tool to bulge-form the tube or pipe. Depending on your application, elastomeric swaging can be a good fit.
Patented for military aircraft use in the late 1970s, internal elastomeric swaging is a cold-forming process. The linear travel of a high-strength drawbolt compresses an elastomeric expander against a static split ring that surrounds the drawbolt (see Figure 1 and Figure 2).
The process has two distinct uses. First, it can expand the tube or pipe into a fitting. During the swaging cycle, the tooling develops a high radial force that displaces the tube wall, expanding it into the grooves of the external connector. When complete, this process creates a work-hardened double layer of material. Second, it can be used to create forms such as beads (see Lead Image).
The internal elastomeric swaging process uses constant pressure, so it doesn’t cause local stress concentrations; because of this, it doesn’t result in distortion, wall thinning, and strength loss of the tube or fitting. Regarding inspection, an elastomerically swaged assembly needs only visual and dimensional inspection, externally and internally. Because the process simply moves the metal, it doesn’t introduce subsurface inclusions or other flaws, and therefore doesn’t require radiographic or ultrasonic inspection.
It is useful for swaging tube and pipe made from the manufacturing industry’s most common material—steel—and it’s also suitable for stainless steels, aluminum, titanium, and INCONEL® alloys.
Conventional end forming may be the best choice for many applications; in other cases, welding or brazing a connector onto a length of tubing is the best option. However, elastomeric swaging can fill a niche in which conventional methods produce less-than-satisfactory results. While it can be used for low-pressure applications up to about 3 in. diameter, such as ductwork, its characteristics make it a strong contender for small-diameter, high-pressure applications.
“This process is most commonly used on sizes smaller than 1½ inch,” said Joe Fortin, an aircraft industry veteran. A retired machine rebuild specialist at The Boeing Co. for 28 years, Fortin has specialized in tube forming processes for many years.
“When applied to two- and three-groove fittings, this double-walled construction provides a strong, leakproof mechanical attachment for fluidtight connections,” he said. “It makes a robust connection that stands up to vibration, which makes it suitable for many aircraft applications. Aircraft use many tubing connections that are 3/8 to ½ in. in diameter and have pressures from 3,000 to 5,000 pounds per square inch, and this process works well for these applications.”
Elastomeric swaging also eliminates some of the risks associated with other methods. For example, welding requires great care to prevent tungsten inclusions and hydrogen embrittlement. Likewise, roller swaging tooling must be cleaned thoroughly when switching from one material to another to prevent contamination. If the tooling picks up tiny particles of steel during one production run, some of this material might get transferred to aluminum workpieces during a subsequent run, potentially contaminating the entire lot.
The key to the elastomeric swaging process is the tooling. Elastomers are dense and don’t compress much. When the drawbolt squeezes the material laterally, it reacts by expanding radially with substantial force.
“For one aircraft project, I tested elastomeric swaging on some lengths of small-diameter tubing with 0.070 wall thickness,” Fortin said. “This material was so thick that it couldn’t be orbital welded because the heat didn’t penetrate the wall thoroughly, resulting in partial welds. However, after the evaluation, we recognized the potential of elastomeric swaging.”
The characteristics that make it work well for aircraft make it a good option for nearly any application that uses fluids under pressure, such as transportation, heavy equipment, military, marine, and refrigeration. This process can be used for air lines, low-pressure water lines, electrical conduit, and wastewater handling systems.
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