Making a 20-degree Marman bead
Rotary or ram forming an airtight seal
Two forming methods can make a bead for an airtight seal on a metal tube: rotary (spin) forming and progressive ram forming. Each has advantages and disadvantages in bead profile, cycle time, amount of wall thinning, and so on.
Metalworking has a history that goes back thousands of years and, if you dig deep enough, you can find interesting stories about products and processes that are commonplace today. One such item is the Marman clamp (see Figure 1), a heavy-duty band clamp that allows two cylindrical interfaces to be clamped together using a ring clamp (called a Marman ring). It is a quick-disconnect clamp often used on large-diameter fuel lines. It also is used in space vehicles, such as the Cassini plasma spectrometer on the Cassini orbiter.
The Marman clamp was devised decades ago and first produced by Herbert Marx, one of the Marx Brothers and better known by his stage name, Zeppo Marx. He had left show business, founded Marman Manufacturing, and found success in the 1940s manufacturing components to support the war effort. The clamp's inventor approached Marx about producing it, which was designed to secure cargo during transport. It later was used in transporting the atomic weapons used at the end of World War II.
In addition to securing cargo, it is suitable for nearly any air management tubing application that requires a leakproof seal. Modern applications include diesel truck exhaust and diesel particulate filter (DPF) canister components, aircraft bleed air ducting, and turbocharger inlets and outlets. A primary advantage is that component removal and replacement is fast and easy. The clamp also allows adjustments in component orientation for applications in which alignment is critical.
Making the Bead
Several processes can form this 20-degree bead form and variations of it. Two main methods are rotary (spin) forming and ram forming. A third method, segmented tooling, can form a bead; however, the tooling marks on the bead do not provide for a smooth sealing surface, so it's not a contender for air management applications.
Rotary, or Spin, Forming. Rotary forming large-diameter beads is fast, easy, and economical; furthermore, it produces a very smooth finish on the mating surfaces. The tooling consists of a set of clamp jaws that maintain the nominal diameter of the tube and a rotary forming head that contains a profiled roller that matches the ID profile of the bead. The tooling segments expand while rotating inside the tube, forcing the rollers and material into the OD bead groove of the clamp jaws. While machine cycle times are typically 8 to 10 seconds or less, the disadvantage is that the process stretches that material,thinning it at the peak of the bead (see Figure 2). This makes the process unsuitable for applications that require a uniform material thickness.
It has other disadvantages. First, it doesn't form corner radii as tight as the progressive ram forming process can. Second, if a reduced-diameter lead-in lip or pilot is required, a separate operation is needed to reduce the tube. Third, if the tube is bent, at least 1 inch of straight length is needed for forming the bead for the Marman clamp. If less than 1 in. of material is available at the end of the tube, it's necessary to make an adapter that features a Marman bead and weld the adapter to the tube end.
Progressive Ram Forming. The main advantage of progressive ram forming is that it doesn't change the wall thickness (see Figure 3). Also, it forms tighter corner radii and, if necessary, it can reduce the lead-in lip in the same process that forms the bead.
Ram forming has two drawbacks. First, the cycle time is double that of the rotary forming process. Second, it doesn't provide any internal support for the bead, which allows some variation of the inside bead profile. This can be overcome by using a secondary rotary finishing tool to complete the process.
Rotary or Ram? Or Both?
The material, wall thickness specifications, and tolerances dictate the method you use. The best results come from a combination of both methods: Ram forming to shape the bead and perform any reduction or expansion of the lead-in, followed by rotary forming to iron out any variations in the bead profile and sealing surface.
Before you start making parts, check the prints and material specifications carefully. If no spec is present, check with your customer to explain the difference—your customer and you will both benefit.
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