Forming a Marman bead

Whether bent or straight, tube’s usefulness in industry increases exponentially when the ends are modified for a specific task. Threading, expanding, flaring, and reducing are some of the ways tube ends are changed to adapt the tube for specific uses.

One end form is named for the company that developed it for commercial use: the Marman bead, named after the Marman Products Co. The initial invention was the Marman clamp, a clamp that secures two cylindrical shapes end-to-end, such as fuel lines. The Marman bead takes advantage of the Marman clamp’s ability to fasten two tubes together.

Elegant and effective, the Marman connection design comprises a male form and a female form; the male is flared and female has the bead. A Marman clamp joins them by pulling them together. It creates a tight seal designed to pass a specific leak test or meet an industry specification.

Although they have been made for decades, Marman beads aren’t necessarily easy to form. They stretch the limits of tube end forming equipment, tooling, and design.

Easier Said than Done

“The Marman bead requires a huge amount of expansion,” said Jack Jacobs, the engineering manager for Tube Form Solutions. “Standard, run-of-the-mill tools can’t produce it. We’ve had to come up with new ideas to do it. Part of it is how you flow the material to keep it from stretching to the point where it wants to break.

“We’ve actually developed an electric machine and call it a nibbler,” Jacobs said. “It uses segmented tools, but you just put the part in, it clamps it up, and you can program the sequencing. It basically nibbles the material while rotating and flowing it out until it’s done and then brings the tool back out.”

This is different from most machines that use segmented tools in that, traditionally, the machines are limited to about 1⁄8 inch of material movement per setting. Because this refers to 1⁄8 in. around the entire circumference, the maximum expansion is about 1⁄4 in.

“For example, if you had a 3.25-in. outside diameter, the peak of the bead would be about 3.5 in. OD,” Jacobs said. Traditional segmented tools are limited in this way to ¼-in. maximum increments.”

This limitation isn’t much of a problem for most applications, but Marman beads don’t fit this mold because they far exceed the ¼-in. limit.

“If you trap the material, it will start to stretch,” Jacobs said. Stretching equates to thinning, which leads to a weak bead.

“The trick is to keep the material so that it’s not trapped or pinched so the material can continue to gather without stretching,” Jacobs said. “If it draws and then starts stretching, you could start out with in an 0.065-in. wall thickness and end up with 0.010-in. wall thickness at the peak. If it stretches that much, the risk of a part failure goes way up.”

Striking a Balance

Like many processes, forming a Marman bead successfully requires quite a bit of understanding of the capabilities and limitations of the machine, the tooling, and the metal.

The Process. The company’s Marman process is based on an electric forming system. It gathers the material without thinning it and has a few other advantages compared to conventional machines: It’s quieter and easier to cool, achieves higher accuracy on small parts, can run at higher speed, and uses no hydraulic oil. That said, it does have a few drawbacks. Most notably, the system costs more than its hydraulic counterparts. Also, it’s a dedicated system, meaning that it does beads only. It doesn’t do other forms.

However, the electric system does permit tool changes and has the versatility that comes with CNC. It also allows for a variety of bead shapes, including shallow beads, conventional hose beads, and Marman style.

The Diameter. The tube diameter matters also. As time goes on, Marman beads are used on a larger variety of diameters. Increasing and decreasing diameters brings new forming challenges. For example, many modern end forms require several strikes, and Marman beads are no exception. As the tube diameter increases, so does the likelihood that the form will need a multistrike process to form it. At the other end of the spectrum, as tube diameter decreases, the Marman bead becomes more challenging to form. As the working area gets smaller, two distinct steps are used: moving the material with segmented tools, then finishing the job with a rolling tool.

The Tooling. Regardless of size, one of the biggest challenges of working with a Marman bead concerns the surface profiles.

“It’s like a profile because it is a mirrored image. The male and female halves must fit snugly and you can’t have any gaps between the two, which would become leak paths. You have to have the same angles and the same size features.”

While moving the material is preferred over stretching it, the latter can’t be eliminated. The forming process attempts to move the material; if the material moves only partway, stretching takes over. This means that when the tooling designer is developing the tooling, one consideration is the amount of stretch and the resulting springback when the part is removed from the machine. The tooling usually is different from the finished part shape.

“You have to compensate your tools,” Jacobs said. “If you were to look at it geometrically you’d think, ‘Wow, this part’s going to be too big.’ Actually, when it rebounds, it’s just right.”

The final finish means striking a balance between quality and time. Increasing the number of forming strikes increases the finish quality, but the tradeoff is that it takes more time to form and finish the part.

“The key specification is the leak test,” Jacobs said. “We have had customers who get a good finish using segmented tools. They can use it as is—the profile is good enough and it passes a leak test.”

Other customers need a secondary operation—the rolling process that improves the part shape and the surface finish. The benefit of a CNC system is that it allows the operator to make adjustments to the process quickly. If the material doesn’t form properly, CNC allows the operator to increase the number of hits or slow the process without a lot of time-consuming setup and trail-and-error work. By making sight adjustments to the program, the operator can experiment to improve the seal’s integrity.

As always, the tube material is a critical factor. For a difficult form, the alloy choice might make the difference between a part that forms successfully and one that doesn’t. Even within a family of steels, each has its own unique minimum yield strength and stretch rate.

“When you’re doing extreme part shapes, like a Marman bead, you’re going to want every advanced technology you can use,” Jacobs said. “We always recommend getting a really good grade of steel just for the formability of it. The same thing can be said for any extreme part shape. If you’re going to move material a half inch or greater, you’re better off having a better, more formable type of material.”