Metal Bellows

Bellows Design
Metal bellows and bellows expansion joints are used in piping systems to absorb thermal growth. Each time the piping system heats up and cools down, the bellows go through one complete cycle. The manufacturing of a metal bellows begins with the design process. The desired life cycle of the bellows or expansion joint must be determined to specify the bellows' thickness and the number of plies required. A multi-ply bellows is generally a better choice for a longer cycle life. In applications with minimal movement or infrequent heating and cooling cycles, a single-ply metallic bellows may be satisfactory. A multi-ply metal bellows is preferable when vibration is present in the piping system, such as in marine exhaust, generator exhaust, blower exhaust, and pumps. To ensure you are purchasing bellows expansion joints from a reputable manufacturer, always request a design data sheet that follows the Expansion Joint Manufacturers Association (EJMA) 10th edition guidelines.

Making the Bellows Tube
The first step in manufacturing a metal bellows is making the tube or tubes that are to be corrugated. The coil or sheet stock is precision sheared to the predetermined size. For multi-ply bellows, each outer tube will be slightly larger than the inner tube. Typically, multi-ply bellows consist of 2 or 3 tubes (plies), but depending on the application, there may be 5 tubes (plies) or more.
Once sheared, the sheets are rolled into tubes. Care must be taken to ensure the tubes are perfectly round so they will fit over the bellows forming dies. For multi-ply bellows, each tube is carefully marked so they will fit together after welding.
After rolling, each tube is seam-welded using an automatic longitudinal seam welder. This is a critical step in the manufacturing of the bellows, as a poor seam weld may result in splitting during the forming process. After welding, the tubes are inspected, cleaned, and telescoped together.

Forming the Bellows
The process of corrugating the tube can be done in several ways, with the most common being mechanical die forming and hydroforming. Triad Bellows incorporates a mechanical forming process that is much less expensive than hydroforming and achieves similar results in terms of metal thinning. The tube is placed over the die on the bellows forming machine. The machine is programmed to form the bellows corrugation to the exact engineered diameter. The tube is rotated as the dies expand and contract, creating an even expansion without concentrated thinning. Once the preprogrammed number of convolutions is formed, this stage of the process is complete.

Roll Seam Welding
The roll seam welding process involves a continuous series of overlapping spot welds used on multi-ply bellows to join the tubes together at the bellows neck. Depending on the material thickness and the experience of the welder, this may or may not be necessary. A reinforcing band can be incorporated into the roll seam weld process as well. For example, if a welder needs to weld a 0.016" thick single-ply bellows to a pipe, this can be challenging. However, if a 0.062" thick reinforcing band is roll seam welded to the 0.016" thick bellows neck, the welder is effectively welding a 0.078" tube to the pipe, which simplifies the weld joint and reduces the chance of leaks.

Trimming the Bellows Necks
A special rotary shear is used to trim the bellows necks to the exact length. A circumferential line is scribed on the tangent of the bellows. The technician carefully guides the bellows into the rotary shear blades, following the scribed line.

Expanding the Bellows Necks to Size
Using special bellows-expanding machines, the bellows necks can be sized to fit over a pipe or inside a flange. Welders who fabricate metal expansion joints will attest that this is a crucial step for achieving the best welds. A tight fit allows the welder to produce a consistent weld size with minimal potential for leaks. A sloppy fit with gaps increases the likelihood of leaks significantly.