TPT May 2007

T ube B ending, H ydroforming & E nd- Fo rming

Scrap rates can be as high as 50 per cent depending on material, bend radii, and complexity of the tube assembly. The process is slow and labour intensive with an average two-day processing time. This can be hard on delivery schedules when unpredictable scrap rates necessitate repeated runs.

Advances in double wall tube bending

Techniques for manufacturing double wall tubing have been around for a long time, yet they are not widely practiced and shops experienced in the manufacture of double wall tubing are uncommon. Wolfbend LLC, USA, is the manufacturer of a patented lightweight system that enables bending of tube-within-tube. Most traditional double wall tube methods rely on a medium that fills the gap between tubes to provide support during bending, which is then removed afterwards. Commonly used mediums are hot wax, sand, small metal shot, and metal alloys such as Cerrobend that melt in boiling water. The processes for all these materials are similar. Hot wax is the most commonly used method in aerospace. A typical hot wax process starts with cutting inner and outer tubes to rough length and applying corrosion protection such as iridite or alodine inside and outside both tubes. Long thin phenolic spacers are cut to size and epoxied onto the inner tube with a template to ensure that they are not in bend areas. After a four-hour cure the inner tube/spacer assembly is slid into the outer tube and capped off at one end. Molten wax is poured into the gap filling 2ft sections at a time, with a two hour cooling between pours to allow for contraction. Bends of 5º or more are X-ray inspected from 2 directions. The tube assembly is then uncapped and hung vertically in a hot

water bath or oven for 2½ hours to remove the wax. Tubes 6ft and longer are flipped over and processed a second time. Pressurized steam is then blown through the assembly to remove most of the remaining wax. After degreasing and cleaning, tube ends are trimmed to length and fittings are installed. The hot wax method has drawbacks, primarily due to the characteristics of the wax. With long tube assemblies, the wax can cool before reaching the bottom. Resultant air pockets can lead to lack of support and bend failures. To help compensate for this, a minimum working gap is typically ¼" (ie a 1.00 OD inner tube requires a 1.50 OD outer). Bending applies varied forces and wax is by no means a perfect medium. It behaves fairly well in compression but tends to squeeze out which can influence tube ovality. In tension wax can separate leading to inadequate support and bend failure. Proper function also depends on the wax being completely cooled before bending. The added weight of the wax usually requires the bender to be slowed down due to increased swinging weight and momentum. 100 per cent removal of the wax is very difficult to achieve and verify, which is a contamination risk in aerospace applications such as fuel lines. There are also potential burn risks in the pouring and removal steps, particularly when using steam.

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Bending applies varied forces and wax is by no means a perfect medium. It tends to squeeze out which can influence tube ovality

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Wolfbend™ is a relatively new double wall manufacturing method. It relies on a bending sleeve that fills the gap between tubes but remains in place after bending to support the tubes. For aerospace applications, where weight is critical, the sleeve is only used in the bends and straight sections remain empty. Metallic grounding spacers are used at both ends of the assembly to keep tube ends concentric and provide an electrical bonding path. A typical Wolfbend process starts with cutting inner tube rough length, outer tube final length, and applying iridite or alodine inside and outside both tubes.

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fi (Below left, middle and right) Cutaways and end view showing the effects of using hot wax as a filler medium for double wall tube bending (Bottom left, middle and right) Cutaways and end view showing the effects of using the Wolfbend method for double wall tube bending

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M ay /J une 2007