TPT May 2007

ube

ending,

ydroforming &

nd-

rming

110

une

2007

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.

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.

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.

Advances in double wall tube bending

(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

Bending applies

varied forces and

wax is by no means a

perfect medium. It

tends to squeeze

out which

can influence

tube ovality

❱