TPT September 2009

Wall thinning Figure 10 reports the OD wall thinning and the ID wall thickening results for the booster bending trials. Table 1 contains the actual percentage thinning or thickening values as determined at the specific bend locations. Photo 4 shows the cross section of sample 10, where the outer and inner walls of the bend are directly displayed. The

sample 7

sample 8

sample 11

FE axial load

trial data indicates a direct connection between the OD wall thinning and the ID wall thickening. It can be observed that initially high boost pressure (up to 102°) provides improved wall thinning ratios.  Photo 4 : Cross section of sample No.8 showing the outer and inner walls of the tube bend

Axial Load, lb

Bend Angle, Degree

 Figure 9 : Axial load settings for booster bending trials

The FEA simulation indicated that minimum OD wall thinning should result, by decreasing the boost pressure after the bend arm angle reaches 66°, and again at 102° and 145°. Trials were completed using the concept of a declining axial load per the simulation schedule. Figure 9 shows the load vs bend angle schedule used in these trials, including the load schedule determined from simulation. Making bends Automatic separation of tools is essential to allow the tube, now bent to 180°, to be removed from the die. CNC machines particularly need to be able to easily manipulate the tube after bending. Photo 3 shows the tubing before insertion between the upper and lower halves of the bending dies.

Wall Thinning & Thickening

Wall Thickness Change, %

Bend Arm Angle

 Figure 10 : Percentage wall thickness change data

Ovality Besides outer wall thinning, ovality and any reduction in cross sectional area of the tube are important. Ovality was measured at 45° and 125° around the degree of bend and is reported in Table 2. Ovality is difficult to predict because most wall thinning specifications include an ovality requirement, in this case < 4%, and a minimum flow reduction (inside diameter area) of not less than 85%. The following data indicates that the ratio was satisfactory in samples 10 and 11 but not 9. Sample 9 appears to be an anomaly, most likely caused by a misaligned clamp die.

 Photo 3 : Split tooling used for tube bending

 Table 1 : Percentage wall thickness change data

Changes in Wall Thickness

Sample 7

Sample 8

Sample 9

Sample 10

Sample 11

WALL

0% 1%

0% 1%

0% 1%

0% 7%

0%

0% 6%

0% 0%

0% 8%

0% 1%

0% 5%

0°

-1% -6%

45° 90°

-13% 13%

-7%

23%

13%

-4% -8% -2%

16% 22% 13%

-3% -8% -6% -4% -1%

15% 24% 22% 21%

-10% 22% -10% 24% -11% 19%

135° 180°

-11% 21%

-8%

24%

-9% -7%

19% 19% -4%

-5%

4% 3%

2% 4%

3% 1%

4% 5%

0%

Bend Arm Angle

WALL

1% 9% Thinning Thickening Thinning Thickening Thinning Thickening Thinning Thickening Thinning Thickening 1% -1%

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S eptember 2009

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