Tube Bending. ENGR 201 Group Orientation

Transcription

1 Tube Bending ENGR 201 Group Orientation

2 Outline A History of Tube Bending & The Tricycle Project Basic Bending Theory Basic Tube Bending Equipment & Processes Compression Bending Rotary Draw Bending Press Bending Roll Bending Fabrication/Process Considerations Hands on Project

3 Tube Bending & The Tricycle Project Objective Provide an appropriate, cost effective method to replace welded joints used in tricycle production.

4 Traditional Joint Fabrication : Hand Cut & Weld Drawbacks to the Cut & Weld Method Time Consuming & Labor Intensive; each joint must be notched by hand Each joint must be individually welded Fuel to operate the welder/generator is expensive and not locally available Maintenance and repair of welder/generator set is expensive, unreliable and results in significant downtime. Local fabricator is experiencing heath problems due to the cumulative affects of years of welding with improper protective equipment.

5 Joint Fabrication Comparison

6 Basic Bending Theory: Stresses in Pure Bending A tube in pure bending will: Be in tension on the outside of the bend Be in compression on the inside of the bend Contain a neutral axis where material fibers are unaffected.

7 Basic Bending Theory: Material Behavior/ Failure Modes Tubing wall thickness on the outside of the bend will thin-out due to tensile stresses. Thinned outer wall tends to be pulled in toward the centerline of the part causing distortion of the tube cross-section. If the ultimate tensile stress of the material is exceeded, the tube will tear along the outside bend surface. Tubing wall thickness on the inside of the bend will thicken due to compressive stresses If the compressive strength of the material is exceeded, the tube will experience localized buckling resulting in excessive wrinkling of the inside bend surface.

8 Basic Bending Theory: Material Behavior/ Square vs. Round Round: Less material in the most highly stressed regions. The plane of highest tension or compression is tangent to the cross section. Less prone to distortion Square The entire tube wall is subjected the maximum tensile and compressive stress at the outside and inside of the bend. More prone to distortion Plane of max tension Plane of max compression Center line of bend

9 Basic Bending Theory: Material Behavior/ Square vs. Round Round Allows metal to flow evenly in all directions during bend. Easier to control distortion. Square Material on the inside bend surface tends to expand vertically and flow horizontally along tube face. These forces tend to draw in the vertical edges of the tube distorting the cross section into a Keystone shape. More difficult to control distortion.

10 Basic Bending Theory: Material Behavior/ Square vs. Round Round Round cross section allows for good transmission of clamping pressure between clamp and bending die Less prone to slip Square Square cross section does not lend itself to good transmission of clamping force between clamp and bending die. Tube tends to slide along the die at the start of the bend. More prone to slip

11 Basic Tube Bending Equipment: Tube bending equipment and tooling functions: Provide a means for controlling the form and path of the bend. Provide the force needed to plastically deform the tubing Provide a means for controlling distortion of the tube cross-section

12 Basic Tube Bending Equipment: Compression Bending Hossfeld #2 Bender Tube is bent around a stationary bending die. Stationary Bending Form Tube

13 Basic Tube Bending Equipment: Rotary Draw Bending Pro Tools HMP 200 Tube is pulled along a rotating die HMP-200-low.wmv

14 Basic Tube Bending Equipment: Rotary Draw Bending More sophisticated bending equipment uses internal mandrels and external wiper dies to control cross sectional distortion of the tube shape during the bending process.

15 Basic Tube Bending Equipment: Press Bending

16 Basic Tube Bending Equipment: Roll Bending

17 Fabrication/Process Considerations: Minimum Bend Radius Bend Radius Limiting Factors Material Properties Material Processing (% Cold Work) Tubing Diameter Tubing Wall Thickness Bending Equipment & Process

18 Fabrication/Process Considerations: Minimum Bend Radius Tubes become weaker as the OD increases and the wall thickness decreases. Decreasing bend radius effectively increases the forces acting on the tube. Rule of Thumb: Centerline Radius (CLR)= 4 x Tube OD

19 Fabrication/Process Considerations: Springback Springback describes the tendency of metal which has been formed to return to its original shape. Springback causes the tube to unbend from two to ten percent depending on the radius of bend, and this can increase the radius of the tube after bending. The smaller the radius of bend, the smaller the springback.

20 Fabrication/Process Considerations: Material Length Ball Park Calculation of Material Length Needed to Form a Bend L A = RA Where: L A = arc length, in. (mm) R= centerline radius, in. (mm) A= included bend angle, deg.