English
日本語
한국어
Español
русский
Tube Stretch Bending Machine is an industrial cold-forming process in which an axial tensile force is applied to a metal profile or tube while it is simultaneously bent around a forming die or between bending rolls. By pre-tensioning the material beyond its yield point before and during bending, the entire cross-section of the workpiece is placed in a state of plastic deformation—rather than the elastic-plus-plastic combination that occurs in conventional roll bending or rotary draw bending. This fundamental difference in the stress state produces dramatically reduced springback, excellent shape accuracy, and minimal cross-sectional distortion.
Tube Stretch Bending Machine equipment uses a fully hydraulic control system to precisely manage both tensile force and bending progression. It can process a wide range of metallic profiles including steel pipes, square tubes, H-beams, I-beams, channel steel, angle iron, and aluminum extrusions. Material utilization exceeds 95% in most applications, and the process can achieve bending arc lengths of up to 12 meters on a single machine pass—making it the technology of choice for large architectural, transportation, and infrastructure structures.
How Tube Stretch Bending Machine Works: The Process Mechanism
The Tube Stretch Bending Machine process involves three coordinated phases, all managed by the hydraulic control system:
- Pre-tensioning: Both ends of the profile are gripped by hydraulically actuated jaw chucks. The hydraulic cylinders apply an axial tensile force that stretches the entire profile past its yield strength—typically to 2–8% elongation depending on material ductility and target bend radius. This ensures the entire cross-section is in the plastic zone before bending begins.
- Wrap-forming: While maintaining axial tension, the forming die (on a rotary table type) or the profile (on a double-arm type) moves to wrap the tensioned material progressively around the die contour. The combination of tension and bending forces causes the outer fiber to stretch further while the inner fiber is restrained from buckling or wrinkling—resulting in a clean, uniform bend profile.
- Post-stretching and release: After completing the wrap, an additional increment of tensile force is applied to equalize residual stress across the cross-section. The grips then release, and the springback is minimal—typically less than 0.5° of angular error on aluminum profiles, compared to 3–10° with conventional bending methods.
The result is a bent profile with precise curvature, negligible twist, and a cross-section that retains 90–98% of its original dimensional accuracy—a critical requirement for architectural curtain wall frames, train body structures, and stadium roof arches where visual and structural consistency across many identical components is mandatory.
Machine Types in the Tube Stretch Bending Machine Series
Rotary Table Type Tube Stretch Bending Machine
The most common configuration for large-radius bending. The profile is clamped at both ends by swing arms mounted on a central rotary table. As the table rotates, the profile wraps around the fixed forming die at its center. Rotary table machines excel at producing consistent circular arcs on long profiles (up to 12 m arc length) and are widely used for architectural aluminum curtain wall frames, stadium roof beams, and train roof arch members. Maximum tensile force on large models reaches 2,000–5,000 kN, accommodating heavy structural steel sections.
Double-Arm Type Tube Stretch Bending Machine
Two independently controlled hydraulic arms grip the ends of the profile and move in a coordinated arc around a central forming die. The double-arm configuration allows asymmetric bending (different bend radii on the two sides of the die) and is more flexible for non-circular arc forms. It is preferred for automotive structural extrusions (door sill beams, roof rails), elevator guide rail arches, and complex aircraft fuselage frame sections.
Industry-Specific Variants
Specialized Tube Stretch Bending Machines have been developed for specific industry applications:
- Shower room and door frame machines: Compact, lower-force models for bending thin-wall aluminum extrusions to tight radii for framed glass enclosures and architectural door surrounds.
- Train roof beam machines: Heavy-duty, long-table models capable of Tube Stretch Bending Machine stainless steel or aluminum profiles up to 20 meters in length into the precise curves required for high-speed rail car roof structures.
- Elevator guide rail bending machines: Designed for the precise, distortion-free bending of T-section elevator rails used in high-rise building lift systems.
Key Advantages of Tube Stretch Bending Machine Over Conventional Methods
| Performance Factor | Tube Stretch Bending Machine | Roll Bending | Press Bending |
|---|---|---|---|
| Springback after forming | <0.5° (aluminum) | 3–8° | 2–6° |
| Cross-section distortion | Minimal (<2%) | Moderate (3–8%) | High (5–15%) |
| Wrinkling risk (inner radius) | None (tension prevents) | Possible on thin walls | Present on thin sections |
| Max arc length per operation | Up to 12 m | 3–6 m typical | Limited by press width |
| Material utilization | >95% | 85–92% | 80–90% |
Materials Suitable for Tube Stretch Bending Machine
Tube Stretch Bending Machine is applicable to all metallic materials with sufficient ductility to withstand the pre-tensioning and wrap-forming without fracture. Material elongation at break should generally exceed 8–10% for reliable Tube Stretch Bending Machine results:
- Aluminum extrusions (6061-T4, 6063-T4, 7075-T0): The most common Tube Stretch Bending Machine material. The T4 or annealed (T0) temper provides the ductility needed for Tube Stretch Bending Machine; the part may be aged post-bending to recover T6 strength properties.
- Carbon steel and HSLA steel profiles: H-beams, channel steel, and hollow sections in grades up to S355 are routinely stretch bent for structural applications. Higher-strength grades (S460+) require higher tensile forces and more careful ductility management.
- Stainless steel profiles: Used in architectural and food-processing applications. The higher work-hardening rate of stainless steel is actually beneficial in Tube Stretch Bending Machine—it increases yield strength during forming, improving shape retention.
- Copper and brass extrusions: Applied in architectural decorative elements and electrical bus bars. Excellent ductility allows large Tube Stretch Bending Machine deformations without fracture.
Major Industry Applications
High-Speed Rail and Metro Structures
Train car roof arches, window frame extrusions, and side wall structural members are stretch bent from aluminum or stainless steel profiles to the precise curvatures of the vehicle body. Each component must match its mating parts with dimensional tolerances of ±0.5 mm over a 15-meter arc length—achievable only through Tube Stretch Bending Machine's minimal springback characteristics.
Automotive Structural Extrusions
Body-in-white structural members including door sill beams, A-pillar reinforcements, roof rails, and bumper beams are stretch bent from high-strength aluminum extrusions (6xxx or 7xxx series) to match the vehicle's body curve. The post-bending aging treatment restores full T6 temper strength, providing the structural performance required for crash energy management.
Airports and Stadiums
Large-span curved steel structures in airport terminal roofs, stadium canopies, and exhibition hall frames require the bending of heavy steel hollow sections and H-beams to specific radii. Tube Stretch Bending Machine of sections up to HEA 300 or RHS 300×150 to radii of 10–50 meters is standard practice for these architectural applications.
Curtain Wall and Façade Systems
Architectural aluminum curtain wall profiles—T-sections, U-sections, and custom extrusions—are stretch bent to form the curved vertical and horizontal grid members of building façades and atrium roofs. Precise radius control ensures uniform glazing joint widths across the full façade height, a critical aesthetic and weatherproofing requirement.
Common Questions About Tube Stretch Bending Machine
What is the minimum bend radius achievable with Tube Stretch Bending Machine?
Tube Stretch Bending Machine is optimized for large-radius, gentle curvature applications (radius-to-section-height ratios typically greater than 5:1). For tighter radii where the outer fiber elongation would exceed the material's fracture limit, conventional rotary draw bending or press bending is more appropriate. Aluminum extrusions can typically achieve R/H ratios (bend radius to section height) as low as 3:1–5:1 through Tube Stretch Bending Machine; steel sections require R/H above 10:1 in most cases.
Does Tube Stretch Bending Machine affect the mechanical properties of the profile?
The pre-tensioning and wrap-forming in Tube Stretch Bending Machine introduce work hardening throughout the cross-section, which typically increases yield strength by 5–15% and slightly reduces ductility. For aluminum alloys in T4 temper, the part can be artificially aged after bending to achieve T6 mechanical properties. For steel, the work hardening is generally beneficial or negligible for structural applications.
Can non-symmetric profiles such as H-beams and I-beams be stretch bent without twisting?
Yes, provided the profile is bent about its strong axis (bending the web) and the jaw chucks grip the profile symmetrically to prevent eccentric loading. Anti-twist guide fixtures mounted close to the die contact zone are often used for asymmetric or complex profiles. Modern Tube Stretch Bending Machines include hydraulic clamping torque compensation that actively counteracts any tendency for the profile to rotate during bending.
How is the forming die designed for a Tube Stretch Bending Machine application?
The forming die (also called the bending form or stretch block) is machined to the target bend radius, with the die face profiled to match the cross-section of the workpiece. For simple circular arcs, the die is a segment of a cylinder. For compound curves or non-circular arcs, the die profile may be CNC-machined to match a point-cloud or spline curve from the architectural design. Die materials are typically cast iron, aluminum alloy, or hardwood (for short-run prototype work), and are usually custom-made for each profile and radius combination.
