Pipe-rotating stretch-bending forming apparatus and method

Abstract

The application discloses a pipe rotating stretch-bending forming device and method, and belongs to the field of mechanical processing. The device comprises two sets of symmetrical rotating stretch-bending mechanisms installed on a workbench, a bending die located in the middle of the workbench, a hydraulic system and a numerical control system. The rotating stretch-bending mechanism is driven by a motor to clamp and rotate the end of the pipe by a chuck, and a steering mechanism is arranged at the bottom of the rotating stretch-bending mechanism and can be deflected during the stretch-bending process. The bending die is driven by a hydraulic cylinder to feed radially. The method comprises the following steps: clamping the pipe; after the pipe is clamped and the bending die is loaded radially to make the pipe adhere to the die, the two motors are started to rotate synchronously and in the same direction to drive the pipe to rotate around its own axis; and the pipe is unloaded and reset. According to the application, the pipe is introduced to rotate during the stretch-bending process, the cross-section stress distribution is homogenized, the cross-section distortion is effectively inhibited, the springback is significantly reduced, the forming precision and the mechanical properties of the component are improved, and the application is suitable for the high-quality bending forming of various pipes.

Description

Technical Field

[0001] This invention relates to the field of machining, and in particular to a device and method for rotary bending forming of pipe fittings. Background Technology

[0002] Tube bending is a key process widely used in aerospace, automotive manufacturing, furniture decoration, and other fields. Stretch bending, as a common tube bending method, offers the advantage of high forming accuracy. However, traditional stretch bending processes still have the following three prominent problems: First, severe cross-sectional distortion occurs. During traditional tension bending, the outer side of the pipe is under tension and the inner side under compression, resulting in extremely uneven stress distribution across the cross-section. This leads to distortion phenomena such as ellipticization, wrinkling, and even collapse of the pipe cross-section. Especially under thin-walled pipes or large bending angles, cross-sectional distortion becomes difficult to control, seriously affecting the appearance quality of the formed parts and subsequent assembly performance.

[0003] Secondly, the springback is significant, resulting in low forming accuracy. After bending and unloading, the internal elastic strain of the pipe recovers, causing a large springback that leads to deviations in the actual bending angle and shape from the target value. To compensate for the springback, repeated trial molding or additional calibration processes are usually required, which not only prolongs the production cycle but also increases manufacturing costs. For complex-shaped pipes, the springback problem is particularly prominent, making it difficult to meet high-precision forming requirements.

[0004] Third, uneven residual stress distribution leads to decreased mechanical properties. Traditional tension bending processes create unfavorable residual stress fields inside the pipe fittings, with significant local stress concentrations. This uneven residual stress reduces the fatigue life and resistance to alternating loads of the components. Especially in fields with extremely high safety requirements, such as aerospace and automotive, this problem has become a key bottleneck restricting the application of pipe bending components.

[0005] To address the aforementioned problems, existing technologies have attempted methods such as heated bending, filling with media, and using anti-wrinkle blocks. However, these methods often suffer from drawbacks such as complex processes, high costs, limited applicability, or incomplete improvement. Therefore, there is an urgent need for a novel forming process and apparatus that can fundamentally improve the stress-strain state during pipe bending. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a tube rotation stretch bending forming device and method, which is applicable to the stretch bending forming of various tubes. By designing a tube rotation device and applying it in stretch bending, tubes and solid or hollow optical shafts can be bent. The equipment is simple. Based on the traditional stretch bending, an axial rotation process is introduced. By coordinating the motion relationship between bending and rotation, the stress distribution of the cross section is effectively improved, the cross section distortion and springback are significantly reduced, and the residual stress distribution is made more uniform. The dimensional accuracy, shape consistency and mechanical properties of the formed parts are all improved.

[0007] Specifically, on the one hand, the present invention provides a pipe fitting rotary bending device, including two sets of rotary bending mechanisms installed on a workbench, a bending mold located in the middle of the workbench, a hydraulic system for providing power, and a CNC system for control and command; Each rotary bending mechanism includes a chuck, a bearing housing assembly, a coupling, a motor, a motor base plate, and a steering mechanism. The motor is connected to the chuck via the coupling, and the bearing housing assembly is fixed to the motor base plate to support the connection between the coupling and the chuck. The bottom of the motor base plate is equipped with a slider that can slide along a first linear guide rail fixed to a flat plate. The steering mechanism is located at the bottom of the rotary bending mechanism and in the circular through holes on both sides of the worktable, allowing the rotary bending mechanism to deflect during the rotary bending process. The bending die is fixedly mounted on the bending die base plate, which can slide along the second linear guide rail fixed on the worktable under the drive of the second hydraulic cylinder; the bending die is driven by the second hydraulic cylinder to feed radially along the clamped pipe, so that the bending die can apply a radial bending load to the pipe to shape it. The motors of the hydraulic system and the two sets of rotary bending mechanisms are all connected to the CNC system.

[0008] Furthermore, the working surface where the bending die contacts the pipe fitting is a concave arc surface that matches the outer diameter of the pipe fitting.

[0009] Furthermore, the steering mechanism includes a slewing bearing located at the bottom of the plate, and the center of the lower part of the plate is hinged to a circular through hole opened on the worktable through the steering mechanism.

[0010] Furthermore, the CNC system is configured to control the drive motors on both sides to rotate synchronously in the same direction and at the same speed.

[0011] Furthermore, it also includes a first hydraulic cylinder set on both sides of the plate. The first hydraulic cylinder is fixed to the plate by bolts, and the piston rod head of the first hydraulic cylinder is fixed to the base plate of the motor by a buckle, for driving the base plate of the motor to slide along the first linear guide rail.

[0012] Furthermore, it also includes an inner support assembly, which is inserted into the inner diameter of the pipe and expands outward to support the inner wall of the pipe.

[0013] Furthermore, the first linear guide and the second linear guide are respectively fixedly connected to the plate or worktable by threads, and the guides are provided with a number of threaded holes at intervals.

[0014] On the other hand, the present invention provides a method for forming a pipe by rotation and bending, comprising the following steps: Step 1: clamping; adjust the two chucks to be parallel and place the two end faces vertically, with the two axes coinciding, and place the two ends of the pipe to be clamped into the two chucks respectively and clamp them; Step 2: Bending Loading and Rotary Bending; First, the second hydraulic cylinder drives the bending die to apply a radial load to the clamped pipe, causing it to bend and deform and gradually conform to the arc surface of the bending die. The bending of the pipe causes the chuck and the rotary bending mechanism to deflect under the action of the steering mechanism. When the pipe is completely in contact with the bending die, the loading of the second hydraulic cylinder is stopped, and then the two motors are started to rotate synchronously in the same direction at the same speed, causing the pipe to rotate around its own axis. Step 3: Unloading and Resetting; After the bent pipe has rotated a certain number of times, the motor stops rotating, the bending die is controlled to unload the radial load and retract, then the chuck is released to remove the formed part, and the first hydraulic cylinder is controlled to drive the motor base plate to slide along the first linear guide rail. At the same time, the steering mechanism rotates back, so that the two rotating bending mechanisms on both sides return to the initial parallel alignment position.

[0015] Preferably, for hollow pipe fittings, before clamping in step one, the inner support assembly is first inserted into the inner diameter of the pipe fitting and then spread outwards so that the hollow pipe fitting tightly supports the inner wall of the pipe fitting around its perimeter, and then the chuck is used for clamping.

[0016] Preferably, in step two, the rotational angular velocity of the motor is a constant value, and the radial loading speed of the bending mold and the rotational speed of the motor are linked and controlled by a CNC system.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention provides a rotary bending forming apparatus and method for pipe fittings, which can significantly suppress cross-sectional distortion. At the apparatus level, by setting up two symmetrical rotary bending mechanisms, each mechanism includes a chuck driven by a motor and a deflectable steering mechanism, allowing the pipe fitting to actively rotate around its own axis during bending. At the method level, by first radially loading the pipe fitting to fit the mold, and then initiating synchronous rotation in the same direction, the material points on the pipe fitting cross-section are periodically alternately subjected to tension and compression, transforming the fixed stress distribution in traditional bending into a dynamically uniform stress field. Compared with the defects of traditional bending in the prior art, where cross-sectional distortion is caused by stress gradients, this invention fundamentally improves the stress state of the cross-section, effectively suppressing ellipticization, wrinkling, and collapse, significantly improving the shape retention of the formed pipe fitting, and is particularly suitable for thin-walled pipe fittings and large bending angle forming.

[0018] This invention significantly reduces springback and improves forming accuracy through the synergistic effect of the device and method. Regarding the device, a CNC system precisely controls the synchronous, unidirectional, and constant-speed rotation of the motors on both sides, and links this rotation with the radial loading of the bending die. Simultaneously, the steering mechanism allows the chuck to deflect freely during bending, avoiding additional elastic deformation caused by additional constraints. Regarding the method, rotation is initiated after the tube is fully in contact with the die, causing the tube to experience both bending moment and torque simultaneously. This results in bending moments in two directions on the cross-section, thereby altering the residual curvature distribution after springback.

[0019] The apparatus and method provided by this invention can homogenize the distribution of residual stress, effectively improving the mechanical properties of components. In terms of the apparatus, the internal support assembly can be inserted into the inner diameter of the pipe and expands outwards, providing uniform support to the pipe end during chuck clamping, preventing end collapse, and enhancing the stability of torque transmission. In terms of the method, the rotary tension bending process ensures that each material point on the cross-section undergoes multiple tensile-compressive stress cycles during the plastic deformation of the pipe, promoting more uniform plastic deformation and thus forming a more uniformly distributed residual stress field circumferentially after unloading. Attached Figure Description

[0020] Figure 1 This is a front view of the rotary bending device of the pipe fitting rotary bending forming device of the present invention; Figure 2 This is an isometric view of the rotary bending device of the pipe fitting rotary bending forming device of the present invention; Figure 3 This is a simplified schematic diagram of the pipe fitting rotary bending forming device of the present invention after clamping. Figure 4 This is a simplified schematic diagram of the pipe fitting film application in the pipe fitting rotary bending forming device of the present invention; Figure 5 This is a simplified schematic diagram of the pipe fitting rotation bending forming device of the present invention, showing the pipe fitting starting to rotate after the film is applied. Figure 6 This is a simplified schematic diagram of the pipe fitting rotary bending forming device of the present invention after unloading; Figure 7 This is a simplified side view of the inner support component provided for the pipe fitting in the pipe fitting rotary bending forming device of the present invention; Figure 8 This is a simplified cross-sectional view of the inner support component provided for the pipe fitting in the pipe fitting rotary bending forming device of the present invention; Figure 9 This is a flowchart of the pipe rotation bending forming method of the present invention.

[0021] Key reference numerals: 1-Bent pipe fitting, 2-Chuck, 3-Coupling, 4-Bearing housing assembly, 5-Motor, 6-Motor base plate, 7-Snap fastener, 8-First linear guide rail, 9-First hydraulic cylinder, 10-Plate, 11-Workbench, 12-Steering mechanism, 13-Bending mold, 14-Second hydraulic cylinder, 15-Base plate, 16-Second linear guide rail, 17-Inner support assembly. Detailed Implementation

[0022] The following will provide a detailed description in conjunction with the accompanying drawings.

[0023] like Figure 1 and Figure 2As shown, a pipe fitting rotary bending forming device includes: two sets of rotary bending mechanisms symmetrically mounted on a worktable 11, a bending mold 13 located in the middle of the worktable 11, a hydraulic system for providing power, and a CNC system for control and command.

[0024] Each set of rotary bending mechanism includes: chuck 2, bearing housing assembly 4, coupling 3, motor 5, motor base plate 6, first linear guide rail 8, first hydraulic cylinder 9, plate 10, and steering mechanism 12.

[0025] Chuck 2 is used to clamp the end of the bent pipe fitting 1. Chuck 2 is a three-jaw or four-jaw self-centering chuck, which can be clamped and released manually or hydraulically.

[0026] The motor 5 is fixedly mounted on the motor base plate 6. The output shaft of the motor 5 is connected to the rotating shaft of the chuck 2 via a coupling 3. A bearing housing assembly 4 is provided between the coupling 3 and the chuck 2. This bearing housing assembly 4 is fixed on the motor base plate 6 and is used to support the rotating shaft of the chuck 2 to ensure rotational accuracy and rigidity.

[0027] A slider is installed at the bottom of the motor base plate 6. A linear guide groove is formed on the upper surface of the plate 10, and the first linear guide 8 is fixedly installed on the plate 10 by a threaded connection. The slider at the bottom of the motor base plate 6 slides with the first linear guide 8, so that the motor base plate 6 can perform linear reciprocating motion along the first linear guide 8.

[0028] The first hydraulic cylinder 9 is symmetrically arranged on the left and right sides, and its cylinder body is fixedly connected to the plate 10 by bolts. The piston rod head of the first hydraulic cylinder 9 is fixedly connected to the motor base plate 6 by a buckle 7. When the piston rod of the first hydraulic cylinder 9 extends or retracts, it can drive the motor base plate 6 to slide along the first linear guide rail 8, thereby adjusting the axial position of the chuck 2 and realizing the axial stretching or unloading of the pipe fitting 1.

[0029] A steering mechanism 12 is located at the lower center of the plate 10. The steering mechanism 12 is installed in the circular through holes on the left and right sides of the worktable 11. The steering mechanism 12 can adopt a cross roller slewing bearing or a hinge structure with a vertical pivot, allowing the plate 10 and the components it carries, such as the motor 5 and chuck 2, to freely deflect around the hinge point in the horizontal plane. After the tube 1 and the bending mold 13 have finished applying the film, the CNC system controls and restricts the steering mechanism's rotation. The steering mechanism 12 can also actively rotate back to its original position hydraulically or electrically after unloading.

[0030] The bending die 13 is located in the middle of the worktable 11. The bending die 13 is fixedly mounted on the bending die base plate 15 by bolts. The working surface of the bending die 13 that contacts the pipe fitting 1 is a concave arc surface that matches the outer diameter of the pipe fitting 1 to provide uniform radial pressure and prevent local indentation.

[0031] A slider is installed at the bottom of the bending die base plate 15. A second linear guide rail 16 is fixed on the worktable 11. The slider at the bottom of the bending die base plate 15 slides in conjunction with the second linear guide rail 16, allowing the bending die base plate 15 to move linearly along the second linear guide rail 16. The extension direction of the second linear guide rail 16 is perpendicular to the initial axis direction of the pipe 1, i.e., the radial direction.

[0032] The cylinder body of the second hydraulic cylinder 14 is fixedly connected to the worktable 11, and the piston rod head of the second hydraulic cylinder 14 is connected to the base plate 15 of the bending die. When the piston rod of the second hydraulic cylinder 14 extends, it drives the bending die 13 to feed radially toward the pipe 1 and apply a bending load; when the piston rod retracts, the bending die 13 disengages from the pipe 1.

[0033] The hydraulic system provides power for the clamping actions of the first hydraulic cylinder 9, the second hydraulic cylinder 14, and the chuck 2. The CNC system is electrically connected to the control valve group of the hydraulic system, the driver of the motor 5, and various sensors such as displacement sensors and force sensors. The CNC system is configured to: control the synchronous, same-direction, and constant-speed rotation of the two motors 5; control the loading and unloading of the first hydraulic cylinder 9 and the second hydraulic cylinder 14; control the deflection and reset of the steering mechanism 12; and coordinate the timing of each action.

[0034] like Figure 3 As shown, the present invention has two chucks 2, both with their end faces vertically parallel and their axes coinciding. At the same distance outside the axes of the two chucks 2, there is a reference point that can perform a certain horizontal displacement. Adjust the size of the clamping diameter of the chucks 2 so that the clamping diameter is larger than the diameter of the pipe to be bent 1, and then place both ends of the pipe to be bent 1 into the two chucks and clamp them.

[0035] like Figure 4 As shown, the bending die 13 located in its radial direction, driven by the second hydraulic cylinder 14, applies a radial load to the rotating pipe 2 at a constant speed until the curvature of the pipe 2 completely conforms to and matches the theoretical surface of the bending die 13, at which point the bending die 13 stops radial loading.

[0036] like Figure 5 As shown, during the loading process, the bent pipe component 1, driven by the left and right motors 5, begins to rotate continuously and synchronously in the same direction around its own axis at an angular velocity of 6.28 rad / s. At the same time, the chuck 2 maintains the same magnitude and direction of movement around its own axis as during the loading process.

[0037] like Figure 6As shown, after the bent pipe 1 rotates a certain number of times, the motor 4 stops rotating, controls the bending die 13 to unload the radial load and retract, and then controls the chucks 2 on both sides to release to remove the formed part; after the part is removed, the first hydraulic cylinder 9 is controlled to slide the motor base plate 6 along the first linear guide rail 8, and at the same time the steering mechanism 12 rotates back, so that the rotating stretching mechanism on both sides is restored to the initial parallel alignment position, preparing for the next work cycle.

[0038] like Figure 7 and Figure 8 As shown, for thin-walled pipe fittings or hollow shaft parts, the present invention may further include an inner support assembly 17. This inner support assembly 17 can be inserted into the inner diameter of the pipe fitting 1 and expanded outwards by mechanical expansion such as wedge blocks, threaded expansion, or hydraulic expansion, so that it tightly supports the inner wall of the pipe fitting from all sides. The inner support assembly 17 provides uniform support force to the pipe end when clamped by the chuck 2, preventing end collapse and distortion, and enhancing clamping stability and torque transmission capability.

[0039] This invention also provides a method for forming pipe fittings by rotational stretch bending, such as... Figure 9 As shown, it includes the following steps: Step 1: Clamping preparation.

[0040] Initial state confirmation: The end faces of the two chucks 2 are parallel and vertically placed, and the axes of the two chucks coincide. The left and right rotating bending mechanisms are in the initial centering position through the steering mechanism 12, the first hydraulic cylinder 9 is in the retracted state, and the bending die 13 is in the retracted state away from the initial position of the pipe fitting.

[0041] Chuck opening adjustment: Adjust the radial position of the jaws of chuck 2 according to the outer diameter of the pipe fitting 1, so that the clamping diameter enclosed by the jaws is slightly larger than the outer diameter of the pipe fitting, that is, the clamping diameter is usually 0.5~2mm larger than the outer diameter of the pipe fitting, so that the end of the pipe fitting can be smoothly inserted.

[0042] Pipe placement: Insert both ends of the pipe to be bent 1 between the jaws of the two chucks 2 from the left and right sides respectively, and make the midpoint of the pipe in the length direction roughly aligned with the center of the bending die 13.

[0043] Clamping fittings: For solid bars or thick-walled fittings, the chuck 2 can be directly driven to clamp the end of the fitting; for thin-walled fittings or hollow shafts, before clamping, the inner support assembly 17 is inserted into the inner holes at both ends of the fitting, and the inner support assembly is operated to expand outward, so that it fits tightly against the inner wall of the fitting, and then the chuck 2 is driven to clamp. The inner support assembly can prevent the clamping force from causing the pipe end to collapse or elliptical deformation, while improving the torque transmission capacity.

[0044] Step 2: Bending Loading.

[0045] Radial feed bending: The CNC system issues a command, the second hydraulic cylinder 14 is activated, its piston rod extends, driving the bending die base plate 15 to move at a constant speed along the second linear guide rail 16 toward the pipe 1. The concave arc surface of the bending die 13 first contacts the outer wall of the pipe 1, and then applies a radial bending load to the pipe.

[0046] Pipe bending deformation: As the bending die 13 feeds radially, the middle of the pipe 1 undergoes bending deformation, and the bending angle gradually increases. During this process, the chucks 2 at both ends of the pipe are passively deflected with the permission of the steering mechanism 12, always keeping the chuck end face perpendicular to the axis of the pipe end, thus avoiding the generation of additional bending moment. The deflection angle is determined by the bending curvature of the pipe, and the steering mechanism 12 provides low-damping rotation without applying active torque.

[0047] Mold Fitting Judgment: When the bending die 13 is fed to a preset radial displacement (which corresponds to the die position required for the target bending angle), the contact state between the pipe and the die is detected by a displacement sensor or force sensor. When the bent portion of the pipe 1 completely fits the concave arc surface of the bending die 13 and the contact force reaches a stable value, the CNC system determines that the mold fitting is complete. At this time, the second hydraulic cylinder 14 stops feeding and maintains a locked position to sustain the bending load.

[0048] Step 3: Rotate and bend. Initiating rotation: With the pipe fully coated with the mold (the CNC system determines full coating based on sensors on both sides of the bending mold) and the bending mold held in a compressed state, the CNC system simultaneously sends a start command to both motors 5. Motors 5 accelerate to a preset constant angular velocity with the same angular acceleration and maintain synchronous rotation in the same direction. The rotation direction can be clockwise or counterclockwise, selected according to process requirements.

[0049] Stress homogenization during rotation: When pipe 1 rotates around its own axis, material points on its cross-section that were originally in the outer tension zone move into the inner compression zone, and vice versa. This periodic stress change causes plastic deformation to be evenly distributed throughout the entire cross-section. At the same time, since the bending die 13 still maintains radial pressure, the pipe continuously undergoes slight relative sliding with the die's arc surface during rotation, further promoting die contact and stress release.

[0050] Rotation count and time control: The number of rotations is preset by the CNC system based on parameters such as pipe material, wall thickness, and bending angle.

[0051] Typically, the number of rotations ranges from 3 to 10. The rotational speed (angular velocity) is related to the pipe diameter and the material yield strength, and is generally controlled between 0.5 and 10 rad / s. The CNC system monitors the motor encoder feedback in real time to ensure that the synchronization error on both sides does not exceed the set threshold (e.g., 0.1%).

[0052] Axial force maintenance during rotation (optional): During rotation, the first hydraulic cylinder 9 can continue to maintain a set axial tensile force to further improve the stress distribution of the cross-section. The direction of the axial force is perpendicular to the bending direction, and the superposition of the two can create a more complex stress state within the pipe, which is beneficial for refining grains and reducing residual stress.

[0053] Step 4: Unload and reset.

[0054] Stop rotation: After the pipe 1 completes the preset number of rotations, the CNC system issues a command, and the two motors 5 on both sides decelerate synchronously to a stop. The deceleration process should be smooth to avoid inertial impact that could cause scratches between the pipe and the mold.

[0055] Relieving radial load: The piston rod of the second hydraulic cylinder 14 slowly retracts, and the bending die 13 detaches from the surface of the pipe fitting 1. The retraction speed should not be too fast to avoid impact caused by the elastic rebound of the pipe fitting.

[0056] Release the chuck: The jaws of chuck 2 release the clamping force on the end of the pipe fitting. If the internal support assembly 17 is used, retract the internal support assembly before removing the pipe fitting.

[0057] Remove parts: Operators or robots remove the formed tubes from the device and check the surface quality and bending angle.

[0058] Reset of the rotary bending mechanism: The CNC system controls the piston rod of the first hydraulic cylinder 9 to retract or extend (depending on the initial position), driving the motor base plate 6 to slide along the first linear guide rail 8, so that the left and right rotary bending mechanisms return to the initial axial spacing. At the same time, the steering mechanism 12 actively rotates under the action of the reset drive (such as a hydraulic motor or spring), so that the plate 10 and the chuck 2 on it return to the initial parallel alignment state (parallel end faces and coincident axes).

[0059] Preparation for the next cycle: Check whether each component of the device has returned to its initial position. Once confirmed to be correct, the clamping and forming of the next pipe fitting can proceed.

[0060] Optional steps: Operational details of the internal support components.

[0061] For thin-walled pipe fittings (wall thickness to outer diameter ratio less than 0.1), it is recommended to perform the following sub-steps before clamping in step one: After the inner support assembly 17 is fully retracted, it is inserted into the inner hole at the end of the pipe fitting, so that the midpoint of the assembly in the length direction is aligned with the chuck clamping position.

[0062] The inner support assembly 17 is linked with the CNC system and adopts an automatic operation mode. The control method is as follows: the expansion and contraction of the inner support assembly is driven by the CNC system (such as electric push rod or hydraulic actuator), and the expansion mechanism of the inner support assembly (such as rotating screw, hydraulic push wedge, etc.) is automatically driven to expand it radially outward until it generates uniform contact pressure with the inner wall of the tube (usually controlled at 1~5 MPa), which facilitates its automated production.

[0063] After confirming that the internal support assembly has reliably supported the pipe wall, clamp the pipe with the chuck.

[0064] During the rotation and bending process in step three, the inner support assembly rotates together with the pipe (if there is no relative sliding between the inner support assembly and the chuck or pipe).

[0065] After unloading, first shrink the internal support assembly, then remove the pipe fitting.

[0066] Taking a pipe fitting with a diameter of 40mm, a wall thickness of 10mm, and a length of 600mm as the bending pipe fitting 1 as an example, the mechanical properties of this pipe fitting are: elastic modulus 71GPa, plastic modulus 2000MPa, yield strength not less than 503MPa, tensile strength 572MPa, and Poisson's ratio 0.33.

[0067] The following section, in conjunction with the accompanying drawings, presents a theoretical analysis and numerical simulation verification of a pipe fitting rotary bending forming device and method according to the present invention.

[0068] In the traditional tension bending process, the pipe fitting is simultaneously subjected to the axial tension of the hydraulic cylinders on both sides and the bending moment of the bending die. The stress distribution of the pipe fitting cross section can be expressed as: ; Where F is the axial tensile force, A is the cross-sectional area, M is the bending moment, y is the distance from the neutral layer, and I is the moment of inertia of the section. This distribution results in tension on the outer side and compression on the inner side of the pipe section, forming a large stress gradient, which is an important reason for the cross-sectional distortion.

[0069] In the rotary bending process of this invention, the pipe fitting rotates around its own axis at an angular velocity ω while bearing the axial tension of the hydraulic cylinders on both sides and the bending moment of the bending die. According to elastoplastic mechanics, its stress state can be expressed as: ; in, The circumferential position angle of the cross section. Let be time. Comparing equations (1) and (2), it can be seen that the stress distribution in traditional tension bending depends only on the axial tension, bending moment, and cross-sectional position, while the stress distribution in rotational tension bending is a function of the axial tension, bending moment, rotational angular velocity, time, and cross-sectional position. Rotation causes the stress at each point on the cross-section to change periodically with time and position, making the stress distribution of the entire cross-section tend to be uniform.

[0070] In traditional tension bending processes, only one bending moment exists within the workpiece cross-section. After rotation is applied, two bending moments exist within the cross-section. The curvature after springback is then: ; in, , These are the residual curvatures after springback in the yoz and xoz planes, respectively.

[0071] The final residual curvature is the vector sum of the residual curvatures in the two planes mentioned above, and the expression for the radius of curvature after springback is: ; in, The radius of curvature after springback. The curvature after springback.

[0072] Because there are more Therefore, the radius of curvature after springback... Smaller size significantly reduces rebound.

[0073] A three-dimensional elastoplastic finite element model was established using ABAQUS software for simulation verification. The pipe fitting was simplified as an elastoplastic beam, the chuck clamping end was simplified as a constrained end that could rotate around its own axis and freely deflect, providing axial tension, and the bending die was simplified as a concentrated force F moving radially along the pipe fitting. The workpiece was discretized as a deformable body with a mesh type of C3D8I non-conforming mode elements. The bending die was discretized as a rigid body.

[0074] Two sets of comparison conditions were set up: Condition A (Traditional stretch bending): The pipe does not rotate, the bending die is fed radially, and the first hydraulic cylinders on both sides apply axial load; Condition B (Rotary stretch bending of this invention): The bending die is fed radially, the first hydraulic cylinders on both sides apply axial load, and after the pipe is coated with film, the motor is started to rotate the pipe. The bending stroke of the bending die is the same in both conditions.

[0075] Analysis of springback simulation results: Simulations were performed in ABAQUS for the above-mentioned working conditions, generating Mises stress distribution cloud maps for the springback of the pipe fitting after bending under two sets of conditions. Under the traditional tension bending process, the stress distribution of the cross-section exhibits a significant stress gradient, with a large stress difference between the outer (tension zone) and inner (compression zone). Under the rotary tension bending process of this invention, the stress distribution of the cross-section is significantly homogenized. According to the ABAQUS finite element simulation results, under the same loading conditions (target bending angle 65.53°), after unloading, working condition A springs back to 60.46° with a springback angle of 5.07°; while working condition B using the rotary tension bending process of this invention springs back to 63.94° with a springback angle of 1.59°. The comparison shows that the rotary process reduces the springback by approximately 68.6%, verifying the effectiveness of this invention in dynamically homogenizing the cross-sectional stress and significantly reducing springback.

[0076] Cross sections at the same location in the ABAQUS springback models under two different working conditions were selected, and the nodal coordinates of the cross sections were extracted. Verification was performed using MATLAB spatial circle fitting analysis. The results showed that the relative roundness of the clamping end cross section of the pipe fitting after adopting the rotary bending process of this invention decreased from 0.4507% in the non-rotational case to 0.3715%, an improvement of approximately 17.6% in roundness accuracy; the flatness improved from 0.0201 mm to 0.0174 mm, an improvement of approximately 13.4%. This indicates that the pipe fitting exhibits better cross-sectional retention and higher end-face flatness during rotary bending, effectively suppressing cross-sectional distortion.

[0077] In summary, this invention, by introducing rotation of the pipe during the bending process, uniformizes the stress distribution across the cross section, effectively suppresses cross section distortion, significantly reduces springback, and improves forming accuracy and mechanical properties of the component. It is suitable for high-quality bending forming of various types of pipes.

[0078] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A device for rotary bending and forming of pipe fittings, characterized in that: It includes two sets of rotary bending mechanisms installed on the workbench (11), a bending die (13) located in the middle of the workbench (11), a hydraulic system for providing power, and a numerical control system for control and command; Each rotary bending mechanism includes a chuck (2), a bearing housing assembly (4), a coupling (3), a motor (5), a motor base plate (6), and a steering mechanism (12); the motor (5) is connected to the chuck (2) through the coupling (3), and the bearing housing assembly (4) is fixed on the motor base plate (6) to support the connection between the coupling (3) and the chuck (2); the bottom of the motor base plate (6) is provided with a slider, which can slide along the first linear guide rail (8) fixed on the plate (10); the steering mechanism (12) is set at the bottom of the rotary bending mechanism and in the circular through holes opened on both sides of the worktable (11), which is used to allow the rotary bending mechanism to deflect during the rotary bending process; The bending die (13) is fixedly installed on the bending die base plate (15), which can slide along the second linear guide rail (16) fixed on the worktable (11) under the drive of the second hydraulic cylinder (14); the bending die (13) is driven by the second hydraulic cylinder (14) to feed radially along the clamped pipe (1), so that the bending die (13) can apply a radial bending load to the pipe (1) to shape it; The electric motors (5) of the hydraulic system and the two sets of rotary bending mechanisms are all connected to the CNC system.

2. The pipe fitting rotary bending forming device according to claim 1, characterized in that: The working surface of the bending die (13) that contacts the pipe fitting (1) is a concave arc surface that matches the outer diameter of the pipe fitting (1).

3. The pipe fitting rotary bending forming device according to claim 1, characterized in that: The steering mechanism (12) includes a slewing bearing located at the bottom of the plate (10), and the center of the lower part of the plate (10) is hinged to a circular through hole on the worktable (11) through the steering mechanism (12).

4. The pipe fitting rotary bending forming device according to claim 1, characterized in that, The CNC system is configured to control the drive motors (5) on both sides to rotate synchronously, in the same direction and at the same speed.

5. The pipe fitting rotary bending forming device according to claim 1, characterized in that: It also includes a first hydraulic cylinder (9) set on both sides of the plate. The first hydraulic cylinder (9) is fixed to the plate (10) by bolts. The piston rod head of the first hydraulic cylinder (9) is fixed on the motor base plate (6) by buckle (7) and is used to drive the motor base plate (6) to slide along the first linear guide rail (8).

6. The pipe fitting rotary bending forming device according to claim 1, characterized in that, It also includes an inner support assembly (17), which is inserted into the inner diameter of the pipe fitting (1) and spreads outward to support the inner wall of the pipe fitting.

7. The pipe fitting rotary bending forming device according to claim 1, characterized in that, The first linear guide (8) and the second linear guide (16) are fixedly connected to the plate (10) or the worktable (11) by threads, and a number of threaded holes are provided on the guide rails at intervals.

8. A method for forming a pipe fitting by rotational stretch bending, characterized in that: It includes the following steps: Step 1: Clamping; Adjust the two chucks (2) to be parallel and place them vertically with their two ends aligned, and place the two ends of the clamped pipe fitting (1) into the two chucks (2) respectively and clamp them; Step 2: Bending loading and rotational bending; First, the second hydraulic cylinder (14) drives the bending mold (13) to apply a radial load to the clamped pipe (1), causing it to bend and deform and gradually conform to the arc surface of the bending mold (13). The bending of the pipe (1) causes the chuck (2) and the rotational bending mechanism to deflect under the action of the steering mechanism (12); When the pipe (1) and the bending mold (13) are completely in contact, the loading of the second hydraulic cylinder (14) is stopped, and then the two motors (5) are started to rotate synchronously in the same direction at the same speed, causing the pipe (1) to rotate around its own axis. Step 3: Unloading and Resetting; After the bent pipe (1) rotates a certain number of times, the motor (5) stops rotating, controls the bending mold (13) to unload the radial load and retract, then releases the chuck (2) to take out the formed part, and then controls the first hydraulic cylinder (9) to drive the motor base plate (6) to slide along the first linear guide rail (8), while the steering mechanism (12) rotates back, so that the two sides of the rotating bending mechanism return to the initial parallel alignment position.

9. The method for forming pipe fittings by rotational stretch bending according to claim 8, characterized in that: For hollow pipe fittings, before clamping in step one, first insert the inner support assembly (17) into the inner diameter of the pipe fitting and spread it outward so that the hollow pipe fitting tightly supports the inner wall of the pipe fitting around its perimeter, and then clamp it with the chuck (2).

10. The method for forming pipe fittings by rotational stretch bending according to claim 8, characterized in that: In step two, the rotational angular velocity of the motor (5) is constant, and the radial loading speed of the bending mold (13) and the rotational speed of the motor (5) are linked and controlled by the CNC system.

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