The manufacturing technology for the segmented portal frames of large bucket-wheel reclaimers primarily covers the introduction and analysis of manufacturing challenges, the fabrication plan, and the machining plan. Details are as follows:
Introduction and Challenge Analysis of Segmented Portal Frames
Portal Frame Assembly Composition: The assembly primarily consists of the portal frame itself (divided into three segments connected by high-strength bolts), a balance beam, slewing pin/pivot assemblies and their retaining plates, rail assemblies and their retaining plates, and connection plates.
Manufacturing Challenges
High Precision Requirements for Connections: The portal frame features a circular box-girder structure with diameters of φ10,000 mm and φ7,240 mm. The segments and connection plates are circular; each connection interface contains 600 bolt holes corresponding to 600 sets of ASTM-standard high-strength bolts, totaling 1,800 sets across the three joints. Post-installation, the clearance between the bolt and the hole wall must not exceed 1 mm. Controlling deformation in the circular box structure after segmentation is difficult; furthermore, long-distance transport makes it impossible to predict deformation at the joints, complicating final on-site assembly.
Strict Tolerances for Key Dimensions: The portal frame supports the upper slewing mechanism of the bucket-wheel reclaimer and connects downward to the travel mechanism (gantry). The tolerance for the clear span between slewing pin holes must be 0 to -1.78 mm, while the tolerance for the φ120 mm pin holes themselves is 0 to +0.035 mm (across a total of 129 holes). Tolerances for the clear span of the slewing bogie rails are -3 to +3 mm, and for the horizontal wheel tracks, -2 to +2 mm; coaxiality requirements apply to both the pin hole layout and the horizontal wheel tracks. For segmented portal frames, dimensions at key locations must be guaranteed both before segmentation and after on-site assembly, ensuring compliance with drawings and client specifications.
Portal Frame Fabrication Plan
Material Cutting
Box-Girder Bottom and Top Plates: Cuts are made at the segmentation points, ensuring these locations do not coincide with plate-splicing seams. When joining plates, adjust the gap at the segment joints to within 1 mm and secure the assembly with fixtures before welding.
Box-girder webs: Cut the material for the segments as continuous pieces, ensuring the cuts do not coincide with the assembly joints. After cutting the connecting plates (for the webs), tack-weld them to the webs and roll the assembly into shape; pre-machine the holes in the inner and outer connecting plates before rolling.
Slewing pin-gear top cover plate: Cut using the same method as the portal frame's bottom and face plates, ensuring the material is cut with a break at the segment joint locations.
Horizontal wheel rails and slewing bogie rail backing plates: Leave a 15 mm machining allowance on the thickness when cutting the horizontal wheel rails, and an 8 mm allowance for the slewing bogie rail backing plates.
Portal frame assembly and fabrication
Plate joining: Based on drawing dimensions and the process-specific cutting/joining plan, place the box-girder face plates, webs, and bottom plates onto a pre-constructed horizontal assembly jig. Weld according to the approved Welding Procedure Specification (WPS) while strictly controlling the flatness of the face plates.
Assembly and forming: Assemble and form the portal frame box girder according to standard fabrication procedures; during the forming of the three-sided box structure, position the internal connecting plates at the segment joints and secure them with fixtures.
Rail and backing plate installation: After welding and forming all four sides of the box girder, mark the layout lines for the horizontal wheel rails and slewing bogie rail backing plates. Install these components, ensuring the gaps at the segment joints meet specifications; weld the rails symmetrically to minimize welding deformation.
Measures to prevent welding deformation
Shrinkage allowance method: When cutting the webs, face plates, and bottom plates, incorporate a welding shrinkage allowance at critical locations based on on-site measurements, practical experience, and theoretical calculations.
Weld reduction method: Cut the face plates and bottom plates with breaks at the segment joints to reduce deformation caused by welding and to improve production efficiency. Vibratory Stress Relief (VSR): Utilizing the principle of resonance, an exciter applies dynamic stress to the component to reduce peak residual stresses and eliminate residual stress. This method offers simple equipment, low costs, and short processing times, while avoiding the oxidation issues associated with high-temperature tempering. Vibratory stress relief is performed after the fabrication of the portal frame box structure.
Sectional Processing: Level the portal frame and mark section lines according to the drawings. Secure the pre-machined outer connecting plates and match-drill the bolt holes at the section joints. First, match-drill the bolt holes at the joints of the upper and lower box-girder webs, securing them with assembly bolts (tightened to over 70% of rated torque), then match-drill the remaining bolt holes on the webs. After machining, remove the outer connecting plates and use semi-automatic cutting equipment to section the box-girder webs. Finally, secure all sections using connecting plates and assembly bolts before proceeding to the next stage.
Portal Frame Machining Plan
Inspection and Marking: After bolting is complete, verify key dimensions and ensure the portal frame is properly leveled. Use a laser total station to mark the portal frame's centerlines (crosshairs) and inspection lines. Using these as a reference, mark the machining allowance, machining, and inspection lines for the slewing bogie track surface; ensure overall levelness and minimum plate thickness requirements during alignment adjustments. Next, mark the machining allowance and inspection lines for the horizontal wheel support track circumference, ensuring overall track roundness and verticality. Finally, mark the locations of the pin holes and use a profile cutter to machine the pilot holes to a diameter of 90 mm. Once the slewing bogie track surface is machined to specification, mark the slewing track position lines and track clamp bolt hole locations using the slewing center as the reference.
Machining Equipment and Tooling: As the company's existing standard machine tools cannot meet the machining requirements, a specialized box-girder fixture is installed at the center of the portal frame-leveraging the frame's structural characteristics-to secure a custom-built slewing turntable assembly, ensuring the assembly's center aligns with the portal frame's slewing center. Subsequently, based on the portal frame's dimensions and specifications, a second box-girder fixture is mounted on the turntable assembly to serve as an arm for holding the machining tool; the portal frame must be leveled prior to this installation. Machining Process: Prior to machining, the rotary turntable assembly was adjusted and used to mark out lines according to the drawing specifications; these markings were then verified using Leica measuring equipment. Once the accuracy of the layout dimensions was confirmed, the cutting tools were installed to sequentially machine the inner surface of the horizontal wheel track, the bogie track backing plate, and the bogie track itself, followed finally by the machining of the pin holes. After each machining step, dimensions were re-checked; any deviations were promptly corrected to ensure the smooth execution of subsequent steps. Upon completion of all machining, a comprehensive inspection was conducted in the presence of the supervisor and the client, confirming that all critical dimensions fell within the specified tolerances.
Pre-assembly and Trial Run: To ensure the quality of the final on-site assembly and compliance with technical specifications, the slewing structure and portal frame underwent pre-assembly and a trial run in the manufacturing workshop. The successful trial run demonstrated the feasibility of the fabrication and machining plans, ensured the project remained on schedule, and provided a strong guarantee for the quality of the final product assembly on-site.
