The Production Process of Sprocket Bolts

1. Raw Material Selection and Cutting

    

   Sprocket bolts need to have high tensile and shear resistance. Common raw materials are medium carbon steel (e.g., 45# steel) or high-strength alloy structural steel (e.g., 40Cr).

    

   First, round steel supplied by steel mills is cut into blanks of equal length using a CNC sawing machine or shearing machine, with a machining allowance reserved for subsequent processes.
2. Cold Heading Forming

    

   This is the core process for forming the bolt head. A cold heading machine applies high pressure to the blank at room temperature, causing plastic deformation to directly form the bolt head shape (such as hexagonal head, round head).

    

   The cold heading process preserves the continuity of metal fibers. Compared with cutting processing, the strength of the bolt head is higher, and the material utilization rate can reach more than 95%.
3. Thread Processing

    

   The threads on the bolt shank are crucial for connection and fastening. There are two mainstream processing methods:
   • Thread Rolling: Put the cold-headed blank into a thread rolling machine, and extrude the shank through two rotating thread rolling wheels to form threads by metal plastic deformation. The strength of rolled threads is 20%~30% higher than that of cut threads, making it the preferred process for sprocket bolts.
   • Turned Threads: A small number of non-standard or large-size bolts are processed by lathe cutting, which has high precision but low efficiency.
4. Heat Treatment Strengthening

    

   To meet the high-strength requirements of construction machinery, the bolts need to be fully heat-treated, with the process as follows:
   • Quenching: Heat the bolts to 800~900℃ (adjust according to the steel grade), hold the temperature, and then quickly cool them in water or oil, transforming the internal structure of the steel into high-hardness martensite.
   • Tempering: Heat the quenched bolts to 300~500℃, hold the temperature, and then air-cool them to eliminate quenching stress, improve the toughness of the bolts, and avoid brittle fracture during use.
      
     After heat treatment, the hardness of the bolts usually reaches HRC 28~35, and the tensile strength is ≥ Grade 8.8.
5. Surface Treatment

    

   Sprocket bolts are mostly used in outdoor, humid, or dusty construction site environments. The core purpose of surface treatment is rust and corrosion prevention. Common processes include:
   • Hot-Dip Galvanizing: Immerse the bolts in molten zinc solution to form a zinc layer on the surface, which has strong rust resistance and is suitable for harsh working conditions.
   • Electrogalvanizing: The coating is thin and uniform, with low cost and bright appearance.
   • Blackening/Bluing: Form a black oxide film on the surface through chemical oxidation, with general rust resistance, mostly used in indoor or supporting protection scenarios.
   • Dacromet Coating: It has excellent rust resistance and no hydrogen embrittlement risk, suitable for high-strength bolts.
6. Finishing and Deburring

    

   After heat treatment and surface treatment, burrs and flash may appear on the bolt surface, which need to be removed by tumble grinding or manual polishing. At the same time, slight deformation that may occur is corrected to ensure the perpendicularity and dimensional accuracy of the bolts.
7. Quality Inspection

    

   100% inspection or sampling inspection is a key link to control product qualification. The inspection items include:
   • Dimension Inspection: Use calipers, thread gauges, and plug gauges to check the length, diameter, thread precision, and head size of the bolts.
   • Performance Inspection: Sampling tests for tensile strength, yield strength, and shear resistance; inspect the thickness and adhesion of the surface coating.
   • Appearance Inspection: Check for defects such as cracks, burrs, and coating peeling.