As a key fastener in mechanical assembly, the end structure design of set screws directly affects the positioning accuracy and anti-loosening performance. There are four typical types: flat end, concave end, cone end, and cylindrical end, which adapt to various engineering needs through different contact forms. Taking the M8 specification as an example, the flat end structure provides the largest contact area (about 19mm²), which is suitable for fixing soft materials that need to disperse the load. For example, the compressive stress applied on the surface of aluminum alloy can be stably controlled in the range of 80-120MPa, which can not only ensure the fixing effect but also avoid material crushing.
The concave end set screw adds a central depression structure on the basis of the flat end to form an annular contact zone. This design increases the contact stress by about 25%, and the measured anti-slip torque is 15-20N·m higher than that of the flat end, which is particularly suitable for positioning scenarios that require regular fine-tuning. The cone end structure is most common with a cone angle of 60°, which generates high locking force through the wedge effect. When the cone angle matches the mating hole, a single M10 cone end screw can generate an axial holding force of more than 4kN while maintaining uniform stress distribution in the contact area.
The cylindrical end structure is essentially a miniature locating pin, and the diameter of the cylindrical end is usually 0.3-0.8mm smaller than the major diameter of the thread. This design controls the diameter interference within the range of 0.05-0.12mm when the interference fit is achieved, which can achieve precise radial positioning (runout ≤ 0.015mm) and facilitate disassembly and maintenance. The key parameters of the four structures are compared as follows:
| Type | End diameter tolerance (mm) | Recommended contact hardness (HB) | Maximum reuse times |
| Flat | ±0.2 | 80-120 | 20 |
| Concave | ±0.15 | 120-180 | 15 |
| Conical | ±0.1 | 200-250 | 10 |
| Cylindrical | ±0.05 | 220-280 | 8 |
The strength and toughness of the material must be considered comprehensively. 45# steel with quenching and tempering treatment (hardness HRC28-32) is suitable for most flat-end and concave-end screws; alloy steel 42CrMo (hardness HRC35-40) is mostly used for high-load cone-end and cylindrical-end products. Pay attention to torque control during installation. For example, the recommended torque for M6 cone-end screws is 8-12N·m. Exceeding 15N·m may cause plastic deformation of the cone surface. Surface treatment is also critical. Blackening treatment can provide basic anti-rust ability, while galvanizing or Dacromet coating can increase the corrosion resistance time to 2000 hours of salt spray test without rust.
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