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Pan-Head Torx Triple-Combo Screw - GB2672

Pan-Head Torx Triple-Combo Screw - GB2672
  • Product description：Torx combo screws facilitate installation and save labor costs. Based on the pan-head torx screw design, they are integrated with a flat washer and a spring washer. During installation, there is no ne
I. Core Relationship Between Triple-Combo Structure and Dimension Standards
The "triple-combo" feature of pan-head torx triple-combo screws dictates that their dimension standards must balance the dimensional coordination of three components: the screw body, flat washer, and spring washer. This prevents assembly jams and anti-loosening failure caused by mismatched component dimensions:
Structural Composition
It consists of three parts: 1) pan-head torx screw (core load-bearing component), 2) flat washer (for pressure distribution and protection of the connected component surface), and 3) spring washer (for providing elastic preload and preventing vibration-induced loosening). These three parts are pre-assembled into a single unit via stamping or riveting processes at the factory, ensuring coaxiality (tolerance ≤ 0.1mm) to avoid washer misalignment during installation.
Standard Basis
The overall design complies with GB/T 9074.17 Cross Recess, Hexagon Socket Pan Head Screw, Spring Washer and Flat Washer Assemblies (with torx slots extended from the "hexagon socket" specifications in this standard). Specifically:
  • The screw body dimensions adhere to GB/T 65 (Pan Head Screws);
  • The torx slot dimensions follow GB/T 2671.1 (Hexagon Socket Slots);
  • The flat washer complies with GB/T 97.1;
  • The spring washer meets GB/T 93.
II. Analysis of Dimension Standards for Core Components
1. Dimension Standards for Pan-Head Torx Screw Body
As the core of the triple-combo structure, the screw body dimensions directly determine load-bearing capacity and installation compatibility:
Thread Specifications
Common sizes range from M2 to M8 (mainstream for household and small industrial equipment), with thread dimensions complying with GB/T 196 Basic Dimensions of Metric Screw Threads:
  • Major Diameter (Nominal Diameter): For example, the major diameter of M3 thread is 3mm (tolerance: +0.084/-0), and that of M5 is 5mm (tolerance: +0.108/-0). This ensures precise engagement with the threaded hole of the connected component, preventing thread stripping (from excessive looseness) or thread damage (from excessive tightness).
  • Thread Length (L): Refers to the length from the bottom of the screw head to the end of the thread. It is determined by the thickness of the connected component (typically, thread length = thickness of the connected component + 1-2 thread pitches). For instance, if the connected component is 3mm thick, an M3 screw with a 5mm thread length is selected. Common thread lengths for M3 include 4mm, 5mm, and 6mm; for M5, they are 6mm, 8mm, and 10mm.
Pan Head Dimensions
  • Head Diameter (dk): 1.5-1.8 times the major thread diameter. For example, the head diameter of M3 is 5.5mm (tolerance: +0.2/-0.1), and that of M5 is 9mm (tolerance: +0.3/-0.1). This ensures the flat washer fully covers the area under the head, avoiding uneven pressure distribution caused by the washer extending beyond the head.
  • Head Height (k): 0.5-0.6 times the major thread diameter. For example, the head height of M3 is 1.8mm (tolerance: +0.15/-0.1), and that of M5 is 2.5mm (tolerance: +0.2/-0.12). Excessive height may lead to insufficient installation space, while insufficient height affects the torx slot depth (which needs to be deep enough to accommodate the wrench teeth).
Torx Slot Dimensions
  • Slot Size Matching: Corresponding to thread specifications—M2-M3 matches T5-T6 slot sizes, M4-M5 matches T8-T10, and M6-M8 matches T15-T20. For example, an M3 screw uses a T6 slot, with a slot depth ≥ 1mm (to ensure sufficient wrench insertion depth and prevent slipping during torque transmission).
  • Slot Bottom Fillet: The slot bottom requires a fillet of R0.1-R0.2mm to avoid stress concentration-induced slot wall cracking (especially for heat-treated high-strength screws, such as Grade 8.8).
2. Dimension Standards for Flat Washers
Flat washer dimensions must be compatible with the screw head and the connected component surface, with core parameters including inner diameter, outer diameter, and thickness:
  • Inner Diameter (d1): 0.1-0.3mm larger than the major thread diameter. For example, the inner diameter of an M3 flat washer is 3.2mm (tolerance: +0.1/-0), and that of M5 is 5.3mm (tolerance: +0.15/-0). This ensures the screw shank passes through easily while preventing washer misalignment from an excessively large inner diameter.
  • Outer Diameter (d2): 2-2.2 times the inner diameter. For example, the outer diameter of an M3 flat washer is 7mm (tolerance: +0.3/-0.2), and that of M5 is 10mm (tolerance: +0.4/-0.2). This ensures sufficient contact area (the contact area of an M3 flat washer is approximately 30mm2) to distribute the tightening pressure on soft or thin connected components (e.g., plastic, thin metal sheets) and avoid substrate indentation.
  • Thickness (h): Typically 0.3-0.8mm. The thickness of an M3 flat washer is 0.5mm (tolerance: +0.1/-0.05), and that of M5 is 0.8mm (tolerance: +0.15/-0.08). Excessive thickness increases the overall installation height, while insufficient thickness may cause the washer to be flattened and lose its pressure-distribution function.
3. Dimension Standards for Spring Washers
Spring washer dimensions must be compatible with the screw shank and flat washer, with core parameters including inner diameter, outer diameter, and free height:
  • Inner Diameter (d1): Consistent with the flat washer’s inner diameter. For example, the inner diameter of an M3 spring washer is 3.2mm, and that of M5 is 5.3mm. This ensures tight fit with the screw shank, preventing spring washer misalignment during vibration from excessive clearance.
  • Outer Diameter (d2): Slightly smaller than the flat washer’s outer diameter (difference: 0.5-1mm). For example, the outer diameter of an M3 spring washer is 6.5mm (vs. 7mm for the flat washer), and that of M5 is 9.5mm (vs. 10mm for the flat washer). This prevents the spring washer from extending beyond the flat washer and interfering with other components of the connected part.
  • Free Height (H): 2-2.5 times the thickness. For example, the free height of an M3 spring washer is 1mm (thickness: 0.5mm), and that of M5 is 2mm (thickness: 0.8mm). Insufficient free height results in inadequate elasticity and failure to provide continuous preload, while excessive free height requires higher torque during tightening and may cause screw head deformation.
III. Key Impacts of Dimension Standards on Usage & Selection Recommendations
1. Core Impacts of Dimension Standards
  • Assembly Compatibility: If the thread length is too short, the thread will not fully engage the connected component, reducing load-bearing capacity. If the flat washer’s inner diameter is too small, the screw shank cannot pass through, causing assembly jams and requiring re-selection of specifications, which delays production efficiency.
  • Anti-Loosening Reliability: Non-standard spring washer free height (e.g., insufficient height) prevents the washer from generating adequate elastic deformation after tightening. In vibrating scenarios (e.g., washing machine motors), loosening may occur within 1-3 months, requiring additional adhesive for locking.
  • Substrate Protection: An excessively small head diameter or insufficient flat washer outer diameter concentrates pressure on local areas of the connected component (e.g., plastic phone casings), leading to cracking during tightening. In such cases, combo screws with larger head sizes must be replaced.
2. Selection & Dimension Verification Recommendations
  • Selection Basis:
  • Determine thread length based on the thickness of the connected component;
  • Determine major thread diameter based on the threaded hole specification;
  • Determine total height (head height + total washer thickness) based on installation space. For example, if the installation space ≤ 5mm, the total height of an M3 combo screw must be ≤ 4.3mm (1.8mm head + 0.5mm flat washer + 1mm spring washer).
  • Dimension Verification: After receiving goods, sample-test key dimensions—use calipers to measure the major thread diameter (e.g., M3 should be within 3mm±0.084), flat washer outer diameter (M3 should be 6.8-7.3mm), and spring washer free height (M3 should be 0.9-1.1mm). This avoids assembly issues caused by non-standard components.
  • Scenario Adaptation: Select M3-M4 sizes (light load) for appliance casings (e.g., rice cookers) and M5-M6 sizes (medium load) for automotive interior components (e.g., instrument panels). Prioritize Grade 8.8 high-strength screws, as they offer more precise thread tolerances and consistent component dimensions.
The dimension standards for pan-head torx triple-combo screws are the foundation for ensuring "smooth assembly, reliable fastening, and long-term durability." Whether in production procurement or on-site installation, only by strictly following national standards and precisely matching the needs of connected components can the advantages of their "integrated assembly" be fully realized, and equipment failures or safety hazards caused by dimensional deviations be avoided.