Введение
DIN 7991 specifies countersunk head cap screws with hexagon socket drives, commonly known as flat head hex socket screws. These fasteners feature a 90-degree conical head that sits flush with or below the surface of the mated part, providing a clean, aerodynamic profile ideal for applications where protrusion must be minimized. The internal hex drive allows for high torque transmission without external wrenching features, enhancing security and reducing the risk of tampering.
These screws are extensively used in mechanical engineering, aerospace, automotive, and mold-making industries due to their ability to provide strong, reliable joints in confined spaces. Manufactured through cold forging and threading processes, they ensure high precision and consistency. This guide details the standard dimensions, material compositions, mechanical properties, and torque recommendations, drawing from DIN 7991 and related ISO standards like ISO 10642, to assist engineers in selection, installation, and quality assurance.
Understanding these specifications is crucial for ensuring structural integrity, as improper sizing or torquing can lead to joint failure under load. Factors such as material grade, thread pitch, and surface finish influence performance, and adherence to standards minimizes risks in critical assemblies.
Dimensions and Specifications
DIN 7991 defines precise dimensional tolerances for countersunk hex socket screws to ensure interchangeability and fit. Key parameters include nominal diameter (d), pitch (p), head diameter (dk), head height (k), socket size (s), and countersink angle (typically 90°-92°). Lengths range from short to long, with full or partial threading options. The table below summarizes standard dimensions for sizes M3 to M24, based on verified DIN 7991 data. Note that for larger sizes like M22 and M24, the countersink angle remains 90°-92°, correcting any potential misconceptions from non-standard sources.
These dimensions facilitate flush mounting, with the head fully recessed into a countersunk hole. Engineers should verify countersink diameters match dk max values to avoid gaps or interference. Tolerances are class 10.9 or equivalent unless specified otherwise.
| Nominal Diameter d | М3 | М4 | М5 | М6 | М8 | М10 | М12 | М14 | М16 | М18 | М20 | М22 | М24 | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Pitch p | 0.5 | 0.7 | 0.8 | 1 | 1.25 | 1.5 | 1.75 | 2 | 2 | 2.5 | 2.5 | 2.5 | 3 | |
| Length Range l | 8-40 | 8-40 | 10-60 | 12-60 | 16-100 | 20-100 | 25-100 | 25-100 | 30-100 | 30-100 | 30-100 | 35-100 | 35-100 | |
| Head Diameter dk | макс | 6 | 8 | 10 | 12 | 16 | 20 | 24 | 27 | 30 | 33 | 36 | 36 | 39 |
| мин | 5.7 | 7.64 | 9.64 | 11.57 | 15.57 | 19.48 | 23.48 | 26.48 | 29.48 | 32.38 | 35.38 | 35.38 | 38.38 | |
| Socket Size s | nominal | 2 | 2.5 | 3 | 4 | 5 | 6 | 8 | 10 | 10 | 12 | 12 | 14 | 14 |
| макс | 2.1 | 2.6 | 3.1 | 4.12 | 5.14 | 6.14 | 8.175 | 10.175 | 10.175 | 12.212 | 12.212 | 14.212 | 14.212 | |
| мин | 2.02 | 2.52 | 3.02 | 4.02 | 5.02 | 6.02 | 8.025 | 10.025 | 10.025 | 12.032 | 12.032 | 14.032 | 14.032 | |
| Head Height k | макс | 1.2 | 1.8 | 2.3 | 2.5 | 3.5 | 4.4 | 4.6 | 4.8 | 5.3 | 5.5 | 5.9 | 8.8 | 10.3 |
| мин | 0.95 | 1.55 | 2.05 | 2.25 | 3.2 | 4.1 | 4.3 | 4.5 | 5 | 5.2 | 5.6 | 8.44 | 9.87 | |
| Countersink Angle α | макс | 92° | 92° | |||||||||||
| мин | 90° | 90° | ||||||||||||
For complete specifications, including thread lengths and tolerances, consult DIN 7991 or ISO 10642. Note that lengths are measured from the top of the head to the tip, and full threading is standard for shorter lengths.
Materials and Chemical Composition
DIN 7991 screws are typically made from stainless steel grades for corrosion resistance or alloy steel for high strength. Common materials include A2 (SUS304) and A4 (SUS316) stainless steels, which offer excellent durability in harsh environments. The chemical composition ensures mechanical properties like tensile strength and hardness are maintained.
Stainless steel variants provide non-magnetic properties and resistance to oxidation, making them suitable for food processing, marine, and medical applications. Alloy steel versions, often in property classes 8.8, 10.9, or 12.9, are heat-treated for enhanced strength in structural uses. Surface finishes like black oxide or zinc plating further protect against wear.
| Материал | Химический состав (%) | |||||||
|---|---|---|---|---|---|---|---|---|
| C | Мн | Си | П | С | Ни | Мо | Кр | |
| SUS304 (A2) | ≤0,08 | ≤2,00 | ≤1,00 | ≤0,045 | ≤0,03 | 8.00-11.00 | – | 17.00-19.00 |
| SUS316 (A4) | ≤0,08 | ≤2,00 | ≤1,00 | ≤0,045 | ≤0,03 | 10.00-14.00 | 2.00-3.00 | 16.00-18.00 |
For carbon steel options, compositions vary by grade (e.g., 10.9: C 0.20-0.55%, Mn 0.40-0.90%). Select based on environmental conditions; A4 for marine exposure due to molybdenum content enhancing pitting resistance.
Механические свойства
Mechanical properties of DIN 7991 screws are classified per ISO 3506 for stainless steel and ISO 898 for carbon steel. Stainless grades A2-50, A2-70, and A4-80 offer tensile strengths from 500 to 800 MPa, with yield strengths at 210-450 MPa. These ensure ductility for vibration resistance while providing sufficient hardness (HV 150-300) for drive integrity.
Carbon steel variants in classes 8.8, 10.9, and 12.9 achieve higher strengths (800-1200 MPa tensile), ideal for load-bearing applications. Properties include elongation min 12% and impact toughness for low-temperature use. Testing per DIN EN ISO 6892-1 confirms compliance, with surface hardness controlled to prevent hydrogen embrittlement in plated screws.
Key guidance: Match property class to application stress; A4-80 for corrosive environments with high strength needs. Regular inspections for fatigue cracks are recommended in cyclic loading scenarios.
Torque Standards
Torque values for DIN 7991 screws ensure proper clamping without stripping or head failure. Minimum breaking torques (in Nm, converted from kgf.cm for accuracy: 1 kgf.cm ≈ 0.098 Nm) are specified for stainless steel grades. These are destructive tests per DIN EN ISO 3506-1, indicating the torque at which the screw fails in torsion.
For installation, use 70-80% of breaking torque as seating value, adjusted for lubrication (μ=0.125 for plain, 0.094 for plated). Preload calculations via VDI 2230 ensure joint security. The table below lists minimum breaking torques, verified against standard data.
| Нить | Класс свойств | ||
|---|---|---|---|
| A2-50 | A2-70 | A4-80 | |
| Min Breaking Torque (Nm) | |||
| М1.6 | 0.15 | 0.2 | 0.24 |
| М2 | 0.3 | 0.4 | 0.48 |
| М2.5 | 0.6 | 0.9 | 0.96 |
| М3 | 1.1 | 1.6 | 1.8 |
| М4 | 2.7 | 3.8 | 4.3 |
| М5 | 5.5 | 7.8 | 8.8 |
| М6 | 9.3 | 13 | 15 |
| М8 | 23 | 32 | 37 |
| М10 | 46 | 65 | 74 |
| М12 | 80 | 110 | 130 |
| М16 | 210 | 290 | 330 |
For carbon steel, recommended tightening torques (MA in Nm) for class 10.9 are higher: M3=1.4, M4=3.4, M5=6.8, M6=11, M8=28, M10=55, etc., per ISO 898-1. Use torque wrenches calibrated to ±4% accuracy, and consider friction coefficients for precise preload.
Manufacturing Process
Production of DIN 7991 screws typically involves cold heading on multi-station machines, forming the conical head and hex socket in one or two strokes. This process ensures high material utilization and precise geometry. Subsequent threading is performed using automatic rolling machines, creating uniform threads with minimal burrs.
Heat treatment for alloy steels involves quenching and tempering to achieve desired hardness (e.g., 39-44 HRC for 10.9). Stainless steel variants undergo annealing for workability. Quality controls include dimensional inspections per DIN EN ISO 4759 and thread gauging to GO/NO-GO standards. Surface treatments like passivation for stainless or electroplating for carbon steel enhance corrosion resistance.
Advanced manufacturing employs CNC for custom lengths and non-standard features, ensuring traceability through lot numbering. This process yields fasteners with consistent performance, reducing variability in assemblies.
Приложения
DIN 7991 screws excel in scenarios requiring flush surfaces, such as aircraft panels, automotive chassis, and precision molds. In high-speed rail, they secure components without aerodynamic drag. Mechanical assemblies benefit from their high torque capacity, while electronics use them for EMI shielding enclosures.
Selection tips: Use A4 grades in moist environments; 12.9 for high-stress joints. Combine with countersunk holes per DIN 74 for optimal fit. In vibration-prone areas, apply thread lockers to maintain preload. These applications highlight the screws’ versatility in achieving secure, aesthetic fastenings.
Часто задаваемые вопросы (FAQ)
- What is the standard countersink angle for DIN 7991 screws?
- The countersink angle is 90° minimum to 92° maximum for all sizes M3 to M24, ensuring proper seating in matching holes without protrusion.
- How do I select the appropriate material grade?
- Choose A2-70 for general corrosion resistance, A4-80 for marine or acidic environments due to molybdenum. For high strength, opt for carbon steel 10.9 or 12.9 with protective coatings.
- What torque should I use for installation?
- Use 70-80% of the minimum breaking torque, adjusted for friction. For example, M6 A2-70: install at 9-10 Nm. Always calibrate tools and consider lubrication.
- Are these screws suitable for high-vibration applications?
- Yes, with thread lockers or prevailing torque features. Their internal drive and flush design reduce loosening risks in dynamic loads like machinery or vehicles.
- How does the manufacturing process affect quality?
- Cold heading ensures precise head formation; improper threading can cause stress risers. Insist on certified manufacturers adhering to DIN EN ISO 9001 for consistent dimensions and strength.
- Can DIN 7991 screws be used with non-metallic materials?
- Yes, in composites or plastics with appropriate inserts, but verify compatibility to avoid crushing. Lower torque values may be needed to prevent material damage.
