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FULLERKREG ISO 7379 Grade 12.9 Black Socket Head Shoulder Screws are precision fasteners with a unique three-part geometry: a cylindrical socket head, a precision-ground smooth shoulder (body), and a reduced-diameter threaded end. The shoulder — not the thread — is the functional element of this screw. Manufactured from SCM440 (42CrMo4) chromium-molybdenum alloy steel with a black oxide surface treatment, the shoulder is precision-ground to h6 tolerance, providing an accurate bearing surface for pivots, guides, linkages, and rolling element bearings. Grade 12.9 heat treatment delivers a minimum tensile strength of 1,220 MPa for demanding dynamic load applications.
A shoulder screw has three distinct sections, each with a specific function:
| Section | Description | Function |
|---|---|---|
| Head | Cylindrical socket head (knurled) | Drive and clamping — seats against mating surface |
| Shoulder (Body) | Precision-ground smooth cylinder, h6 tolerance | The functional element — pivot axis, guide pin, bearing race |
| Thread | Reduced diameter, shorter than shoulder | Fastening into tapped hole — pulls head down to clamp |
Key principle: The thread diameter is always smaller than the shoulder diameter. The shoulder bears the load; the thread only provides clamping force. The component mounted on the shoulder (bearing, bushing, link) rotates or slides on the shoulder, not on the thread.
Per ISO 7379, each shoulder diameter corresponds to a specific thread size. The thread is always smaller than the shoulder:
| Shoulder Dia. (d1) | Thread Size (d2) | Head Dia. (d3) | Head Height (l2) | Key Size (s) | Key Depth (t) |
|---|---|---|---|---|---|
| φ 5 mm | M4 | 8.0 | 4.5 | 3 | 2.0 |
| φ 6 mm | M5 | 10.0 | 5.5 | 4 | 2.5 |
| φ 8 mm | M6 | 13.0 | 7.0 | 5 | 3.0 |
| φ 10 mm | M8 | 16.0 | 8.5 | 6 | 4.0 |
| φ 12 mm | M10 | 18.0 | 10.0 | 8 | 4.5 |
| φ 16 mm | M12 | 24.0 | 13.0 | 10 | 5.5 |
| φ 20 mm | M16 | 30.0 | 16.0 | 14 | 7.0 |
The shoulder diameter is precision-ground to h6 tolerance per ISO 286-1. This is a close-clearance fit tolerance suitable for rotating and sliding applications:
| Shoulder Dia. | h6 Upper Deviation | h6 Lower Deviation | h6 Tolerance Band |
|---|---|---|---|
| φ 5 mm | 0 | −8 μm | 8 μm |
| φ 6 mm | 0 | −9 μm | 9 μm |
| φ 8 mm | 0 | −15 μm | 15 μm |
| φ 10 mm | 0 | −15 μm | 15 μm |
| φ 12 mm | 0 | −18 μm | 18 μm |
| φ 16 mm | 0 | −18 μm | 18 μm |
| φ 20 mm | 0 | −21 μm | 21 μm |
For rotating applications (bearings, bushings), pair with H7 bore tolerance for a standard clearance fit (H7/h6). For sliding/guiding applications, H8 or H9 bore provides more clearance.
SCM440 is a chromium-molybdenum alloy steel with excellent hardenability, high fatigue strength, and good toughness — the preferred material for precision shoulder screws subject to dynamic and cyclic loads. All values are maximum unless a range is indicated.
| Grade | C | Si | Mn | P (max) | S (max) | Cr | Mo |
|---|---|---|---|---|---|---|---|
| SCM440 (42CrMo4) | 0.38–0.45 | 0.15–0.35 | 0.60–0.90 | ≤0.030 | ≤0.030 | 0.90–1.20 | 0.15–0.30 |
| Property | Requirement | Unit |
|---|---|---|
| Tensile Strength (Rm) | Min. 1,220 | MPa |
| Yield Strength / Proof Load Stress (Rp0.2) | Min. 1,100 | MPa |
| Core Hardness | 39–44 | HRC |
| Surface Hardness (max) | ≤50 | HRC |
| Elongation after fracture (A) | Min. 8 | % |
| Reduction of area (Z) | Min. 44 | % |
| Shoulder Tolerance | h6 (precision ground) | — |
| Surface Treatment | Black oxide (blackening) | — |
FULLERKREG ISO 7379 Grade 12.9 black socket head shoulder screws are precision components used wherever a precision pivot, guide, or bearing shaft is required:
When ordering ISO 7379 shoulder screws, specify the following parameters:
Example specification: ISO 7379 — 8 × 20 — 12.9 (shoulder diameter 8 mm, shoulder length 20 mm, Grade 12.9)
Q: What is the difference between a shoulder screw and a standard socket cap screw?
A standard socket cap screw (DIN 912) has a uniform thread along its full body length. A shoulder screw (ISO 7379) has a precision-ground smooth shoulder (body) that is larger in diameter than the thread. The shoulder is the functional element — it acts as a precision shaft for bearings, bushings, and pivots. The thread only provides clamping force.
Q: What bore tolerance should I use with an h6 shoulder screw?
For rotating applications (bearings, bushings): use H7 bore tolerance for a standard clearance fit (H7/h6). For sliding/guiding applications: use H8 or H9 bore for more clearance. For press-fit applications: use p6 or r6 bore (not recommended for shoulder screws).
Q: Why is SCM440 used for shoulder screws instead of standard carbon steel?
SCM440 (42CrMo4) provides superior hardenability, fatigue strength, and toughness compared to standard carbon steels. These properties are critical for shoulder screws subject to dynamic, cyclic, and impact loads in pivot and bearing applications.
Q: Does the black oxide coating affect the shoulder diameter tolerance?
Black oxide coating is only 1–2 μm thick and has negligible effect on the h6 shoulder diameter. The shoulder remains within h6 tolerance after blackening.
Q: Can I get dimensional inspection reports for the shoulder diameter?
Yes. FULLERKREG provides shoulder diameter inspection reports, hardness test reports, material certificates, and PPAP documentation upon request.
All FULLERKREG ISO 7379 Grade 12.9 black socket head shoulder screws are manufactured under strict quality control procedures including:
For wholesale pricing, bulk orders, custom shoulder lengths, or technical inquiries, please contact our sales team.
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