Precision Shoulder Bolts (ISO 7379 / Stripper Bolts)
In precision machinery and tool-and-die applications, a standard bolt cannot function as a running surface. Shoulder Bolts (or Stripper Bolts) bridge the gap between structural fasteners and precision axles. They feature a precision-ground, unthreaded shoulder that exceeds the diameter of the threads, providing a smooth, concentric bearing surface for rotating parts like pulleys, gears, and cams.
Our manufacturing process ensures the shoulder diameter is ground to tight tolerances (ISO f9 or h8), enabling a perfect slip fit with standard H7 reamed holes. By integrating the axle and fastener into a single component, these bolts eliminate the need for separate clevis pins and retaining rings, reducing assembly complexity in punch press dies, plastic injection molds, and linear guide systems.
- Precision ground f9 shoulder tolerance.
- High shear strength (Alloy Steel).
- Concentricity within 0.05mm TIR.
- Reduces part count (Axle + Bolt).
- Knurled head for manual grip.
- Available in Metric & Imperial.
Technical Specifications
Product Name
Hexagon Socket Head Shoulder Bolt / Stripper Bolt
Common Standards
ISO 7379, ANSI/ASME B18.3 (Inch Series), DIN 9841
Material
High Tensile Alloy Steel (SCM435 / 42CrMo4), Stainless Steel (A2-70 / A4-70)
Hardness (Steel)
32-43 HRC (Shoulder), 30-40 HRC (Thread)
Shoulder Tolerance
f9 (Standard Precision), h8 (High Precision), -0.025/-0.060 mm
Diameter Range
Shoulder Ø: 6mm – 24mm (Metric) / 1/4″ – 1″ (Inch)
Surface Finish
Black Oxide (Thermal), Zinc Plated (with Hydrogen De-embrittlement), Electropolished
Certifications
ISO 9001:2015, RoHS Compliant, Mill Test Reports (MTR)
1. The “Pivot Point” Challenge (Concentricity)
Pain Point: Using standard hex bolts as axles leads to “wobble” because thread tolerances are loose (6g), and the threaded section damages the mating part’s bore.
The Engineering Solution: Our Shoulder Bolts feature a precision-ground unthreaded shoulder. We control the Total Indicator Reading (TIR) between the head, shoulder, and thread to ensure perfect concentricity. This minimizes radial runout, ensuring smooth rotation for idler pulleys and cam followers.
2. Shear Strength vs. Tensile Strength
Pain Point: In die sets (“stripper plates”), the bolt is subjected to repeated transverse impact loads (shear), not just tension. Standard Grade 8.8 bolts may shear off at the thread root.
The Engineering Solution: We utilize SCM435 Alloy Steel heat-treated to 36-43 HRC. Crucially, the shear plane acts on the solid, ground shoulder, not the threads. This maximizes the shear area and fatigue resistance, essential for high-cycle stamping operations.
3. Tolerance Management (The f9 Fit)
Pain Point: A shoulder that is slightly oversized will seize in the bushing; one that is undersized causes vibration and premature wear.
The Engineering Solution: We manufacture to ISO Tolerance Class f9 (e.g., -0.013mm to -0.049mm for a 10mm shoulder). This is engineered to pair with standard H7 reamed bushings, providing a “running fit” that retains lubricant without excessive play.
Dimensions for standard ISO 7379. Note: The “Nominal Size” typically refers to the Shoulder Diameter, not the Thread Diameter.
| Shoulder Ø (ds) | Thread Size (d1) | Head Dia (dk) | Head Height (k) | Socket Size (s) | Shoulder Lengths (l) |
| 6 mm | M5 x 0.8 | 10 mm | 4.5 mm | 3 mm | 10, 12, 16, 20… 60 |
| 8 mm | M6 x 1.0 | 13 mm | 5.5 mm | 4 mm | 12, 16, 20, 25… 80 |
| 10 mm | M8 x 1.25 | 16 mm | 7.0 mm | 5 mm | 16, 20, 25, 30… 100 |
| 12 mm | M10 x 1.5 | 18 mm | 9.0 mm | 6 mm | 16, 20, 25, 30… 120 |
| 16 mm | M12 x 1.75 | 24 mm | 11.0 mm | 8 mm | 25, 30, 40, 50… 120 |
| 20 mm | M16 x 2.0 | 30 mm | 14.0 mm | 10 mm | 30, 40, 50, 60… 120 |
1. Torque Warning (Crucial)
Engineers often make the mistake of torqueing shoulder bolts based on the shoulder diameter.
Risk: The thread is always smaller than the shoulder (e.g., a 12mm shoulder has an M10 thread). Applying M12 torque to this bolt will strip the threads or snap the neck.
Rule: Always set torque limiting drivers based on the Thread Diameter (d1) and property class (usually 12.9 or 8.8 equivalent).
2. Hole Preparation
Running Fit: For pivot applications, ream the housing hole to H7. The bolt’s f9 tolerance ensures a clearance fit for rotation.
Counterbore: The mating hole for the threaded end should be countersunk or counterbored to allow the shoulder to seat firmly against the “step” (datum face), ensuring perpendicularity.
3. Lubrication
Since the shoulder acts as a bearing surface, proper lubrication is required to prevent galling, especially with stainless steel. Apply a thin film of lithium grease or anti-seize to the shoulder section before installation in bushings.
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FAQ
What is the difference between a Shoulder Bolt and a Stripper Bolt?
There is no functional difference; the terms are often used interchangeably. “Shoulder Bolt” is the general engineering term describing the geometry. “Stripper Bolt” is a specific industry term derived from their common use in tool-and-die machinery to hold “stripper plates” in punch presses.
How do I measure a shoulder bolt correctly?
Shoulder bolts are specified by the Shoulder Diameter and Shoulder Length, not the thread size. For example, a “10mm x 50mm” shoulder bolt features a 10mm diameter precision shoulder that is 50mm long. The thread will be smaller (typically M8) and is not included in the primary size designation.
What is the standard tolerance for ISO 7379 shoulder bolts?
The standard manufacturing tolerance for the shoulder diameter in ISO 7379 is f9. For a 12mm shoulder, this translates to a deviation of -0.016mm to -0.059mm. This negative tolerance ensures the bolt will always fit into a standard “zero” or positive tolerance (H7) hole without jamming.
Can I use stainless steel shoulder bolts for high-load applications?
It depends on the load type. Stainless Steel (A2/A4) shoulder bolts have significantly lower tensile and shear strength compared to Alloy Steel (SCM435) versions. While they offer corrosion resistance, they are prone to galling if used as a dynamic pivot without proper lubrication or bushings. For high loads, specify Alloy Steel with a Zinc Flake coating.
Why is the thread diameter smaller than the shoulder?
The “necking down” design allows the bolt to be tightened until the bottom of the shoulder seats firmly against the sub-surface (datum). This locks the bolt in place rigidly while leaving the shoulder section above free to act as an axle or guide pin for moving parts.