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Shoulder screws feature a precision-ground unthreaded shoulder between the head and thread, functioning as a pivot pin, axle, or locating pin in mechanical assemblies. The shoulder diameter controls the bearing fit, typically to h9 tolerance. DIN 923 and DIN 924 define slotted and socket-head variants. Common in linkages, pulleys, and precision jigs where rotational alignment matters. Supplied M3–M10 in carbon steel, stainless A2, and alloy steel.
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A shoulder screw is a fastener with a precision-ground cylindrical shoulder between the head and a smaller-diameter threaded end. The shoulder is not just a spacer—it is a bearing surface, a locating pin, or a pivot axle machined to a tolerance measured in microns. The thread only holds the screw into the tapped hole; the shoulder does the mechanical work.
Strip the name down and you will hear them called shoulder bolts, stripper bolts, or axle screws depending on the shop floor. The naming difference is regional, not technical—DIN 923 calls them “Schulterbolzen” and ISO 7379 standardizes the dimensions globally. The function is the same: a precision-ground cylindrical body that a mating part slides, rotates, or locates against.
The thread locks the screw into a tapped hole. The shoulder is the working surface—a precision-ground bearing journal, locating pin, or pivot axle.
An undercut relief at the shoulder-thread transition prevents the mating part from riding on the incomplete thread run-out. Standard shoulder tolerance is h8: +0 / -0.022 mm at 10 mm nominal. Paired with an H7 or H8 reamed hole, radial play sits at 0.01–0.03 mm for smooth rotation.
The shoulder face is ground square to the axis within 0.05 mm runout, with a surface finish of Ra 0.4–0.8 µm. Any rougher and the mating plate galls under cyclic load.
Socket Head (Hex Socket) The standard across DIN 923 and ISO 7379. A cylindrical head with a recessed hex socket delivers high torque for tightening the thread without marring the head top. The head diameter is smaller than the shoulder—this is deliberate, allowing the shoulder to pass through a clearance hole from the head side. A standard M10 shoulder screw has an 8 mm hex drive and a 10 mm shoulder, so the entire screw passes through a hole reamed slightly over 10 mm.
Low Head (Low-Profile Socket) Half the standard head height, used when the screw head must clear a moving slide or cam that passes over it. Torque capacity drops roughly 30% because the hex socket is shallower—compensate with a thread-locker on the thread rather than over-torquing the head.
Button Head (Round Top) The domed profile eliminates sharp edges in operator-accessible areas. Button head shoulder screws are common in consumer equipment and lab instruments where safety and aesthetics matter. Same hex socket drive, lower torque ceiling than standard socket head due to reduced hex depth.
Flat Head (Countersunk) The head sits flush or below the mounting surface. Rare in shoulder screw catalogs because countersinking removes material from the bearing area—only use when clearance above the head is zero and a counterbore is impossible.
Undercut / Relief Type (Standard) A turned relief groove between the shoulder and thread. This is the standard configuration—the undercut prevents the mating part from bottoming on an incomplete thread.
Full-Thread-to-Shoulder No relief undercut; the thread runs to within one pitch of the shoulder face. Used only when axial space is tight and the mating part must seat as close to the thread as possible. One drawback: the last partial thread acts as a stress riser and can initiate fatigue cracks under cyclic bending.
Alloy Steel (42CrMo4 / 4140) Quenched and tempered to 32–40 HRC, tensile 1,000–1,200 MPa. This is the default material for DIN 923 and ISO 7379 shoulder screws. The shoulder is ground after heat treatment to maintain h8 tolerance through the hardened case. Alloy steel shoulder screws handle the highest shear loads—a 10 mm shoulder in 4140 at 40 HRC can survive roughly 60 kN in single shear before yield. Black oxide finish provides light corrosion resistance for indoor tooling.
Stainless Steel 303 Tensile 500–700 MPa. Free-machining austenitic stainless; the sulfur addition makes it easier to turn and grind the shoulder. Non-magnetic in the annealed condition, but light cold-working from the grinding process can induce trace magnetism. 303 shoulder screws serve in food equipment, cleanrooms, and outdoor assemblies where alloy steel would rust. Shear capacity is roughly half of 4140 alloy steel at the same shoulder diameter—size up one diameter step if substituting 303 for alloy steel.
Stainless Steel 316 Tensile 500–700 MPa, identical to 303 in strength but with molybdenum for chloride resistance. Use 316 shoulder screws in marine pivot applications, chemical plant valve linkages, and offshore instrumentation. The sulfur-free composition machines slower than 303, adding about 15–20% to the part cost.
Stainless Steel 416 Martensitic stainless, heat-treatable to 28–35 HRC. The only stainless shoulder screw material that can be hardened to approach alloy steel wear resistance. 416 provides roughly 70% of the shear strength of 4140 alloy steel with moderate corrosion resistance—better than carbon steel, below 303/316. Use 416 when the application needs both wear resistance and some corrosion protection, such as a stripper bolt in a die set exposed to water-based lubricants.
Brass Tensile ~350 MPa. Low-friction bronze-on-brass rotation for light instrument bearings and electrical-contact pivots. Brass shoulder screws are non-magnetic and spark-resistant, meeting ATEX requirements for explosive-atmosphere equipment. Shear capacity: roughly 20% of alloy steel at the same diameter.
The part that rides on the shoulder determines the tolerance class. Get this wrong and a precision pivot becomes a rattling sloppy joint.
| Tolerance | Deviation (10 mm nominal) | Fit Type | Use Case |
|---|---|---|---|
| h6 | +0 / -0.009 mm | Precision slide | Die stripper bolts, metrology fixtures |
| h8 | +0 / -0.022 mm | Close running | Standard rotating pivots, cam followers |
| h9 | +0 / -0.036 mm | Free running | Rough guide pins, manual-adjustment links |
| h11 | +0 / -0.090 mm | Loose clearance | Alignment guides with thermal expansion allowance |
Standard catalog shoulder screws ship with h8 tolerance. For h6, specify a precision-ground shoulder screw with the tolerance called out on the order line—the difference in grinding time between h8 and h6 adds about 25–35% to the unit cost but buys you a pivot with roughly one-third the radial play.
Mating hole fit: Pair an h8 shoulder with an H8 reamed hole for a close running fit (~0.01–0.03 mm radial play at 10 mm nominal). H7 is tighter and may bind if misalignment exceeds 0.01°. H9 is looser and allows the shoulder to rotate freely but adds about 0.02 mm of additional radial play.
Shoulder length tolerance: Typically ±0.13 mm (±0.005 in.) for standard catalog screws, tighter on precision-ground grades. The shoulder length determines the stack-up of parts it positions—if the shoulder is too short, the assembly binds when tightening the thread; if too long, the parts rattle.
| Parameter | Metric (DIN 923 / ISO 7379) | Imperial (ANSI B18.3) |
|---|---|---|
| Shoulder diameter | 3, 4, 5, 6, 8, 10, 12, 16, 20 mm | 1/8, 3/16, 1/4, 5/16, 3/8, 1/2, 5/8, 3/4 in. |
| Shoulder length | 5 mm to 100 mm | 1/4 in. to 4 in. |
| Thread size | M3 to M16 (smaller than shoulder) | #4-40 to 5/8-11 |
| Head type | Socket head, low head, button, flat | Same |
| Drive | Hex socket (1.5–12 mm Allen) | Hex socket (1/16–3/8 in.) |
| Shoulder tolerance | h8 standard; h6, h9, h11 on request | 0.000 to -0.001 in. (≈ h8) |
| Standards | DIN 923, ISO 7379, JIS B 1176 | ANSI/ASME B18.3.4M |
Thread-to-shoulder ratio: The thread diameter is smaller than the shoulder by design—an M10 shoulder screw carries an M8 thread. This diameter step creates the bearing face that resists axial pull-through. If the thread matched the shoulder diameter, there would be no shoulder face to seat against, and the part would function as an ordinary cap screw.
Die Tooling and Stamping Shoulder screws serve as stripper bolts in stamping and forming dies. A set of four shoulder screws suspends the stripper plate above the die face, compressing springs that eject the finished part. The shoulders cycle millions of times against hardened bushings—h6 tolerance and 4140 alloy steel at 38–42 HRC are standard here. Die shops replace stripper bolts on a schedule, not on failure; a snapped shoulder screw in a progressive die stops a press line billing $500–$1,500 per hour.
Injection Molds Ejector plates, guide pins, and return pins in plastic injection molds run on shoulder screws that align the moving mold halves. The shoulder must withstand thermal cycling from 20°C to 120°C per shot, hundreds of thousands of times. Thermal expansion of the mold steel can close the running clearance by 0.01–0.02 mm at operating temperature—specify h9 on the shoulder instead of h8 if the mold runs above 100°C to prevent binding.
Robotics and Automation Robot gripper linkages, cam followers, and articulated joints use shoulder screws as pivot axles. A 6 mm shoulder screw in 4140 alloy steel, torqued to 15 N·m on an M5 thread, handles roughly 12 kN of clamp preload while allowing the gripper arm to rotate freely on the shoulder. For collaborative robots where pinch points are a safety concern, button head shoulder screws eliminate sharp hex corners.
Precision Fixtures and Jigs CNC machining fixtures use shoulder screws as locating dowels that also clamp. The shoulder aligns the fixture plate relative to the machine table; the thread pulls it tight. This dual function eliminates a separate dowel pin and bolt at each fixture point—reducing part count and assembly time by roughly 30% on a 6-point fixture.
Medical Devices and Lab Instruments Surgical instrument hinges and microscope stage adjustments run on 303 or 316 stainless shoulder screws. Autoclave sterilization at 134°C for 3 minutes is the acceptance test—any shoulder screw that galls, pits, or loses tolerance after 1,000 autoclave cycles fails qualification. Electropolished 316 with a surface finish of Ra 0.2 µm or better is the baseline spec for reusable surgical tools.
Bicycle and Sports Equipment Suspension pivots, brake lever axles, and derailleur jockey wheels use miniature shoulder screws (M4–M6, #6–1/4 in.) in stainless or alloy steel. The shoulder is the axle; the thread just holds it in the frame. A bicycle suspension pivot sees roughly 5,000–10,000 cycles per hour of riding—a shoulder screw with an Ra 0.4 µm finish and moly grease lasts roughly 300–500 hours before measurable wear.
| Decision | What to Ask | Guidance |
|---|---|---|
| 1. Shoulder function | Is it a pivot, a locator, or a guide? | Pivot → h8 shoulder + H8 reamed hole, lubricated. Locator → h6 shoulder + H7 reamed hole, dry. Guide pin → h9 shoulder + H9 reamed hole, clearance for thermal expansion if needed. |
| 2. Load | What shear, axial, and bending loads? | Shear load rides on the shoulder cross-section—calculate single-shear capacity at 60% of material tensile × shoulder cross-section area. For 4140, a 10 mm shoulder handles ~60 kN single shear. Axial load compresses the shoulder face against the mounting surface—4130 alloy steel shoulder screws in a die set run face pressures over 200 MPa without yielding. |
| 3. Environment | Corrosion, temperature, cleanliness? | Indoor tooling → alloy steel + black oxide. Food/lab/outdoor → 303 or 316 stainless. High wear + some corrosion → 416 stainless (hardened). Explosive atmosphere → brass (spark-resistant). Cleanroom → electropolished 316. |
| 4. Head clearance | Space above the installed screw? | Standard head height fine → socket head. Overhead clearance < 5 mm → low head. Operator-accessible, no sharp edges → button head. Zero clearance above → flat countersunk (last resort). |
Match the shoulder length to the stack-up thickness with 0.1–0.2 mm of axial play. A shoulder shorter than the mating part stack-up binds the assembly when the thread tightens. A shoulder longer than the stack-up leaves the parts loose—add a shim washer under the head to take up excess length, but never expect the thread torque alone to hold precision alignment.
What is the difference between a shoulder screw and a regular bolt? A shoulder screw has a precision-ground cylindrical body between head and smaller-diameter thread—a bearing surface at h8 or tighter. A bolt shank is not ground to bearing tolerance. Use a shoulder screw when a part must pivot, slide, or locate precisely on the screw body.
What does h8 tolerance mean on a shoulder screw? ISO tolerance where shaft is nominally exact with negative-only deviation. For 10 mm shoulder: +0 / -0.022 mm. The shoulder never exceeds nominal—always fits a 10.00 mm reamed hole. Default catalog tolerance. h6 is tighter (+0 / -0.009 mm), h9 looser (+0 / -0.036 mm).
Can I use a socket head cap screw as a shoulder screw? Not for precision use. SHCS shank tolerance is ±0.13 mm—six times looser than h8—with no undercut relief. Works as temporary substitute for rough pivots only. For any cyclic or wear-sensitive application, use a proper shoulder screw.
What material should I use for a stripper bolt in a stamping die? 4140 (42CrMo4) alloy steel, hardened to 38–42 HRC, with h6 tolerance. Stripper bolts in progressive dies cycle millions of times at 60–120 strokes per minute. The shoulder wears against hardened tool-steel bushings—a Rockwell mismatch of 4–6 points between the shoulder and bushing minimizes adhesive wear. Replace stripper bolts every 500,000–1,000,000 cycles on a preventive schedule.
Do shoulder screws need lubrication? If the shoulder functions as a rotating pivot—yes. A dry 4140 shoulder running in a steel bushing galls within hours. Moly-disulfide grease or a light machine oil reduces friction and wear by roughly 60–80% versus dry. For food equipment, use a food-grade PTFE-based lubricant. For vacuum or cleanroom applications where lubricant outgassing is unacceptable, specify a 416 stainless shoulder with a nitrided surface running in a bronze bushing—the dissimilar metals provide some natural lubricity.
How do I prevent a shoulder screw from loosening? Shoulder screws rarely loosen—the thread is under tensile preload, separated from cyclic shear. For added security: medium thread-locker, prevailing-torque insert, or jam nut. Never use a split lock washer—it tilts the shoulder off-axis, accelerating wear.
What is the undercut for on a shoulder screw? The relief groove ensures the mating part seats against the flat shoulder face, not the incomplete last thread. Without it, the partial thread brinnels into the mating part. The rolled radius at the undercut bottom eliminates sharp corners that initiate fatigue cracks.
Our shoulder screw inventory covers DIN 923 and ISO 7379, with shoulder diameters from 3 mm to 20 mm (1/8 in. to 3/4 in.) and shoulder lengths from 5 mm to 100 mm (1/4 in. to 4 in.). Standard tolerance: h8. Precision grade: h6 on request. Materials stocked: 4140/42CrMo4 alloy steel (black oxide), 303 stainless, 316 stainless, 416 hardened stainless, and brass. Head styles: socket head, low head, button head, and flat countersunk.