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Set Screws
A comprehensive engineering reference for mechanical design engineers, maintenance specialists, OEM manufacturers and procurement teams — covering set screw tip geometry, holding force mechanics, tip selection for shaft and bore applications, dimensional standards, torque-holding capacity, material grades, drive types and full project documentation.
Set Screw Tip Types, Holding Mechanics
& Drive Style Engineering
Cup · Cone · Flat · Dog · Oval · Hex Socket · Torx
Definition and Engineering Function
A set screw (also called a grub screw or headless screw) is a fully-threaded fastener with no head — the entire body is threaded, and the top face is either flush or recessed with a drive socket (hex, Torx or slotted). The set screw is threaded into a tapped hole in a collar, hub, coupling, pulley or housing and tightened until its tip bears against a shaft, bore surface or flat, locking the component in angular and axial position on the shaft. The set screw is the most compact possible shaft-locking fastener — it sits entirely within the outer diameter of the clamped component, leaving no protruding head to snag clothing, guards or adjacent components during rotation.
The holding mechanism of a set screw is entirely different from that of a standard bolt. A bolt develops its clamping force through the tension in the bolt body — a set screw develops its holding force primarily through the hardness and geometry of its tip pressing against the shaft. The tip indents or bears on the shaft surface, creating a local deformation or friction point that resists both torque (rotational slip) and axial thrust (sliding along the shaft). The geometry of the tip determines the balance between holding force, shaft damage and releasability — the core engineering design decision in set screw selection.
A set screw resists two types of relative motion between the collar/hub and the shaft: (1) Axial sliding — the shaft slides through the hub in the axial direction; and (2) Torsional slip — the hub rotates relative to the shaft. The axial holding force is generated by the normal force of the tip against the shaft surface (for cup, cone and dog points) or the friction force across a flat (for flat points). The torsional holding force is the axial force multiplied by the shaft radius multiplied by the friction coefficient at the tip-shaft contact. Cup point set screws provide the highest torsional holding capacity because the sharp cup rim cuts into the shaft surface, creating mechanical interlock in addition to friction. For temporary or non-damaging connections (precision shafts, frequently re-adjusted components), flat point or nylon patch set screws are selected — at the cost of lower holding capacity.
Set Screw Tip Types — Engineering Descriptions
Drive Types
Dimensions, Standards
& Tightening Torque & Holding Force Reference
Socket Size · Cup Dia. · Tightening Torque · Axial Holding Force
Set Screw Dimensions — ISO 4029 (Cup Point, Hex Socket)
| Thread | Pitch (mm) | Hex Socket s (mm) | Min Length (mm) | Cup Dia. dk (mm) | Cup Depth (mm) | Dog Dia. dp (mm) |
|---|---|---|---|---|---|---|
| M2 | 0.40 | 0.9 | 2 | 1.0 | 0.5 | 1.0 |
| M2.5 | 0.45 | 1.3 | 3 | 1.5 | 0.6 | 1.5 |
| M3 | 0.50 | 1.5 | 3 | 2.0 | 0.7 | 2.0 |
| M4 | 0.70 | 2.0 | 4 | 2.5 | 1.0 | 2.5 |
| M5 | 0.80 | 2.5 | 5 | 3.5 | 1.25 | 3.5 |
| M6 | 1.00 | 3.0 | 6 | 4.0 | 1.5 | 4.0 |
| M8 | 1.25 | 4.0 | 8 | 5.5 | 2.0 | 5.5 |
| M10 | 1.50 | 5.0 | 10 | 7.0 | 2.5 | 7.0 |
| M12 | 1.75 | 6.0 | 12 | 8.5 | 3.0 | 8.5 |
| M16 | 2.00 | 8.0 | 16 | 12.0 | 4.0 | 12.0 |
| M20 | 2.50 | 10.0 | 20 | 15.0 | 5.0 | 15.0 |
| M24 | 3.00 | 12.0 | 24 | 18.0 | 6.0 | 18.0 |
ISO 4029:2003 (hexagon socket cup point set screws). Dog diameter dp applies to ISO 4028 dog point set screws of the same thread size. Thread tolerance class 6H (internal tapped hole) / 6g (set screw external thread). Minimum length equals the nominal thread diameter — shorter set screws are non-standard. Standard lengths available in steps: 2, 3, 4, 5, 6, 8, 10, 12, 16, 20, 25, 30, 35, 40, 45, 50 mm (size-dependent availability). All dimensions in mm.
| Thread | Install. Torque (N·m) | Axial Holding Force (kN) — Steel Shaft | Torsional Holding Moment (N·m) — 20mm shaft | Flat Point Torque (N·m) | Cone Point Torque (N·m) |
|---|---|---|---|---|---|
| M3 | 0.3 | 0.6 | 6 | 0.2 | 0.5 |
| M4 | 0.7 | 1.2 | 12 | 0.5 | 1.1 |
| M5 | 1.4 | 2.1 | 21 | 1.0 | 2.2 |
| M6 | 2.5 | 3.4 | 34 | 1.8 | 3.8 |
| M8 | 6.0 | 6.7 | 67 | 4.3 | 7.5 |
| M10 | 12.0 | 11.0 | 110 | 8.6 | 13.2 |
| M12 | 20.0 | 17.0 | 170 | 14.0 | 21.0 |
| M16 | 47.0 | 32.0 | 320 | 32.0 | 45.0 |
| M20 | 92.0 | 53.0 | 530 | 62.0 | 85.0 |
Installation torque values per standard manufacturer recommendations for Grade 45H (ISO 898-5) alloy steel cup-point set screws in a steel shaft without locking compound. Axial holding force and torsional holding moment are indicative values for cup point tip engaging a hardened steel shaft — actual values depend on shaft material hardness, surface roughness, lubrication and whether the shaft has a flat or dimple. Flat point torques are typically 35–50% of cup point for the same installation torque due to the absence of mechanical interlock. Cone point torques may exceed cup point for pre-machined cone recess shafts. Always verify with the applicable machinery standard or OEM specification.
μ ≈ 0.12–0.15 // Cup point on smooth steel shaft (no lubricant)
μ ≈ 0.20–0.25 // Cup point with shallow shaft dimple / flat
μ ≈ 0.08–0.10 // Flat point on smooth shaft — friction only
// WORKED EXAMPLE: M8 cup point, Grade 45H, 25mm dia. shaft, steel-on-steel, no dimple
F_axial = 6,700 N // From Table 2 above at recommended installation torque
T_hold = 6,700 × 0.13 × 0.0125 = 10.9 N·m // Torsional holding moment on 25mm shaft
// For higher holding capacity: add shaft flat / dimple, use two set screws 90° apart, or use keyway + set screw
Governing Standards
ISO 4026 — Hexagon socket set screws with flat point. ISO 4027 — Hexagon socket set screws with cone point. ISO 4028 — Hexagon socket set screws with dog point. ISO 4029 — Hexagon socket set screws with cup point. Together these form the ISO set screw series covering M1.6–M24. DIN 913 (flat point), DIN 914 (cone point), DIN 915 (dog point) and DIN 916 (cup point) are the historical German equivalents, now aligned with the ISO series. ASME B18.3 covers socket set screws in inch series (#0–1" UNC and UNF) with equivalent tip designations. The mechanical property class for set screws is defined by ISO 898-5 (property class 14H and 45H) rather than ISO 898-1 (which covers bolts) — because set screws are loaded in torsion and compression rather than tension.
Material Grades, Mechanical Properties
& Surface Treatments
Black Oxide · Zinc · Passivation · Nylon Patch · Brass Tip
| Grade | Standard | Hardness | Min Tensile (MPa) | Corrosion | Key Application |
|---|---|---|---|---|---|
| Class 14H | ISO 898-5 | 140–200 HV | — | Low | Light-duty; low clamping force; not for structural shaft locking |
| Class 45H | ISO 898-5 | 450–560 HV | — | Low | Standard industrial; high holding force; can indent most shaft materials |
| Alloy Steel 12.9 | ISO 898-1 | 380–435 HV | 1220 | Low | High-strength set screw; precision machinery; use where 45H hardness causes shaft galling |
| SS 304 (A1/A2) | ISO 3506-1 | ≤220 HV | ≥500 | High | Food, pharma, outdoor, marine-adjacent, medical instruments |
| SS 316 (A4) | ISO 3506-1 | ≤220 HV | ≥500 | Very High | Offshore, chloride, chemical, pool, marine direct exposure |
| Brass (CuZn37) | BS 2872 | ~130 HV | ~370 | Good | Non-sparking; electrical/electronic; delicate shaft surfaces (softer than most shafts) |
| Titanium Gr.5 (Ti-6Al-4V) | ASTM F1472 | 30–36 HRC | 895 | Extreme | Aerospace, motorsport, medical devices; weight-critical |
| Nylon (PA66) | ISO 527 | ~80 HRR | ~80 | Excellent | Non-metallic, non-conductive, chemical-resistant tip element or full nylon screw (low load) |
Material Selection Guidance
ISO 898-5 Grade 45H is the standard material for set screws in industrial machinery, power transmission, and general mechanical applications. The 45H designation refers to the Vickers hardness range (450–560 HV) — significantly harder than most shaft materials (mild steel shafts are typically 140–200 HV; hardened steel shafts 400–550 HV). The high hardness of the 45H set screw tip is what enables the cup rim to indent the shaft surface, creating the mechanical interlock that gives cup-point set screws their characteristic high holding torque. However, the 45H tip is too hard for use against precision-ground shafts where surface damage is unacceptable — in those cases, specify nylon patch, flat point, or brass tip set screws.
Stainless steel set screws (SS 304 or SS 316) have a maximum hardness of 220 HV — significantly softer than Grade 45H alloy steel set screws. This means SS set screws provide lower holding force in cup-point applications because the softer SS cup tip does not indent the shaft as deeply as a hardened alloy steel tip. For SS set screw applications where maximum holding force is required, use dog-point or cone-point SS set screws engaging a pre-machined shaft feature, or supplement with thread-locking compound (Loctite 243 or equivalent). Anti-galling compound (MoS₂ paste or PTFE) on SS set screw threads is recommended for SS-to-SS thread combinations to prevent seizing during installation and removal.
| Finish / Option | Standard | Shaft Damage | Self-Locking | Notes & Application |
|---|---|---|---|---|
| Plain / Bare | — | High (cup) | No | Indoor dry; use thread locker for vibration resistance |
| Black oxide + oil | MIL-DTL-13924 | High (cup) | No | Standard for alloy steel 45H; corrosion-inhibiting oil applied; dry environments |
| Zinc electroplate | ASTM B633 | High (cup) | No | Light corrosion protection; general outdoor; not for SS shafts (galvanic) |
| SS passivation | ASTM A380 | Medium (cup) | No | All SS set screws; mandatory post-machining; food/pharma/outdoor |
| Nylon patch / insert | Manufacturer spec. | None | Yes | Self-locking; no shaft damage; food/pharma/vibrating equip.; limited re-use cycles |
| Brass tip insert | Manufacturer spec. | Very low | No | Soft tip protects hardened/polished shafts; lower holding than steel tip |
| Loctite/thread lock pre-applied | Loctite spec. | High (cup) | Yes | Micro-encapsulated adhesive on threads activated by installation torque; single-use |
| PVD / TiN coating | Proprietary | High (cup) | No | Wear and corrosion resistant; high-cycle or abrasive environments; minimal dimension change |
Tip Selection Guide, Applications
& Quality Control and Documentation
Power Transmission · Instrumentation · Food Grade · Aerospace
Tip Selection Reference — Application Decision Matrix
| Requirement | Recommended Tip | Reason | Holding Level |
|---|---|---|---|
| Max holding force, shaft can be marked | Cup Point | Cup rim creates mechanical interlock; highest torsional capacity | Highest |
| Permanent assembly, max axial hold, recess in shaft | Cone Point | Cone seats in pre-drilled recess; excellent axial and torsional lock | Highest (axial) |
| Precision shaft, no damage permitted, frequent adjust | Flat Point | No indentation; friction only; shaft surface preserved | Low |
| Positive location, removable, pre-drilled shaft | Dog Point | Cylindrical pin engages shaft hole; positive mechanical stop; re-usable | High (with hole) |
| Precision shaft, moderate hold, round contact | Oval Point | Spherical tip reduces marking vs cup; higher friction than flat | Medium |
| Vibration, no shaft damage, self-locking needed | Nylon Patch | Prevailing torque from nylon; no shaft marking; re-usable limited times | Medium |
| Hardened/polished shaft protection | Brass Tip | Soft brass tip deforms; shaft surface protected; lower holding than steel | Low–Medium |
| Set screw in keyway (on key) | Cup or Flat Point | Cup for permanent; flat for re-adjustable. Always install over keyway centre | High (cup) |
| Set screw on shaft flat (milled flat) | Cup or Flat Point | Flat point bears across the full flat area — preferred; cup also acceptable | High |
Applications by Industry
Installation Best Practices
The torsional holding capacity of a set screw can be increased by up to 100% by milling a small flat on the shaft surface directly under the set screw location. The flat allows the set screw tip to bear across a larger area and prevents the screw from riding up the curved shaft surface under torque. For cup point set screws, the flat should be milled to the same width as the cup diameter and approximately 0.5 mm deep. When installing in a keyway, position the set screw directly over the top centre of the key — this locks the key in place and transmits the clamping force into the key material rather than creating a point contact at the keyway edge.
For shaft collars and couplings in high-torque or reversing load applications, install two set screws 90° apart on the same collar — not 180° (diametrically opposite). Two screws at 90° provide more uniform radial load distribution on the shaft, reduce the risk of shaft wobble under the collar, and increase the total torsional holding moment by approximately 1.6× compared to a single screw. The first screw is tightened to specification, then the second screw is installed and tightened — then re-check the first screw torque as the shaft may have moved slightly.
For applications subject to vibration or reversing loads where vibration loosening of the set screw is a risk, apply an appropriate thread-locking compound before installation. Loctite 222 (purple — low strength) for set screws that require frequent adjustment; Loctite 243 (blue — medium strength) for standard shaft-locking set screws subject to vibration; Loctite 270 (green — high strength) for permanent installations. Never apply thread-locking compound to nylon patch set screws — the adhesive prevents the nylon from working correctly and may degrade the nylon insert.
Inspection and Quality Control
Set screw QC per ISO 4026–4029 covers: (1) Dimensional inspection — thread diameter, pitch, length and hex socket size per the applicable standard; (2) Hardness verification — Vickers hardness (HV) on the screw body and tip confirmed within the Grade 45H range (450–560 HV) — critical for cup and cone point set screws where tip hardness directly controls holding force; (3) Tip geometry — cup diameter and depth, cone angle (90° ± 1°), dog diameter and length; (4) Thread gauge — Go/No-Go per ISO 1502; (5) Torsional strength test — drive-to-fracture torque per ISO 898-5 (must exceed minimum break torque for the size). For stainless steel set screws, PMI (XRF) is performed on 100% of production lots. For nylon patch set screws, patch adhesion and Shore A hardness of the nylon insert are verified.
Export Packaging and Preservation
- Set screws packed in sealed polypropylene bags or cardboard boxes by type (tip), size, material and grade — clearly labelled with ISO standard reference (e.g. ISO 4029), thread size, length, grade (45H / A4), and batch/lot number
- VCI desiccant sachet inside each bag for alloy steel set screws destined for sea freight or storage exceeding 3 months
- SS set screws in clean sealed bags — segregated from carbon steel fasteners to prevent ferrous contamination
- Nylon patch set screws must not be stored above 40°C or exposed to direct sunlight for extended periods — UV and heat degrade the nylon patch and reduce the prevailing torque
- Bags packed in double-wall corrugated master cartons; pallets on ISPM-15 heat-treated timber with stretch wrap
- MTC (EN 10204 3.1), hardness test report (Grade 45H: HV range confirmed), dimensional inspection report, thread gauge certificate and all project documents in waterproof sealed envelope with each shipment
| # | Document | Standard / Reference | Minimum Requirement |
|---|---|---|---|
| 01 | Material Test Certificate (MTC) | EN 10204 3.1 | 3.1 for OEM / industrial; grade, heat/lot, chemical analysis |
| 02 | Dimensional Inspection Report | ISO 4026–4029 / B18.3 | Thread, length, socket size, tip geometry (cup dia., cone angle, dog dia.) per AQL |
| 03 | Hardness Test Report (HV) | ISO 6507 / ISO 898-5 | Mandatory for Grade 45H; body HV 450–560 confirmed per heat/lot |
| 04 | Thread Gauge Certificate | ISO 1502 / ASME B1.2 | Go/No-Go per lot; tolerance class 6g (external thread) |
| 05 | Torsional Strength Test Report | ISO 898-5 / ASTM F606 | Drive-to-fracture torque ≥ minimum per size — mandatory for Grade 45H |
| 06 | PMI Report (XRF / OES) | Project specification | 100% of SS 304/316, duplex, titanium and exotic grade set screws |
| 07 | Nylon Patch Test Report | Manufacturer / ISO 2320 | Required for nylon patch set screws; patch Shore A hardness, adhesion, prevailing torque |
| 08 | ISO 9001 Manufacturer Certificate | ISO 9001:2015 | Current; scope must include set screw manufacture |
| 09 | ISPM-15 Phytosanitary Certificate | IPPC / FAO | All wood packing for international export |
RR Hydraulics manufactures and exports set screws (grub screws) in all tip types — cup point (ISO 4029), flat point (ISO 4026), cone point (ISO 4027), dog point / half-dog point (ISO 4028), oval point — in alloy steel Grade 45H and 12.9, stainless steel SS 304 and SS 316, brass, titanium Grade 5, and with nylon patch, brass tip and pre-applied thread-locking compound options. Drive types: hex socket, Torx, slotted, square. M2–M24 metric and #0–1" UNC. Finishes: black oxide, zinc plate, SS passivation, PVD TiN. EN 10204 3.1 MTC, hardness test certificates, torsional strength reports, PMI, nylon patch test reports. 48-hour express dispatch on standard in-stock sizes.