All-thread Hex Locking Screw: High Clamping Force Selection

Terminology Note: American vs. British English

Before diving into the technical content, one key terminology issue needs clarification, because it directly affects selection and communication accuracy.

In American English, a "set screw" refers to a headless, fully threaded screw that holds an object in place within or against another object by pressure or friction—for example, securing a pulley or gear to a shaft. It typically is not used with a nut; instead, it threads into a tapped hole in only one of the two objects being secured.

In British English, "set screw" or "setscrew" refers only to a fully threaded bolt—a bolt with no smooth unthreaded shank, which is what American English calls a "cap screw."

The full-thread hex head set screw discussed in this article corresponds to DIN 933 / ISO 4017 in the metric system. It is a "set screw" in the British sense (a fully threaded bolt), not the American headless set screw. It has a hex head and is typically used with a nut or threaded into a tapped hole to achieve clamping.

Standard Overview

The DIN 933 standard was withdrawn in October 1991, and its technical content has been superseded by ISO 4017. Technical documentation from Böllhoff and PGB Europe confirms: "DIN standard withdrawn | Wrench widths for M10, M12, M14, and M22 changed | Bolts can mate with 6h tolerance internal threads."

In actual purchasing, DIN 933 is still widely used as a product name, but note that DIN 933 and ISO 4017 differ in width across flats for M10, M12, M14, and M22. DIN 933 specifies fully threaded hex head bolts, with threads extending all the way to the underside of the head, corresponding to the partially threaded DIN 931. The standard covers metric thread sizes, commonly from M3 to M30, with manufacturers supplying up to M64.

Key Technical Features

Full Thread Design: The Engineering Logic of Flexibility and Clamping Force

The essential difference between DIN 933 and DIN 931 is the full thread structure. The thread extends from the end of the shank all the way to the head bearing surface, which means:

Adjustable along the full length, threads never "run out": The bolt can mate with a nut at any position, with clamping length not limited by an unthreaded shank. In thin panel connections or where multiple washers are needed for adjustment, full thread ensures sufficient thread engagement at all times—no risk of the nut bottoming out on an unthreaded shank and being unable to tighten further.

Advantage under tensile load: Fully threaded bolts perform well in axial tension-dominated clamping. With threads distributed along the full length, the tension applied to the bolt is spread across the entire shank, making it suitable for connections requiring high grip force.

Trade-off under shear load: On a fully threaded bolt, the shear plane falls in the threaded section, and the thread root is a stress concentration point. Under transverse force, the thread root can become a fatigue crack initiation site. Therefore, structural connections carrying significant transverse shear loads should preferentially use DIN 931 partially threaded bolts—their unthreaded shank sits at the shear plane, and the full cross-section provides higher shear strength.

Clamping Mechanisms for Shafts/Gears/Pulleys

In shaft-hub connections, full-thread hex head bolts are typically used in the following ways:

Axial face clamping: The bolt passes through a through-hole in an end cap or pressure plate and threads into a tapped hole in the hub face. Tightening axially clamps the gear or pulley against the shaft shoulder. Full thread ensures sufficient engagement even when hub thickness varies. This is the most common axial positioning method for gears, sprockets, and pulleys on shafts.

Clamping bolts for locking assemblies/wedges: In locking assemblies or wedge-type shaft-hub connections, the bolt threads into a tapped hole in the wedge or sleeve. Tightening drives the wedge along a conical surface, pressing the sleeve onto the shaft. Industrial power transmission documentation lists DIN 931/933 Grade 10.9 bolts as standard clamping bolts for such locking devices, with sizes from M5 to M6 depending on transmitted torque and structural dimensions.

Set function (American context): For shaft securing, if using the American definition of "set screw" (headless), the screw threads into a tapped hole drilled in the hub, with the tip pressing directly against the shaft surface, securing by pressure/friction. The hex head version discussed here achieves similar axial clamping through a mating nut or tapped hole, but with a head that allows greater tightening torque.

Engineering Considerations for Clamping Force: Torque and Friction

Clamping force is not simply determined by "tightening harder." In engineering, clamping force is estimated by the relationship:

F = T / (K × D)

Where T is applied torque, K is the nut factor (friction coefficient), and D is the nominal bolt diameter. Only about 10%-15% of input torque converts to actual clamping force; the rest is consumed by friction in the threads and at the head bearing surface.

This means:

• Dry steel bolts (K ≈ 0.20) produce about 25% less clamping force at the same torque than lightly oiled bolts (K ≈ 0.15)

• Coated bolts (K ≈ 0.10) have higher clamping force efficiency, but the specific effect of the coating on friction coefficient must be considered

• For shaft-hub connections, consistent lubrication state is more important than simply pursuing higher torque to ensure predictable clamping force

In gear/pulley securing, if torque is transmitted solely by the bolt's axial clamping force (keyless connection), the friction generated by clamping force must be sufficient to overcome working torque. For high-torque transmission, keyways or locking assemblies are typically used to share the load, with the bolt providing only axial clamping force.

Property Grades and Materials

Per the ISO 898-1 system, DIN 933 covers grades from general to ultra-high strength:

Grade 8.8 carbon steel is the most common grade for shaft-hub connections, with 800 N/mm² tensile strength balancing strength and cost. Grade 10.9 alloy steel (such as SCM435, hardness HRC 32-39) is used for locking assemblies and other devices requiring higher clamping force. Grade 12.9 uses ML40Cr alloy steel with 1220 N/mm² tensile strength for high-stress clamping and molds/tooling. Stainless steel A2-70 (304) and A4-70 (316) are used where corrosion risk exists.

Dimensional Parameters (Common Sizes)

Per Böllhoff and Fastenal dimensional data:

Bolded sizes are those where width across flats differs from ISO 4017. Product grade is A (≤M24) and B (>M24), with Grade A having tighter tolerances.

Surface Finishes

Hydrogen embrittlement boundary conditions: For electroplated steel parts with tensile strength ≥1000 MPa or hardness ≥320 HV, there is a risk of hydrogen-induced brittle fracture. Grade 10.9 and above should avoid electroplated zinc, preferring zinc flake coating.

Application Areas

Shaft Securing and Power Transmission Components

Full-thread hex head bolts are used for axial clamping and positioning in shaft-hub connections. Gears, pulleys, sprockets, and other transmission components are clamped against shaft shoulders by bolts, or secured to the shaft by radial force from locking assemblies. Industrial locking device documentation lists DIN 931/933 Grade 10.9 bolts as standard clamping bolts for transmitting torque and axial force.

Axial Positioning of Gears and Pulleys

Where adjustable axial position is required, the continuous thread of a full-thread bolt allows locking at any position. For example, when a pulley's axial position on a shaft needs fine adjustment for belt alignment, a full-thread bolt with nut can be locked immediately after adjustment—no concern about mismatched unthreaded shank length.

Fixtures, Jigs, and Molds

Full-thread hex head bolts are used in fixtures and molds where full-depth thread engagement is required. When clamping thickness varies widely, or when full-depth threading into a tapped hole is needed for maximum clamping force, the full thread design avoids the "can't reach" problem of an unthreaded shank. Grade 12.9 versions (1220 N/mm² tensile strength, HRC 39-44 hardness) suit high-stress tooling applications such as mold clamping and ejection mechanisms.

General Machinery and Equipment Assembly

Grade 8.8 full-thread hex head bolts are widely used for component connections in equipment manufacturing, construction machinery, and agricultural machinery. The full thread design is particularly convenient in assemblies requiring clamping length adjustment or multiple washer adjustments. Hot-dip galvanized versions (marked 8.8U) suit outdoor and corrosive environments, but note the mating nut requirement (6H/6G)—the U marking indicates the bolt thread was reduced to 6az before coating and must mate with standard nuts after coating; if the bolt has no U marking (6g/6h before coating), the nut must be oversize-tapped 6AZ/6AX after coating (marked Z or X). The two paths must not be mixed—mixing causes thread stripping or assembly interference.

Selection Guide

Summary

Full-thread hex head set screws (DIN 933/ISO 4017) feature threads extending to the head as their core characteristic, providing flexible clamping length and maximum thread engagement in shaft, gear, and pulley securing. Their advantage lies in accommodating varying clamping thickness and full-depth threading into tapped holes, making them suitable for axial tension-dominated clamping. In shaft-hub connections for transmission components, they are often used as clamping bolts for locking assemblies or wedges, with Grade 10.9 alloy steel versions meeting higher clamping force requirements. The actual efficiency of clamping force is significantly affected by friction coefficient—only about 10%-15% of torque converts to clamping force, so consistency of lubrication state is more important than simply pursuing higher torque. Note when selecting: full-thread bolts have lower shear strength than partial-thread bolts; where significant transverse load exists, use a keyway or switch to DIN 931. DIN 933 has been superseded by ISO 4017, with M10/M12/M14 differing in width across flats—verify wrench sizes when purchasing.