Metric Socket Set Screws

2. Relevant ISO Standards:

Several ISO standards work together when specifying metric socket set screws.

ISO 898-5 specifies the mechanical and physical properties of set screws and similar threaded fasteners manufactured from carbon steel or alloy steel. It covers fasteners having ISO metric threads and classifies them according to specified hardness.

The standard covers:

• Hardness requirements.
• Torque strength.
• Decarburization requirements.
• Surface hardness.
• Surface integrity.
• Material and heat-treatment requirements.
• Coarse and fine-pitch metric threads.

ISO 261 Metric thread sizes establishes the general plan for ISO metric screw threads. It defines the nominal diameter and pitch combinations used for general-purpose metric threads. that ISO 261:1998 remains current following its

ISO 965 Metric thread tolerance establishes the tolerance system for ISO general-purpose metric screw threads. The tolerance class combines a tolerance grade with a tolerance position.
For example: M10 x 1.5 – 6g.
Means an M10 external thread with a 1.5 mm pitch and a 6g tolerance class.

3. ISO 898-5 Property Classes:

ISO 898-5 uses four principal hardness classes:

This distinction is important. A designation such as 45H is not equivalent to an ISO 898-1 “45” tensile property class. Set screws are classified differently because their principal function is to withstand compressive contact forces and transmit torque without unacceptable deformation or failure.

4. Vickers Hardness Specified by ISO 898-5:

These values provide the principal basis for the hardness class designation.
The hardness range is significant in practical applications because increasing hardness generally improves resistance to indentation and wear on the set screw point. However, excessive hardness can reduce toughness and increase susceptibility to cracking under unfavorable loading or installation conditions.

4.1 Brinell Hardness:

The Vickers test is generally the more fundamental reference for the property-class designation, while Brinell and Rockwell values provide alternative hardness measurement methods.

4.2 Rockwell Hardness:

Where applicable, ISO 898-5 gives the following Rockwell limits:

The Rockwell values should be read carefully because the standard specifies different scales and acceptance provisions, particularly for 22H. In case of doubt, the Vickers’ hardness requirements provide the decisive basis for acceptance.

5. Torque Strength:

Torque strength is an important characteristic of a socket set screw because installation requires torque to be applied through the internal hexagon socket.
ISO 898-5 does not specify a single universal torque value for all four hardness classes. The applicable torque requirements are given as a function of the fastener dimensions in the standard’s torque table.
In particular, the torque requirement applies to the higher hardness classes, while the lower classes do not have the same torque-strength requirement
This distinction is important in engineering specifications. A manufacturer or purchaser should not infer an allowable installation torque simply from the hardness class.

Installation torque can also be affected by:

• Thread friction.
• Surface coating.
• Lubrication.
• Point configuration.
• Mating-material hardness.
• Thread engagement.
• Installation speed.

Consequently, an installation torque specification used in production should be established for the actual fastener/coating/lubricant combination rather than inferred solely from the ISO hardness class.

6. Metric Thread Classes:

The term thread class should not be confused with property class.

• Property/hardness class describes the mechanical condition of the fastener material.
• Thread tolerance class describes the dimensional tolerances of the thread.

For an external metric set-screw thread, the most common general-purpose tolerance class is 6g.

7. Understanding a Thread Tolerance Designation:

A designation such as, 6g contains two elements:

• 6 = tolerance grade.
• g = tolerance position.

A designation such as M10 x1.5 x 20 mm 5g6g describes separate tolerances for the pitch and major diameter of an external thread.

• 5g = tolerance class refers to the pitch diameter.
• 6g = tolerance class for the major diameter.
• 4g = tighter tolerance on the pitch diameter.
• 6g = tolerance on the major diameter.

ISO 965 defines tolerance grades and positions for metric threads. External threads use lower-case letters such as g and h, while internal threads use upper-case letters such as G and H.
The fundamental deviation of g places the external-thread tolerance zone below the basic size. This provides clearance for plating and accommodates
assembly requirements.

8. Internal Thread Classes and Thread Fits

Although the set screw itself has an external thread, the mating tapped hole also has a thread tolerance class. Common internal thread classes include:

• 4H
• 5H
• 6H

9. Example Specification:

A complete engineering specification for a metric hexagon socket set screw could look like:

Hexagon socket set screw, M10 × 1.5 x 20mm 6g, 45H,
This specification communicates several different characteristics:
Hexagon socket set screw — fastener type.

• M10 — nominal thread diameter.
• 1.5 mm pitch — coarse metric pitch for M10.
• 20 mm—screw length.
• 6g — external thread tolerance class.
• 45H — ISO 898-5 hardness/property class.

A mating tapped hole could typically be specified as:
M10 × 1.5 – 6H
The complete assembly specification therefore distinguishes between the product geometry, material class, and thread tolerance.

10. Selection of Property Class:

Selection of the appropriate ISO 898-5 hardness class depends heavily on the application.

Property Class 14H:
14H is the lowest hardness class and can be appropriate where relatively modest mechanical demands are imposed.
Its lower hardness can also be advantageous where minimizing damage to a mating component is important.

Property Class 22H:
22H provides greater hardness than 14H and is suitable for applications requiring increased resistance to indentation and deformation.

Property Class 33H:
33H provides substantially higher hardness and is appropriate for more demanding applications where the set screw must resist significant contact stresses and wear.

Property Class 45H:
45H is the highest hardness class in ISO 898-5 and is manufactured using quenched-and-tempered alloy steel. It is intended for demanding applications where high hardness and torque resistance are required.
However, selecting 45H simply because of its “hardness” is not always correct. The high hardness of the screw can increase the likelihood of damaging a relatively soft shaft or mating surface.

11. Considerations for Shaft and Mating-Material Design:

The set screw and the component against which the set screw point bears against should be considered as a system.
For example, a hardened 45H set screw installed against a soft aluminum shaft can produce substantial local contact pressure. The shaft may deform, allowing the assembly to loosen or alter the shafts geometry.
The point style is particularly important. Cup points, flat points, cone points, dog points, and other configurations distribute or concentrate the contact force differently.

12. Important Distinction: Hardness Is Not Tensile Strength!

One of the most common errors is to treat the 14H/22H/33H/45H designations as if they were ordinary bolt property classes.

They are not: ISO 898-1 uses property classes such as 8.8, 10.9 and 12.9 for conventional bolts, screws and studs. ISO 898-5 instead establishes hardness classes for set screws and similar fasteners.

Therefore: 45H, 45 MPa, 450 MPa, or property class 4.5.
The “45” in 45H is fundamentally associated with a minimum Vickers hardness of approximately 450 HV.
This distinction is essential when creating engineering drawings, purchasing specifications, material specifications, or quality-control documentation.