METRIC NUT GRADES, IDENTIFICATION MARKINGS AND STRENGTH


Metric nut grades, identification markings and strength calculator was developed to show proof strength and Vickers and Brinell hardness values of metric nuts made of carbon steel and alloy steel. Identification marking symbols of metric nuts are given in the following charts for grades 5, 6, 8, 9, 10 and 12.

This page has been developed according to ISO 898 - 2 : 2012 "Nuts with specified property classes – Coarse thread and fine pitch thread" standard.

Some important notes from ISO 898 - 2 standard:

- Standard specifies proof load values of nuts when tested at an ambient temperature range of 10 °C to 35 °C.

- Nuts conforming to ISO 898 - 2 have been used in applications ranging from –50 °C to +150 °C.

- ISO 898 applies to nominal thread diameters up to and including M39 with triangular ISO thread.

- ISO 898 applies to different nut styles: thin nuts, regular nuts and high nuts.

- Bolt - Nut assemblies with threads tolerances wider than 6H/6g, there is an increased risk of stripping.

- Hex nuts shall be marked by an indentation on the side or the bearing surface or by an embossment on the chamfer of the hex nut. Embossed markings must not protrude beyond the bearing surface of the nut.

Root contour for ISO general purpose metric screw threads
Hex Nut Marking Examples

- The symbol for property classes of regular nuts (style 1) and high nuts (style 2) consists of one number and given in the following chart.

Nut Property
Class
5 6 8 9 10 12
Marking alternative 1
Designation Symbol
5 6 8 9 10 12
Marking alternative 2
Code Symbol
Hex Nut Property Class 5 Hex Nut Property Class 6 Hex Nut Property Class 8 Hex Nut Property Class 9 Hex Nut Property Class 10 Hex Nut Property Class 12
Identification marking symbols  for nuts

- Failure of bolt - nut assemblies due to over-tightening can occur by shank fracture of bolt or by thread stripping of the nut and/or bolt. Shank fracture is immediate and therefore easily observed. Stripping is slow and therefore difficult to catch and this causes the risk of partly failed fasteners being left in bolt and nut assemblies. It would therefore be preferred to design bolt - nut assemblies whose mode of failure would always be by shank fracture of bolt.

- Following chart is intended to provide a bolt - nut assembly which is capable of being tightened to the bolt proof load without thread stripping occurs.

Nut Property
Class
5 6 8 9 10 12
Mating Bolt
Property Class
 5.8 6.8 8.8 9.8 10.9 12.9

- Higher property class nuts may replace lower property class nuts in bolt - nut assembly.

- For metric thin nuts, property class of thin nuts is designated as 04 and 05. Thin nuts (style 0) have a reduced loadability compared to regular nuts or high nuts, and are not designed to provide resistance to thread stripping.

- Thin nuts used as jam nuts should be used with a regular nut or a high nut. In assemblies with jam nut, the thin nut is first tightened against the assembled parts and then the regular or high nut is tightened against the thin nut.

Calculator:

INPUT PARAMETERS
Metric Thread
Property Class

 


RESULTS
Parameter Symbol Value Unit
Designation -- --- ---
Property Class - ---
Thread Series -- ---
Proof Load - --- N
Max. Vickers Hardness HVmax --- ---
Min Vickers Hardness HVmin ---
Max. Brinell Hardness HBmax ---
Min. Brinell Hardness HBmin ---


Note 1: For the application of thin nuts, it should be considered that the stripping load is lower than the proof load of a nut with full loadability.

Note 2: HVmin, HVmax, HBmin and HBmax results which are given in the table for nuts with property class 8 and thread diameter M16 < D ≤ M39 coarse pitch thread are for regular nuts (style 1). For high nuts (style 2), HVmin: 180 , HVmax:302 , HBmin:171 , HBmax:287.

Note 3: HVmin and HBmin results which are given in the table for nuts with property class 12 and thread diameter M5 ≤ D ≤ M16 coarse pitch thread are for regular nuts (style 1). For high nuts (style 2), HVmin: 272 , HBmin:259.

Note 4: HVmin, HVmax, HBmin and HBmax results which are given in the table for nuts with property class 8 and thread diameter range M8×1 ≤ D ≤ M16×1.5 fine pitch thread are for regular nuts (style 1). For high nuts (style 2), HVmin: 192 , HVmax:302 , HBmin:185 , HBmax:287.

Note 5: HVmin and HBmin results which are given in the table for nuts with property class 10 and thread diameter range M8×1 ≤ D ≤ M16×1,5 fine pitch thread are for regular nuts (style 1). For high nuts (style 2), HVmin: 250 , HBmin:238.

Definitions:

Alloy Steel: A ferrous (or iron-based) alloy that contains appreciable concentrations of alloying elements (other than C and residual amounts of Mn, Si, S and P). These alloying elements are usually added to improve mechanical and corrosion resistance properties.

Nut styles Thin nuts (style 0) - 0,45D ≤ minimum height < 0,8D, regular nuts (style 1) - minimum height ≥ 0,8D, high nuts (style 2) - minimum height ≈ 0,9D or minimum height > 0,9D. (according to ISO standards)

Proof Strength: Specified loads or, in the case of proof stresses, specified stresses that the fastener must withstand without any permanent deformation.

Supplements:



Link Usage
Metric Thread Size and Tolerance Calculator Calculates basic major, minor and pitch diameters of the metric external (bolt) thread and internal (nut) thread according to ISO 724:1993 standard. In addition to basic size calculations, tolerance calculations of different tolerance classes can be done according to ISO 965-1:1998 and ISO 965-2:1998 standards.

Reference:
  • ISO 898-2:2012 – Mechanical properties of fasteners - Part 2: Nuts with specified property classes –Coarse thread and fine pitch thread