Bolt Standards

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In North America bolts are classified according to ASTM standards (American Society for the Testing of Materials) while in Europe, they are classified according to national standards of the individual states. Each state has its own national standards institute BSI, DIN, AFNOR, etc. with each operating independently.

To establish common standards within the European Union, CEN (European Committee for Standardization) was set up with the participation of member states to unify the separate standards organizations under the umbrella of the EU by preparing the EN (Euro-Norm) standards.

Once agreed, member states of the EU are obliged to adopt the EN standard and withdraw any conflicting pre-existing national standard. Thus DIN XXXX would be re-published as DIN EN XXX

Then we move on to ISO – International Organization for Standardization, which is an international umbrella standards organization whose aim is to provide a single worldwide standard – which on the face of it makes a lot of sense – but to accomplish this in Europe, the EU must adopt the relevant ISO standard – which becomes an EN ISO standard – and following the EU protocols, will be referred back to the national standards institutes to be published with the ISO appendix. Therefore, it will reappear as DIN EN ISO XXXX

Confused? – you’re not alone. But on the plus side, as a detailing professional you don’t have to worry about any of this – the important thing to consider is the Grade or Property Class, or simply Class

Bolt Grades and Property Classes Explained

In Europe and beyond, metric bolts are classified according to their Property Class, or Class, while in the United States and wherever else in the world inch series bolts are used, they are classified according to their Grade or ASTM rating

Both of these standards refer to the strength of the bolt which is measured in terms of its:

Proof load: which is the axial tensile load which the bolt must withstand before any evidence of a permanent set.
Yield strength: which is the maximum load before the bolt exhibits a permanent deformation
Tensile strength: which is the maximum load in tension (pulling apart) that the bolt can withstand before breaking or fracturing.

There are numerous grades and classes to which bolts may be manufactured, but in practical terms the following tables represent the most common bolt Grades and Classes used for structural applications

Metric Series Bolts

Property Class Size range Min. Proof Strength (MPa) Min. Yield Strength (MPa) Min. Tensile Strength (MPa) Material
4.6 M5 – M36 225 400 Low carbon steel
8.8 M16 – M36 600 640 830 Medium carbon steel Q & T *
10.9 M5 – M36 830 872 1040 Low carbon martensite steel, Q & T *
12.9 M16 – M36 970 960 1220 Alloy steel, Q & T *

Class 4.6 bolts are used in non-critical structural applications
Class 8.8 bolts are heat-treated- high-strength, or Precision bolts, and are generally considered the standard for structural steel connections.
Class 10.9 and 12.9 are also heat treated high-strength bolts and are commonly found in slip-critical connections

The class identification is defined as follows: the first number multiplied by 100 is the minimum tensile strength of the bolt material in MPa and the second number multiplied by 10 is the ratio (expressed as a percentage) between the stress at permanent set of 0.2% (Yield stress) and the minimum tensile strength.

Therefore, a class 8.8 bolt has a minimum tensile strength of 800 MPa and a minimum yield stress of (80% x 800) = 640 MPa.

Inch Series Bolts

ASTM Grade Size range Min. Proof Strength (ksi) Min. Yield Strength (ksi) Min.  Tensile Strength (ksi) Material
A307 1/4” – 4” 60 Low or medium carbon steel
A325 1/2” – 1-1/2” 85 92 120 Medium carbon steel Q & T *
A490 1/2” – 1-1/2” 120 130 150 min. 173 max. Medium carbon alloy steel, Q & T *

A307 bolts are standard run-of-the-mill bolts and are rarely used in critical structural connections – they come in two grades:
Grade – is for general purpose structural applications with a minimum tensile strength of 60 ksi
Grade – is used for cast iron flanged joints in piping systems

A325 heavy hex bolts are classed as high-strength bolts and are most commonly used for structural connections. They have shorter thread lengths than standard hex bolts and the heavy hex head provides a large load-bearing area for added strength. They come in two grades, or types
Type – Carbon or alloy steel
Type – Weathering steel for added corrosion resistance
(Type 2 was withdrawn in 1991)

A 490 bolts are also high strength bolts and are commonly used in slip-critical connections. They come in two grades, or types:
Type 1 – Carbon or alloy steel
Type 3 – Weathering steel for added corrosion resistance
(Type 2 was withdrawn in 2002)

Specifying Bolts

The size of a metric screw or bolt is specified in terms of its: Diameter, Thread pitch and Length, in millimeters.
Example: M20-2.5x 50 (If pitch isn’t specified, the fastener is assumed to be coarse threaded)

Unified Thread series (Inches) are noted in a similar manner 3/4-10-2

For Nuts, size will appear as Diameter and Pitch.

Tightening Bolts

There are 2 methods for tightening bolts:

  1. The turn of the nut method
  2. The use of a direct tension indicator (Torque wrench)

Turn of the Bolt method

This method requires a prescribed rotation of the bolt-head or nut. To be effective, the head and nut must have a solid base to react against, it’s essential that, before applying the prescribed turn, all plies of the connection, at all bolt positions are pulled into close contact, called the ‘Snug Tight’ condition. At this point, suitable marks are made so that the subsequent rotation may be observed as the head or nut is subjected to the appropriate rotation, i.e. 1/3 or 3/4 turns depending on the grip length.

This is probably the most common method because of its reliability.

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