Metric Fastener Classes 1

[Philrock]

Supposedly, metric fastener classes work like the following example:

Class 12.9: The 12 means the minimum ultimate tensile strength of the material is 12 x 100 MPa = 1,200 Mpa. The 9 means the minimum yield strength is 9 x 100 = 90% of minimum ultimate tensile. 90% x 1,200 = 1,080 MPa.

The above is widely posted on manufacturers and distributor web sites. However, also posted in many places are tables citing ISO standards giving slightly different values than those calculated by the above formulas. For example, ultimate for 12.9 is 1,220 MPa. Yield is 1,100 MPa. Grades 10.9 and 8.8 have similar differences, though the discrepenacies are in the other direction.

Anybody have an explanation for the inconsistencies? Which is right - the formulas or the tables?

 

[diamondjim]

The tables are correct.
The 12 10 8 etc are rounded
values. Likewise on the .9 .8
values.

 

[CoryPad]

The tables are correct. The nomenclature is a guide, but the exact numbers for yield and tensile stresses may deviate from them.

For example, 10.9 has a nominal tensile stress of 1000 MPa, but the specified minimum tensile stress is 1040 MPa. Likewise, the nominal yield stress is 900 MPa, but the minimum yield stress is 940 MPa. All of this is from ISO 898-1.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[drawoh]

Philrock,

According to my Machinery's Handbook 19th edition, the 12 means 120kgf/mm^2. See if this makes your numbers work out exactly.

This has disappeared from the 26th edition, which uses SI units properly.

JHG