SEED Guides Bolted Joints and Threaded Fasteners < >
6. Selection of Fastener MaterialThe strength of many standard metric threaded fasteners is specified according to a system set out in ISO Standard R/898. Its provisions have been incorporated in the relevant British Standards. The strength of bolts is specified using a two digit grading system. The first digit is one tenth of the ultimate tensile strength (UTS) of the bolt material in kgf/mm2 or kilopounds/mm2 (1 kgf/mm2 = 9.81 MPa). The second is ten times the ratio of the yield strength to the UTS. A grade 6.8 bolt thus has a UTS of 60 kgf/mm2 (589 MPa) and a yield strength of 0.8 x UTS = 48 kgf/mm2 (471 MPa).
A single digit grading system is used for nuts, the number indicating one tenth of the minimum UTS of the bolt, in kgf/mm2, with which the nut should be used.
Table 6.1 gives details of the bolt property classifications specified in ISO R/898, together with the type of steel normally used to achieve the required properties.
|
ISO R/898 Strength Grading
|
Minimum UTS (N/mm2)
|
Yield Strength (N/mm2)
|
Typical Material
|
|
3.6 4.6 4.8 5.6 5.8 |
294 490 490 |
177 294 392 |
Low carbon steel (Free-cutting grades allowed only where less than 12% ductility is permissible) |
|
6.6
6.8 6.9 |
589
589 589 |
353
471 530 |
Medium carbon steel. (Free-cutting grades allowed for 6.8 strength grading only) |
|
8.8 10.9 12.9 14.9 |
785 981 1177 1373 |
628 883 1060 1236 |
Medium carbon, alloy steels hardened
and tempered. (Straight carbon steels permissible for grade 8.8 in small diameters.) |
Table 6.1 Bolt properties as specified in ISO R/898
Table 6.2 gives similar details of nut properties, as specified in ISO R/898. The proof stress values given in the table represent the highest stress levels that can safely be sustained by the nut threads.
| ISO R/898 Strength Grade |
4
|
5
|
6
|
8
|
10
|
12
|
14
|
| Proof Stress (N/mm2) |
392
|
490
|
589
|
785
|
981
|
1177
|
1373
|
Important Note: Tables 6.1 and 6.2 refer to give details of static properties only. Where varying loads are applied to the fastener, a specific material of known fatigue strength should be called for.
The foregoing paragraphs should enable a nut to be selected whose threads ought theoretically to fail simultaneously with the other critical sections of the fastener assembly. Where standard nuts are to be used, the main choice open is of material. For non-standard nuts or female threads tapped in one of the components to be fastened, the necessary length of thread will depend on the strength of the chosen material. The following rules of thumb will give an initial guide to the length of female thread for use with steel bolts:
Steel threads - length at least equal to the major thread diameter.
Cast Iron, brass or bronze threads - at least 11/2 times the major diameter.
Aluminium or Zinc Alloys - at least twice the major diameter.
These values should always be checked by calculation, using equations 5.9 and 5.10. Note that other modes of failure are also possible - for example shear failure by pulling of the bolt head through one of the components being clamped.