About JIS screw strength/ torque
About JIS screw strengtha
JIS specifies 10 steps of strength categories for bolts.
JIS specifies strength categories for bolts, and the table is attached below. (stainless steel is specified separately.)
For the strength category 4.8, the first numeric character 4 indicates the nominal tensile strength 400N/mm2.
The second numeric character 8 indicates the yielding point where it does not break even if 80% of 400 is applied. Therefore, the yielding point of this bolt is around 400 x 0.8 = 320N/mm2.
| Strength category | 3.6 | 4.6 | 4.8 | 5.6 | 5.8 | 6.8 | 8.8 | 9.8 | 10.9 | 12.9 |
| Nominal tensile strength (N/mm2) （N/mm2） |
300 | 400 | 400 | 500 | 500 | 600 | 800 | 900 | 1000 | 1200 |
The nuts also have strength categories.
Nuts also have strength categories.
The combinations between the nuts whose nominal height is 0.8d or above (e.g., 8mm or above when M10) and their bolts are listed below.
| Nut strength category |
Combine bolt | Nut | ||
| Strength category | Range of nominal sizes of the screw |
Style 1 | Style 2 | |
| Range of nominal sizes of the screw |
||||
| 4 | 3.6,4.6,4.8 | ＞Ｍ16 | ＞Ｍ16 | ― |
| 5 | 3.6,4.6,4.8 | ≦Ｍ16 | ≦Ｍ39 | ― |
| 5.6,5.8 | ≦Ｍ39 | |||
| 6 | 6.8 | ≦Ｍ39 | ≦Ｍ39 | ― |
| 8 | 8.8 | ≦Ｍ39 | ≦Ｍ39 | ＞Ｍ16 ≦Ｍ39 |
| 9 | 9.8 | ≦Ｍ39 | ― | ≦Ｍ16 |
| 10 | 10.9 | ≦Ｍ39 | ≦Ｍ39 | ― |
| 12 | 12.9 | ≦Ｍ39 | ≦Ｍ16 | ≦Ｍ39 |
The bolt strength varies depending on the material used and the heat treatment.
The most general steel materials are carbon steel and alloy steel, and the strength category varies according to their type and heat treatment.
The materials and heat treatments corresponding to strength categories are listed below.
| Strength category |
Material and heat treatment |
Chemical components (check analysis) % | Tempering temperature °C |
||||
| C | P | S | B | ||||
| Min | Max | Max | Max | Max | Min | ||
| 3.6(1) | Carbon steel | ― | 0.20 | 0.05 | 0.06 | 0.003 | ― |
| 4.6(1) | ― | 0.55 | 0.05 | 0.06 | 0.003 | ― | |
| 4.8(1) | |||||||
| 5.6 | 0.13 | 0.55 | 0.05 | 0.06 | 0.003 | ― | |
| 5.8(1) | ― | 0.55 | 0.05 | 0.06 | 0.003 | ― | |
| 6.8(1) | |||||||
| 8.8(2) | With additives (e.g., B, Mn, Cr) Carbon steel, hardening/ tempering Carbon steel, hardening/ tempering |
0.15(7) | 0.40 | 0.035 | 0.035 | 0.003 | 425 |
| Carbon steel, hardening/ tempering | 0.25 | 0.55 | 0.035 | 0.035 | |||
| 9.8 | With additives (e.g., B, Mn, Cr) Carbon steel, hardening/ tempering |
0.15(7) | 0.55 | 0.035 | 0.035 | ||
| Carbon steel, hardening/ tempering | 0.25 | 0.55 | 0.035 | 0.035 | |||
| 10.9(3) | With additives (e.g., B, Mn, Cr) Carbon steel, hardening/ tempering |
0.15(7) | 0.35 | 0.035 | 0.035 | 0.003 | 340 |
| 10.9(4) | Carbon steel, hardening/ tempering | 0.25 | 0.55 | 0.035 | 0.035 | 0.003 | 425 |
| With additives (e.g., B, Mn, Cr) Carbon steel, hardening/ tempering |
0.20(7) | 0.55 | 0.035 | 0.035 | |||
| Alloy steel (6), hardening/ tempering | 0.20 | 0.55 | 0.035 | 0.035 | |||
| 12.9(4)(5) | Alloy steel (6), hardening/ tempering | 0.28 | 0.50 | 0.035 | 0.035 | 0.003 | 380 |
(1) Free cutting steel may be used as the materials of these strength categories. However, the maximum content of sulfur (S), phosphorus (P) and lead (Pb) shall be as per the following: S: 0.34%, P: 0.11%, Pb: 0.35%.
(2) If adequate hardening is required for any screws whose nominal diameter is over 20mm, it is good to use materials for Strength category 10.9.
(3) For the products of this strength category, the symbol of strength category has to be distinguished by underlining.
(4) For the materials of this strength category, it is necessary to consider adequate hardenability so that approx. 90% of the center of the cross section of the threaded portion becomes martensitic structure in the state of hardening before tempering.
(5) For strength category 12.9, there should be no white concentrated phosphorus layer which is visible with an optical microscope on the surface where tensile stress works.
(6) At least 1 alloying element among chromium (Cr), nickel (Ni), molybdenum (Mo) or vanadium (V) should be included for alloy steel of this strength category.
(7) For boron steel whose C is 0.25% or less (ladle analysis value), the content of Mn should be 0.6% or above for the strength category 8.8, and 0.7% or above for the strength categories 9.8 and 10.9
(8) When the male threaded fasteners of the strength categories 8.8 to 12.9 are tested, there should not be difference of the average hardness which exceeds 20 in Vickers hardness value.
Torque of screws
10% of the tightening torque is transferred to the axial force.
The amount of tightening torque transferred to the axial force is 10%, and 90% of it is lost by abrasion. Pay attention because it may become loose even if you think it was tightened firmly. It is possible to reduce the abrasion loss by using lubricant.
Management of tightening force is important.
Screws break if they are tightened with too much force, and they become loose if they are not tightened enough.
Therefore, it is very important to manage the tightening force, but since there are variations caused by the conditions of abrasion and tightening works as described above, management is very difficult.
As the managing method of tightening force, the calibrated wrench tightening method and the turn-of-nut tightening method are well known.
Calibrated wrench tightening method
he tightening torque is managed during tightening works using tools such as a torque wrench.
Turn-of-nut tightening method
This is a method to acquire the target axial force by turning the bolt or the nut by the set turning angle (degrees) from the state in which the bolt and the nut are completely seated.
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