Bolt strength

(Example: Find a suitable size for a single hexagon socket head cap screw that will be subjected to repeated (pulsating) tensile loads of P=200 kgf. (Hexagon socket head cap screw material: SCM435, 38~43 HRC, strength class 12.9)
  From formula (1) :

2) For stripper bolts and others which are subjected to tensile impact loads, the
    selection is made based on the fatigue strength. (The bolt is subjected to
    200 kgf loads in the same way. Stripper bolt material: SCM 435 33~38 HRC,
    strength class 10.9.)

    From the table at right, for a strength class of 10.9 and a maximum allowable
    load of 200 kgf, the suitable bolt is a 318 [kgf] M8.Therefore we select a 10 mm
    MSB10 with a M8 thread section. When the bolt is subjected to shear load, also
    use a dowel pin.

Screw plug strength

Dowel pin strength

Find a suitable size for a single dowel pin which is subjected to repeated (pulsating)
shear loads of 800 kgf. (Dowel pin material: SUJ2 hardness 58 HRC or higher)

∴For an MS dowel pin, select a size of D8 or larger.
   In addition, selecting a single size for all dowel pins makes it possible to reduce items such as
   tools and inventory.

Unwin safety factor α based on tensile strength

Material Static
Repeated load Impact
Pulsating Alternating
Steel 3 5 8 12
Cast iron 4 6 10 15
Copper, soft metals 5 5 9 15

Standard strength: For ductile materials=Yield stress
                               For brittle materials=Fracture stress

Yield stress for strength class 12.9 σb=112 [kgf/mm2]
Maximum allowable stress σt=σb / safety factor
(From table above, safety factor  =5)
                                               =22.4 [kgf/mm2]

Bolt fatigue strength (For threads: fatigue strength = count of 2 million)

area As
Strength class
12.9 10.9
Fatigue strength* Maximum
allowable load
Fatigue strength* Maximum
allowable load
kgf/mm2 kgf kgf/mm2 kgf
M 4 8.78 13.1 114 9.1 79
M 5 14.2 11.3 160 7.8 111
M 6 20.1 10.6 213 7.4 149
M 8 36.6 8.9 326 8.7 318
M10 58 7.4 429 7.3 423
M12 84.3 6.7 565 6.5 548
M14 115 6.1 702 6 690
M16 157 5.8 911 5.7 895
M20 245 5.2 1274 5.1 1250
M24 353 4.7 1659 4.7 1659
Fatigue strengths* have been excerpted from "Estimated values of fatigue limits for metal threads of small screws, bolts, and nuts" (Yamamoto) and modified.

Do not use in such a way that load is applied to the threads.
The information provided here is only an example of calculating the strength. For actual selections, it is necessary to consider the hole pitch accuracy, hole perpendicularity, surface roughness, circularity, plate material, parallelism, with or without heat treatment, accuracy of the press machine, production volume, tool wear, and various other conditions. Therefore the strength calculation value should be used only as a guide. (It is not a guaranteed value.)


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