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Bolt / Screw plug / Dowel pin Strength

Content shown here is only an example of how to determine strength. In practice, various conditions must be taken into account, including inter-hole pitch accuracy, hole flatness, surface coarseness, roundness, plate material, parallelism, use of hardening, press machine accuracy, production volume of products, and tool wear. Therefore, the strength calculation values should be used as a guide. (They are not guaranteed values)

Bolt Strength
Screw Plug Strength
Dowel Pin Strength

Bolt Strength

1) When the bolt is subjected to tensile load

Pt: ＝σt×As ……（1）
＝πd2σt/4 … （2）

Pt: : Tensile load in axial direction [N]

σb: : Bolt yield stress [N/mm²]

σt: : Bolt allowable stress [N/mm²]
(σt＝σb / safety factor α)

As: : Bolt effective cross-sectional area [mm²]
As＝πd2/4

d: : Bolt pitch diameter (root diameter) [mm]

Unwin safety factor α based on tensile strength
Material	Static load	Repeated load		Impact load
Material	Static load	Pulsating	Alternating	Impact load
Steel	3	5	8	12
Cast iron	4	6	10	15
Copper, soft metals	5	6	9	15

(Example) Find a suitable size for a single hex socket head cap screw that will be subjected to repeated (pulsating) tensile loads of Pt = 1960 N {200 kgf}. (Hex socket head cap screw materials: SCM435, 38 to 43HRC, strength class 12.9.)

From equation (1)

As: ＝Pt/σt
＝1960/219.6
＝8.9［mm²］

∴ The effective cross-sectional area of values larger than this should be found from the following table, and M5 of 14.2 [mm2] should be selected.

In consideration of fatigue strength, M6 with an allowable load of 2087 N {213 kgf} is selected from the strength class 12.9 in the table.
Yield stress for strength class 12.9 is σb = 1098 [N/mm²] {112 [kgf/mm²]}
Allowable stress σt＝σb / safety factor (from table above, safety factor 5)
＝1098/5
＝219.6［N/mm²］{22.4［kgf/mm²］}

2) When it is subjected to tensile impact load, like a stripper bolt, select from fatigue strength.
(Similarly, assume the condition of receiving a load of 1960 N {200 kgf}, and stripper bolt materials: SCM435, 33 to 38 HRC, strength class: 10.9.)
In accordance with the below table, M8 with 3116[N] {318[kgf]} is selected when the allowable load of strength class 10.9 is 1960 N {200 kgf} or more. Consequently, MSB10 with a 10 mm shaft diameter and an M8 thread section is selected. When receiving a shear load, use together with a dowel pin.

Bolt fatigue strength (For screws: Fatigue strength is 2 million cycles)
Nominal thread size	Effective cross-sectional area As mm²	Strength class
		12.9		10.9
		Fatigue strength ^*1	Allowable load	Fatigue strength ^*1	Allowable load
		N/mm² {kgf/mm²}	N{kgf}	N/mm² {kgf/mm²}	N{kgf}
M4	8.78	128{13.1}	1117{114}	89{9.1}	774{79}
M5	14.2	111{11.3}	1568{160}	76{7.8}	1088{111}
M6	20.1	104{10.6}	2087{213}	73{7.4}	1460{149}
M8	36.6	87{8.9}	3195{326}	85{8.7}	3116{318}
M10	58	73{7.4}	4204{429}	72{7.3}	4145{423}
M12	84.3	66{6.7}	5337{565}	64{6.5}	5370{548}
M14	115	60{6.1}	6880{702}	59{6}	6762{690}
M16	157	57{5.8}	8928{911}	56{5.7}	8771{895}
M20	245	51{5.2}	12485{1274}	50{5.1}	12250{1250}
M24	353	46{4.7}	16258{1659}	46{4.7}	16258{1659}

Screw Plug Strength

Find the allowable load P when the screw plug MSW30 receives an impact load.
(MSW30 materials: S45C, tensile strength σb of 34 to 43HRC is 637 N/mm2 {65 kgf/mm2}.)
If it breaks due to shear at the MSW root diameter portion,

Allowable load P: ＝τt×A
＝38×1074
＝40812［N］{4164［kgf］}

Shear cross-sectional area A＝ Root diameter d1 × π × L
(Root diameter d1≒M-P)
A＝（M－P）πL＝（30－1.5）π×12
＝1074［mm²］
Yield stress ≒0.9 × tensile strength σb＝0.9 × 637＝573［N/mm²］
Shear stress ≒0.8 × yield stress
＝459［N/mm²］
Allowable shear stress τt = shear stress / safety factor 12
＝459/12［N/mm2］＝38［N/mm²］{3.9［kgf/mm²］}

When the power strip is made of soft material, the allowable shear is found from the root diameter of the female thread.

Dowel Pin Strength

Find the appropriate size when there is a repetitive (pulsating) shear load of 7840N{800kgf} for one dowel pin. (Dowel pin material is SUJ2, hardness from 58HRC)

SUJ2 yield stress response σb＝1176［N/mm2］{120［kgf/mm²］}
Allowable shear strength τ＝ σb × 0.8 / safety factor α
＝1176×0.8/5
＝188［N/mm²］{19.2［kgf/mm²］}

∴ If it is an MS dowel pin, select a size of D8 or larger.
Also, if the size of the dowel pins is unified to a larger size, tools and stock, etc., can be reduced.

Do not use in such a way of placing a load on the thread section.

Notes

*1 Fatigue strengths have been excerpted from “Estimated values of fatigue limits for metric threads of small screws, bolts, and nuts” (Yamamoto) and modified.