STRENGTH OF BOLTS, SCREW PLUGS, AND DOWEL PINS

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 load	Repeated load		Impact load
		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)                                                =112/5                                                =22.4 [kgf/mm2]

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

Nominal thread size	Effective cross-section area As mm2	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.