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Screw Type / Appropriate Tightening Axial Force of Bolt / Appropriate Tightening Torque (Fundamentals of Screw)

Screw Type

Thread shape

Triangular screw thread

Common fastening parts

Square thread

It is used when strong force is applied to the screw thread
Mainly used in power transmission area (jacks, vices, etc.)

Trapezoidal lead screw

Used in precise motion transmission areas (lathe feed screws, etc.)

Male thread and female thread

Standard	Symbol	Example
Metric coarse thread	M	M3
Metric fine thread	M (with pitch)	M3 x 0.35
Unified coarse thread	UNC	3/4-10UNC
Unified fine thread	UNF	1/2-20UNF
Parallel pipe thread	G, PF	G1/2
Tapered pipe thread	R, PT	R3/4

* Metric threads are according to JIS B 0205 (1997) for coarse threads and JIS B 0207 (1982) for fine threads.

Male thread

Those with threads on the outside of a cylindrical body or conical body

Female thread

Those with threads on the inside

Screws are used in combination with male and female threads, so the dimensions of each must match.
That is to say, fastening is not possible unless the nominal numbers are the same.

Material

Requirements for screw materials

Low cost and easy to obtain
With strength and toughness, and machining is possible
Properties can be changed widely by appropriately selecting components and heat treatments

Typical materials

Steel alloy SCM (chrome molybdenum steel): Standard
Stainless steel SUS: It is used in locations where corrosion and outlet gas emissions are undesirable

Surface treatment

Black oxide coating
Bright chromate plating

Appropriate Tightening Axial Force of Bolts / Appropriate Tightening Torque

The torque method is widely used because of its simplicity

Tightening axial force and fatigue limit when fastening with bolts

Calculation of the appropriate tightening axial force when tightening bolts shall be within the elastic range, that is up to 70% of the standard yield strength in the torque method.
Fatigue strength of bolts due to repeated load shall not exceed the allowable value.
The fastened object shall not be recessed on the bolt and nut bearing surfaces.
The fastened object shall not be damaged by tightening.

Calculation of tightening axial force and tightening torque

Tightening axial force: F_f＝0.7・σ_y・A_s

Tightening torque: T_fA＝0.35k（1＋1/Q）σ_y・A_s・d

k: : Torque coefficient

d: : Bolt nominal diameter [cm]

Q: : Tightening coefficient

σ_y: : Yield strength (112 kgf/mm² when strength class 12.9)

A_s: : Bolt effective cross-sectional area (mm²)

<Calculation examples>

When tightening mild steel to mild steel with M6 hex socket head cap screw (strength class 12.9) under oil lubrication

Axial force is

F_f: ＝0.7・σ_y・A_s; ＝0.7・112・20.1; ＝1576［kgf］

Appropriate torque is

T_fA: ＝0.35k（1＋1/Q）σ_y・A_s・d; ＝0.35・0.175（1＋1/1.4）112・20.1・0.6; ＝142［kgf・cm］

**Torque coefficient based on bolt surface treatment and combination of the fastened object material and the female thread material**
Bolt surface treatment lubrication	Torque coefficient ｋ	Combination Material of the fastened object (a) - female thread material (b)
Steel bolt Black oxide coating Oil lubrication	0.145	SCM-FC　FC-FC　SUS-FC
	0.155	S10C-FC　SCM-S10C　SCM-SCM　FC-S10C　FC-SCM
	0.165	SCM-SUS　FC-SUS　AL-FC　SUS-S10C　SUS-SCM　SUS-SUS
	0.175	S10C-S10C　S10C-SCM　S10C-SUS　AL-S10C　AL-SCM
	0.185	SCM-AL　FC-AL　AL-SUS
	0.195	S10C-AL　SUS-AL
	0.215	AL-AL
Steel bolt Black oxide coating No lubrication	0.25	S10C-FC　SCM-FC　FC-FC
	0.35	S10C-SCM　SCM-SCM　FC-S10C　FC-SCM　AL-FC
	0.45	S10C-S10C　SCM-S10C　AL-S10C　AL-SCM
	0.55	SCM-AL　FC-AL　AL-AL

S10C: : Untempered mild steel

SCM: : Heat-treated steel (35JRC)

FC: : Cast iron (FC200)

AL: : Aluminum

SUS: : Stainless steel (SUS304)

**Standard value of tightening coefficient Q**
Tightening coefficient Q	Tightening method	Surface condition		Lubrication condition
Tightening coefficient Q	Tightening method	Bolt	Nut	Lubrication condition
1.25	Torque wrench	Manganese phosphate	Not treated or phosphate	Oil lubrication or MoS2 paste
1.4	Torque wrench	Not treated or phosphate
1.4	Wrench with torque limit
1.6	Impact wrench
1.8	Torque wrench	Not treated or phosphate	Not treated	No lubrication
1.8	Wrench with torque limit	Not treated or phosphate	Not treated	No lubrication