Minimum coefficient of friction in the thread

Symbol	μ_Gmin
Unit	-
Attribute-ID	vdi_2230_minimum_coefficient_of_friction_in_the_thread
Numeric ID	13021
Value Type	PMDouble

The minimum friction coefficient in the thread µ_Gmin is used to calculate the permissible assembly preload (R7) or the moment in the thread for the working stress (R8), for example.

The aim is to achieve coefficients of friction which fit into friction coefficient class B in order to apply as high a preload as possible with low scatter.

Friction coefficient class	Range for µ_G and µ_K	Selection of typical examples for
Friction coefficient class	Range for µ_G and µ_K	material/surfaces	lubricants
A	0.04 bis 0.10	metallically bright black oxide phosphate galvanic coatings such as Zn, Zn/Fe, Zn/Ni Zinc laminated coatings	solid lubricants, such as MoS₂, graphite, PTFE, PA, PE, PI in lubricating varnishes, as top coats or in pastes; liquefied wax; wax dispersions
B	0.08 bis 0.16	metallically bright black oxide phosphate galvanic coatings such as Zn, Zn/Fe, Zn/Ni Zinc laminated coatings Al and Mg alloys	solid lubricants, such as MoS₂, graphite, PTFE, PA, PE, PI in lubricating varnishes, as top coats or in pastes; liquefied wax; wax dispersions, greases; oils; delivery state
		hot-galvanized	MoS₂; graphite;wax dispersions
		organic coatings	with integrated solid lubricant or wax dispersion
		austenitic steel	solid lubricants or waxes; pastes
C	0.14 bis 0.24	austenitic steel	wax dispersions, pastes
		metallically bright phosphated	delivery state (lightly oiled)
		galvanic coatings such as Zn, Zn/Fe, Zn/Ni Zinc laminated coatings adhesive	none
D	0.20 bis 0.35	austenitic steel	oil
D	0.20 bis 0.35	galvanic coatings such as Zn, Zn/Fe; hot-galvanized	none
E	≥ 0.30	galvanic coatings such as Zn/Fe, Zn/Ni austenitic steel Al, Mg alloys	none

Table from: VDI 2230 Part1, VDI Verlag Düsseldorf, 2015, p. 114, "Table A5. Friction coefficient classes with guide values for different materials/surfaces and lubrication states in bolted joints."