The yield control method of tightening can obtain the maximum available preload and which distribution is relatively uniform. However

it is difficult to observe the tightening process and measure the preload once the bolt entering the plastic zone. By using the yield control method for large flange bolts of fan tower flange

the quantitative relationship between the bolt lubrication and the yield control methods are studied by using the precise finite element method (FEM) and the tightening experiment. The research shows that the relationship between the bolt preload and the torque are linear throughout the entire tightening process

and the nut factor is linearly related to the friction coefficient. The increase in friction coefficient will lead to increase the torsional effect of the bolt and the shear stress at the thread

while resulting in the earlier bolt yield and reduces the preload after tightening. There is not a correlation between the bolt elongation and the friction coefficient

and the preload in the elastic section can be accurately controlled according to the nut rotation angle. In addition

the correction factor has been introduced to measure the bolt preload based on the elongation method with the yield control method

which is independent of the bolt lubrication state

while accelerates to decrease with the increasing of bolt yield level.