Guidelines for tightening factors and tolerances
According to VDI 2230 1986 edition (The Association of German Engineers bolt calculation theory).
|Setting procedure for screwdriver
|±5 to ±12
|Yield point controlled motorised tightening.
|The fluctuation of the preload is primarily governed by the fluctuation of the yield load in the screw to be assembled. In this case, the screws are designed for minimum preload.
|±5 to ±12
|Angle of rotation (turn of nut) controlled motorised or manual tightening.
|Experimental determination of the snug torque (pre-tightened and rotation angle (steps).
|±9 to ±23
|Setting is established by measuring length elongation or applied pressure, respectively.
|Low values for long screws high values for short screws.
|±17 to ±23
|Torque control tightening with manual torque control.
|Experimental determination of the nominal tightening torque required on original screw, e.g. by elongation measurements of screw.
Lower values for: A large number of torque tests e.g. 20 minimal fluctuations of the output torque. Electronically controlled torque during assembly using precision drivers.
Lower values for: Precision torque wrenches (e.g. with dial indicator) consistent tightening precision screwdrivers. Higher values for torque wrenches with accoustic signalling or release mechanism.
Lower values for: Small turning angles that is relatively stiff joints. Joint members with smooth surfaces (threads and bearing surfaces) surfaces that do not tend to gall i.e. phosphated.
Higher values for: Large turning angles for that is, relatively flexible joints as well as fine threads Joint members with rough/hard surfaces. Form distortions (head to shank not perpendicular thread distortions).
|±23 to ±28
|Determination of required torque by estimating the friction coefficient (surface and lubricating conditions).
|±26 to ±43
|Torque controlled tightening with precision power screwdriver.
|Pre-setting of power driver with post tightening torque, which is established from the required torque (for estimated friction conditions) plus post torquing.
|Lower values for: A large number of torquing tests and post torquing screw driver with cut-off coupling.
|±43 to ±60
|Impulse controlled tightening with impact wrench.
|Pre-setting of power driver with post torquing as above.
|Lower values for: A large number of torquing tests (with post torquing) on the horizontal axis of the screw characteristics. Play-free impulse transmission.