Principle of Bolt Thread Fastening and Torque Setting
Fastening torque and pre-tightening force
Rotating nuts or screws cause the screw to elongate
The clamping force generated by the elongation of the screw clamps the joint
What we need is the clamping force in the joint
Deformation relationship of fastened bolts
Clamping force is generated through threads to clamp the connected parts
Relationship between pre-tightening force and frictional force
The tightening torque must be controlled within a certain range
Make sure that the applied torque reaches the minimum required torque
The clamping force must be higher than the external load
Influencing factors of safety margin load: vibration,change of frictional force,change in joint size,tightening precision.
The applied torque should not exceed the use limit.
Applying excessive torque will cause the bolt to overstretch.
Safety margin depends on factors such as tightening accuracy and material grade.
Setting the Fastening Torque
Firstly, determine the required preload based on the loading conditions of the fastened joint.
Select fasteners with appropriate specifications and performance grades based on the preload.
Calculate the tightening torque based on the preload, friction coefficient, and other factors.
Determining Fastening Torque through Experiments
Schatz Multi-Function Bolt Fastening Analysis System
Tightening device: includes torque/angle sensor.
Attachment of bolts and nuts: a socket drives the nut's rotation, while the sensor continuously measures the tightening torque.
· Refer to international standard "GB-T 3098.1-2000 Mechanical Properties of Fasteners - Bolts, Screws, and Studs" and look at Table 7 or Table 9 to obtain the bolt's guaranteed load based on nominal diameter, pitch, and performance grade.
· Follow the international standard "ISO 16047 - Fasteners - Torque/Preload Test Standard" and use 75% of the guaranteed load as the preload for testing.
· Use the bolt's nominal diameter, pitch, performance grade, and preload as input parameters for the equipment.
· The testing equipment can measure the following parameters from the beginning of tightening to reaching the set preload:
(1) Tightening torque (2) Thread torque.
· Through software analysis, other coefficients can be calculated, such as bearing face torque, thread torque, bearing face friction parameters, thread friction coefficient, and torque coefficient.