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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.

Related Industrial High-Strength Fasteners & Precision Machining Parts
916 Wode Blvd, Changqing District, Jinan, Shandong, China, 250300
916 Wode Blvd, Changqing District, Jinan, Shandong, China, 250300