Background
The threaded torque angle method is to tighten the bolt or nut to the initial torque and then rotate it at a specific angle. Depending on the magnitude of the target axial force, the bolt can be screwed to the elastic area or to overyield. The advantage is that the initial torque tightening phase is affected by the coefficient of friction, but the axial force generated during this phase is negligible and negligible.
In the elastic region, the axial force is proportional to the elongation of the bolt, and the axial force obtained by rotating the bolt to a specific angle is not affected by the friction coefficient, and the dispersion of the axial force of the bolt is small, so the torque angle method is more and more widely used in the assembly of automobiles and construction machinery.
In order to standardize the design of torque corner process tightening technology and improve the reliability of connection, today, Screw Jun introduces the commonly used strength grades to all partners. 9 metric bolts, in the elastic zone and the yield zone torque angle tightening method connection, recommended yield zone and elastic zone tightening recommended process parameters, for your reference, but more accurate tightening process parameters should be determined according to the test results.
1. The principle of ultrasonic measurement of axial force
According to the formula for calculating the axial force, the axial force f of the bolt is proportional to the elongation of the bolt. The time difference between the ultrasonic transmission of the free state and the tightening state of the bolt is received by the sensor, and the change of the elongation of the bolt in the free state and the tightening state can be calculated from the time difference, and the axial force f of the bolt can be calculated.
f - bolt axial force.
e - modulus of elasticity of the bolt material.
s - bolt cross-sectional area.
l - the amount of deformation of the bolt.
l - Bolt clamping length.
2. The torque angle process is divided into elastic zone tightening and yield zone tightening
In the torque angle tightening process of the elastic section, the final value range of the process is controlled in the elastic zone under this assembly method, and the bolt utilization rate is 65%.
Torque angle tightening process in the yield zone, under this assembly method, the final value range of the process is controlled in the yield zone, and the bolt utilization rate is 100%.
3. The scope of application of the torque turning angle process
a.After assembly, the parts that need to be disassembled, repaired and replaced frequently are not suitable for the yield zone tightening process.
b.When the reinforced part is a thin plate (clamping thickness < 1d), the torque angle tightening process is not recommended.
Fourth, two key terms of torque angle technology
1.Threshold torque ms—the torque used to fit the joint part. Before this torque, the torque control method is adopted, and after this torque, the switch is switched to corner control.
2.Process Rotation Angle wa – The value of the rotation angle controlled by the second stage of the Torque Rotation Angle, which is completed after the installation of this rotation is completed, and a complete assembly process is completed.
5. Setting of torque and angle process parameters
The torque angle tightening process needs to determine two process parameters according to the results of the torque angle tensile force test - threshold torque ms and process angle wa. In the test, ultrasonic axial force testers, electric tightening machines and friction coefficient testers and other equipment are used.
At the beginning of the test, the initial torque is generally about 30% of the yield torque, and the angle can be designed according to the yield of the bolt. When the curve is obtained, the threshold torque can be adjusted, and the angle of the process angle can be obtained according to the target axial force value.
Fig.1. Curves obtained from the torque-turn tensile force test.
6. Recommended value of torque and angle process parameters in the yield zone
The commonly recommended yield value, torque, corner, process torque and preload parameter values are shown in Table 1 and Figure 2
Table 1 Axial force torque parameters.
Process corner wa at different clamping lengths
wa<90, 45 recommended; WA 90, 180 recommended
Fig.2. Process rotation angle wa at different clamping lengths
l<1*d; wa<90° 1*d≤1≤4*d; wa=90°
l>4*d; wa>90°
In addition to bolt strength, there are other variables that affect the final torque value, including the initial coefficient of friction and the friction radius of the head. Therefore the final torques given in the table are reference values and they are only available for GB T16674A small series of hexagonal flange bolts mentioned in 1.
When using the large-head flange face bolts mentioned in GB T5789 5790, the final torque value is increased by about 20%.
7. Recommended value of torque and angle process parameters in the elastic zone
The torque angle process parameters of the elastic zone are determined according to the test results of the joint and the design requirements of the target preload (axial force), and the following experience can be referred to:
Sill torque ms:The threshold torque recommended in Table 1 is used.
Process corner wa:45 is used as the process corner.
8. Calculation principle
1. The initial torque mentioned in the above table is determined according to the experience and test results, and the axial preload and final torque of the assembly in the yield zone are calculated and determined according to the following calculation formula.
Formula for calculating axial preload in the yield zone assembly:
a0 — minimum stress cross-sectional area of the bolt.
Bolt yield strength utilization factor, yield zone assembly =1
rp0.2 — Bolt yield strength.
d2 — Thread middle diameter.
d3 — Minimum cross-sectional diameter of the screw
g — coefficient of friction of the threaded pair
Formula for calculating the final torque of the assembly in the yield zone:
fy — axial force of bolt yield.
ges — composite coefficient of friction of the thread.
p — thread pitch.
d2 — Thread middle diameter.
dw — Outer diameter of the support surface under the head of the fastener
DH — Inner diameter of the support surface under the head of the fastener.
2. The over-yield threaded connection is tightened, and the preload and torque are determined according to the strength and friction coefficient of the bolt, and the maximum preload appears when the bolt strength is the largest and the thread friction coefficient is the smallest, and the maximum torque appears when the bolt strength is the largest and the friction coefficient is the largest.
Fig.3. Axial preload and torque under superelastic bolt assembly.
fv — Axial preload
rp0.2 — Bolt yield strength.
fm — Assembly axial preload
fh — Auxiliary variable
m — torque.
Coefficient of friction. Today's topic is shared here, and criticism and correction are welcome if it is inappropriate; If you have any questions or suggestions, or need to communicate with your partners, you can pay attention to WeChat***gaf-luosijun