Classification of tightening methods:

1. Torque control method (T)
The torque control method was initially used and is also the simplest control method. It is a control method that immediately stops tightening when the tightening torque reaches a set control value Tc.
It is based on the proportional relationship between the axial pre tightening force F of the bolt and the tightening torque T applied during tightening when threaded connections are made. The relationship between them can be:
T=KF
K: the torque coefficient, whose value is mainly determined by the frictional resistance F between the contact surfaces and the thread teeth μ To decide:
(K=0.161p+0.585) μ D2+0.25 μ (De+Di)
p: thread pitch
μ：comprehensive friction coefficient
D2: the center diameter of the thread
De: effective outer diameter of the supporting surface
Di: inner diameter of the supporting surface
In general, the value of K is approximately between 0.2 and 0.4, however, some may even be between 0.1 and 0.5.

2. Torque angle control method (TA)
Also known as hyperelastic control method.
The torque angle control method is developed based on the torque control method. To apply this method, first tighten the bolt to a small torque, and then start from this point to tighten a specified angle control method.
It is based on a certain angle, which causes the bolt to produce a certain axial elongation and the connector to be compressed, resulting in a certain axial pre tightening force of the bolt.
The purpose of setting the initial torque (TS) when applying this method of tightening is to tighten the bolt or nut to the tight contact surface and overcome some initial uneven factors such as surface unevenness. The axial pre tightening force of the bolt is mainly obtained in the later corner.
The difference in frictional resistance only affects the starting point of the turning angle and continues its influence until the end. After calculating the turning angle, the influence of frictional resistance on it no longer exists, so its effect on the axial pre tightening force of the bolt is not significant. Therefore, its accuracy is higher than that of a simple torque control method.
The torque angle control method has the greatest impact on the axial pre tightening force of bolts at the starting point of measuring the angle and the S1 (or S2) point corresponding to Ts along the way.
Therefore, in order to achieve high tightening accuracy, attention should be paid to the study of point S.
The biggest difference between the torque angle control method and the torque control method is that the torque control method usually limits the maximum axial pre tightening force of the bolt to 90% of the elastic limit of the bolt, that is, at point Y in Figure 2; The torque angle control method is generally based on the Y-M zone, and the most ideal control is to control it behind the yield point.
The accuracy of the axial pre tightening force of the bolt using the torque angle control method is very high. As can be seen from Figure 2, the same angle error in its plastic zone results in the axial pre tightening force error of the bolt Δ Error in axial pre tightening force of bolts in F2 elastic region Δ F1 is much smaller.

3. Yield point control method (TG)
The yield point control method is a method of stopping tightening a bolt after tightening it to the yield point.
It is a high-precision tightening method developed by utilizing the phenomenon of material yielding.
This control method confirms the yield point by continuously calculating and judging the slope of the tightening torque/angle curve.
The torque/angle variation curve of the bolt during tightening is shown in Figure 3.
At the beginning of true tightening, the slope rises significantly, and then remains constant after a simple slowdown. After passing point b, its slope slowly decreases briefly and then rapidly decreases again.
When the slope decreases by a certain value (generally defined as when it drops to half of the maximum value), it indicates that the yield point (i.e. point Q in Figure 3) has been reached, and a stop tightening signal is immediately issued.
The tightening accuracy of yield point control is very high, and the error of its pre tightening force can be controlled within ± 4%, but its accuracy mainly depends on the yield strength of the bolt itself.