Bolted joints are widely used detachable joints. The bolt tightening torque is a very important parameter in bolted joints. This paper analyzes the stress of bolted joints and establishes the linear relationship between bolt preload and tightening torque. Then, from the bolt strength, thread strength, gasket characteristics, flange strength and other aspects, the appropriate pre-tightening force values ​​in the bolt connection are studied to determine the tightening torque of the bolt, and the calculation method of the tightening torque is improved. The theoretical calculation of the tightening torque is described, which provides a reference method for determining the tightening torque of the bolt for production.

Bolted joints are one of the most widely used detachable connections in equipment installation. In order to enhance the rigidity, tightness, and anti-loose ability of the bolted joint of the bolt and to prevent the sliding of the joint of the lateral load bolt, most of the threaded joints need to be pre-tensioned during assembly, and the application of the pre-tightening force is achieved by a certain tightening torque. A suitable tightening torque is beneficial to the life of the bolted coupling and the coupled component.

Excessive tightening torque will often lead to joint failure, especially in the case of a sealed coupling, the bolt tightening torque is too large, the gasket will be crushed and lose its elasticity, and even the bolt will be broken; too small tightening torque will make The residual pressing force on the surface of the pressed gasket does not reach the working seal specific pressure, resulting in leakage of the connection system.

Therefore, how to determine the tightening torque of the bolt is a problem that must be paid attention to in actual production.

1. The force of bolt connection

In most cases, the bolts are used in groups. When designing, the force transfer mode, the number of bolts and the arrangement of the joints are determined according to the structure of the joints and the load of the joints.

In general, there are several types of stress:

(1) pure axial force, that is, only in the axial direction of the bolt;

(2) Lateral force, that is, the radial force of the bolt;

(3) Rotational moment, such as wheel flange connection;

(4) The turning moment, also called the overturning moment.

The first and fourth kinds of forces are connected by tension bolts, and the second and third types of force can be connected by tension bolts or shear bolts.

In the production, the bolts are connected by tension bolts. The bolts selected according to the national standard are also the bolts (GB3098.1~3098.3-82 fastener mechanical properties). Therefore, this paper mainly discusses the connection problem of the tension bolt.

2. The relationship between axial force and moment of bolt

Generally, the tightening torque is applied by tightening the wrench, and the tightening wrench torque T is used to overcome the thread resistance torque T1 of the thread pair and the end surface frictional moment T2 between the nut and the bearing surface of the connected member (or washer).

Where: d—nominal diameter of the thread, mm;

F0—preload, N;

K—tightening torque coefficient;

T—tightening torque;

among them:

D2—the diameter of the thread, mm;

Ω—the angle of the thread;

Ρv—thread equivalent friction angle;

摩擦—the friction coefficient between the nut and the bearing surface of the connected piece;

Dw—the diameter of the nut or washer that is connected to the support plane;

D0—the outer diameter of the thread;

Therefore, in the case of determining the size of the bolt, the axial pre-tightening force of the bolt is proportional to the tightening torque, and the proportional coefficient is the tightening torque coefficient K.

The value of K is more complicated and it is difficult to get the correct value. For ordinary coarse teeth M12~M64 threads, the tightening torque coefficient K generally varies from 0.1 to 0.3, and Table 1 is the reference value.

It can be considered that when the model of the bolt, nut, washer, connected parts, etc. is selected, the tightening torque coefficient is fixed under the determined working environment and working condition. That is to say, in this case, the magnitude of the required preload force directly determines the amount of tightening torque that should be applied.

According to the linear relationship between the tightening torque and the bolt pre-tightening force, the pre-tightening force value can be calculated experimentally or theoretically by controlling the tightening torque.

Due to the influence of friction coefficient and geometric parameter deviation, the pre-tightening force value is relatively large under a certain tightening torque. Therefore, the precision of controlling the bolt pre-tightening force by tightening torque is not very high, and the error is about ± About 25%, the maximum is even about ±40%.

This creates a contradiction: we can determine the required preload force according to the design conditions and requirements, but we can't accurately apply the required preload force in reality, which can be controlled. It is only the amount of tightening torque applied.

However, the discussion on this contradiction is not the focus of this article, mainly to illustrate the existence of this reality. At the same time, the discussion in Section 4 of this paper only considers the pure linear relationship between tightening torque and preload and ignores this error.

3. Method for determining bolt pre-tightening force (tightening torque)

Figure 1 shows a typical example of a flange bolt connection of a pressure vessel while being subjected to preload and axial working loads.

Usually, the flange coupling body, which is coupled by N bolt and nut pairs, gives a certain tightening force F, and the internal pressure Fx is generated in the coupling body. For such a coupling body for a fluid having a certain internal pressure, the design bolt is designed. When tightening the torque, the following four points must be considered:

(1) Ensure the strength of the bolt, that is, the function of losing the joint due to the yield failure of the bolt due to excessive tightening torque is not allowed.

(2) Ensure the strength of the thread, and the thread cannot be tripped due to excessive tightening torque.

(3) Ensure the strength of the flange, and the flange cannot be damaged due to excessive tightening torque.

(4) Ensure the performance of the gasket and gasket, and ensure that the leakage is not caused by the tightening torque being too small, and the gasket is not crushed due to excessive tightening torque, so that it leaks after the internal pressure is generated.

3.1 Consider the strength of the bolt

It is generally stipulated that the pre-tightening stress of the threaded coupling after tightening shall not exceed 80% of the yield point σs of its material. For steel bolts for general jointing, the recommended preload force limits are as follows:

Carbon steel bolt F0=(0.6-0.7)σsAs

Alloy steel bolt F0=(0.5-0.6)σsAs

Where: σs—the yield point of the bolt material, Mpa;

As—the nominal stress cross-sectional area of ​​the bolt, mm2;

When using the torque wrench to measure the tightening torque regularly, generally take the pre-tightening factor Q=1.4, then the required value of the torque wrench should be:

This method of calculating the tightening torque simply considering bolt strength has certain limitations, but it can be used as the actual tightening torque in the absence of sealing requirements and the strength of the coupling is sufficiently high, especially It is more common to apply the tightening treatment of anchor bolts, and it is also suitable.

3.2 Consider the strength of the thread

Proper tightening torque is required to ensure that the threads are tightened without damaging the threads or nuts.

This requires that the tightening torque is necessarily between the screwing torque and the limit torque. There are many factors that affect the rotating torque and the limiting torque. The more important factors are the type of thread, the size, the nut composition and the hardness, thickness and so on.

However, the standard threaded and nut thread and other parts are specified according to the principle of equal strength and experience.

When standard parts are used, these parts do not need to be calculated for strength, but when the non-standard parts are used together or the material is changed, the strength of the threads needs to be further calculated to determine the ultimate tightening torque. The calculation method can be found in the relevant manual.

3.3 Consider the strength of the flange (or other connected parts)

When bolting flange parts (or other joints), the strength of the joint to be joined is also an important factor to consider, taking the flange as an example.

The flanged joint structure is an assembly that is generally composed of a pair of flanges, a number of bolts, nuts and a gasket. The strength calculation of the flange is described in detail in the GB150 or ASME specifications, and will not be described in detail in this paper.

In practical applications, it is rare for the pressure vessel to fail due to the strength of the coupling or the joint to be broken, and more often because the seal is not good, that is, the gasket first has problems.

3.4 Consider the sealing performance of the gasket

In many cases, especially in the case of bolted joints using gaskets, due to the higher strength of the bolts, more consideration is given to the characteristics of the gaskets in determining how to determine the preload.

The pressing force required for the gasket is divided into the pre-tightening state and the operating state, and the two are calculated as follows:

In the formula:

DG—the gasket pressing force acts on the center circle diameter;

y, m—the specific pressure of the gasket and the gasket factor;

B—the effective sealing width of the gasket;

Pc—calculate pressure;

At this time, the minimum bolt load (ie, bolt preload) required for the bolt in the preloaded state and the operating state is calculated as follows:

Of course, this pre-tightening force is the minimum pre-tightening force required for the bolt, and the calculated tightening torque is also the minimum tightening torque. In a suitable size range, the tightening torque should take a larger value, therefore, according to the most basic Sealing requirements, the minimum tightening torque of the bolt is obtained.

At the same time, when the gasket is subjected to excessive pressing force when the bolt is pre-tightened, it may be compressed into plastic deformation and lose the resilience. When the flange sealing surface is separated under the action of the medium pressure, the gasket cannot be produced back. Bounce to “close” the sealing surface so that it does not maintain sufficient contact force (ie, the gasket pressing force) to cause leakage.

Therefore, when the gasket is pre-tightened, it must be pressed so that the pressing force on the unit effective sealing area is not less than the y value (ie, the specific pressure of the gasket), and the pressing force cannot be excessively large to prevent being Pressed into plastic deformation.

In the case of a flat seal, in order to prevent the gasket from being plastically deformed, the pre-tightening force of the gasket should be controlled to be no more than 4y. When the gasket is pre-tightened, if the pressing force on the effective sealing area of ​​the unit is less than y, the “leakage passage” will not be eliminated, and the pre-sealing requirement will not be reached; on the contrary, when the gasket pre-tightening force is too large (>4y) When the gasket loses its elasticity, it will cause a leak when the gasket acts under internal pressure. The maximum bolt preload force Wm is obtained as follows:

3.5 Determination of tightening torque

A suitable tightening torque is very important for the bolt connection. The first step in determining the tightening torque is to calculate a suitable preload force F. After the above analysis, it can be determined that the appropriate range of F is F1~Min (F2, F3, F4, F5).

Among them, F1—gasket guarantees the minimum pressing force of the seal;

F2—the pressing force that causes the gasket or gasket to break;

F3—the pressing force that causes the thread to break;

F4—the maximum pre-tightening force of the bolt;

F5—The pulling force that breaks the flange or other pressing surface.

In the extreme case of considering the use of standard fasteners and neglecting the failure of the flange, it can be considered that the pre-tightening force should be in the range of F1~Min (F2, F4) in the sealing bolt connection of the pressure vessel.

In general, the tightening torque should be taken to a large value within the appropriate range, because the greater the axial force of the bolt, the better its resistance to looseness and fatigue, and the higher the efficiency of the bolt, so it is most suitable. The preload should be Min (F2, F4).

That is to say, in the calculation process of the pre-tightening force, the pressing force of the maximum resistance of the gasket is mainly considered, and the maximum allowable pressing force of the bolt is calculated, and the two take small values.

4 Conclusion

At present, in many documents and even standard specifications, the calculation of the tightening torque is often only one or two cases. It is not based on the calculation of the minimum and maximum tightening torques, and the bolt tightening torque under various conditions is fully considered. Calculation method.

In this paper, a more comprehensive theoretical calculation method for bolt tightening torque is obtained through discussion, which has certain reference significance. It is worth noting that in order to obtain the most suitable bolt tightening torque, theoretical calculations still have large defects and errors. Under certain conditions, more accurate values ​​can be obtained through experiments.

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