High-strength bolts stand as one of the most crucial products in the industrial sector, playing a vital role in assembling various types of machinery. Do you know the methods for securing high-strength bolts? And how do you go about selecting the right fastening method? In this article, TorcStark will provide a detailed answer to these two questions.

Now, let’s get started!

High-strength bolts are divided into two types: large hexagonal head and torque-shear type. They differ only in shape and fastening methods, with identical stress performance. Each type of high-strength bolt has its applicable fastening method, and we will delve into a specific analysis.

1. Fastening Method for Large Hexagonal Head High-Strength Bolts

Large hexagonal head high-strength bolts fall under regular-type bolts and are widely used in Europe, America, and China. They find extensive usage, especially in railway bridges, with China exclusively adopting this type.

The fastening methods for large hexagonal head high-strength bolts include the torque and angle methods.

1.1 Torque Method

The torque method involves tightening the nut using a torque wrench that displays torque values.

The operation generally consists of two steps: preliminary tightening and final tightening. For large joints, it should involve three steps: preliminary tightening, repeated tightening, and final tightening.

The purpose of preliminary and repeated tightening is to bring the connected plates close together. The torque for preliminary tightening should be around 50% of the construction torque, and the torque for repeated tightening equals the torque for preliminary tightening.

After preliminary or repeated tightening, high-strength bolts should be marked with color on the nut. Then, repeat the tightening according to the construction torque value. Once repeated tightening is completed, mark the nut with another color.

Tip:

Preliminary Tightening: When screwing the bolt into the part hole, use an electric torque wrench to apply appropriate force for pre-tightening. This ensures that the bolt snugly fits against the fixed object without excessive force.

Repeated Tightening: After the preliminary tightening of the bolt, due to the relaxation of the bolt itself and the deformation of the fixed object, it is necessary to apply the appropriate torque again to bring the bolt to the predetermined tension state.

Final Tightening: After completing the preliminary and repeated tightening, apply a strong torque once again to bring the bolt to the predetermined final sealing state.

The construction torque T should be determined according to the following formula:

T=kd(P+δP) (Formula 1-1)

Where:

  • d is the nominal diameter of the bolt.
  • P is the pre-tension of the bolt.
  • δP is the value of pre-tension loss caused by bolt relaxation after compensation tightening, generally around 10% of the pre-tension.
  • k is the torque coefficient.

As evident from the above formula, the applied tightening torque is not only proportional to the nominal diameter of the bolt and the specified pre-tension but also dependent on the torque coefficient.

The torque coefficient must be determined through prior experimentation, and factories typically provide this value for each batch of bolts upon delivery. This coefficient represents the fundamental value of the torque coefficient, reflecting the manufacturing quality of the bolts. Factors such as the treatment of the bolt surface (blackening, phosphating), surface cleanliness, thread precision, lubrication, etc., contribute to this coefficient, indicating the bolt’s manufacturing quality. However, it is determined under specific conditions in the factory.

Various external factors also influence the torque coefficient, such as transportation, storage, whether the threads are damaged during use, whether they are contaminated with dust or oil, and whether they have been exposed to moisture and rusted.

Even if these factors are avoided, other elements such as the number of layers, thickness, and flatness of the connected plates, the sequence of tightening the bolt group, the speed of tightening, the presence of lubricant (grease) between the nut and bolt or between the nut and washer, the temperature during tightening (which decreases as the temperature rises), and the timeliness of the final tightening, all have some impact on the torque coefficient.

If the torque coefficient is large with high variability, it indicates that the construction torque is high, and the pre-tension of the bolt is unstable. This will inevitably make it challenging to guarantee quality. Moreover, an increase in construction torque not only increases the shear stress generated during tightening but also raises the equipment and labor intensity during tightening. Therefore, when using the torque method, it is necessary to determine the torque coefficient based on the actual engineering conditions before construction to ensure the quality of the project.、

1.2 Angle Method

The angle method does not require the use of a specialized wrench. It controls the rotation angle of the nut, effectively managing the bolt’s strain to achieve the specified pre-tension. Therefore, it is a simple and effective fastening method, widely used in the United States.

The angle method also requires two steps: preliminary tightening and final tightening.

For preliminary tightening, a short wrench (30~50cm long, tightened by one person, achieving approximately 20%~30% of the pre-tension) can be used to tighten the nut until the plates are snug together, and a mark is made. Then, using a long wrench (or electric, pneumatic wrench), the nut is tightened from the marked position for an additional 1/3 to 1/2 turn (120° to 180°) to reach the final position.

The final tightening angle needs to be determined based on the relationship with the pre-tension and is related to the plate stack thickness (h) and the bolt diameter (d). The current regulations in Japan and the United States are as follows:

Japan:

  • h≤8d or h≤200mm 1/2 circle
  • h>8d or h>200mm 2/3 circle

USA:

  • h≤4d 1/3 circle
  • 4d<h≤8d 1/2 circle
  • 8d<h≤12d 2/3 circle

2. Fastening Method for Torque-Shear Type High-Strength Bolts

The torque-shear type high-strength bolts were originally introduced in Japan.

Due to the bolt’s inherent design, it features a groove and a cam head at the end of the bolt. The groove depth is determined based on the relationship between the torque required to finally break the cam head and the pre-tension. Therefore, when the cam head breaks off, the bolt reaches the specified pre-tension value.

The tightening of torque-shear-type high-strength bolts also involves preliminary tightening and final tightening, with an additional repeated tightening in the middle for large joints.

Preliminary tightening and repeated tightening can be performed using an electric torque wrench, with the torque value also set at 50% of the formula (1-1) and a torque coefficient of 0.13. Thus, the torque for preliminary tightening/repeated tightening can be expressed as:

T0=0.065d(P+δP)(Formula 2-1)

After preliminary or repeated tightening, high-strength bolts should be marked with color on the nut. Then, a specialized TC-type electric wrench is used to tighten the nut and cam head until the cam head breaks off.

The fastening of torque-shear type high-strength bolts is simple and facilitates the inspection of whether bolts are under-tightened or over-tightened. Therefore, it is highly favored by construction units. The drawback is that the cam head requires a bit more steel consumption. Additionally, if there is a lack of strict control over the manufacturing error of the groove depth, it may introduce some variability in the pre-tension of the bolts.

3. Analysis of Two High-Strength Bolt Tightening Methods

3.1 Analysis of Torque-Shear Type High-Strength Bolt Fastening Method

The fastening method of torque-shear-type high-strength bolts is simple and easy to inspect. Therefore, as long as there is a reliable supply and access to electric torque wrenches, and the quality of the bolts meets the national standards, it is a viable option for use in construction structures.

3.2 Analysis of Fastening Method for Large Hexagonal Head High-Strength Bolts

The manufacturing of large hexagonal head high-strength bolts is relatively straightforward, with a wide supply availability. However, its fastening process requires certain specifications, especially regarding the determination of the torque coefficient and fastening torque. Although there are several factors affecting the torque coefficient, by enhancing the quality control of bolt manufacturing and construction technology management, the reliability of the connection quality can be achieved at a higher level.

Tip:

Ensuring the quality of bolts is crucial, and the key lies in reducing the torque coefficient and minimizing its variability in bolt manufacturing.

Surface phosphating treatment for high-strength bolts, nuts, and washers can significantly reduce the torque coefficient and its variability. The torque coefficient can be reduced by approximately 50%, typically ranging from 0.115 to 0.120, with a standard deviation of 0.0026 to 0.0098, and a pre-tension variability of 0.016 to 0.096.

The fastening method for large hexagonal head high-strength bolts, using the angle method, is simple and practical. However, it is crucial to strictly control the final tightening angle; otherwise, there is a risk of under-tightening or over-tightening, with over-tightening being more likely.

Under normal circumstances, the pre-tension values obtained using the torque method are generally stable. However, various factors during construction can influence the torque coefficient, making it more challenging to make quantitative analyses. Factors such as the impact of temperature fluctuations during construction (change approximately k=士6.15X10-4~士6.6X10-4/℃, with an average change rate of approximately 0.48% to 0.50% per degree Celsius) and the effect of untimely final tightening can increase or decrease the torque coefficient, often making it difficult to provide a quantitative analysis.

In such situations, it is advisable to consider using the angle method for the final tightening, combining both the torque and angle methods. When using the angle method, after the final tightening, it is recommended to check by tapping each bolt with a small hammer weighing 0.3 to 0.5kg. Additionally, torque wrench checks should be conducted by backing off the tightened nut by 30° to 50° and then tightening it back to its original position to verify if there is a significant deviation (not exceeding 10%).

This approach allows the two fastening methods to complement each other. The torque check can be calculated using the following formula:

Tzb=kdP

Summary:

As high-strength bolts are commonly used in the connection of bridges, railways, and high-pressure, and ultra-high-pressure equipment, it is essential to consider all factors that may affect the fastening process comprehensively. The chosen fastening method should ensure the accuracy of applying torque to high-strength bolts. This guarantees the smooth completion of the project and addresses safety concerns for construction personnel.