# 6 FAQs on Fatigue Calculation of Bolts: Answers You Need to Know

## 1. What are the factors affecting the fatigue strength of bolts?

There are various factors that can impact the fatigue strength of a connection, including the material used, structural design, size, manufacturing process, thread-to-thread fit, load distribution, stress amplitude, mechanical properties, and more.

Related reading: Metal Mechanical Properties Chart

To begin with, selecting suitable materials and heat treatment processes is crucial to ensure that the materials’ strength and plasticity index meet the standards required.

It’s essential to ensure that there are no defects in the materials that may impact their strength, particularly low magnification intergranular defects.

Nonetheless, the strength of the bolt connection primarily relies on the bolt’s strength.

During installation, the bolt will experience stretching, while the nut will be compressed. The difference in extension and contraction of the thread pitch is highest at the first circle near the bearing surface, resulting in maximum strain and stress. The remaining circles (pitch P) decrease accordingly.

Here are the recommended strength increases for various types of nuts:

a) Suspension nut – strength increases by 40% (the nut is also under tension, which helps distribute the load evenly with the bolt deformation)

b) Ring groove nut – strength increases by 30% (the nut is tensioned near the bearing surface)

c) Inner inclined nut – strength increases by 20% (contact ring decreases, load moves up)

d) Bond nut (combination of b and c) – strength increases by 40%

e) Use of different materials for bolt and nut – strength increases by 40%.

### 2. Reduce bolt stress amplitude

1) Reduce bolt rigidity

Measures: vertical center bar, slender bar, flexible bolt connection, etc.

2) Increase flange stiffness

Measures: Use high hardness gasket or screw it directly on cast iron.

### 3. Reduce stress concentration

Stress concentrations can arise at the root of the thread, the end of the bolt, and the transition between the bolt head and the bolt rod.

To alleviate these stress concentrations, you can consider the following options:

1. Increase the fillet at the transition point.
2. Use an undercut ↑ of 20~40% for the thread end.
4. Remove the transition structure’s load.

### 4. Adopt reasonable manufacturing process

1. The bolts are manufactured using the extrusion (rolling) method, resulting in a fatigue strength increase of 30-40%.
2. Applying techniques such as cold work hardening, surface treatments like cyaniding, nitriding, shot peening, or introducing residual stress (compression) can improve the fatigue strength.
3. For even better results, thread rolling after heat treatment can increase strength by 70-100%. This method offers high-quality, high-yield, and low-consumption benefits.
4. It is crucial to control both single pitch error and cumulative pitch error.

## 2. What are the reasons for the reduction of bolt fatigue strength?

Bolt connections are widely used in mechanical manufacturing and equipment installation. However, due to the difficulty in detecting and preventing fatigue damage, there have been frequent incidents of serious accidents caused by fatigue fractures of bolts over the years. Therefore, there is increasing attention being paid to the study of bolt failure.

The reduction of the fatigue strength of bolts can be attributed to the following reasons:

(1) When turning the thread, the metal with good external quality of the blank is removed, while the remaining metal with poor quality is used as the bolt rod. This results in the high-quality metal crystal being underutilized, which ultimately reduces the thread strength.

(2) Due to the existence of small machining fillet and large stress gradient at the root of the thread, stress concentration is caused.

(3) The surface roughness value at the root of the thread is higher than that at the bevel of the thread.

(4) Tool marks parallel to each other and perpendicular to the thread axis, and micro-cracks can be found between the tool marks. Since the thread of the turned bolt is at its root, these factors affecting the fatigue strength also exist.

In the presence of alternating loads, the fatigue source will be generated first, thereby accelerating the fatigue failure of the bolt.

## 3. Why can increasing the bolt length increase the fatigue strength of the bolt?

For high-strength bolts (pre-tensioned bolts) only, it is recommended to increase the bolt length, reduce the bolt stiffness, decrease the working force FSA shared by the bolt when bearing the load, reduce the alternating stress, and subsequently increase the fatigue strength.

## 4. What is the difference between the connection stress of high-strength bolts and ordinary bolts?

In terms of connection force, which is mainly tensile force, there is no difference between high-strength bolts and ordinary bolts.

However, the stress experienced by steel structure bolts and torsional shear bolts differs from that of ordinary bolts. This is because steel structure bolts and torsional shear bolts are subject not only to tensile force, but also to shear force.

## 5. What are the types of high-strength bolts? What are the advantages and disadvantages of each?

When high-strength bolts are subject to shear stress, they can be classified into two types: friction-type high-strength bolts and bearing-type high-strength bolts, depending on their design and stress requirements.

Friction-type high-strength bolt connections have good integrity and stiffness, resulting in small deformation, reliable stress, and fatigue resistance.

This type of connection maintains friction between the contact surfaces of plates, which prevents relative slip. It is primarily used for installing and connecting structures that bear dynamic loads, as well as some components and high-altitude installations.

On the other hand, bearing-type high-strength bolt connections have a higher design bearing capacity than friction-type bolts, as their bearing capacity continues to increase after friction is overcome.

Consequently, the number of bolts required can be reduced. However, their integrity and stiffness are poor, with large deformations, poor dynamic performance, and small actual strength reserves. They are only suitable for connections that allow certain slip deformations in structures subject to static or indirect dynamic loads.

One of the drawbacks of high-strength bolt connections is that they have special technical requirements for materials, wrenches, manufacturing, and installation, which makes them relatively expensive.

## 6. What is the strength of high-strength bolt?

Grade 8.8 is considered a high-strength bolt.

Currently, high-strength bolts of 8.8S and 10.9S are being used.

The number before the decimal point, either 8 or 10, represents the approximate minimum tensile strength value of the bolt after heat treatment, which is 100Mpa.

The actual tensile strength of 8.8S is between 830Mpa and 1030Mpa, while that of 10.9S ranges between 1040Mpa and 1240Mpa.

The number after the decimal point, either 0.8 or 0.9, represents the yield ratio of the bolt after treatment. Yield ratio is the ratio of the conditional yield tensile strength of the bolt to its minimum tensile strength. The letter “S” represents the bolt, and the letter “H” represents the nut. Nuts are divided into two grades: 8H and 10H.

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Author

#### Shane

##### Founder of MachineMFG

As the founder of MachineMFG, I have dedicated over a decade of my career to the metalworking industry. My extensive experience has allowed me to become an expert in the fields of sheet metal fabrication, machining, mechanical engineering, and machine tools for metals. I am constantly thinking, reading, and writing about these subjects, constantly striving to stay at the forefront of my field. Let my knowledge and expertise be an asset to your business.

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