6 FAQs in Fatigue Calculation of Bolts

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

There are many factors that affect the fatigue strength of the connection, such as material, structure, size, process, thread to thread, load distribution, stress amplitude, mechanical properties, etc.

Related reading: Metal Mechanical Properties Chart

First of all, it is important to select appropriate materials and heat treatment processes to ensure that the strength and plasticity index of the materials meet the standards.

There are no defects in the materials that affect the strength, such as low magnification intergranular defects, which is the most important.

However, the strength of the bolt connection mainly depends on the strength of the bolt.

6 FAQs in Fatigue Calculation of Bolts 1

1. Improve uneven load distribution between threads

During the work, the bolt tooth shall be stretched, and the nut tooth shall be compressed short.

The difference between the extension and contraction of the thread pitch is the largest at the first circle close to the bearing surface, with the maximum strain and stress.

The rest of the circle (pitch P) decreases in turn.

a) Suspension nut – strength ↑ 40% (the nut is also under tension, which is in harmony with the bolt deformation, so that the load is evenly distributed);

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

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

d) (b) (c) Bond nut – strength ↑ 40%;

e) Matching of different materials – strength ↑ 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 concentration may occur at the root of thread, end, and the transition between bolt head and bolt rod.

1) Increase the fillet at the transition;

2) Change to undercut ↑ 20~40% (thread end);

3) Unloading tank;

4) Unload the transition structure.

4. Adopt reasonable manufacturing process

1) The bolts are manufactured by extrusion method (rolling method), and the fatigue strength is ↑ 30~40%;

2) Cold work hardening, with residual stress (compression), cyaniding, nitriding, shot peening, etc. on the surface, can improve the fatigue strength;

3) After heat treatment, the thread is rolled, with a better effect. The strength is ↑ 70~100%. This method has the functions of high quality, high yield and low consumption;

4) Control single pitch error and cumulative pitch error.

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

Bolt connection is a kind of connection form widely used in mechanical manufacturing and mechanical equipment installation.

However, because it is difficult to find and prevent the fatigue damage of bolts, serious accidents caused by fatigue fracture of bolts have occurred frequently for many years.

Therefore, more and more attention has been paid to the study of bolt failure.

The reasons for the reduction of fatigue strength of bolts are as follows:

(1) When turning the thread, the metal with good external quality of the blank is turned off, while the remaining metal with poor quality is used as the bolt rod.

The high quality metal crystal is not used reasonably, which reduces the thread strength.

(2) Due to the existence of small machining fillet and large stress gradient at the root of 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) There are tool marks parallel to each other and perpendicular to the thread axis, and there are micro cracks between the tool marks.

As the thread of the turned bolt is at its root, the above factors affecting the fatigue strength also exist.

If there is an alternating load, the fatigue source will be generated first, accelerating the fatigue failure of the bolt.

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

Only for high-strength bolts (pre-tensioned bolts), increase the bolt length, reduce the bolt stiffness, reduce the working force FSA shared by the bolt when bearing the working force, reduce the alternating stress, and increase the fatigue strength accordingly.

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

There is no difference between high-strength bolts and ordinary bolts in connection force, which is mainly tensile force.

However, the stress of steel structure bolts and torsional shear bolts is different from that of ordinary bolts.

Steel structure bolts and torsional shear bolts are subject to shear force in addition to tensile force.

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

When high-strength bolts bear shear, they can be divided into friction-type high-strength bolts and bearing-type high-strength bolts according to different designs and stress requirements.

Friction-type high-strength bolt connection has good integrity and stiffness, small deformation, reliable stress and fatigue resistance because it always keeps the friction between the contact surfaces of plates from being overcome and relative slip.

It is mainly used for the installation and connection of structures directly bearing dynamic loads, as well as some parts of component field splicing and high-altitude installation and connection.

When bearing shear, the bearing capacity of bearing type high-strength bolt connection continues to increase after friction is overcome, so its design bearing capacity is higher than that of friction type bolt, so the total amount of bolts can be saved;

But compared with friction-type high-strength bolt connection, its integrity and stiffness are poor, its deformation is large, its dynamic performance is poor, and its actual strength reserve is small;

It is only used for connections that allow certain slip deformation in structures subject to static or indirect dynamic loads.

The disadvantage of high-strength bolt connection is that there are some special technical requirements in materials, wrenches, manufacturing and installation, and the price is relatively expensive.

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

Grade 8.8 is already a high-strength bolt.

At present, high-strength bolts of 8.8S and 10.9S strength are used.

The number 8 or 10 before the decimal point represents the approximate value of the minimum tensile strength “100Mpa” of the bolt after heat treatment.

The actual tensile strength of the two is 830Mpa – 1030Mpa and 1040Mpa – 1240Mpa respectively;

0.8 or 0.9 after the decimal point represents the yield ratio of the bolt after treatment (i.e. the ratio of the conditional yield tensile strength of the bolt to the minimum tensile strength), S represents the bolt, H represents the nut, and the nut is divided into two grades: 8H and 10H.

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