1. What are the factors that affect the fatigue strength of bolts?
There are several factors that can affect the fatigue strength of a connection, including the material used, structural design, size, manufacturing process, thread-to-thread fit, load distribution, stress range, mechanical properties and more.
Related reading: Chart of mechanical properties of metals
To begin with, the selection of suitable materials and heat treatment processes is crucial to ensure that the strength and plasticity index of the materials meet the required standards.
It is essential to ensure that there are no defects in the materials that could affect their strength, especially low-magnification intergranular defects.
However, the strength of the screw connection mainly depends on the strength of the screw.
1. Improve uneven load distribution between threads
During installation, the bolt will stretch, while the nut will compress. The difference in thread pitch extension and contraction is greatest in the first circle closest to the bearing surface, resulting in maximum stress and tension. The remaining circles (step P) decrease accordingly.
Here are the recommended stamina boosts for various types of nuts:
a) Suspension nut – resistance increases by 40% (the nut is also under tension, which helps distribute the load evenly as the screw deforms)
b) Ring nut – resistance increases by 30% (nut is tensioned close to bearing surface)
c) Internal inclined nut – resistance increases by 20% (contact ring decreases, load increases)
d) Connection nut (combination of b and c) – resistance increases by 40%
e) Use of different materials for screws and nuts – resistance increases by 40%.
2. Reduce screw tension range
1) Reduce screw stiffness
Measurements: vertical center bar, slender bar, flexible screw connection, etc.
2) Increase flange rigidity
Measurements: Use high hardness gasket or screw directly into the cast iron.
3. Reduce stress concentration
Stress concentrations can arise at the thread root, at the end of the screw, and at the transition between the screw head and screw shank.
To alleviate these concentrations of stress, you may consider the following options:
- Increase the fillet at the transition point.
- Use an undercut ↑ of 20~40% for the thread end.
- Use a flush tank.
- Remove the load from the transition structure.
4. Adopt a reasonable manufacturing process
- Screws are manufactured using the extrusion (rolling) method, resulting in an increase in fatigue strength of 30-40%.
- The application of techniques such as cold work hardening, surface treatments such as cyanidation, nitriding, shot peening or the introduction of residual stress (compression) can improve fatigue resistance.
- For even better results, thread rolling after heat treatment can increase strength by 70-100%. This method offers benefits of high quality, high yield and low consumption.
- It is crucial to control both single tone error and cumulative tone error.
2. What are the reasons for the reduced fatigue strength of bolts?
Bolted 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 bolt fractures over the years. Therefore, more and more attention is paid to the study of screw failure.
The reduction in 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 screw shank. This causes the high-quality metal crystal to be underutilized, which ultimately reduces the strength of the wire.
(2) Due to the existence of a small machining fillet and a large stress gradient at the thread root, stress concentration is caused.
(3) The surface roughness value at the thread root is greater than that at the thread chamfer.
(4) Tool marks parallel to each other and perpendicular to the thread axis, and microcracks can be found between the tool marks. Since the thread of the turned screw is at its root, these factors that affect fatigue resistance also exist.
In the presence of alternating loads, the fatigue source will be generated first, thereby accelerating bolt fatigue failure.
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 bolt length, reduce bolt stiffness, decrease the FSA working force shared by the bolt when carrying the load, reduce alternating tension, and subsequently increase fatigue resistance.
4. What is the difference between the connection voltage of high strength bolts and ordinary bolts?
In terms of connection force, which is mainly tensile force, there is no difference between high-strength screws and ordinary screws.
However, the stress experienced by steel structure bolts and torsional shear bolts differs from that of ordinary bolts. This is because steel framing bolts and torsional shear bolts are subject to not only tensile force but also shear force.
5. What are the types of high strength screws? What are the advantages and disadvantages of each?
When high-strength bolts are subjected 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 tension requirements.
Friction-type high-strength screw connections have good integrity and rigidity, resulting in small deformation, reliable tension and fatigue resistance.
This type of connection maintains friction between the contact surfaces of the plates, which prevents relative sliding. It is mainly used to install and connect structures that support dynamic loads, as well as some components and installations at high altitudes.
On the other hand, bearing-type high-strength bolt connections have a greater design bearing capacity than friction-type bolts, as their bearing capacity continues to increase after friction is overcome.
Consequently, the number of screws required can be reduced. However, its integrity and rigidity are poor, with large deformations, poor dynamic performance and small real strength reserves. They are only suitable for connections that allow certain sliding deformations in structures subject to static or indirect dynamic loads.
One of the disadvantages of high-strength bolted connections is that they have special technical requirements for materials, keys, manufacturing and installation, which makes them relatively expensive.
6. What is the strength of high strength screw?
Grade 8.8 is considered a high strength bolt.
Currently, 8.8S and 10.9S high strength screws are being used.
The number before the decimal point, 8 or 10, represents the approximate minimum value of the screw's tensile strength 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, 0.8 or 0.9, represents the screw flow rate after treatment. The yield rate is the ratio of the bolt's conditional yield tensile strength to its minimum tensile strength. The letter “S” represents the screw and the letter “H” represents the nut. Nuts are divided into two grades: 8H and 10H.
