Development of high strength bolts
The application history of riveting bolts is quite long, and it has been widely used until the 1950s. However, the riveting bolts need to be heated to a red (about 900 °C), and then the skilled workers will transfer the riveting bolts to the riveting bolt installers. The tapered barrel is prepared, and then the technician wears a cone to shape the rivet.
The construction process of the riveting bolt is not only dangerous but also difficult for professional technicians. It will also produce noise resistance during construction, and the construction quality is difficult to control due to the strength and clamping efficiency of the riveting bolt. In recent years, due to the rapid advancement of welding technology and the use of high-strength bolts, the riveting bolts have been completely replaced by high-strength bolts due to their limited limitations.
The final representative of the later use of domestic riveting bolts is the circular roof frame structure of the Linkou Stadium, which was donated by plastics enterprises in the Republic of China for 72 years (now the National Institute of Physical Education).
Commonly used bolts are divided into ordinary bolts and high-strength bolts. Common bolts are also called mechanical bolts (common bolts or machine bolts). They are produced according to ASTM A307 and are generally used for lightweight structures and secondary structures that are not related to seismic design. Objects, or temporary fixed use (such as process beams), and should not be used in structures subject to repeated loads, vibrations or fatigue loads. The bolts can be tightened to the close state during installation.
High-strength bolts are used for the main structures. Most designers understand that the price difference between high-strength bolts and ordinary bolts is not large, and there are not many common bolts. Therefore, in order to avoid the mistakes and misuse of the constructor, the overall use is high. Strength bolts. This article focuses on high-strength bolt applications.
High strength bolt specifications
High-strength bolts commonly used in China are classified into ASTM and JIS specifications. The commonly used ASTM high-strength bolts are A325 and A490. The specific use is shown in Table 1.
Table 1 General Conditions of ASTM High Strength Bolts
The main components of A325 bolts are TYPE1 and TYPE3. TYPE1 is for general structure. Hot-dip galvanizing can be used if necessary. Weather-resistant steel should be used with TYPE3 bolts. When using TYPE3 bolts, the design should be specially marked. The mechanical properties of A325 bolts. As shown in Table 2.
Table 2 Mechanical properties of ASTM high strength bolts
A490 bolts have a higher material strength than A325, but A490 bolts are not hot dip galvanized. The A490 bolts are divided into 3 types of TYPE. The same weather-resistant steel is used with the TYPE3 type bolts. The standard diameters of the A325 and A490 bolts are measured in feet, in increments of 1/8 feet, and are between 1 and 1.5 feet, with diameters of around 1 foot being more common.
In some special cases, the required bolt diameter exceeds 1.5, and A490 bolts can be used. The use of A490 bolts is shown in Table 1. The material strength is shown in Table 2. The screw heads and nuts of ASTM high-strength bolts are hexagonal heads, and the nuts must be heavy-duty hexagon nuts.
The JIS-specified high-strength bolts are divided into the F-series of hexagonal bolt heads and the S-series of round bolt heads developed to reduce material usage and aesthetic requirements (see Figure 1). It is difficult to remove the round bolt heads after installation.
Figure 1 JIS F10T round head torque control high strength bolt
The material strength is different. The F series is divided into F8t, F10t and F11t. The s series is divided into s8t and s10t. The F11t material is difficult to control due to its quality. Only some manufacturers have manufacturing qualifications. Three are shown.
Table 3 JIS high strength bolt material strength
JIS bolt diameters are in mm and are generally used in diameters between 12mm and 24mm. In the past, the experience of hot dip galvanizing shows that high-strength bolts absorb hydrogen and oxygen during the pickling process before hot dip galvanizing, so delayed fracture occurs. Delayed damage is a sudden break after high-strength bolt tightening for a period of time. Phenomenon, this is because the pressure of hydrogen and oxygen stored in the metal crystal is too large.
High-strength bolts with a delayed damage above F10t will not occur, and F8t will not occur below. Therefore, the hot-dip galvanized high-strength bolts used for friction bonding can only be used below F8t.
Because the "Hot-Dip Galvanized Steel Bridge Design and Construction Manual" edited by the China Institute of Civil and Hydraulic Engineering clearly indicates that the fatigue strength of F8t bolts, F10 nuts, F35's washers and galvanized test pieces is about the same as that of black lacquer base materials. Low 12kgf/mm2, so the high-strength nut F8t for hot dip galvanizing should be considered as appropriate.
High strength bolt marking
The appearance of the bolt is shown in Figure 2. The bolt will identify its type in the bolt head, as shown in Figures 3 and 4, for ease of use and construction management.
Figure 2 Bolt appearance and size
Figure 3 ASTM high strength bolt marking
Figure 4 JIS high-strength bolt marking
Bolts are to be used with nuts and washers. Bolts, nuts and washers are used to form bolt sets. Tables 4 and 5 are combinations of ASTM and JIS high-strength bolt sets. Weather-resistant steel should be used with weather-resistant bolts, nuts and washers.
Table 4 ASTM High Strength Bolt Set
Note: The nuts except the A563-B are all heavy-duty hexagon bolts.
Table 5 JIS high strength bolt set
In addition, the washer can prevent the bolt or nut locking process from injuring the steel plate due to the rotation, and the pressure on the top of the dispersing bolt can be too concentrated. In addition, the friction of the rotating surface can be reduced at the rotating end (the bolt head end or the nut end). .
High strength bolt length and determination of thread
Generally, only the standard diameter d of the bolt and the length L of the bolt are specified. The required length of the bolt is the sum of the clamp distance (thickness of the lock) and L1 (the thickness of the nut plus the thickness of the washer and the length of the bolt), as shown in Figure 5 and Figure 6. Shown.
Length required for bolt installation 1 The length of the screw is not specified in the steel structure design specification and construction specifications. If the length is too short, it cannot be used. If it is too long, the length of the screw will not be enough, so the nut will be stuck when the nut is tightened. At the screw head, the steel plate cannot be clamped, or the protrusion is too long, which may affect the appearance and interference of other parts.
The length required for the installation of high-strength bolts at 2000 PCSC is defined as the bolt length (from the inside to the end of the bolt head) after proper installation. In general, L1 is approximately equal to 1.5d plus washer thickness; or nut thickness plus The thickness of the washer is increased by the total length of the screw to the distance of 2 to 3 screws. As for the length of the screw, the length of the thread provided by the bolt on the market is independent of the length of the bolt. The same diameter, the length of the bolt and the length of the thread are fixed. Figure 6.
If the length of the bolts purchased is too long, the steel plate will not be clamped when the nut is tightened due to insufficient thread length. Therefore, in order to have sufficient pre-tightening force when locking high-strength bolts, it is necessary to have sufficient thread length.
Bolt shaft tension-axial deformation curve
The bolt tightening process is first locked to the close state. The so-called close-fitting state is to use a special wrench to screw a few turns of the bolt to be the fastening state of the joint surface. The bolts in the tight state have already received some pre-tensioning force.
Figure 8 shows the curve of the axial deformation of the bolt when the bolt is self-adhesive. The abscissa is the number of turns of the nut, which is equivalent to the axial deformation. The ordinate is the bolt tension, and the curve starts to be linear. The relationship, then the first to yield at the thread, the curve enters the nonlinear phase, followed by necking at the thread and the strength begins to decline, and finally the thread breaks.
Figure 8. Axial tension-axial deformation curve of bolt
When the bolt is subjected to tension, the critical section occurs at the thread. The maximum tensile strength of the thread is the product of the tensile strength of the material and the ascending force surface As.
. among them, 
=0.75 to 0.79Abn = number of teeth per cm d = standard diameter of bolt Ab = standard sectional area of ​​bolt (cm2) = πd / 4 The yield of the material occurs at the dent of the thread, and the yield of the thread will be limited. The local area, and the axial tensile deformation curve of the bolt is not clear at that point in time, the load of the bolt when the screw is yielded is proof load, which is equal to the product of the yield strength of the bolt material and the tension area, A325 bolt The proof load is about 70% of its own tensile strength, and the proof load of the A490 bolt is about 80% of its own tensile strength. Therefore, the high-strength bolt needs to be added with a pre-tightening force Tb during installation. The pre-tightening force is 80% of the bolt tensile strength. Table 8 shows the minimum pre-tightening force of the A325 and A490 bolts, and Table 9 shows the minimum pre-tightening force of the JIS bolt.
The minimum pre-tension of the A572 bolt is approximately equal to its proof load. The minimum pre-tightening force of the A490 bolt is also close to its proof load. The actual pre-tension will be higher than the minimum preload force, so the bolt will basically yield when the pre-tightening force is applied.
In principle, the pre-tightening force of the bolt is better as long as the bolt is not broken and the bolt is not permanently plastically deformed to be prone to slack.
Table 8 Minimum pre-tightening force of A325 and A490 bolts
Table IX JIS bolt minimum preload
Bolt fastening method
The degree of tightening of ordinary bolts is not clearly defined, and the force applied during locking is generally as long as the connection is tight. However, since the tension is not applied when the lock is applied, the nut is prone to loosening. To prevent loosening, the inner nut can be pressed by the outer nut by a spring washer or a double nut to prevent the two nuts from being synchronized. Rotate and relax.
When the double nut is used, the inner nut needs to use the heavy hex nut, and the outer nut can be the general nut (see Figure 9). Generally, the high-strength bolt has tension due to the preloading type or the friction type. The tight force causes the normal stress of the contact surface to generate anti-torsion friction resistance, so that the nut can be released without loosening, so it is not necessary to attach an anti-off device.
Figure IX
However, if special chucks (such as vibrating machinery or important structural positions of the car body) must be used to prevent loosening of the nut, the collet of the bolt determines the length or form of the bolt and the thread to prevent loosening. See Figure 10 for details on the cap clamp.
Figure ten Â
Conclusion
High-strength bolts are currently used in the automotive field. They are mainly used in the connection of the vehicle's strength and its important parts, such as the wheel hub of the car, which plays an irreplaceable role in the safe and stable driving of the car.
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