What’s the difference between high strength bolt and common bolt?
High strength bolt is the most common construction content in steel structure construction. All steel structure engineers will feel that they are too familiar with it.
However, this may not be the case. Today, we start with the most basic concepts and take you to re understand the high-strength bolt, which may overturn your most basic understanding.
- What is high strength bolt
- Specification of high strength bolt
- Bolt axis tension axial deformation curve
- Comparison between high strength bolt and common bolt
- Bolt tightening method
High strength friction grip bolt (HSFG for short). It can be seen that the high-strength bolt in our Chinese construction is the abbreviation of high-strength friction pre tightening bolt. In daily communication, only the words “friction” and “grip” are abbreviated, but many engineers and technicians have misunderstood the basic definition of high strength bolt.
Myth 1: bolts with material grade over 8.8 are “high strength bolts”?
The core difference between high strength bolt and common bolt is not the strength of the material used, but the form of stress. The essence is whether to apply preload and use static friction to resist shear.
In fact, there are only 8.8 and 10.9 grades of hsfg bolt (BS EN 14399 / astm-a325 & astm-490) mentioned in British standard and American Standard, while common bolts include 4.6, 5.6, 8.8, 10.9, 12.9, etc. (Table 2 of BS 3692 11); therefore, material strength is not the key to distinguish high-strength bolts from common bolts.
Myth 2: the bearing capacity of high-strength bolt is higher than that of ordinary bolt, which is “high strength”?
It can be seen from the calculation of single bolt that the design strength of high-strength bolt in tension and shear is lower than that of ordinary bolt. The essence of its high strength is that no relative sliding is allowed in normal operation, that is, the elastic-plastic deformation is small and the joint stiffness is large.
It can be seen that under the given design node load, the nodes designed with high-strength bolts may not necessarily save the number of bolts, but they have small deformation, large stiffness and high safety reserve. It is suitable to use the main beam and other positions with high rigidity, which conform to the basic seismic design principle of “strong node, weak member”.
The strength of high-strength bolt is not due to its design value of bearing capacity, but due to its large rigidity, high safety performance and strong anti-damage ability.
High strength bolts commonly used in China are classified into ASTM and JIS specifications. Generally, there are two kinds of ASTM high-strength bolts, A325 and A490.
Table I. General situation of ASTM high strength bolts
- American made A325 type1 medium carbon, carbon boron or medium carbon alloy steel, suitable for general steel structure, hot-dip galvanized.
- American made A325 type2 has been out of service since 1991 / 11.
- American made A325 type3 weathering steel, the weathering and corrosion resistance is equivalent to A242 / a566 / a709 and other steel. It is suitable for use with weathering steel.
It can be hot dip galvanized.
- American A490 type 1 bolt is made of alloy steel with nominal diameter of 1 / 2 “~ 1 1 / 2”. It is suitable for general steel structure and cannot be hot-dip galvanized.
- American A490 type2 bolt is usually made of low carbon Martian loose iron steel, with nominal diameter of 1 / 2 “~ 1 1 / 2”, which cannot be hot-dip galvanized.
- American made A490 type3 weather resistant steel, the weather resistance and corrosion resistance are equivalent to A242 / a588 / a709 and other steel. It is suitable for use with weather resistant steel. Hot galvanizing is not allowed.
- American a449 type1 medium carbon content, nominal diameter in 1 / 4 “~ 3”. Suitable for general steel structure, can be used for high temperature resistance, can be hot-dip galvanized.
- Made of American a449 type2 low carbon Martian loose iron steel, nominal diameter 1 / 4 “~ 1”, not hot-dip galvanized.
The main components of A325 bolt are type1 and type3. Type1 is used for general structure. If necessary, it can be hot-dip galvanized. Weather resistant steel should be used with type3 bolt. When type3 bolt is used, it should be specially indicated on the design drawing. The mechanical property of A325 bolt is shown in Table 2.
Table II. MAechanical properties of ASTM high strength bolts
The material strength of A490 bolt is higher than that of A325, but A490 bolt cannot be hot-dip galvanized. A490 bolts are divided into 3 types, and type 3 bolts are used together with weather resistant steel. The standard diameters of A325 and A490 bolts are in feet and 1 / 8-foot increments, and they are all between 1-1.5 feet. The diameter of about 1 foot is commonly used.
In some special cases, when the required bolt diameter is more than 1.5, A490 bolt can be used. The use of A490 bolt is shown in Table 1, and the material strength is shown in Table 2. The screw head and nut of ASTM high-strength bolt are hexagon head, and the nut must be heavy hexagon nut.
JIS high strength bolts are divided into F series of hexagon bolt heads and S series of round bolt heads developed to reduce material consumption and aesthetic requirements (see Figure 1). It is difficult to disassemble the round bolt heads after installation.
Fig. 1 JIS F10t round head torque control high strength bolt
The material strength is different. F series is divided into F8t, f10t and F11t, while s series is divided into S8t and S10t. Because the quality of F11t material is not easy to control, only some manufacturers have manufacturing qualification, and the material strength is shown in Table 3.
Table III. Material strength of JIS high strength bolt
JIS bolt diameter is in mm, and the commonly used diameter specification is between 12mm and 24mm. According to the past experience of hot-dip galvanizing, high-strength bolts will absorb hydrogen and oxygen in the process of pickling before hot-dip galvanizing, so there will be delay failure. Delay failure is the sudden fracture of high-strength bolts after fastening for a period of time, which is the result of excessive hydrogen and oxygen pressure in the metal crystal.
The high-strength bolt with delay failure above f10t will not occur, but below F8t, so the hot-dip galvanized high-strength bolt used for friction bonding can only be used below f8t.
Because the design and Construction Manual of hot-dip galvanized steel bridge compiled by China Society of civil and hydraulic engineering clearly indicates that the fatigue strength of f8t bolt, F10 nut, F35 Huasi combination and galvanized test piece is about 12kgf / mm2 lower than that of black paint base metal, therefore, the amount of bolt should be increased for f8t high-strength nut of hot-dip galvanizing as the case may be.
The appearance of the bolt is shown in Figure 2, and the bolt type will be marked on the bolt head, as shown in Figure 3 and Figure 4, so as to facilitate the use and construction management.
Figure 2 Appearance and dimension of bolt
Fig. 3 mark of a STM high strength bolt
Fig. 4 marking of JIS high strength bolt
Bolts shall be used with nuts and washers. Bolts, nuts and washers form bolt groups. Table 4 and table 5 are the combination of ASTM and JIS high-strength bolt groups respectively. Weather resistant steel shall be used with weather resistant bolts, nuts and washers.
Table IV. ASTM high strength bolt set
Note: except A563-b, all nuts are heavy hexagonal bolts.
Table V. JIS high strength bolt set
In addition, the washer can prevent the bolt or nut from damaging the steel plate due to rotation during the locking process, and can also disperse the too concentrated pressure on the top of the bolt. In addition, the friction of the rotating surface can be reduced by placing the washer on the rotating end (bolt head end or nut end).
The bolt tightening process will be locked to the snug tight state first. The so-called snug tight state is that the special wrench is used to screw the bolt for several turns, which is the tight state of the joint surface. In the tight condition, the bolt has been partially pretensioned.
Fig. 8 shows the curve relationship of axial deformation of bolt after tightening. The abscissa is the number of nut rotation turns, which is equivalent to axial deformation. The ordinate is the bolt tension. The curve begins to show a linear relationship when it reaches the snug state. Then it will yield at the screw teeth first, and the curve enters into the nonlinear phase. Then the screw teeth will produce necking and the strength will decline, and finally the screw teeth will break. Split.
Fig. 8 relation curve between axial tension and axial deformation of bolt
When the bolt is under tension, the critical section occurs at the screw teeth. The maximum tensile strength of the screw teeth is the product of the material tensile strength and the tension surface as.When the bolt is under tension, the critical section occurs at the screw teeth. The maximum tensile strength of the screw teeth is the product of the material tensile strength and the tension surface as. Among them, = 0.75 to 0.79abn = number of threads per centimeter d = standard diameter of bolt AB = standard sectional area of bolt (cm2) = π D / 4 material yield occurs in the dent of the thread, and the yield of the thread will occur in the local area, while the relationship curve of the axial tensile deformation of the bolt is not clear at that time point, the load of the bolt at the yield of the thread is proof load, which is equal to the bolt. The product of yield strength and tension area of material, the proof load of A325 bolt is about 70% of its own tensile strength, and the proof load of A490 bolt is about 80% of its own tensile strength.
Therefore, a pre tightening force TB shall be added to the installation of high-strength bolts. This pre tightening force is 80% of the bolt tensile strength. Table 8 shows the minimum pre tightening force of A325 and A490 bolts, and table 9 shows the minimum pre tightening force of JIS bolts.
The minimum pretension of A572 bolt is about equal to its proof load, and the minimum pretension of A490 bolt is also close to its proof load. In actual construction, the pretension will be higher than the minimum preload, so the bolt has basically yielded when applying the preload.
In principle, under the premise of not using bolts to damage and not making the bolts produce permanent plastic deformation to easily produce relaxation, the greater the pre tightening force of the bolts, the better.
Table VIII. Minimum preload of A325 and A490 bolts
Table IX. Minimum pre tightening force of JIS bolt
- Common bolts can be reused, and high-strength bolts can not be reused.
- The high-strength bolt is generally made of high-strength steel (45 steel (8.8s), 20mtib (10.9s), which is a prestressed bolt. The friction type uses a torque wrench to apply the specified prestress, and the pressure type screws off the plum head. Ordinary bolts are generally made of ordinary steel (Q235) and only need to be tightened.
- Ordinary bolts are generally 4.4, 4.8, 5.6 and 8.8. High strength bolts are generally grade 8.8 and grade 10.9, most of which are grade 10.9.
- The screw hole of ordinary bolt is not necessarily larger than that of high-strength bolt. In fact, ordinary bolt holes are relatively small.
- Generally, the screw holes of ordinary bolts A and B are only 0.3-0.5mm larger than those of bolts. Class C screw holes are generally 1.0-1.5mm larger than bolts.
- Friction type high-strength bolt transmits load by friction force, so the difference between screw and screw hole can reach 1.5-2.0mm.
- The force transmission characteristics of high-strength bearing bolt is to ensure that the shear force does not exceed the friction force in normal use, which is the same as that of friction type high-strength bolt. When the load increases again, there will be relative slippage between connecting plates. The connection depends on the screw shear resistance and the hole wall pressure to transmit the force, which is the same as the common bolt, so the difference between screw and screw hole is slightly smaller, which is 1.0-1.5mm.
There are great differences in construction inspection methods between ordinary bolts and high-strength bolts due to their different design principles.
The mechanical performance requirements of common bolts of the same grade are slightly higher than those of high-strength bolts, but high-strength bolts have one more impact energy acceptance requirements than common bolts.
Identity style comparison
The identification of common bolts and high strength bolts is the basic method for field identification of bolts of the same grade. Because the value of torque value of high-strength bolts calculated in British and American standards is not the same, it is necessary to identify the two standards of bolts.
The price of ordinary bolts is about 70% of the price of high-strength bolts. Combined with the comparison of their acceptance requirements, it can be concluded that the premium part should be to ensure the impact energy (toughness) performance of materials.
The tightening degree of ordinary bolts is not clearly specified, and the force applied during locking is generally only connected tightly. However, due to no tension applied during locking, the nut is easy to loose and fall off. In order to prevent loosening, the spring washer or double nut method can be used to compress the inner nut with the outer nut to prevent the two nuts from rotating synchronously and loosening.
When double nuts are used, heavy hex nuts shall be used for inner nuts and general nuts (see Fig. 9) for outer nuts. For general high-strength bolts, no matter load-bearing or friction type, there is tension after pre tightening. The normal stress of contact surface caused by this pre tightening force can produce torsional friction resistance, which can make nuts not loose, so it is not necessary to attach anti stripping device.
Figure 9 general nuts can be used for external nuts
However, if special chuck must be used for special purpose (such as vibration machinery or important structural position of vehicle body) to prevent nut from loosening, the chuck of bolt determines the length or form of bolt and screw tooth, so as to prevent loosening. See Figure 10 for details of nut clamping device.
Figure 10 nut clamping device
At present, high-strength bolts are widely used in automobile field, mainly used in automobile strength connection and its important parts, such as the wheel hub of automobile, which plays an irreplaceable role in the safe and stable driving of automobile.
Source: China High Strength Bolts Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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