Locking of high strength bolts

The metal structure of amusement facilities adopts four connection modes: welding connection, rivet connection, ordinary bolt connection and high-strength bolt connection. Among them, high-strength bolt connection is commonly used, and the relaxation mode of high-strength bolt is mainly double nut relaxation.

The description of bolt connection in the relevant specifications of amusement facilities is too simple, so that in the actual operation process, the operation of amusement equipment enterprises is not very standardized. Due to the incorrect use of high-strength bolts, the fatigue fracture and connection failure of high-strength bolts have become a more hidden accident form of amusement equipment, and even overturning occurs under normal operation, Cause sudden accidents. It is necessary for us to have an in-depth understanding of high-strength bolts. Based on this, a special article is written to discuss the pre tightening of high-strength bolts.

What is the preload of high strength bolt?

The bolt pre tightening force is the pre tightening force generated between the bolt and the connected part along the axis of the bolt under the action of tightening torque in the process of tightening the bolt. For a specific bolt, the preload is related to the tightening torque of the bolt, the friction between the bolt and the nut, and the friction between the nut and the connected part. The so-called pre tightening force is the pre tightening force when various threads are combined. When tightening nuts or bolts, if the preload used is too small, the parts cannot be tightened; If the pre tightening force is too large, it is easy to cause bolt breakage, nut sliding buckle, and deformation of the fixed parts, but there will be no so-called bolt elongation by the landlord.

Effect of bolt pre tightening

Pre tightening can improve the reliability, anti loosening ability and fatigue strength of bolt connection, and enhance the tightness and rigidity of connection. In fact, a large number of tests and application experience have proved that a high preload is beneficial to the reliability of the connection and the service life of the connected, especially for the connection with sealing requirements. Of course, as the saying goes, “when things reach the extreme, they will turn back”. If the preload is too high, it will often lead to the failure of the connection if it is improperly controlled or accidentally overloaded. Therefore, it is very important to accurately determine the preload of bolts.

Method for controlling thread preload

Whether the pre tightening force of bolts or nuts meets the requirements can generally be judged by torque wrench. The torque display panel is generally marked on the torque wrench. When tightening a bolt or nut with a torque wrench, the torque can be directly displayed on the display panel to prevent damage to the bolt or nut due to excessive force, and ensure the preload of the bolt or nut. The force shall be uniform in use, and the impact force shall not be used to prevent inaccurate reading, etc. In actual use, there are the following control methods:

  • 1) Use special assembly tools. Such as force measuring wrench, torque wrench, electric and pneumatic wrench, etc.
  • 2) The elongation of the bolt can be measured. Before tightening the nut, the original length of the bolt and the length elongation of the thread after tightening can determine whether the tightening torque is accurate.
  • 3) Torsion angle method. The principle is the same as that of measuring elongation, except that the elongation is converted into an angle at which the screw is screwed again at the original tightening position.

Installation method of double nut

Double nuts are used. Generally, the vibration in the use environment is relatively large and anti loosening is required. In another case, the use strength is relatively large and one nut is not enough. Locking type: flat washer + double nut (the steel structure is mainly static, and the commonly used locking type: high-strength bolt + 1 nut + 2 gaskets for large hexagonal steel structure, generally without double nuts).

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Double nut anti loosening principle: after the two nuts are tightened, the axial force generated between the nuts increases the friction between the nut teeth and the bolt teeth to prevent the nut from loosening automatically. As shown in Figure 1.

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Fig. 1 double nut locking principle
The correct installation method is: first tighten the lower nut with 80% of the specified tightening torque, and then tighten the upper nut with 100% tightening torque; The lower nut thread is only subject to the jacking force, and its height can be reduced. Generally, thin nuts are used; The upper nut is a type 1 standard nut; In order to prevent wrong installation and ensure sufficient strength of the following nuts, two equal height nuts (type 1) are used. The nuts have the advantages of simple structure, good locking effect, low cost and high quality. They are mostly used in low-speed, heavy-duty or stable load situations (Mechanical Design Manual [connection and fastening]), as shown in Figure 2.

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Fig. 2 Thickness double nut locking
There is a solution to the problem of looseness of high-strength bolts in the technology of high-strength fasteners for construction machinery and equipment. It is said that “when using grade 8.8 or 9.8 high-strength bolts, spring washers are generally not allowed to be used as components to prevent looseness, and double nuts are recommended to prevent looseness.
Locking method of slewing ring: double nut locking method is adopted, and the two slewing ring nuts should be the same. Therefore, in the connection of important parts of amusement facilities (bolted connection above grade 8.8 is usually important connection), flat washer + double nut type is adopted, which is usually used in slewing ring, boom flange connection and other parts.

Matching and pre tightening of bolts, nuts and washers

If the user chooses the nut, please ensure that the strength level of the nut is matched with the bolt. There are two levels commonly used in amusement facilities: level 4.8 and level 8.8. As shown in Table 1:
Table.1 matching grades of bolts, nuts and washers

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The performance grade code of carbon steel bolts, studs and screws in GB / t3098.1-2000 mechanical properties of fasteners bolts, screws and studs consists of two parts separated by “•”:
The first part of the number (“•”) indicates the tensile strength( σ b) 1 / 100 of; the second part of the number (“•”) represents the nominal yield point( σ s) Or nominal specified non proportional elongation stress( σ P0.2) and nominal tensile strength( σ b) Ratio (yield ratio)( σ s/ σ b) The product of these two parts is the nominal yield point( σ s) Or nominal specified non proportional elongation stress( σ P0.2).
For the preload of high-strength bolts, please refer to tables C1 and C2 in gb8408-2018, in which table C1 refers to ordinary bolts, including ordinary high-strength bolts (for the difference between ordinary high-strength bolts and high-strength bolts, please refer to my other article, but in this article, in order to facilitate everyone’s understanding, the high-strength bolts referred to in this article include high-strength ordinary bolts) , the value given in the table is the maximum allowable value, that is, the upper critical value. In the design of amusement equipment, if the bolt is subjected to additional tension (such as the bolt connection of cantilever flange of large pendulum), it shall not be applied according to the preload written in the table, but shall be applied after calculation according to the actual working conditions.

Some misunderstandings in the use of high strength bolts

Spring washer shall be used for locking of high-strength bolt

Technical conditions for high strength fasteners of construction machinery and equipment JG / t5057.40-1995 stipulates that “when grade 8.8 or 9.8 bolts are used, spring washer locking is generally not allowed. When bolts of other performance grades are used, spring washer locking is not allowed. It is recommended to adopt the following locking method: double nut locking, and the two nuts should be the same” The connecting pair of large hexagon high-strength bolt is composed of a bolt, a nut and two washers. During installation, the installation direction is from bottom to top. Each side of the bolt head and nut is equipped with a washer. The chamfered side of the washer must face the bolt head or nut.
However, many installation units have added an additional spring washer. The high-strength bolt acts by applying a large preload and transmits the external force through the friction between the connectors. If the spring washer is added, the spring washer itself can not bear such a large preload and may be crushed. This will not play a role in preventing looseness, but will make the high-strength bolt Bolt looseness and connection failure due to insufficient preload, resulting in accidents

Reuse of high strength bolts

Technical conditions for high strength fasteners of construction machinery and equipment JG / t5057.40-1995 stipulates: “high strength bolts and nuts shall be removed and reused after use, generally not more than twice. Moreover, the removed bolts and nuts must be free of any damage, deformation, sliding teeth, missing teeth, rust corrosion, large change in thread roughness, etc. otherwise, they shall not be used for the connection of stressed components.” High strength bolt connection is to transfer external force through the friction generated between the connectors by applying a large pre tightening force to the bolt itself. In this case, the high-strength bolt bears a large force. Therefore, the high-strength bolt is generally not allowed to be reused. Only when the bolt and nut are free of any damage, deformation, sliding tooth, missing tooth, corrosion, large change in thread roughness, etc It can only be used under the condition of, but not more than twice.
However, in the actual use of amusement equipment, there is a common phenomenon that its high-strength bolts have been reused for many times since the equipment was purchased and used, so that the high-strength bolts have lost their due efficacy. The installation and use units should strengthen the understanding of the correct use of high-strength bolts, so as to avoid the reuse of high-strength bolts, the use of high-strength bolts without manufacturer marks and asexual grade marks High strength bolts.

Disassembly and inspection of high-strength bolts

Some installation units have weak safety awareness. In order to save effort during the disassembly of amusement equipment, high-strength bolts are not connected for disassembly and inspection, resulting in complex alternating stress under the action of bending moment with changing size and direction. When the number of alternating stress reaches a certain amount, fatigue cracks appear on the bolts, resulting in the fracture of high-strength bolts Therefore.

When installing high-strength bolts, use an ordinary wrench for installation

We know that amusement equipment bears various stresses and loads during use, such as self weight, passenger weight, wind load, running impact, acceleration, etc. Therefore, in addition to its high material strength, high-strength bolts also apply the specified preload to the bolts (generally 0.6-0.7 times the yield strength of the bolt material) However, the installation unit basically uses an ordinary wrench for installation in the installation process, and rarely uses a special torque wrench, resulting in insufficient preload and inconsistent preload of each connecting bolt.

Unqualified bolts marked with asexual grade

JG / T5057.40-1995 technical requirements for high strength fasteners of construction machinery and equipment Regulations: the top or side of the head of high-strength bolt shall be marked with performance grade and manufacturer’s mark. It shall also be provided with quality guarantee and certificate. The quality guarantee includes: specification, quantity; performance grade; material, heat number, chemical composition; mechanical performance test data; inspection date. At present, many leasing enterprises do not have performance grade and manufacturer on the equipment in order to save cost The use of these unqualified bolts poses a serious hidden danger to the safety of amusement equipment.

Bolts and fasteners of installation equipment are not tightened in time after loosening

After the threaded fastener is loosened, it will generate huge kinetic energy, which directly acts on the fastener and equipment, resulting in the damage of the fastener. After the fastener is damaged, the equipment cannot work under normal conditions, further leading to the damage of the equipment. For the fastener under axial force, the thread is damaged and the bolt is pulled off. For the fastener under radial force, the bolt is cut off and the bolt is tightened The bolt hole is made into an ellipse.
The installation unit shall strengthen daily inspection. After the first installation, it shall generally check and tighten it after 100h, and then tighten it every 500h.

Source: Network Arrangement – High Strength Bolts Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.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.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@metallicsteel.com

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