Discussion on the usage of high strength fastener

Fastener is the most common type of standard parts in mechanical products due to its variety, specification, large usage and wide application. The reasonable selection of fasteners has also become an important link in the process of product design. Whether the fasteners are used correctly or not is directly related to the functional realization and safe operation of the product. Compared with general fasteners, high-strength fasteners have different requirements in product design, manufacturing, assembly, inspection and maintenance, and play a more critical role in the operation of equipment. With the development of mechanical products to large-scale, automatic, intelligent and other diversified situations, the use of high-strength fasteners as key functional parts is increasing, especially in key occasions and important parts. At present, there are still some problems in the selection and use of high-strength fasteners, such as different opinions and insufficient understanding. It is necessary to do further research work. This paper mainly discusses the use methods of high-strength fasteners for mechanical products, and does not discuss the use of high-strength fasteners instead of general fasteners. 

1. Characteristics and application of high strength fasteners 

1) high strength fasteners refer to fasteners with high performance grade, including high-strength bolts, high-strength nuts, high-strength washers, high-strength screws and high-strength studs, etc. high strength bolts generally refer to bolts with performance grade marking system in accordance with GB / T 3098.1-2010 mechanical properties of fasteners bolts, screws and studs and mechanical property grade > = 8.8; high strength nuts pass It usually refers to the nuts that conform to the performance grade marking system of GB / T 3098.2-2015 mechanical performance nuts for fasteners and the mechanical property grade is > = 8; high strength washers usually refer to the high strength washers for steel structures (GB / T 1230-2006 high strength washers for steel structures) and the special washers for high strength bolts (heavy machinery standard JB / ZQ 4080-2006 high strength washers) Bolt special washer), the former hardness is 35hrc-45hrc, the latter tensile strength is 900MPa (< = M90) and 700MPa (> = M100). The performance grade of high-strength fastener reflects the strength of its material. The higher the performance grade is, the higher the strength of the material is, the greater the clamping force can be generated. The Executive Standards of high-strength screw, high-strength stud and high-strength bolt are the same, the characteristics are basically the same, and the use is basically the same. For the convenience of description, only high-strength bolts, high-strength screws and studs can be used by reference. 

2) high strength fasteners have high mechanical strength. Compared with general fasteners (mainly refers to bolt mechanical property grade < = 6.8), under the same working condition, smaller specifications or fewer high-strength fasteners can be used instead to fully meet the use requirements of the working condition. However, there are more requirements for testing items of high-strength bolts, such as: reduction of area, failure torque, minimum height of non decarburized layer of thread, maximum depth of fully decarburized layer, hardness after re tempering, etc. generally, these items are not tested for bolts. 

3) according to the use requirements of the product, select the performance grade of high-strength fastener that meets the safety factor, not the higher the performance grade, the better. 

4) different high-strength fasteners are suitable for different products and working conditions, and appropriate fastener varieties are selected according to product structure and material. For example, the fastener used for steel structure shall be selected according to GB / t1228-2006 (high-strength large hexagon head bolt for steel structure), GB / T 1229-2006 (high-strength large hexagon nut for steel structure), GB / T 1230-2006 (high-strength washer for steel structure), which is composed of one high-strength bolt, one high-strength nut and two high-strength washers, which shall be used together. For the use of gasket, it is generally used when the material strength is low and plastic deformation may occur on the compression surface. 

5) the thread of high-strength bolt has coarse thread and fine thread, generally coarse thread is selected. Fine thread can be selected for special or high anti loose occasions and parts. 

6) high strength bolts and nuts meet higher product grade, generally grade A or grade B products, and have higher requirements on the shape of thread bottom. 

7) torque coefficient refers to the coefficient of all influencing factors that consider the relationship between tightening torque and pre tightening force when tightening threaded fasteners, including thread diameter, friction coefficient, pitch, profile, etc. For the high-strength thread fastening of important parts, the use of torque coefficient should be considered to ensure that the clamping force is more uniform and the screw stress is more consistent. 

2. Selection of high strength fasteners 

1) in the same product or component, the variety of high-strength fastener shall be as few as possible, the specification shall be relatively centralized, and the layout form shall be consistent, which is conducive to construction operation, cost reduction and better connection quality. 

2) nuts and washers used with high-strength bolts shall have corresponding performance level with bolts. There are two types of nuts, i.e. type 1 and type 2. The appropriate nut type shall be selected according to the performance grade and thread specification of high-strength bolt. If washers are used, washers with high hardness larger than the size of bolt or nut head must be used, and their performance meets the requirements of GB / T 1230 or JB / ZQ 4080. 

3) the through-hole of high-strength bolt is generally selected according to the through-hole size of medium assembly specified in GB / T 5277-1985 fastener bolt and screw through-hole. When the edge of the through hole interferes with the fillet under the bolt head, the hole shall be chamfered. 

4) when the ambient temperature of high-strength bolt and nut is too high or too low, the influence of ambient temperature on the connection of high-strength bolt and nut shall be considered. If the ambient temperature is too high, it shall be in accordance with Appendix A of GB / t3098.1. 

3. Assembly of high-strength fasteners 

1) the mechanical property grade of high-strength fastener used for assembly shall be consistent with the performance grade specified in the design drawing, and it is not allowed to replace high-strength fastener with low-strength fastener. Check whether the mechanical property grade of high-strength bolts, nuts and washers is consistent with the design requirements. 

2) high strength fasteners for important occasions and key parts are not recommended to be reused. If it is not reused, it shall be specified in the design documents and operation instructions. For general occasions and common parts, whether high-strength fasteners can be used twice is not regulated by the state at present, and the practices of various industries and units are different. If it is reused, it shall be specified in the design documents and operation instructions, and necessary inspection shall be carried out for it, and the fasteners meeting the requirements can be put into secondary use.

  • A) appearance inspection: no obvious bump, extrusion trace, no damage to thread profile, no bending deformation, no necking phenomenon on bolt rod, no crack on surface, etc.
  • B) thread inspection: check the thread with thread gauge. Check whether the pitch error and half angle error of the thread are qualified on the whole length of the thread.
  • C) magnetic particle inspection: for a batch of fasteners, take samples according to the quantity proportion for each specification, and conduct full inspection when the number of inspection days is small. No cracks are allowed in magnetic particle inspection.
  • D) torque coefficient: when it is necessary to consider the use of torque coefficient, re measure the torque coefficient, and use the measured torque coefficient for secondary tightening.

3) check the quality of high-strength bolts, nuts and washers before installation. The appearance is good, free of dirt, oil stain and bruise; the fastener surface is free of cracks, dents, wrinkles and other defects. If there are such problems, the nature of the defects shall be evaluated according to the corresponding standards, and the unqualified ones shall not be put into use. 

4) for the high-strength washer with chamfer on one side, the chamfer side of the washer shall be placed under the bolt head to avoid interference between the washer and the fillet under the bolt head. 

5) high strength fasteners shall be tightened according to the specified tightening torque. The high-strength fasteners provided with torque coefficient together with the supply shall be kept in the original state, and shall not be cleaned or oiled. Based on the torque coefficient provided, the fasteners shall be tightened according to the specified tightening torque; the fasteners that damage the original state of thread or exceed the warranty period of torque coefficient shall be verified or re calibrated; the tightening torque of fasteners that do not provide torque coefficient shall be in accordance with the phase Implementation of applicable standards. 

6) when tightening the high-strength bolt or nut group, the bolt or nut group with circular distribution shall be tightened symmetrically and crosswise, as shown in Figure 1. The bolt or nut group with linear distribution shall be tightened outward in the middle sequence, as shown in Figure 2. When tightening, the tightening torque shall be applied evenly gradually to avoid sudden force application. The tightening sequence of the method is compared in the quality supervision and testing center of general parts in the mechanical industry, and the effect is the best.

20191117230712 81843 - Discussion on the usage of high strength fastener 

Figure 1. the bolt or nut group with circular distribution shall be tightened symmetrically and crosswise 

20191117230729 18763 - Discussion on the usage of high strength fastener 

Figure 2. the bolt or nut group with linear distribution shall be tightened outward in the middle sequence 

7) the tightening of high-strength bolts is divided into initial tightening and final tightening. The initial tightening torque is about 50% of the specified torque, and the final tightening torque is the specified tightening torque. Finally, check one by one according to the specified tightening torque. The matching parts with tightening method requirements shall be carried out according to their operation instructions. 

8) check the quality of bolt holes and threaded holes. Only when the processing quality is good and there is no burr, dirt, deformation, etc., can high-strength bolts be installed. In case of wrong hole, proper measures shall be taken to correct it, and bolts shall not be forced into the hole. 

9) use appropriate tools (manual or electric, etc.) for fastening, instead of knocking. After fastening, the head of bolt or nut shall not be damaged. The fastening tools shall be verified regularly according to their use regulations, and their accuracy shall be checked to be qualified before each use. 

10) the metal surface shall be kept in the area of the connected parts contacting with the high-strength bolt head, nut and washer and its adjacent area, which shall be clean and free of dirt. If there is oil or paint coating in the area, it shall be removed and then assembled. 

4. Concluding remarks

In fact, there are many factors that affect the assembly performance of high-strength fasteners. Such as: material and hardness of internal and external thread, quality of fastening surface, screwing length, etc. In addition, for the thread coated with MoS2, a relatively stable torque coefficient can be obtained, and the test and research are also needed to ensure the consistency of tightening axial force.

Source: China Fasteners Manufacturer – Yaang Pipe Industry (www.steeljrv.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@steeljrv.com

Please notice that you might be interested in the other technical articles we’ve published:

What is high strength bolting?

The term “high strength” is a somewhat ambiguous fastener term. Most people in the industry would interpret the phrase “high strength” to refer to any bolt that has been quenched and tempered (heat treated) to develop its strength.

What are the strongest bolts?

The strongest commercial-quality bolt is grade 8, marked by six raised dashes; its medium-carbon alloy steel has been quenched and tempered to achieve a tensile strength of 150,000 psi.

How do I know if I have high tensile bolts?

To identify this grade, look out for an ‘8.8’ stamp or a set of three radial lines on the head of the bolt. Most often, customers looking for a high tensile bolt may just be looking for the 8.8 grade.

What is a Grade 9 bolt?

Grade 9 Hex Head. The Grade 9 Bolting system develops the highest clamp force available in a standard bolt/nut assembly and it is frequently referred to by one of the folowing trade names: L9, PFC9, or F911. Special Alloy steels and heat treatments are used to achieve 180,000 psi tensile strength.

Is a 10.9 Bolt stronger than Grade 8?

Class 10.9 is stronger than class 8.8, and is commonly found in high strength automotive applications. Class 10.9 is similar to grade 8. A low carbon steel for general use.

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discussion on the usage of high strength fastener - Discussion on the usage of high strength fastener
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Discussion on the usage of high strength fastener
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Fastener is the most common type of standard parts in mechanical products due to its variety, specification, large usage and wide application.
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www.steeljrv.com
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