Talking about the Standardization of Design and Installation of Bolts, Studs and Nuts

In modern industries, especially in machinery and construction, there are few products that can avoid fasteners such as bolts and nuts. With the rapid development of modern industry, people continue to summarize the design and production experience, and gradually developed a series of standard systems for bolts and nuts, including the mechanical properties, specifications, bolt hole design and tightening methods of common bolts and nuts. Standards bring great convenience to design, processing and installation. However, due to the cumbersome standards and the scattered standards, it is relatively inconvenient for design and installation. This paper mainly summarizes the related design and installation problems of bolts and nuts, and makes the design and installation of bolts and nuts more standardized, and finally ensures the quality of the products.

First, the relevant design of the bolt and nut The thread of the bolt and nut generally adopts a thread with a tooth angle of 60, a high root strength and a good self-locking function. The thread is based on the GB/T192-2003 national standard for basic thread type. This type of coupling thread is divided into coarse thread and fine thread according to the pitch. The diameter of the fine thread is larger than that of the coarse thread, so the bolt with fine thread has high strength and self-locking performance. However, the fine thread is not wear-resistant, has low shear strength, and is easy to slide. Commonly used in the design of coarse thread; for thin-walled parts, subjected to vibration shock and dynamic load, precision need fine-tuning mechanism, generally use fine thread; in high temperature parts and bolts, thread design with hydraulic tensioner installation In order to facilitate the disassembly, the large diameter, medium diameter and small diameter of the bolt are generally designed to be slightly smaller than the 6g tolerance dimension of the corresponding specification, and the thread of the nut is still 6H tolerance, so that the gap increases correspondingly when the bolt and nut are matched. It will be easier to disassemble. This design is reflected in the marine diesel engine, especially the bolts in the exhaust system and the main bolts that are stretched by hydraulic tensioners (see Table 1).

Table 1 Corresponding Thread Requirements for Different Cases Threads Threaded Threaded Threaded Angle Bolts Precision Advantages Disadvantages The root strength is usually 6g high with coarse threaded bolts and has a good self-locking function. The thin-walled parts and the bolts of the fine-threaded thread have high strength, and the self-locking performance of the fine-toothed thread is not impacted by vibration, and the dynamic load and the fine thread are resistant to shearing and strong, and the fine-tuning machine is required to be 60 degrees low. . Easy to slide buckle. Installation at high temperatures and with a hydraulic tensioner.

For example, bolts in the exhaust system and main bolts that are stretched using a hydraulic tensioner. ―When bolts such bolts have large diameters and nuts, and the coarse and fine thread diameters and small diameters are designed to be larger than the corresponding specifications, the clearance will increase accordingly, and the 6g tolerance dimension will be slightly smaller. It is easier to disassemble.

Table 2 Problems and Solutions in the Design of Bolt Through Holes Different assembly situations are prone to problems The solution involves standard medium assembly accuracy design. Because the installed aperture size is smaller than the corresponding standard bolt hex head rounded corner, it is easy. During installation, the two ends of the interference bolt through hole shall be chamfered. GB/T5277-1985 Fastener bolts and screw through holes Mechanical assembly equipment The hex head bolt has a diameter larger than the diameter of the flat washer or spring washer. Easy to install, the interference washer adopts the inner 1987 standard spring washer, GB/T1230-2006 steel structure high-strength washer 44 Shanghai standard bolt hole design bolt hole design is mainly aperture size design, according to assembly precision Use bolt hole sizes of different diameters. The bolt hole diameter is generally designed according to the medium assembly accuracy of fastener bolts and screw through holes in GB/T 5277-1985. Since the installed aperture size is smaller than the corresponding standard bolt hex head rounding, the interference will occur during installation, so both ends of the bolt through hole should be chamfered.

In the assembly process of machinery and other equipment, many bolt heads are used to flatten the washer or spring washer, but according to GB/T5782-2000 hex head bolt, GB/T 97.1-2002 flat washer A grade and GB/T93-1987 standard The spring washer shows that the diameter of the head of the hex head bolt is larger than the inner diameter of the flat washer or the spring washer, so interference occurs. Therefore, the above combined structure should be avoided in the design and installation. This is clearly reflected in GB/T1230-2006 high-strength gasket for steel structure. In this standard, the gasket adopts the inner diameter chamfering structure. Table 2 summarizes the problems and solutions that are easy to occur in the design of bolt through holes.

Design of bolt threaded holes When designing bolted holes, it is mainly the tapping depth and drilling depth of threaded holes. According to different base materials, the depth and depth of the same thread are different. When designing, the tapping depth of the threaded hole is generally calculated as follows: H is the length of the bolt screwed into the threaded hole, and P is the thread pitch of the threaded hole. The drilling depth of the threaded hole is calculated according to the following formula: D is the nominal diameter of the thread of the threaded hole. In order to ensure that the bolt is pre-tightened and subjected to the working load, the thread is not tripped, and the bolting length of different base materials is different. General steel or bronze material: H=d; cast iron material: H=(1.25~1.5)d; aluminum alloy material: H=(1.5~2.5)d, where H is the screwing length of the bolt and d is the nominal name of the bolt thread diameter.

The specific bolt hole design dimensions can refer to B/GQ 0126-1980 coarse tooth stud, screw screwing depth, tapping depth and drilling depth 4247-2006 common thread internal and external thread remaining length, drilling residual depth, bolt The length of the end of the nut is highlighted. These two standards have specific dimensions such as tapping depth, drilling depth and remaining length. In general, the length of the bolt thread extending out of the nut is generally 2 to 3 teeth, that is, 2 to 3 times the pitch length.

Other dimensions of bolt holes (through holes or threaded holes) The distance between the bolt axis and the edge of the part is generally calculated according to the following formula: During installation, most of the threaded joints must be tightened so that the connected parts are compressed and the bolts are subjected to Stretching. This force that is applied before the bolt is subjected to the working load is called the preload. The purpose of the pre-tightening is to improve the reliability, tightness and anti-loose ability of the joint. The pre-tightening force control methods of bolts and nuts mainly include: torque method, bolt and nut angle method, bolt elongation control method, stretching method (hydraulic tensioner stretching) and other specials 3560.3880.4200.4520.4840.5170.549 Note: K The value is pressed, and the corresponding torque coefficient can be directly detected according to different friction coefficients during design. Generally, the tightening torque of the connecting bolts and nuts can also be selected in 5000.10-2007 under normal circumstances, and it is not necessary to perform accurate calculation.

At present, the corner method is more commonly used in bolting systems that are particularly important, especially in the automotive and steel industries. The pre-tightening force must have Fi in order to ensure that the bolt breaks first. According to different use occasions, the appropriate method of controlling the pre-tightening force is selected, and the pre-tightening force is accurately controlled by calculation.

Choose a suitable anti-loose method to make it more reliable and convenient.

The calculations show that the nut material can be selected to have a lower strength than the bolt material to achieve the matching with the bolt, so that the material can be saved to the greatest extent while ensuring safety.

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