How to use high strength fastener
1. Characteristics and application of high-strength fasteners
1) High-strength fasteners refer to fasteners with higher performance levels, including high-strength bolts, high-strength nuts, high-strength washers, high-strength screws, and high-strength studs. High-strength bolts generally refer to GB / T 3098.1-2010 "Fastener Mechanical Performance Bolts, Screws and Studs" performance level marking system and mechanical performance grade> = 8.8 bolts; high-strength nuts usually refer to GB / T 3098.2-2015 "fastener mechanical properties" Nuts "performance level marking system and nuts with mechanical performance level> = 8; high-strength washers usually refer to high-strength washers for steel structures (GB / T 1230-2006" High-strength washers for steel structures ") and high-strength bolts Washer (Heavy Machinery Standard JB / ZQ 4080-2006 "High Strength
Bolt special washer》), the former has a hardness of 35HRC -45HRC, and the latter has a tensile strength of 900MPa (<= M90) and 700MPa (> = M100). The performance level of high-strength fasteners reflects the strength of its material. The higher the performance level, the higher the strength of the material, the greater the clamping force that can be generated. The high-strength screws, high-strength studs, and high-strength bolts implement standards. The same, the characteristics are basically the same, the use is basically the same. For the convenience of description, this article only refers to high-strength bolts, high-strength screws, and studs.
2) High-strength fasteners have high mechanical strength. Compared with ordinary fasteners (mainly referring to the mechanical performance level of bolts <= 6.8), under the same working conditions, it can be replaced with smaller specifications or fewer high-strength fasteners to fully meet the working conditions. Requirements. However, high-strength bolts have more requirements for testing items, such as: reduction of area, torque of failure, minimum height of the non-decarburized layer of the thread, maximum depth of the fully decarburized layer, and hardness after tempering. These items are not detected.
3) According to the requirements of the product, select the high-strength fastener performance level that meets the safety factor, not the higher the performance level, the better.
4) Different high-strength fasteners are suitable for different products and working conditions. Choose the appropriate variety of fasteners according to the structure and material of the product. For example: Fasteners for steel structures are in accordance with GB / T1228-2006 (high-strength large hexagon head bolts for steel structures), GB / T 1229-2006 (high-strength large hexagon nuts for steel structures), GB / T 1230- 2006 (High-strength washer for steel structure) is selected. It consists of a high-strength bolt, a high-strength nut and two high-strength washers. The three are used together. For the use of gaskets, it is generally used when the material being clamped has low strength and the compression surface may undergo plastic deformation.
5) The threads of high-strength bolts have coarse thread and fine thread. Generally, coarse thread is used. For special occasions or places with high requirements for anti-loosening, fine thread can be used.
6) High-strength bolts and nuts comply with higher product grades, which are generally A or B grade products, and have higher requirements on the shape of the thread bottom.
7) Torque coefficient is the coefficient of all influencing factors considering the relationship between tightening torque and pre-tightening force when tightening threaded fasteners. These influencing factors include thread diameter, friction coefficient, pitch, tooth profile, etc. For high-strength thread tightening in 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 same component, the number of high-strength fasteners should be as small as possible, and the selection specifications should be relatively concentrated, and it is best to have a consistent arrangement form, which is conducive to construction operations, reduces costs, and obtains good connection quality. .
2) Nuts and washers used with high-strength bolts should have the corresponding performance level corresponding to the bolts. See Table 1 for the performance of bolts and nuts in general. There are two types of nuts: type 1 and type 2. According to the performance level and thread size of high-strength bolts, a suitable nut type is selected. If a washer is used, a washer with a hardness greater than the size of the head of the bolt or nut must be used, and its performance meets the requirements of GB / T 1230 or JB / ZQ 4080.
5) The through-holes of high-strength bolts are generally selected according to the size of the medium-assembly through-holes specified in GB / T 5277-1985 "Fastener Bolts and Screw Through-holes". When the edge of the through hole interferes with the fillet under the bolt head, the hole should be chamfered.
6) When the ambient temperature of high-strength bolts and nuts is too high or too low, the influence of ambient temperature on the connection of high-strength bolts and nuts should be considered. When the ambient temperature is too high, follow GB / T3098.1 Appendix A.
3.Assembly of high-strength fasteners
1) The mechanical performance level of the high-strength fasteners used in assembly shall be consistent with the performance level specified in the design drawings. It is not allowed to replace high-strength performance fasteners with low-strength performance fasteners. Check that the mechanical properties of the high-strength bolts, nuts and washers are in accordance with the design requirements.
2) High-strength fasteners for important occasions and key parts are not recommended for repeated use. If it is not reused, it should be specified in the design document and instruction manual.
For general occasions and ordinary components, whether the high-strength fasteners can be used again is currently not uniformly regulated by the state, and the practices of various industries and units are also different. If it is used repeatedly, it shall be stipulated in the design document and the instruction manual, necessary inspection shall be carried out, and the fasteners that meet the requirements can be put into secondary use.
a) Appearance inspection: no obvious bumps, squeeze marks, no damage to the thread profile, no bending deformation of the bolt rod, no necking, and no crack defects on the surface.
b) Thread inspection: Check the threads with a thread gauge. Check the thread pitch error and half-angle error over the entire length of the thread.
c) Magnetic particle inspection: For a batch of fasteners, a sample of each specification is sampled according to the quantity ratio and inspection is performed when the quantity is small. There shall be no cracks in the magnetic particle inspection.
d) Torque coefficient: When the torque coefficient needs to be considered, the torque coefficient is re-measured, and the measured torque coefficient is used for secondary tightening.
3) Check the quality of high-strength bolts, nuts and washers before installation. Good appearance, no dirt, no oil, no scratches; no cracks, dents, wrinkles and other defects on the fastener surface. If there is such a problem, the nature of the defect should be evaluated according to the corresponding standards, and those who fail will not be put into use.
4) For high strength washer with chamfer on one side, the chamfered side of the washer should be placed under the bolt head to avoid interference between the washer and the round corner under the bolt head.
5) High-strength fasteners should be tightened according to the specified tightening torque. High-strength fasteners that provide a torque coefficient with the supply should be maintained in their original state, and no cleaning or oiling operations shall be performed. Based on the provided torque coefficient, tighten according to the specified tightening torque. Fasteners that exceed the torque factor guarantee period shall be verified or recalibrated; for fasteners that do not provide a torque factor, the tightening torque shall be implemented in accordance with the corresponding standards.
6) When tightening high-strength bolts or nut groups, the circularly distributed bolts or nut groups should be tightened in a symmetrical and crosswise order, and the linearly distributed bolts or nut groups should be tightened outward from the middle. When tightening, the tightening torque should be applied evenly and gradually to avoid sudden force. This method has been compared with the tightening sequence in the quality supervision and inspection center for general parts and components of the machinery industry, and the effect is the best.
7) Tightening of high-strength bolts is divided into initial tightening and final tightening. The initial tightening torque is about 50% of the specified tightening torque, the final tightening torque is the specified tightening torque, and finally check each one according to the specified tightening torque. The fittings required by the tightening method shall be carried out according to its instruction manual.
8) Check the quality of bolt holes and threaded holes. High-strength bolts can only be installed when the machining quality is good and there are no burrs, dirt, or deformation. When a wrong hole occurs, take appropriate measures to correct it. Do not force the bolt.
9) Use a suitable tool (manual or electric, etc.) for tightening. Do not use percussion. The head of the bolt or nut after tightening must not be damaged. The tightening tools shall be regularly checked according to their use regulations, and their accuracy shall be checked before each use.
10) The area of the connected parts that is in contact with the high-strength bolt heads, nuts and washers and the nearby area should be kept metal surfaces, and this area should be clean and free of dirt. If the area is coated with oil or paint, it should be removed before assembly.
Concluding remarks
There are actually many factors that affect the assembly performance of high-strength fasteners. Such as: the material of the internal and external threads and the hardness of heat treatment, the quality of the fastening surface, the length of the turn and so on. In addition, MoS2 for thread coating can obtain a relatively stable torque coefficient, and the consistency of tightening axial force also needs to be tested and studied.
Through continuous in-depth research and discussion on the use of high-strength fasteners, formulating scientific and reasonable specifications, guiding the design and selection of designers, and ensuring the requirements of manufacturing and assembly are of great significance for improving product quality.
