Discussion on the use of high-strength fasteners

1. Characteristics and applications of high-strength fasteners

1) High-strength fasteners refer to fasteners with relatively high performance levels, including high-strength bolts, high-strength nuts, high-strength washers, high-strength screws, and high-strength studs. High-strength bolts usually refer to the bolts that comply with the performance grade marking system of GB / T 3098.1-2010 “Mechanical properties of fasteners bolts, screws and studs” and have a mechanical performance grade of 8.8; high-strength nuts usually refer to GB / T 3098.2 “2015 leaf fastener mechanical performance nut” performance level marking system and mechanical performance level “is equal to 8 grade nuts;

high-strength washer usually refers to high-strength washer for steel structure (GB / T 1230-2006″ high-strength for steel structure Washers “) and special washers for high-strength bolts (heavy machinery standard JB / ZQ 4080-2006” Special washers for high-strength bolts “), 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 the material.

The higher the performance level, the higher the strength of the material and the greater the clamping force that can be generated. High-strength screws, high-strength studs and high-strength bolts have the same execution standards, basically the same characteristics, and basically the same use. For convenience of description, this article only talks about high-strength bolts, high-strength screws and studs.

2) High-strength fasteners have high mechanical strength. Compared with general fasteners mainly referring to bolt mechanical performance level <= 6.8, under the same working conditions, smaller specifications or a smaller number of high-strength fasteners can be used to replace to fully meet the requirements of the working conditions . However, high-strength bolts have more requirements for testing items, such as: section shrinkage, breaking torque, minimum height of undecarburized layer of thread, maximum depth of fully decarburized layer, hardness after tempering, etc., general bolts These items are not detected.

3) Select the performance level of high-strength fasteners that meet the safety factor according to the requirements of the product. The higher the performance level, the better.

4) Different high-strength fasteners are suitable for different products and working conditions. Choose suitable fastener varieties according to the product structure and material, such as: steel fasteners according to GB / T1228-2006 “Steel High-strength large hexagon head bolts for structural use “, GB / T 1229-2006” High-strength large hexagon nuts for steel structure “, GB / T 1230-2006” High-strength washers for steel structure “, consisting of one high-strength bolt and one The high-strength nut and two high-strength washers are used together. For the use of washers, it is generally used when the clamped material has low strength and plastic deformation may occur on the pressure surface.

5) The thread of the high-strength bolt has coarse thread and fine thread. Generally, coarse thread is used. Fine-thread can be used for special occasions or places with high requirements for anti-loosening.

6) High-strength bolts and nuts comply with higher product grades, generally A-grade or B-grade products, and have higher requirements on the shape of the thread bottom.

7) The torque coefficient is the coefficient of all influencing factors that consider the relationship between the tightening torque and pre-tightening force when tightening threaded fasteners. These influencing factors include thread diameter, friction coefficient, thread pitch, tooth shape, etc. For high-strength thread fastening of important parts, the 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 the same component, the variety of high-strength fasteners should be as few as possible, the selection specifications should be relatively concentrated, and it is best to have a consistent layout form, which is conducive to construction operations, reduces costs, and obtains better connection quality .

2) Nuts and washers used in conjunction with high-strength bolts should have a performance level corresponding to the bolts, and select the appropriate nut type according to the high-strength bolt performance level and thread size. If a washer is used, a washer with a hardness greater than the head size of the bolt or nut must be used, and its performance meets the requirements of GB / T 1230 or JB / ZQ 4080.

3) When the high-strength bolt is directly screwed on the body, the internal thread size should be considered according to the body material and the strength of the heat treatment to fully meet the corresponding strength performance requirements of the high-strength bolt.

4) The high-strength fasteners used on the same structural element (such as the flange for fastening a hydraulic valve) should be bolts, nuts and washers of the same performance level, and fasteners of different performance levels are not allowed to be mixed in one place.

5) The through holes of high-strength bolts are generally selected according to the size of medium-sized through holes specified in GB / T 5277 1985 “Fastener Bolts and Screw Through Holes”. When the edge of the through hole interferes with the rounded corner of the bolt head, the hole should be chamfered.

6) When the ambient temperature used for high-strength bolts and nuts is too high or too low, the influence of the ambient temperature on the connection of high-strength bolts and nuts should be considered. When the ambient temperature is too high, it shall be implemented according to Appendix A of GB / T3098.1.