First, the advantages of the yield point method
The yield point method controls the preload force by controlling the torque applied to the bolt. Only 10% of the torque of the torque method acts on the pre-tightening force, 90% is consumed on the thread and the end friction of the bolt, so the fluctuation of the friction coefficient greatly affects the bolt pre-tightening force. If the fluctuations exceed a certain range, the reliability of the product will arise. If the friction is too small, the torque is too high, the screw will be elongated or tapped, the friction will be too large, the torque will be too low, and the pre-tightening force will be insufficient, which will cause the cylinder head to squirm and cause fluid leakage and the bolt itself or the whole. Fatigue failure of the joints, which may occur during the assembly process or during use or maintenance. Control the bolts after the yield point. It has three major advantages:
1) A relatively stable axial force can be obtained even in the case where the torque coefficient is dispersed.
2) The large axial preload force is obtained by using the bolt strength to a large extent, so that the bolt diameter can be effectively reduced and the weight of the diesel engine can be reduced.
3) The bolts that are tightened to yield are subjected to an alternating load with low force, high fatigue strength and good anti-loose performance.
Second, the determination of the initial torque
The torque angle method is corner control - torque monitoring, and the corner control is not recorded from the beginning of the assembly. Because the length of the bolt has a certain error, the thickness of the coupling member also has a certain error, and when the tightening device is used for assembly, it is not necessary for the device to screw in when the screw is turned on. If the difference is one turn, the difference is 360 degrees, so there is no corner. Starting point, using corner control is an empty talk, and the determination of the starting point of the corner is very important, because the shortcoming of torque control is that it is greatly affected by the friction factor, and the advantage of the torque angle method is that it is less affected by the friction factor. Once the corner control is adopted, the axial preload force of the bolt is hardly affected by the friction force, and is only related to the compression length of the tensile joint of the bolt, that is, the rotation angle of the bolt.
The torque angle method must first determine the starting point of the corner control. In the development of the torque angle method, the initial torque must be sufficient to ensure that the joint is fully fitted and the stiffness is stable so that the elongation of the bolt is stable when the corner is applied. Generally take about 75% of the final torque. After the assembly machine reaches this point in actual operation, pause for 1 2s, change the rotation speed to make the speed of the assembly machine slow down, then start to rotate, and start to calculate the angle until the required angle is reached. The corners generally take an integer control angle of 60o, 90o, 120o, 150o, 180o, etc., and should be as large as possible. Record the torque while calculating the corner. If the torque is too large, the tensile strength of the bolt material is too high or the material after the heat treatment is too hard. If the torque is too small, the tensile strength of the bolt material is too low, or the heat treatment is not good, and the torque is monitored. To find the abnormality to achieve the purpose of monitoring the thread material.
Third, the application
In the late 1970s, Atlas, Sweden, with the assistance of the Fiat Group of Italy, developed the torque method into a torque angle method in order to develop a tightening wrench for high-precision bolts for diesel engines, ie its initial point was determined by a certain The torque value is the starting point, and ideally the control is near the bolt yield point. In the 1980s, Dongfeng Motor Corporation began to apply the corner control method. In addition, the high-precision pre-tightening control methods such as the seating angle control method, the quality assurance method, and the yield point control method are gradually being promoted and applied.
HDT Vicat Softening Point Apparatus
The HDT Vicat softening point apparatus is a testing instrument used to determine the softening point of a material. It is commonly used in the plastics industry to evaluate the heat resistance and thermal stability of polymers.
The apparatus consists of a heating bath, a test specimen holder, and a needle or a flat-ended plunger. The test specimen is placed in the holder and immersed in the heated bath. The temperature is gradually increased at a specified rate, and the needle or plunger is applied to the specimen at regular intervals.
The softening point is determined by observing the point at which the needle or plunger penetrates a specified distance into the specimen. This indicates the temperature at which the material starts to soften and lose its dimensional stability.
The HDT Vicat softening point apparatus is designed to meet international standards, such as ISO 306 and ASTM D1525. It provides accurate and reliable results, allowing manufacturers to assess the suitability of materials for various applications and ensure product quality.
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