The ultimate goal of tightening control is to obtain accurate preload, but the current control torque method has significant drawbacks. This method mainly controls the torque, and there are other influencing factors between the torque and the pre-tightening force, namely friction. The effect of torque accuracy on the preload force accuracy under different friction coefficients is shown in Table 1. The conclusions in Table 1 may be surprising: the accuracy of the applied torque is increased from 10% to 3%, and the accuracy of the pre-tightening force of 3% is increased by 2% to 3%; the accuracy of the applied torque is 5%. Increasing to 3% has no significant effect on the accuracy of the preload.
This is because the effect of the change in the friction coefficient is dominant. However, the applied torque and frictional deviations are not the only errors that affect the use of various wrenches and power tools to control the preload. For example, when tightening with a torque wrench, there are three sources of error: the first is from the estimation of the average friction coefficient; the second is the torque accuracy of the wrench; the third is the friction coefficient between the coupling and the connected member. The change, mainly due to the difference in surface shape, such as the uneven support surface. According to foreign data reports, the pre-tightening force F is generally used when the "torque method" is used. The error range is up to 30% - 40%. The above situation shows that in order to make a meaningful breakthrough in the precision control of the pre-tightening force in the bolt connection, it is not limited to the method of controlling the torque to achieve the value and accuracy of the bolt pre-tightening force.
Tightening torque M and preload force F of the bolt. Apply a tight torque O to the bolt. Then turn a middle angle in the elastic region to obtain a pre-tightening force of the bolt, and the error is still △Fl and △Ll. If it is rotated through a predetermined angle: reach the plastic region of the F-△L curve of the bolt, then Although the error of the bolt elongation is still ΔLl, the error of the axial preload force of the bolt is ΔFZ, so the error of the preload force ΔF: is small, and F. It has improved a lot, and the effect of bolting has also increased.
Practice has shown that this method can only achieve good accuracy when the coupling is in the plastic deformation range. At this time, the error is determined by the horizontal portion of the curve, and the small angular error in the steep elastic portion (as shown in Fig. 1) has a large influence on the preload load. Twist angle control method In order to apply the power wrench in mass production and overcome the influence of friction, the best method adopted at present is the "torque angle control" or "nut rotation" method. This requires a predetermined twist angle, which is an indirect measure of the amount of elongation of the coupling.
Obviously, the amount of elongation includes not only the elastic deformation of the coupling material but also the amount of compression of any gaps and voids in the joint. When applying this method, it is first necessary to determine a "sticking" torque, and then start measuring the angle of rotation. It represents the material variation range of the bolts of this specification is between these two curves. Because of the "torsion angle control", regardless of F.
Mistakes 1 The effect of torque accuracy on the pre-tightening force under different friction coefficients is the accuracy of the tightening torque of the compacted surface (Preloading precision 1) Material steel 37K (AISllol6) steel 37K (AISllol6) steel CI465 (AISllO16) cast iron surface state The coefficient of friction below the head is measured. When this method is implemented in the plastic zone, it is only necessary to find out the value of the corner by the test in the assembly process. The general illusion is that if the fastening is done in the plastic zone, the fatigue strength will decrease slightly. According to the relevant data, Japan Isuzu Automobile uses the plastic zone "torsion angle control method" fastening bolts for connecting rod screws, flywheel screws, crankshaft balance block screws, cylinder head screws, and its experience shows that if the pre-tightening force is accurate, the stress amplitude It is small and the fatigue strength is increased.
Therefore, some units tend to improve the accuracy of the torque wrench, and even stipulate the use of the wrench used by the operator to test, which is the absolute accuracy of the tool, it is very unscientific. Among the factors affecting F;, people also have a large proportion of errors. Although some units use electric wrenches and wrenches to eliminate errors caused by the operator, when these machines are automatically tightened, the tools and methods for retesting the torque should be different.
Overshot go into the heart of the drill pipe, the slider move up, make the tube socket head can be inserted into the center, when on the slider move up tight, using friction lift drill, remove loose again after the slider, by means of the connection string to slide along the block should tighten, set aside the center pipe, but in front of the salvage must first determine the bit in the bottom of the hole position, can be in a thin plate with bolts tight end of drill pipe, Drop the drill pipe to the bottom of the hole and press it on the drill bit. After waiting for a moment, it is proposed that the relative position of the drill bit at the bottom of the hole and the center of the rotary plate of the drill can be determined by observing the indentation left by the center pipe of the drill bit on the board, and then move the drill or rotary plate so that the center of the rotary plate is directly in front of the center pipe of the injured head, so as to ensure that the fishing device can be inserted into the center pipe accurately.
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