Calculate the equivalent design parameter values ​​that meet the design conditions. Since the variables on the right side of the deformation formula can be directly found in the design manual, it is possible to determine the value of each variable on the right side of the deformation formula according to the requirements of the design conditions, and directly calculate Right-hand result. Check the table to determine the better solution according to the right-hand calculation results, check the equivalent design parameter column in the table, and take the closest value to meet the condition, you can directly get the better solution of the design parameters.
The direct solution calculation method for bolt static strength calculation does not control the pre-tightening force and the ordinary bolt strength condition only affected by the pre-tightening force is: Re=1.3F0Pd12/4<1R2(1) where 1R2=RsSs; Rs is the bolt material The yield strength limit (MPa) is determined by the material of the bolt; Ss is the allowable safety factor and is related to the material, the nature of the load, the size of the diameter, and whether or not the preload is controlled. For ordinary bolts that do not control the preload, Ss is determined by the bolt material and diameter and should be selected.
For the deformation of equation (1), the design formula can be obtained as follows: d15.2F0SsPRs(2) Because of the three variables F0, Ss, and Rs on the right side of the formula, Ss is affected by d, that is, it has a correlation with d (see). Therefore, it is not possible to directly apply (2) for calculation, but to use a trial algorithm. That is, first assume the approximate range of the large diameter d of the bolt, from the initial safety factor, and then calculate d1 by the formula (2). Check the manual to determine the large diameter d of the thread according to d1, if the value of d matches the pre-assumed diameter range. , the calculation is completed, otherwise the above calculation process should be repeated until the d value matches Ss.
If (2) is transformed as follows: d1Ss5.2F0PRs(3), the parameters on the left side of the above equation are only related to the design parameters to be sought, and the parameters on the right are related to the working conditions and the selected materials, which can be calculated from known conditions. . In this way, the bolt strength design is used to find 5.2F0SsPRs to determine the bolt diameter d1, which is converted into direct calculation of 5.2F0PRs to determine the d1Ss value (this value is the equivalent design parameter). For bolts of any diameter, a table or spreadsheet can be listed, and the equivalent diameter values ​​under different Ss (as shown) corresponding to different diameters are pre-calculated according to d1Ss.
After making this form, calculate the equivalent diameter value that meets the strength condition according to the design requirements, and then check the table to directly determine the bolt diameter d. This only requires a simple table, which is equivalent to adding the original part of the table about the thread geometry parameters. The content (black body part) can directly obtain a better solution without increasing the complexity of solving the problem, avoiding trial calculation.
Calculation examples The following is an example to illustrate the specific application of this method. It is known that a bolt connection that does not control the preload and is only subjected to the preload force, F0=6800N, the performance grade of the part is 4.6 (carbon), and the diameter of the bolt is determined. Solution: Deform the design formula to d15.2F0SsPRs deformation to d1Ss5.2F0PRs.o Calculate the equivalent diameter values ​​under different diameters according to d1Ss. Find the right-hand result based on known conditions.
According to the performance level 4.6, it can be obtained: Rs=6@400/10=240(MPa) Bring F0=6800N, Rs=240MPa to the right side of the formula d1Ss5.2F0SsPRs, and find 5.2F0PRs: 5.2F0PRs=5.2@6800P@240 ==6.848 (mm) Check the equivalent diameter column d1Ss of the medium carbon steel, take d1Ss=6.912mm.
The direct solution calculation process for bolt fatigue strength calculation When the bolt is subjected to the changing axial working load, the bolt strength calculation formula is: RA=FAPd12/4<1RA2MPa(4) In the formula, FA is the bolt working tension variation amplitude, and Work load, preload, residual preload. The value of the allowable stress amplitude 1RA2 is calculated according to the following formula: 1RA2=RAlimSA=R-1EkmkukRSA(5) is the safety factor in the formula; R-1 is the fatigue limit of the material under the tension and compression cycle, and the materials and stresses used. The condition is related; Km is the material coefficient, ordinary steel is taken as 1, and alloy steel is taken as 1.25; Ku is the uneven coefficient of force on the thread, the pressure nut is taken as 1, and all or part of the tension nut is taken as 1.51.6; KR is the thread stress concentration Coefficient, related to material and manufacturing process; E is the size factor, related to the diameter of the thread.
Bring formula (5) into formula (4), and derive the design formula for d1 as follows: d14FASsKRPR-1KmKuE(6) Equation (6) contains the size factor E related to the diameter sought, so the bolt cannot be directly calculated. Diameter, but a trial algorithm is required.
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