How to correctly select the bolts, nuts, bolts, nuts, mechanical properties of the selection is mainly to meet the requirements of the connected parts. According to the requirements of the connection, calculate the performance and grade of the required bolts, and then design the matching nuts according to this. Regarding the principle of nut design selection, here is a brief talk about the differences between the new and old standards.
The old standard view is that as long as the guaranteed load of the nut is equal to the minimum tensile load of the bolt, the new standard is that the guaranteed load of the nut is greater than the maximum tensile load of the bolt. The old standard point of view is that when the bolt and nut assembly fails in use, it is not the bolt but the nut that is expected to fail. The new standard has the opposite view. When the combination fails, it is not the nut failure, but the bolt screw is broken. Why is it because the national standard specifically proposes that the failure condition of the threaded assembly in use is usually the following: the screw is broken; the thread of the screw is tripped; the thread of the nut is tripped; the thread of the screw and the nut are tripped.
After analysis, the breakage of the screw is sudden and easy to be found during the assembly process, but the tripping phenomenon is gradually occurring and very difficult to find, which increases the possibility of accidents due to failure of the screw assembly, so From the design aspect of the threaded connection, it is hoped that the failure form is fracture, which is the basis of the new standard. It is from the perspective of avoiding or reducing accidents and ensuring safety. The old standard does not take this into consideration. It is only convenient to replace one nut (especially for non-standard parts) and the price is cheaper. Departure. This is also the difference in concept between the original national standard and the ISO international standard.
Bolts and nuts of the same performance class, the latter's stress is not only larger than the former, but also the tensile strength is generally larger than the former (except for the 12-level and individual small-size nuts). The focus of the new standard is to strive for the thread combination to be a screw break in the event of a failure, so that problems can be detected in time and appropriate measures can be taken to avoid accidents.
How can we meet the requirements of the new standard? According to the relevant information, simply improving the strength of the nut material is not acceptable. First of all, we must break the old traditional design concept - the height of the nut is fixed at 0.8D. The new standard is a combination of test and calculation, and the height of the nut of each specification is generally adjusted. The adjusted height has increased a little more than the original 0.8D, although the increase is very small, but it solves the big problem of tripping. In addition, some adjustments and regulations have been made to the effective length of the nut thread (≥0.6D), the screwing length, the accuracy of the bolt and nut thread, and the thread strength of the bolt, thus meeting the requirements of the new standard.
In order to ensure the correct use of standards by designers, we recommend to select and use the performance level of bolts and nuts to solve the combination failure problem caused by improper selection by designers. In the above two tables, it is generally possible to combine nuts with high performance grades instead of nuts with lower performance grades, which is in line with the requirements of the above new standard. Otherwise, it will not work. That is to say, bolts with lower performance grades can be matched with nuts with higher performance grades, and bolts with higher performance grades should not be matched with nuts with lower performance grades.
Through the study and research of the above standards, a common basic standard seems to be very simple. If you further study its performance, scope of application and relationship with related standards, it is not an easy task. To meet the needs of enterprise production, enterprise standardization must change the traditional working methods: when the new standard is released, a standard document is issued within the enterprise, even if the standard is implemented; the technical content of the standard is not carefully studied, and the standard is The situation in use is unclear, so that the problems that arise in the implementation of the standards cannot be solved. Modern enterprise production requires our standardization personnel to master the advanced standard information at home and abroad, master the dynamics of standardization development at home and abroad, in-depth design and production of the first line to understand the characteristics of product performance, purposefully implement the corresponding standards, so that the standard is implemented in actual production. Play a proper role and bring better economic benefits to the company.
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