Analyze the principle of excessive decarburization of fasteners and tolerance zone of thread inspection

The decarburization layer exceeds the standard. The decarburization layer is also a category A project. Due to decarburization, the surface hardness and strength of the parts are greatly reduced, which seriously affects the surface contact strength and fatigue life of the fastener, especially the harm to the threaded part. The main reasons for decarburization are decarburization of raw materials, decarburization in the process of material reforming, and decarburization in the process of product heat treatment.

The size is out of tolerance. Fasteners are general-purpose parts and require high interchangeability. Out-of-size size will directly affect their interchangeability, and even affect the connection strength, anti-loose performance or service life. The main reasons for the unqualified size:

1. Improper process control, in order to save materials, some dimensions are controlled near the lower limit, the equipment is not adjusted in time during the production process, and out-of-tolerance tools and molds are used;

2. Insufficient inspection, lax process inspection and factory inspection, which caused some unqualified batches to pass the customs;

3. It is that the measuring tool cannot be regularly checked, and even the gauge that exceeds the wear limit is used.

Fasteners are general-purpose basic parts with a large quantity and a wide range. The quality and reliability of its products play an important role in the working performance and structural safety of the main engine, especially the quality of high-strength fasteners. .

Principle of tolerance zone for thread inspection: There are tolerance zone diagrams distributed along the thread profile and a table of tolerance values u200bu200bfor each diameter.

The vast majority of technical personnel in China believe that the thread profile of a product that is qualified by the thread gauge inspection should fall within the tolerance range specified by the thread standard, and the thread quality should be no problem. When the thread quality of qualified products was found to be very poor, they were at a loss and did not know where the problem was.

Actually, the thread is a complex geometry, it has many technical parameters. If you want to fully control the quality of the thread (using the principle of tolerance zone), you need to measure all the parameters of the thread, which will take a lot of time and high inspection costs. This is not feasible in actual production. Moreover, in different occasions, the functions and requirements of the various parameters of the thread are different, and some parameters can be guaranteed by the production process without separate inspection.

At present, the thread pass and stop gauges used in my country are derived from the Taylor principle invented by the British in 1905. Its advantage is that it is economical and can guarantee assembly; its disadvantage is that the thread quality assurance level is low and there are many technical loopholes. China should earnestly study and learn from the American thread inspection system technology (standard) to improve the level of thread manufacturing in my country.