Why can't China make good screws?

I think the screws are completely made in China. It is a matter of no dispute. As a matter of fact, many standard parts factories in China now import all their manufacturing machinery and equipment, so it is not difficult to observe the manufacturing quality from the perspective of machinery and equipment.

From the perspective of raw materials, according to the REQUIREMENTS of ISO standards, screws only require mechanical performance level, not raw materials. No matter which raw materials are selected, according to which heat treatment process, if the mechanical performance of a certain level is achieved, screws of a certain level will not have all the problems.

So why is there a real problem with screws coming in and out of China? In fact, there are two dimensions to this problem:

1. In order to get cheaper, some suppliers in Western countries or agents in China purchase screws from small enterprises without qualification certificates in China. The quality of such screws is not guaranteed.

In fact, most of us engaged in mechanical equipment, the understanding of screws is still stuck in our country before the 85 standard understanding, and even in the cultural education of colleges and universities or choose the definition of 85 standard. They don't know the mechanical properties of screws at all, especially in small businesses, they don't have the basic knowledge of the mechanical properties of screws, they only know that screws are a raw material according to a certain type of heat treatment process, which is a certain level of screws. There is no guarantee of the mechanical properties of the screw by looking at the whole process rather than the result.

In addition, small enterprises without qualification certificates do not have excellent manufacturing equipment and are manually processed. In that case, the tightening surface of the top of the screw head and the external thread are not vertical, which will lead to the cracking of the screw heel due to the non-vertical screw after tightening.

2. Not all import and export screws in China have problems, but only a small part of them (which are made by small enterprises without qualification certificates).

For various reasons, that objective fact has been exaggerated! This leads to the illusion that Chinese screws (screws) cannot be made.

Here, I indicate the invalid mode of connection of the external thread. There are three key invalid ways of external thread connection: extruder screw break, blurt break and head top break. The first two are mainly caused by the incorrect design scheme, while the last one is mainly caused by the manufacturing quality.

Let me show you again why some small enterprises make bad screws? Because the manufacturing process does not comply with the regulations.

Now the screw (nut) manufacturing often choose cold extrusion way, to make metal materials of chemical fiber in not ruin, physical performance is good, but the screw manufacturing machinery and equipment (heading, thread rolling) price is more expensive, small businesses have no money to buy at all, they often use several hundred yuan a desktop machine manual type CNC lathe milling screw, as metal materials chemical fiber was destroyed across the physical properties (compressive strength) is poor.

In addition, the screw heel of the cold extruder must be curved, which can prevent the problem of stress; If the screw is made by milling and the CNC blade does not have an arc, then the screw heel of the extruder will have an oblique Angle, which will cause stress and is very easy to break under the condition of tightening. Moreover, raw materials with higher compressive strength are more likely to cause stress, and screws with high toughness are more likely to break.

What's more, such small enterprises do not have the mechanical performance of standard parts testing instruments, they only according to raw materials, heat treatment process to determine the mechanical performance of screws (anchor bolts, nuts), it is not guaranteed.

So how is it that screws made in China don't work? There is no all analysis and discussion, it is unreasonable! It may be a problem of manufacturing quality, but it may also be a problem of design and application. In the early 1970s, the International Maritime Organization (IMO) carried out a lot of tests to show that even if the mechanical performance requirements of screws manufactured under typical processing standards are met, there is no guarantee that there will be no invalidation. Therefore, it is one-sided to make no comprehensive analysis.

I have repeatedly stressed that impact toughness is a probability statistic value, which does not mean the specific compressive strength of raw materials. It is usually possible that the specific compressive strength of a raw material is less than the allowable compressive strength. Therefore, the design scheme must be multiplied by the commercial insurance index.

For the external thread connection, commercial insurance index is 3 ~ 6 times, it is international requirements! It should also be noted here that the higher the mechanical properties of the screw, the higher the commercial insurance index will be.

For the reason of invalid external thread connection, it is mainly caused by tensile and torsional deformation, especially torsion deformation, because the tensile strength of raw materials is greatly lower than compressive strength, if the pretightening force is not appropriate when the external thread connection, the screw will usually be screwed.

We can actually see from the cross section of the screw, the screw extruder fracture will not have a significant convergence, but a skewed azimuth cracking, which is the typical twisting condition.

In addition, in the case of our design scheme, we cannot fully consider the impact and load hazards, especially vibration, which will have a great harm to the fatigue limit of the screw and greatly weaken the service life of the screw.

In fact, from the point of view of many destruction, fatigue destruction is the most critical reason for the inefficiency of mechanical equipment. Accordingly, in order to assure certain exhaustion service life better, must multiply when designing a scheme by certain commercial insurance index.

In fact, even in overseas screw tests, there is no guarantee that 100% of the screws will not be invalid.

This can be obtained from experimental data from the international ISO/TC2 Technical Federation. In the 1970s, the International Maritime Organization (IMO) carried out a lot of experiments, and screws manufactured under typical processing standards, even if the mechanical performance requirements were met, were not guaranteed to be invalid. At this point, it is likely that your netizens have not read the ISO report and do not know the specific situation.

In fact, there are many, even serious, problems with foreign-made goods, but the Chinese can live with them; And Chinese manufactured goods, even if the problem is not as serious as foreign ones, can not tolerate, this is actually a kind of psychological distortion. If it is to demonize China on the pretext of the quality of its products, it is quite another characteristic problem.