【Thought】Can we really make good screws? ? ?

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This post was last edited by luogankongyaji on 2015-12-23 at 14:37. As for whether China can produce high-quality screws, I believe it is absolutely possible to do so; there is no need to argue about this. In fact, many standard parts manufacturers in China use imported manufacturing equipment, and from the perspective of equipment, there are no issues with the quality of production. From a material perspective, according to ISO standards, screws are specified only in terms of their mechanical property grades; the material used is not specified. Regardless of the material employed or the heat treatment methods used, as long as certain mechanical properties are achieved, the screw is considered to be of that grade. Therefore, from a material standpoint, there are no issues at all. Then why are there indeed problems with the screws exported from China? Actually, this issue can be viewed from two perspectives: 1. Some Western purchasers or domestic purchasing agents, in an attempt to save money, purchase screws from small Chinese enterprises that do not meet the required standards; such screws do not have guaranteed quality. In fact, the vast majority of us in the machinery industry still have an understanding of screws based on standards from before 1985; even in university education, concepts from that earlier era are still used. They have no idea about the mechanical property grades of screws, especially small businesses; they simply lack any basic knowledge regarding the mechanical properties of screws. All they know is that a screw is made from a certain material through a certain heat treatment process, and that determines its grade. By adopting this approach of focusing only on the process rather than the outcome, the mechanical properties of the screws produced cannot be guaranteed. Furthermore, small enterprises that lack the necessary qualifications do not have advanced manufacturing equipment and rely on manual processing; as a result, the mating surface and threads on the head of such screws are not perpendicular to each other. This lack of perpendicularity causes the shaft of the screw to break after it is tightened. 　　2. Not all exported screws from China have problems; only a very small portion of them do (those manufactured by unqualified small enterprises). For various reasons, this fact has been exaggerated! This has created the illusion that China is unable to even manufacture screws. 　　Here, I will explain the failure modes of threaded connections. The failure modes of threaded connections mainly include screw breakage, unthreading, and head fracture. The first two are primarily caused by design errors, while the latter is mainly due to manufacturing defects. 　　Let me explain again why the screws manufactured by some small businesses are of poor quality Because the manufacturing process does not meet the requirements. Today, cold heading is commonly used in the manufacturing of screws (nuts), as this method prevents the disruption of the metal’s fiber structure, resulting in better mechanical properties. However, the equipment needed for screw production (such as heading and threading machines) is expensive, and small businesses cannot afford it. As a result, they often use desktop manual lathes that cost only a few hundred yuan to turn screws, which leads to the disruption of the metal’s fiber structure and consequently lower mechanical properties (strength). Furthermore, the shank of a cold-headed screw must have an arc shape, which helps to avoid stress concentration ； If the screw is manufactured by turning and the cutting tool has no rounded edges, a sharp corner will form at the back of the screw, resulting in stress concentration; this makes it prone to breaking when tightened ； Moreover, the higher the strength of a material, the more likely stress concentration occurs, and high-strength screws are more prone to breaking. Moreover, such small enterprises simply do not have the testing equipment necessary to determine the mechanical properties of fasteners; they rely solely on the material and heat treatment to assess the mechanical properties of screws (bolts, nuts), which is not a reliable method. 　　So what causes the screws manufactured in China to fail? There is no analysis or discussion here; this is unscientific! It could be a problem with the manufacturing quality, or it might be an issue with the design and usage. Furthermore, the International Organization for Standardization conducted numerous experiments in the early 1970s, showing that even screws manufactured under typical processing conditions and meeting the required mechanical properties do not necessarily remain free from failure. Therefore, making generalizations without analysis is one-sided. 　　I have repeatedly emphasized that mechanical strength is a probabilistic statistic; this value does not represent the actual strength of the material. It is often possible for the actual strength of a material to be lower than its nominal strength, so it is essential to apply a safety factor during design. For threaded connections, the safety factor is 3 to 6 times, as specified internationally! It should also be noted here that the higher the mechanical properties of the screw, the larger the safety factor should be. 　　The main cause of failure in threaded connections is the combined tensile and torsional deformation. In particular, in the case of torsional deformation, since the shear strength of the material is **lower than its tensile strength, if the preload applied during threaded connections is not appropriate, the screw often gets twisted off. We can actually see from the cross-section of the screw that there is no significant contraction at the point where the shank breaks; instead, it breaks in a tilted direction, which is a typical sign of torsional fracture. 　　Furthermore, when designing, we are unable to take into account the effects of interference and overload. In particular, vibration has a significant impact on the fatigue strength of screws, **reducing their service life. In fact, considering many types of failures, fatigue failure is the primary cause of mechanical failure. Therefore, to ensure a certain fatigue life, a certain safety factor must be applied during design. 　　In fact, even according to foreign screw experiments, it is not possible to guarantee that screws will not fail 100% of the time. This can be concluded from the experimental reports of the international ISO/TC2 technical committee. In the 1970s, the International Organization for Standardization conducted numerous experiments; screws manufactured under typical processing conditions might not be free from failure, even if they met the mechanical performance requirements. On this point, our netizens may not have read the reports issued by the ISO organization and are therefore unaware of the actual situation. In fact, there are many problems with products made abroad, some even quite serious, but the Chinese can tolerate them ； And for things made in China, even if the problems are not as severe as those in foreign products, they are still not tolerable; this is actually a pathological phenomenon. If China is demonized under the pretext of its product quality, then it is a completely different issue. Source: Heat Treatment Ecosystem