According to the performance characteristics and functions of fasteners used in many fields, stainless steel steel is often used for fasteners. The most common drawback of stainless steel fasteners is the problem of bolts and nuts being unable to be loosened after fitting during use, which is called "bite". In general, we can also refer to it as "lock" or "bite" problem. Why do stainless steel bolts and nuts have this problem? How can we take preventive measures to prevent stainless steel fasteners from biting. Xiaorui shares relevant knowledge based on his understanding of the use of stainless steel fasteners, hoping to be helpful to everyone.
Once bolts and nuts become stuck during use, it becomes difficult to separate them. Usually, we cut off bolts or break nuts to achieve disassembly of accessories, which directly affects the speed of accessory disassembly and the reusability of fasteners. Therefore, we need to first analyze the principle and causes of stainless steel fastener jamming, and then analyze to find the most reasonable preventive measures to prevent fastener bolts and nuts from jamming.
1. The principle of stainless steel bolts and nuts biting
Stainless steel fasteners themselves have rust resistance, and when the surface is damaged, a thin oxide layer (in the case of austenitic stainless steel, it is chromium oxide) is formed on the metal surface to prevent further and deeper corrosion. When stainless steel fasteners are locked, the pressure and heat generated between the teeth will damage and erase the chromium oxide layer between them, causing the metal teeth to directly block/shear, leading to adhesion. When the phenomenon of adhesion continues to occur, it will cause the stainless steel fastener to lock completely and cannot be removed or locked again. Usually, this series of blocking, cutting, sticking, and locking actions occur in just a few seconds.
2. The causes of stainless steel bolts and nuts biting
By analyzing the principle of the phenomenon of stainless steel fastener jamming, it can be seen that the jamming of stainless steel fasteners is caused by the pressure or heat between the grooves, which destroys the oxide layer, causing direct contact between the metal material and further development. Therefore, excessive pressure or heat between the grooves often leads to the jamming of stainless steel fasteners. The following reasons often result in excessive pressure or heat between the tooth lines, leading to the occurrence of stainless steel fasteners biting.
1) Rough tooth lines or foreign objects adhering. If there are solder joints and other metal shavings sandwiched between the teeth, it often leads to biting.
2) Excessive tightening force or tightening speed. Excessive tightening force can easily cause significant pressure between the teeth and lead to bite; Tightening too fast can easily cause the temperature between the teeth to rise rapidly and cause biting.
3) The angle of the force application direction is incorrect. When tightening, the tightening direction is not perpendicular to the axial direction of the bolt, which can easily lead to jamming.
3. Dismantling method for stainless steel bolts and nuts after biting
Once stainless steel bolts and nuts bite, it is difficult to separate and can only be dismantled destructively. The bolts are often cut off and replaced. But in some cases, if the bolt diameter is large, it is extremely inconvenient to cut it off, or if the bolt is a pre embedded structure, once the bolt is damaged, it is extremely inconvenient to replace it. Therefore, finding some methods to quickly separate the bitten fasteners or remove the nuts without damaging the bolts has certain practical significance.
Here are several disassembly methods for stainless steel bolts and nuts in the event of jamming, based on practical application situations.
1) Use thread loosening agents (such as kerosene, lubricant, etc.) for lubrication. Spray loosening agent onto the threaded connection during homework, allowing as much loosening agent as possible to penetrate into the threaded connection, and use tools such as rubber hammers to lightly tap the circumference of the nut to create vibration at the threaded connection. After about 30 minutes of spraying loosening agent, try to remove the nut. Through practical application, it can be found that this method has a significant effect on dismantling corroded fasteners, but its effect on tightening and dismantling threads is relatively limited.
2) For nuts that have been sprayed with loosening agent but still cannot be disassembled, the thermal expansion method can be tried, which uses tools such as welding guns to heat the nuts, causing them to expand, and then remove the nuts. The use of this method is subject to certain limitations, and it is necessary to consider whether there is a space for hot work around the fasteners, and whether the fasteners and the connected parts can withstand high temperatures and heat.
3) For bitten bolts and nuts, special tools such as nut splitters can be directly used for disassembly. When using a nut splitter for operation, place the nut splitter on the nut and use mechanical or hydraulic pressure to push the blade to split the nut, thereby separating the nut from the bolt. This method is convenient and fast, especially when using a hydraulic system to advance the cutting head, saving time and effort. However, it should be noted that in order to retain the bolt from being damaged and only remove the nut, some improvements should be made in the operating method. Because at the moment when the cutting head splits the nut, due to inertia, the cutting head also leaves indentations on the bolt and stud. Even if the feed speed of the cutting head is controlled as much as possible during application, a certain mark is still left on the bolt. Through on-site testing, it was found that only the nut was squeezed, and after slight deformation, it immediately stopped. The nut produced visible deformation and could be removed, with almost no damage to the bolt threads.
4. Preventive measures for stainless steel bolts and nuts biting
From the previous analysis, it can be seen that it is extremely inconvenient to disassemble stainless steel fasteners after being bitten. Even some methods or special tools are time-consuming and laborious. Therefore, it is particularly important to do a good job in preventing stainless steel fasteners from being bitten. The following are several methods to prevent stainless steel fasteners from biting.
1) Choose bolts and nuts of different materials for matching.
2) Improving the performance of fasteners themselves, such as adding film on the threads of fasteners to prevent jamming, is a costly method and is currently rarely used.
3) Clean the threads, check that the tooth patterns are complete and continuous, and that there are no impurities between the tooth patterns.
4) Apply lubricant to the threads, such as molybdenum disulfide, butter, etc., evenly and fully fill the thread base, covering the length of the bolt head by approximately (10-15) mm.
5) When tightening the homework, the nut or bolt should be first screwed into the (2-3) thread by hand, and then tightened with a torque wrench or a wrench. The tightening should be evenly applied, and the tightening direction should be perpendicular to the axial direction of the bolt. Try not to use a wrench or an electric or pneumatic wrench.
6) When using an electric or pneumatic wrench for pre tightening, excessive torque or speed should be avoided. It should be ensured that the nut comes into contact with the gasket and is immediately stopped from use. Then, use a torque wrench to tighten, and the tightening torque should not exceed the specified torque value.
Through analysis and summary, it can be concluded that the main reason for stainless steel bolts and nuts to bite is due to the excessive pressure and high temperature between the threads, which damages the surface oxide layer of the tooth pattern and leads to direct contact with the metal material, further blocking, shearing, and bonding. Therefore, in the process of using stainless steel fasteners, we not only need to understand the symptoms of fastener bite, but also need to understand the principles and causes, as well as how to solve the problem of bite better and take preventive measures for stainless steel fastener bite. This can extend the service life of stainless steel fasteners.
