The connection between bolts and nuts, as well as the connection between bolts and base metal fasteners, focuses on different directions.
When connecting bolts and nuts, without considering the use of twisted hole bolts, all considerations are entirely focused on thread strength, bolt rod strength, and anti loosening. Therefore, when the thread strength is not taken into account, the general rough compression standard bolt nut connection, and the bolt rod generally fails before the thread. At this time, limited consideration should be given to using higher grade bolts with contract grade or slightly higher nuts. This method of use has only one purpose, and artificially causing the bolt rod to break becomes the only way for bolt connections to fail. This can control the form of failure and establish a solution.
When it is necessary to consider thread strength and anti loosening, such as fine thread or anti loosening through thread load balancing, because bolt rod failure is no longer the main form of failure, the key consideration at this time is the even load situation of the thread itself. Under this premise, if higher strength and hardness materials are still used for the internal thread, it will exacerbate the uneven stress on the thread teeth.
Considering 7 teeth, if the situation is severe, the load-bearing capacity of the first thread can even reach 63% of the total load-bearing capacity. This makes it easier for external threads with lower load-bearing capacity than internal threads to be damaged. Therefore, for this situation, we usually downgrade the use of nuts. This degradation does not simply refer to a decrease in the grade of nuts, but rather to the use of slightly lower hardness or strength nuts.
This change sometimes does not represent a decrease in nut grade. For example, SAE J995 grade 8 nuts have a hardness of only HRC24-32 and can be paired with SAE J429 grade 8 bolts, with a hardness of HRC33-39. Both are level 8, but slightly lower in hardness. This measure will reduce the load-bearing percentage of the first tooth in threaded connections, for example, in the above fit, the load-bearing capacity of the first tooth is only 34%.
It must be said here that when using special threads, such as the Schbiler thread, due to its inherent load-bearing capacity, it is completely unnecessary to consider using low-grade nuts.
Similarly, for the connection between bolts and base metal buckles. Due to the consideration of the non replaceable nature of the base material buckle. At this point, it is necessary to choose bolts with strength lower than the base material for connection. Whether it's a slippery thread or a broken rod. The purpose is all based on the premise of protecting the mother buckle from damage. (Haha, no one wants to fix this kind of mother buckle, right?).
Of course, for this type of connection, I personally prefer using fine thread instead of bolt rod failure, making repair and replacement easier. But the specifics depend on the situation.