In most applications, bolts and nuts are used in pairs. However, nut design receives far less attention than bolt design, as nuts are often simply "selected." This article will address ten common misconceptions about nut selection to help users better understand and choose nuts, ensuring safe and reliable fastening connections.
Misunderstand 1: Nut Grade Equals Raw Material Strength?
Bolt grades explicitly indicate their raw material properties, such as tensile and yield strength (e.g., Grade 8.8 bolts). This leads to a misconception that nut grades also directly reflect their raw material strength.
Clarification:
Nut grades are determined based on the maximum performance grade of the matching bolt, not the nut's inherent material strength. For example, a Grade 8 nut can pair with a Grade 8.8 bolt, but this does not mean the nut’s tensile or yield strength matches the bolt's.
Misunderstand 2: Higher Nut Grades Are Always Better?
Higher-grade nuts often exhibit superior mechanical properties, leading to the assumption that "higher is better."
Clarification:
Select nuts based on the bolt's performance grade to avoid waste. If the nut grade is too high for a weaker bolt, the bolt may fail first under load, compromising the connection. Overgrading nuts shifts the weak point to the bolt.
Misunderstand 3: A Nut One Grade Lower Than the Bolt Is Safer?
Some believe using a nut one grade lower than the bolt ensures the nut fails first, making issues easier to detect.
Clarification:
This depends on the application and safety requirements. In critical fields like aerospace or heavy machinery, mismatched grades may reduce reliability. Generally, the nut's grade should not be lower than the bolt's, though excessive overgrading is unnecessary.
Misunderstand 4: Harder Nuts Are More Durable?
Nuts with higher hardness can better resist external forces and reduce wear to a certain extent. Therefore, it is often mistakenly believed that the harder the nut material, the more durable it is.
Clarification:
Nuts with lower hardness can better cooperate with bolts during the tightening process, reducing friction and wear between them.
If the hardness of the nut is too high, it may cause excessive wear to the bolt and shorten the service life of the bolt. Generally, the hardness of the nut material should be 20 - 40HBW lower than that of the bolt material, which can reduce the wear of the bolt and extend the service life of the bolt.
Misunderstand 5: Nuts are Strong Enough to Adapt to Most Usage Environments?
As general - purpose parts, nuts have sufficient safety redundancy. There is often a misunderstanding that nuts can adapt to most usage environments.
Clarification:
Select the appropriate nut material and type according to the actual usage environment.
Different usage environments have different requirements for the performance of nuts. For example, in a corrosive environment, if a common carbon - steel nut is used, it may be corroded quickly, resulting in connection failure. Stainless - steel nuts, on the other hand, have better corrosion resistance and can maintain the stability of the connection in a corrosive environment. In a high temperature environment, ordinary nuts may deform or fail due to high temperatures, while high temperature - resistant nuts can maintain their performance in a high temperature environment. In a vibrating environment, ordinary nuts may loosen due to vibration, while nuts with a locking function can effectively prevent loosening and maintain the reliability of the connection.
Misunderstand 6: Radial Deformation/Expansion of the Nut During Tightening Can Be Ignored?
The deformation of the nut during the tightening process is often small, and for sufficient safety margins, the impact of radial deformation/expansion is relatively small most of the time. Thus, there is a misunderstanding that the radial expansion of the nut can be ignored.
Clarification:
When tightening small nuts, measures should be taken to avoid the impact of radial expansion.
When tightening small nuts, the nut may expand radially, reducing the engagement length and leading to thread stripping. This is because small nuts have a smaller size, and the relative impact of their radial expansion is greater. Increasing the thread engagement length or choosing a higher strength nut can avoid thread stripping.
Misunderstand 7: Lock Nuts Can Be Applied to Most Occasions with Anti - loosening Requirements?
The main function of lock nuts is to prevent nuts from loosening. There is often a misunderstanding that lock nuts can be applied to most occasions with anti loosening requirements.
Clarification:
Select the appropriate type of lock nut according to the actual working conditions.
There are many types of lock nuts, such as nylon lock nuts, metal lock nuts, and toothed lock nuts. Nylon lock nuts rely on the elasticity of the nylon material to provide locking force, but in a high temperature environment, the nylon material may fail, resulting in a decrease in the locking effect. Metal lock nuts have better high temperature resistance and can maintain the locking effect in a high - temperature environment. Toothed lock nuts provide locking force through a special toothed design and are suitable for some occasions that require high locking force.
Misunderstand 8: Mixing Nuts Occasionally is No Problem?
Nuts and bolts of different standards may look similar in appearance, and their performances mostly have redundancy. There is often a misunderstanding that mixing nuts occasionally is no problem.
Clarification:
When choosing nuts and bolts, ensure that the thread standards of nuts and bolts are consistent.
There are different thread standards in different countries and regions, such as Metric, UNC/UNF, etc. The pitch, thread angle, and other parameters of metric threads and American threads are different. If they are used interchangeably at will, it will lead to poor thread fit and may even make it impossible to tighten. This is because different standard threads have differences in design, and their fitting methods and stress conditions are also different.
Misunderstand 9: Nuts Can Be Reused?
After being tightened and disassembled multiple times, there may be no obvious damage to the appearance of the nut. There is often a misunderstanding that nuts can be reused indefinitely.
Clarification
For important connections, it is recommended to replace with new nuts to ensure the reliability of the connection.
After a nut is tightened and disassembled multiple times, the threads may be worn or deformed, resulting in a decrease in the locking effect. This is because the threads are subjected to friction and stress during the tightening and disassembly process. After long term use, the accuracy and strength of the threads will decrease. In addition, the performance of the nut material may also change due to fatigue, further affecting its locking effect.
Misunderstand 10: The Tighter the Nut is Screwed, the Better?
Tightening can improve the reliability of the connection. Since nuts are strong enough, there is often a misunderstanding that for better fastening effects, the tighter the nut is screwed, the better.
Clarification:
Use appropriate tools to tighten the nut according to the specified torque.
Over tightening the nut will cause the bolt to stretch and deform, and even break. This is because when the bolt is over tightened, it will be subjected to excessive stress, exceeding its yield strength, resulting in plastic deformation or fracture of the bolt. At the same time, over tightening the nut may also damage the connected parts, such as causing parts to deform or crack.
