Both rectangular and round nuts serve a fastening function in mechanical connections, but their structural differences lead to significantly different load distribution characteristics. The geometry of a rectangular nut determines the concentration of its load path, while a round nut achieves a more uniform load distribution through its annular symmetry structure. This difference directly affects their performance in vibration environments, spatial adaptability, and anti-loosening designs.
The load distribution of a rectangular nut exhibits a clear axial concentration. Its hexagonal or square structure creates a fixed angle between the contact surface and the bolt axis. When the nut is tightened, the axial load is transferred to the bolt through the beveled contact of the thread teeth, with the contact points concentrated on six or four sides of the nut. This design provides high load-bearing efficiency under static loads, but under dynamic loads or vibration environments, concentrated stress can easily lead to localized wear of the thread teeth and even fatigue fracture. Furthermore, the load distribution of a rectangular nut is significantly affected by the installation direction. If there is an angle between the nut's side and the external load direction, eccentric loads may exacerbate local stress concentration.
The load distribution of a round nut, on the other hand, is characterized by annular symmetry. Its cylindrical structure allows the load to be evenly distributed through the 360-degree annular contact surface of the thread teeth, ensuring a relatively balanced axial force on each thread tooth. This design effectively reduces local stress peaks and extends thread life when subjected to alternating or impact loads. For example, in the pin fixing of amusement rides, when a round nut is used in conjunction with a locking washer, its annular contact surface ensures that the axial load is evenly transferred to the shaft parts, preventing shaft end cracks caused by stress concentration. Furthermore, the symmetrical structure of the round nut makes it easier to adjust the installation angle in space-constrained situations (such as pipe connections) to adapt to load requirements in different directions.
From the perspective of anti-loosening performance, the two types of nuts have different design logics due to the difference in load distribution. The concentrated load-bearing characteristics of rectangular nuts require them to rely on mechanical anti-loosening structures (such as double nuts, spring washers) or frictional anti-loosening (such as self-locking nuts) to counteract loosening caused by vibration. For example, self-locking cap nuts increase friction by setting twisted threads in the hexagonal area to prevent loosening, but their anti-loosening effect is still limited by the stress concentration problem of the rectangular structure. Round nuts achieve passive anti-loosening through structural innovation. Their annular groove and locking washer design, utilizing the interlocking mechanism of the washer's inner and outer teeth, convert axial load into annular constraint force, fundamentally preventing nut rotation. This anti-loosening method complements the uniform load-bearing characteristics of round nuts, forming a more reliable connection system.
In terms of material utilization efficiency, the annular structure of round nuts provides a larger thread contact area for the same outer diameter, thus improving load-bearing capacity. While the angular design of rectangular nuts can provide greater tightening torque for the same opposite side width, localized stress concentration in the thread teeth may lead to premature material failure. Therefore, in high-strength or high-load scenarios, round nuts often further improve load uniformity by optimizing thread profile (such as fine thread design) and material strength (such as using high-strength steel of grade 8.8 or higher), while rectangular nuts need to compensate for structural shortcomings by increasing thickness or using anti-loosening coatings.
The difference in application scenarios further highlights the value of their load-bearing characteristics. Rectangular nuts, due to their high tightening torque and simple structure, are commonly used in static connections requiring high preload (such as steel bridge structures and heavy machinery bases). However, their anti-loosening requirements necessitate additional design considerations.
The difference in load distribution between rectangular nuts and round nuts stems from fundamental differences in their geometry: the former achieves efficient force transmission through concentrated load-bearing, while the latter enhances fatigue resistance through uniform distribution. This difference not only determines their mechanical performance but also shapes their core value in various engineering scenarios. Understanding this distinction helps engineers accurately select nut types based on load type, space conditions, and anti-loosening requirements, thereby optimizing the reliability and economy of mechanical connections.
