01 EU improve load distribution between threads
When the standard nut is used, the axial load distribution is not consistent.
As illustrated in Figure 1(a), the load on the first thread is highest at the nut supporting surface and then decreases.
Theoretical analysis and experimentation have demonstrated that the more turns there are, the more pronounced the uneven load distribution becomes. After the 8th to 10th turns, the thread is essentially free of any load.
As a result, a thicker nut with more turns does not increase connection strength.
If the tension nut in Figure 1(b) is used, the conical mounting section of the nut and bolt shank undergo tensile deformation, which helps reduce the difference in bolt moment between the nut and bolt shank. and makes load distribution more consistent.
Figure 1 (c) shows a ring nut, whose function is similar to that of a mounting nut.
02 Reducing or reducing additional stress
Due to poor design, production or installation practices, bolts may experience additional bending stresses (as shown in Figure 2), which significantly affects their fatigue resistance and should be avoided.
For example, when installing screws into rough surfaces such as castings or forgings, structures such as bosses or countersunk seats are often employed to provide flat bearing surfaces after cutting (as illustrated in Figure 3).
03 R Reduce stress concentration
The thread root and the junction between the screw head and the shank are stress concentration points prone to fracture.
The stress concentration at the thread root has a significant impact on the fatigue strength of the screw.
Stress concentration can be reduced by increasing the fillet radius at the root of the thread, adding a fillet in the screw head transition area (as shown in Figure 4(a)), or cutting a discharge groove (as illustrated in Figures 4 (The)). b) and 4(c)).
04 R reduce voltage amplitude
When the maximum stress in a bolt remains constant, the greater the fatigue strength, the smaller the stress amplitude.
With the same working load and residual preload, reducing bolt stiffness or increasing the stiffness of connected parts can reduce the stress amplitude (as shown in Figure 5), but requires increasing the preload.
Ways to decrease screw stiffness include: extending the screw length appropriately, partially reducing the screw diameter, or creating a hollow structure such as a flexible screw.
An elastic component (as illustrated in Figure 6) installed below the nut can also serve as a flexible screw.
A flexible screw has high deformation capacity, strong energy absorption, and is suitable for dealing with impacts and vibrations.
To increase the rigidity of the connected system, it is not recommended to use a joint with low rigidity. Instead, it is preferable to use a sealing ring for the sealing connection, as illustrated in Figure 7.
05 EU improve the manufacturing process
The production process significantly affects the fatigue strength of bolts, especially for high-strength steel bolts.
When the thread is rolled, the effect of cold work hardening results in residual compressive stress in the surface layer, the metal structure is optimized, and the fatigue strength of the screw is higher than that of turning.
Methods such as carbonitriding, nitriding and shot peening can improve the fatigue strength of bolts.
