Why do threads in screws, nuts, and other fasteners strip?

Why do threads in screws, nuts, and other fasteners strip? What are the solutions?

Thread stripping in screws and nuts occurs when the shear strength of the threads is lower than the axial force during assembly, failing the weaker threads. When the thread's shear strength is relatively high, the final failure mode of the threaded connection, as the assembly torque and axial force increase, is bolt fracture, as shown in the schematic diagram below.

When assembling, due to the thread, Thread stripping” is a standard translation for failure, which is more hidden and not easy to notice; in the assembly process, torque can also reach the target value that has caused slippery teeth. In this state of the threaded vice in the dynamic load, axial force attenuation will fail the threaded connection.

In addition, when the thread appears, thread stripping” is a standard translation for when the thread rework and rework workload is more significant, and assembly line assembly may not meet the production beat's requirements. Therefore, in the design and assembly of threaded connections, thread Thread stripping“ is a common translation that should be avoided as much as possible, even if the overload, the failure mode should be bolt fracture, rather than thread Thread stripping” is a standard translation.

So what are the causes of Thread stripping?” is a standard translation. What can be done about it?

（1）Design Considerations – Thread Engagement Length

For threaded connections made of lightweight metals, such as aluminum alloy, ensuring reliability requires sufficient thread strength in the parts. Therefore, the thread engagement length of threaded holes is subject to specific design requirements. The greater the engagement length,

The higher the shear strength of the internal threads

You might ask, isn’t the load-bearing capacity of each thread different when tightening? Indeed, the first three threads typically bear the highest shear loads, as shown in the diagram below. Under normal assembly conditions, threads from the seventh and beyond generally bear minimal loads. However, if significant deformation or thread stripping occurs in the first few threads, the load-bearing capacity of the subsequent threads increases, preventing further thread stripping.

So, theoretically, what is the appropriate engagement length?

This is usually determined using calculation formulas or reference charts. The diagram below provides theoretical values for engagement lengths corresponding to bolts of different grades paired with internal threads of varying strengths.

The table below shows the minimum engagement lengths for common materials. Aluminum and magnesium alloys require significantly longer engagement lengths than steel-threaded connections.

Additionally, testing is necessary to prevent thread stripping in actual components. Typically, the threads are subjected to 5–10 repeated tightening cycles at the specified torque and angle without experiencing thread stripping.

The diagram below shows a component where thread stripping occurred after several repeated tightening cycles.

(2) Design Considerations – Weld Nuts

When designing the pairing of bolts and nuts, the selection is typically based on the table below, where the maximum bolt grade should not exceed the corresponding nut grade. For example, if an 8-grade nut is used, the maximum allowable bolt grade is 8.8.

Following this design and selection principle, the guaranteed load of bolts is lower than that of nuts of the same grade, making thread stripping unlikely under normal circumstances.

However, the welding process generates heat for weld nuts, which may lead to an uncontrolled heat treatment effect, reducing the nut’s strength and guaranteed load. Additionally, with the widespread use of the torque-turn tightening method beyond the yield point, thread utilization reaches 100%, and the assembly preload becomes significantly higher. As a result, even when bolts and nuts of the same grade are paired, weld nuts are more prone to thread stripping.

Therefore，when designing and selecting weld nuts, choosing nuts one grade higher than the bolts is recommended. For instance, if the bolt is grade 8.8, a grade 10 nut should be used. This design approach effectively reduces the likelihood of thread stripping in weld nuts.

(3) Component Quality – Thread Profile Dimensions, Strength, and Thread Morphology

When thread stripping occurs, possible causes include abnormalities in thread profile dimensions, strength, or morphology. Thread profile dimensions can be analyzed using go/no-go gauges, while thread strength and morphology can be assessed through hardness tests, load retention tests, and metallographic analysis.

In the failure analysis of thread stripping caused by component quality issues, the root causes can typically be traced to these influencing factors:

Low thread strength: Insufficient heat treatment results in inadequate shear load capacity of the threads.

Incomplete thread formation: During the rolling process, incomplete thread profiles reduce the effective load-bearing area of the threads, thereby lowering the guaranteed load capacity.

Dimensional non-compliance: If thread dimensions fail to meet design requirements and cannot pass go/no-go gauge inspection, the load-bearing area of the threads will also decrease.

Thread damage: Severe thread collisions or nicks can lead to thread stripping.

The formula below calculates the minimum engagement length for threaded connections. From this formula, it is evident that factors such as the primary diameter of the bolt and the primary pitch diameter of the internal thread affect the shear strength of the threaded connection, which in turn influences the likelihood of thread stripping:

Where:

• mgesminm_{gesmin}mgesmin: Minimum engagement length

• RmmaxR_{mmax}Rmmax: Maximum tensile strength of the bolt

• A0A_0A0: Minimum cross-sectional area of the bolt

• PPP: Thread pitch

• τBM\tau_{BM}τBM: Shear strength of the internal thread

• ddd: Major diameter of the bolt

• D2D_2D2: Basic pitch diameter of the internal thread

• CCC: Correction factor

(4) Assembly Process Anomalies

The assembly process also involves numerous factors that may lead to thread stripping in threaded connections, such as welding slag in weld nuts, eccentricity during weld nut installation, or misalignment during bolt tightening.

Weld nuts are widely used, and it is almost inevitable for welding slag to remain inside the threads during welding. The presence of slag creates significant resistance when the bolt is screwed in, causing torque to increase rapidly. In such cases, thread stripping is often observed on the bolt's leading threads after the bolt is removed, and the weld nut threads with slag residues exhibit noticeable damage. To prevent this, anti-slag adhesive can be applied to the internal threads of the nut before welding to reduce slag adhesion.

If eccentricity occurs during the welding of a weld nut, the bolt may screw in smoothly for the first half but encounter noticeable resistance in the latter half. Forcing the bolt further in can result in thread damage and stripping. To avoid this, the welding position of the nut should be optimized to ensure proper alignment and prevent eccentricity.

Misalignment during tightening, commonly called "cross-threading," is another frequent issue. Cross-threaded bolts cannot be fully screwed in, and upon removal, the bolt's head typically shows severe damage and thread stripping, while the middle and rear portions remain intact. The nut's threads may also be stripped. If misalignment is detected early, the bolt should be immediately removed, repositioned, and reinserted. Cross-threading damage typically affects the bolt's leading threads, leaving the middle and rear threads intact.

Additionally, over-torquing caused by uncalibrated or improperly used tightening tools can lead to thread stripping.

To address thread stripping during assembly, defects can be effectively identified by setting appropriate torque and angle monitoring windows. The monitoring process typically involves four steps: cap detection, pre-tightening monitoring, process monitoring, and final tightening monitoring. Thread stripping issues can often be detected during the pre-tightening monitoring phase. If excessive resistance is encountered during assembly and torque rises too quickly, the final tightening angle may fall below the lower limit of the monitoring window, triggering an alarm. Thus, establishing a monitoring window is crucial for identifying and preventing thread stripping.

Thread stripping occurs when the shear strength of bolt and nut threads is insufficient to withstand the preload during assembly. Proper thread connection design and assembly practices should aim to prevent this failure mode.

When thread stripping occurs, the causes can be analyzed through the aspects of design, component quality, and assembly process discussed in this article to identify and address the root issues.

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