Factors Leading to Thread Stripping in Threaded Screw

In fastener design, the most common failure mode is bolt fracture, and both a bolt and a nut must be designed to prevent thread stripping. For this reason, when a bolt is paired with a nut, the load-bearing capacity of the nut is typically higher than that of the bolt. For example, a grade 10.9 threaded screw should be matched with at least a grade 10 nut, and an 8.8-grade bolt should be paired with at least an 8-grade nut.

If the mating part is a component with internal threads, the depth of thread engagement in the component is also important. The table below shows the minimum thread engagement requirements for a steel bolt. As seen in the table, when the mating part is made of aluminum alloy, the required depth of engagement is usually between 2d and 2.7d. This design ensures that the failure mode after over-tightening is bolt fracture, not thread stripping between the nut and the bolt.

However, in actual production, thread stripping in bolts and nuts is sometimes inevitable, and the rework caused by thread stripping can be significant. Today, we will discuss the various factors in production that affect thread stripping in a threaded screw and a metal nut, for your reference.

Impact of Excessive Electrophoretic Coating on a Threaded Screw

To enhance corrosion resistance, a layer of electrophoretic paint is often applied to components, and some paint may inevitably adhere to the threaded hole. But does a thick electrophoretic coating affect thread stripping in a threaded screw? When the paint is too thick, it can cause some tightness during tightening, but it generally does not result in thread stripping or severe damage. After the bolt is removed, both the internal threads and the threaded screw will have paint powder residues. The paint is typically applied through cathodic electrophoresis, and due to the process characteristics, the paint thickness is usually not too thick—generally around 60-80 microns—so it usually doesn't cause significant thread tightness or thread stripping.

Weld Slag on a Metal Nut

This issue is more commonly encountered with a welded metal nut. Welding inevitably results in some slag being left in the threads. As the threaded screw engages with the metal nut, a strong resistance to tightening is felt. After the bolt is removed, the front portion of the screw shows signs of thread damage, and the nut's threads where the slag was present will also exhibit visible thread loss.

In this case, whether thread stripping occurs depends on the size and quantity of the slag particles. Larger and more numerous slag particles tend to cause more severe thread stripping. To prevent this issue, it is recommended to take measures to protect against weld slag, such as using protective covers or applying anti-weld-slag coatings.

Thread Deformation in a Threaded Screw

In some cases, excessive welding current or stamping forces can cause thread deformation. When a threaded screw is tightened, misaligned threads can lead to thread damage and stripping. This type of thread deformation should be avoided as much as possible. If thread deformation is detected, production should be stopped, and suitable processes should be explored to ensure that threads are not deformed.

Eccentricity in a Metal Nut

When welding a metal nut, if eccentricity occurs during the welding process, the first half of the engagement may be smooth, but the second half will encounter noticeable resistance. If forced into position, this can result in thread damage and stripping.

It is recommended to optimize the positioning of the welded metal nut to ensure that eccentricity does not occur during the welding process.

Cross-Threading of a Threaded Screw

Cross-threading is a common problem during tightening. When a screw is cross-threaded, it will not engage properly. After removing the screw, the head of the screw will exhibit significant damage and thread stripping, while the middle and rear parts remain intact. The threads on the metal nut will also experience stripping.

If the screw is found to be misaligned at the beginning of tightening, it should be removed and realigned before continuing to tighten. The front part of the screw may show damage, with thread marks visible on the middle section, while the rear part remains undamaged.