How to Avoid Thread Stripping

In the field of mechanical connections, thread stripping failure is a critical issue. Threaded connections are widely used due to their simple structure and easy assembly and disassembly, but thread stripping problems cause difficulties in equipment engineering, leading to mechanical function failure, equipment damage, frequent malfunctions, increased safety risks and maintenance costs, and shortened equipment lifespan. Conducting in-depth research on its causes, preventive measures, and solutions is of great significance for ensuring the reliable operation of mechanical systems, maintaining equipment safety and stability, and promoting sustainable industry development.

1.1 Material factors

When the strength and hardness of the bolt or nut material are low, the threaded surface is prone to plastic deformation during the tightening process due to the inability to withstand significant pressure, resulting in tooth slippage. In addition, there are defects such as impurities, pores, and cracks inside the threaded material, which can reduce the effective load-bearing area and mechanical properties of the thread. When subjected to stress, these defects are prone to become stress concentration points, accelerating thread damage and tooth slippage.

1.2 Thread size deviation

Excessive dimensional deviations such as diameter, pitch, and profile angle of threads can result in loose fit between thread pairs. Insufficient interference fit can lead to relative sliding of threads during tightening, resulting in tooth slippage; Excessive interference fit can make it difficult to screw in the thread, and even damage the thread when forcibly screwed in.

1.3 Foreign objects on the threaded surface

When there are foreign objects on the surface of the thread, such as welding slag and electrophoretic paint, the foreign objects will hinder the normal engagement of the thread, resulting in uneven stress on the thread. On the one hand, foreign objects can generate additional friction during the tightening process of the thread, which may cause excessive local stress and lead to plastic deformation or fatigue cracks on the thread surface. On the other hand, the presence of foreign objects can damage the fitting accuracy of threads, reduce the connection strength of threads, and make threads more prone to slipping when subjected to loads. Meanwhile, foreign objects may also cause scratches on the surface of the thread, further weakening its load-bearing capacity.

1.4 Assembly and Application Issues

(1) Unreasonable tightening process

If the tightening torque is too large, it will cause huge pressure on the threaded pair, leading to severe friction and wear on the threaded surface, and even causing plastic deformation of the threaded material, resulting in a decrease in load-bearing capacity and tooth slippage. If the tightening sequence is disorderly, it will cause uneven force distribution on the threaded connection, local stress concentration, and damage to the normal force distribution of the thread, which can also easily lead to thread slippage.

(2) Tighten the tilt

When tightened normally, the forces on each thread are balanced and jointly bear the load. In the tilted tightening state, some threaded teeth are subjected to excessive force, while others are subjected to insufficient force. Excessive stress on threaded teeth can result in shear stress and friction forces far beyond the design range, leading to increased wear on the surface of the thread and even causing plastic deformation or local damage to the thread profile, ultimately leading to tooth slip failure and compromising the reliability of the threaded connection.

(3) Repeated tightening of threads

After repeated disassembly and tightening, the surface wear of threaded connectors intensifies, the thread profile gradually blurs, the fitting accuracy decreases, and the self-locking performance deteriorates. At the same time, the accumulation of material fatigue damage leads to the formation of small cracks locally, weakening the load-bearing capacity of the threads. When subjected to force again, the thread is prone to slip along the crack, resulting in tooth slippage.

02 Solution for thread slippage

When there is slippage in the threaded connection, it will bring many hidden dangers to the normal operation of the equipment and the quality of the product, and effective solutions must be taken in a timely manner. Here are some common solutions:

2.1 Re threading or threading

If thread slippage is caused by thread size deviation or wear, a tap or die can be used to re tap or thread the thread. Before operation, ensure that the size of the threaded hole or bolt meets the requirements, select the appropriate tap or die, and use the appropriate lubricant or cutting fluid to reduce friction and heat during the machining process. After re tapping or threading, the threads should be cleaned and inspected to ensure that the quality of the threads meets the requirements.

2.2 Thread insertion method

The thread insertion method is an effective thread repair method suitable for repairing internal and external threads. For internal thread sliding teeth, the threaded hole of the sliding tooth can be enlarged, and then a suitable sized threaded insert can be installed in the hole. The external thread of the insert matches the enlarged hole, and the internal thread is used to connect with the bolt. For external thread sliding teeth, a threaded insert can be placed on the bolt of the sliding teeth, and the internal thread of the insert matches the external thread of the bolt. The external thread is used to connect with the nut or threaded hole. Threaded inserts are usually made of high-strength and high hardness materials, which have good wear resistance and anti slip properties.

2.3 Welding repair method

When the degree of thread damage is too severe to be repaired by mechanical processing, welding can be used to weld the threaded connection piece to the connected piece. This method is suitable for situations where the strength requirements for threaded connections are not high, but it will have a certain impact on the material properties of the threaded connection and the connected parts, and it is difficult to disassemble.

2.4 Replacement of connectors

If the problem of thread slippage cannot be repaired by the above methods, or the repair cost is too high, the most thorough solution is to replace the damaged threaded connection. When replacing, choose a new connector that matches the specifications, material, performance, etc. of the original connector, and ensure that it is installed correctly to avoid the problem of slipping teeth again.