Causes And Prevention Of Screw Thread Galling (Seizure)

Most operators have encountered the problem of seized screw threads that cannot be disassembled. Forcible removal of galling screws is highly likely to cause bolt breakage, damage to internal hexagonal holes and other driving structures, and failure of disassembly tools. It not only increases rework costs but also consumes substantial labor and working hours. To effectively avoid thread galling issues, this article systematically explains the core causes and standardized prevention methods of screw thread galling and seizure.

1. Definition of Thread Galling (Seizure)

Thread galling, also commonly known as thread seizure or jamming, refers to a failure state where a threaded pair cannot rotate freely during assembly or service. It is essentially a metal adhesion and fusion failure occurring on thread contact surfaces.

2. Core Causes of Screw Thread Galling

Thread galling is mainly attributed to two categories: metal fusion caused by accumulated thread friction heat, and jamming failure induced by external working conditions and assembly defects.

2.1 Accumulation of Thread Friction Heat (Primary Cause)

During screw tightening, continuous friction on thread contact surfaces generates massive heat, which causes thermal expansion and local softening of the surface metal. The thread flanks adhere and fuse together, eventually leading to galling and seizure. Compared with ordinary carbon steel screws, stainless steel screws are more prone to galling and seizure due to their unique material properties:

(1) High friction coefficient: The friction coefficient of stainless steel is approximately twice that of carbon steel, resulting in intense meshing friction and rapid heat accumulation.

(2) Low thermal conductivity: Stainless steel has only one-third the thermal conductivity of carbon steel, so friction heat cannot dissipate quickly and continues to accumulate locally.

(3) High thermal expansion coefficient: The thermal expansion rate of stainless steel is about twice that of carbon steel. The inner and outer threads fit more tightly after temperature rise, further aggravating friction and fusion risks.

In addition to stainless steel screws, carbon steel screws are also susceptible to thread galling in high-temperature operating environments and require strict prevention.

2.2 External Working Conditions and Assembly Defects (Secondary Causes)

Thread galling of ordinary carbon steel screws is mostly caused by improper assembly operations and defective thread conditions, mainly including the following situations:

(1) Foreign matters such as iron chips, dust and impurities adhere to the thread surface. Fastening with contaminants will scratch and squeeze the thread teeth, resulting in meshing jamming.

(2) Excessive tightening torque or overly fast tightening speed exceeds the bearing limit of threads, causing local thread tooth fracture, deformation and stagnation.

(3) Excessive surface roughness, dents or burrs on threads lead to uneven meshing clearances and sharply increased friction resistance, triggering local adhesive galling.

3. Standardized Prevention Measures for Thread Galling

3.1 Prevention of Galling Caused by External Defects

Before tightening, thoroughly clean foreign substances, burrs and impurities on both internal and external threads, and inspect thread integrity to avoid defective assembly. Follow official assembly standards, apply appropriate torque with uniform tightening speed, and avoid over-tightening and violent rapid fastening to eliminate assembly-induced thread galling from the source.

3.2 Prevention of Galling Caused by Friction Heat

Since the thermal conductivity and thermal expansion properties of stainless steel cannot be changed artificially, the industrial mainstream solution is to reduce the thread friction coefficient to minimize friction heat generation. Two standard prevention methods are widely adopted:

(1) Evenly apply special lubricants on thread contact surfaces to reduce meshing friction resistance.

(2) Implement professional surface treatment on screws to optimize thread surface accuracy and lower the friction coefficient.

Key note: Reducing the thread friction coefficient will significantly increase the axial preload under the same tightening torque. Retaining the original torque parameters will easily cause thread overload damage and secondary galling failure. Therefore, after adopting lubrication or friction-reducing surface treatments, the tightening torque must be calibrated and reduced synchronously to match the optimal assembly parameters.

4. Influence of Surface Treatment on Thread Assembly Performance

This test adopts A2-70 stainless steel cylindrical head hexagon socket bolts (M5×25). All test data are for process reference only and not for quality guarantee purposes. The 0.2% permanent elongation yield strength of A2-70 stainless steel screws is 450N/mm². The standard assembly specification requires that the tightening stress shall not exceed the 0.2% permanent elongation limit, and the assembly axial force shall be controlled within 70% of the yield strength.

Test results show that screws with special surface treatments achieve a greatly reduced thread friction coefficient. Under the same tightening torque, higher and more stable axial preload can be obtained. Based on this feature, properly reducing the tightening torque can effectively lower friction heat generation and significantly inhibit galling and seizure of stainless steel threads.

5. Matching Solutions for Special Working Conditions

Low-temperature black chromium plating and stainless steel blackening treatment can greatly reduce the spectral reflectivity of screw surfaces, which is highly suitable for precision optical equipment requiring elimination of diffuse reflection.

For high-cleanliness precision working environments where lubricants are prohibited and coating peeling is not allowed, SNSL-PN hardened anti-galling screws can be selected. The surface hardening process fundamentally prevents thread galling and meets the assembly and service requirements of special scenarios.