Have you ever faced a situation where screwing together stainless steel components becomes unexpectedly tricky, and then they’re stuck together as if welded? This thread annoying phenomenon can cause frustration and unnecessary delays in manufacturing. It’s widespread in the fast-paced world of sheet metal fabrication, where precision and efficiency are crucial to success.
Thread galling occurs when the pressure and friction of tightening fasteners cause the metals to fuse. This microscopic welding effect occurs without applying any actual heat. The result is a fastener that is difficult to remove or adjust, often leading to damaged threads or the need to replace the entire assembly.
Understanding galling is crucial for anyone involved in the manufacturing and assembly of metal components. In the following few sections, I’ll share insights on preventing thread galling and ensuring the longevity and reliability of your metal assemblies. With knowledge and the proper techniques, let’s tackle this challenge head-on.
What is thread galling?
Definitions and basic principles
In its simplest form, thread galling is a type of wear that occurs when sliding surfaces adhere. Friction causes two threads of mating parts to entangle and fuse at micro-level contacts.
Fasteners made of certain metals or alloys are particularly susceptible to this phenomenon. Unlike typical wear due to gradual removal, galling occurs when material is transferred from one thread onto another, causing the threads to seize and lock.
What makes galling different from other types?
Galling is the adhesion of a material and its subsequent movement between two surfaces when compressed. Unlike other wear types, which are often caused by contaminants or external factors, galling is caused by materials and their mechanical action.
Galling: The mechanics behind it
Friction and pressure: their role
Friction and pressure lead to galling. When threads tighten, pressure between surfaces increases, increasing friction.
When friction exceeds the material’s resistance to shearing, tiny welds can form between the threads. The condition is aggravated by continued tightening and movement, resulting in more material damage.
Materials that influence galling
Hardness and flexibility are the main factors determining a material’s susceptibility to galling. Galling is more common in softer and more pliable metals, as these are more likely to deform or adhere with less force.
Even harder materials may irritate under certain conditions, such as rough surfaces or lack of lubrication.
Standard Materials and Configurations prone to Galling
Metals and alloys most at risk
Stainless steel, titanium, and aluminum are most prone to galling. People favor these metals for their corrosion resistance and strength, and they can adhere when under pressure.
Specific thread designs that increase susceptibility
Due to their tighter fit and high contact area, fine threads are more prone to galling than coarse threads. Threads that are higher pitched and closer together also increase the risk.
Galling can be caused by the design of the thread, including its depth, angle, and smoothness.
Signs and symptoms of thread galling
Early indicators
Initial signs of galling are often subtle. They can begin with an increased resistance to tightening the fastener. It may get to the point that turning the fastener is difficult or impossible without using excessive force. This can cause further thread damage.
Confirming the presence or absence of galling
You can check for galling by looking at the accumulated material on the thread crests or the bolt or nut’s root. In severe cases, threads can appear completely stripped or smeared.
Causes and contributing factors
Material hardness and compatibility
Materials of similar hardness tend to be more susceptible to galling, as they can adhere or deform when under pressure.
Compatibility of materials can also influence the likelihood of galling. Under friction, dissimilar metals are less likely to weld.
Surface roughness: its role
Surface roughness has a significant impact on the likelihood of galling. Asperities on the surface, or high points, can cause material transfer and increase friction.
In contrast, smoother surfaces reduce contact points available for adhesion and decrease the risk.
Coatings and lubricants
Applying lubricants or coatings to threads can reduce friction and thus the likelihood of galling. Coatings like Teflon or zinc can act as a barrier between surfaces.
Lubricants reduce the risk of galling by minimizing metal-to-metal friction and dissipating heat.
Environmental Influences
Effects of corrosion on moisture
Corrosion and moisture on metal surfaces can worsen galling, damaging the protective oxide layer and making them more susceptible to welding.
The impact of temperature on galling
The temperature plays a double role in galling. Metals can be softer at higher temperatures, which makes them more susceptible to adhesion and deformation. Specific lubricants and coatings become more effective when heated, protecting against galling.
Considerations for Load and Speed
Dynamic vs. static loads
The sustained force generated by static loads can lead to material transfer.
Dynamic loads involving movement and pressure variations can reduce adhesion time but cause galling in high-speed conditions.
Safe speed limits for operation
The speed of assembly or operation can influence galling. Installing fasteners at high speeds can, for example, generate heat and friction, which promote galling.
Identifying the safe speed limits for different material combinations and applications is essential. Proper tooling and the control of installation techniques will help manage pressure and speed, reducing the risk of galling.
Preventive measures and solutions
Material and Design Optimization
Anti-galling Materials to Choose
The first step to preventing thread galling is choosing materials with anti-galling characteristics. Nitronic-60 stainless steel, certain bronze alloys, and specially treated metals with higher resistance to weld under pressure are excellent options.
The consideration of materials with different levels of hardness for mating components will also reduce the likelihood of irritation.
Reduced risk through thread design modifications
Modifying the thread design can also be a critical factor in reducing galling. Features like wider thread angles and optimized pitch diameters can reduce the friction and contact pressure between mating surfaces.
Lubrication Strategies
What types of lubricants are suitable to prevent galling?
Choosing the proper lubricant for your vehicle is essential to prevent galling. The anti-seize compound, which contains metals such as copper, nickel, or aluminum in a lubricant base, is particularly effective.
These compounds are not only lubricants, but they also act as a barrier to corrosion and excessive wear. Synthetic lubricants like Teflon products offer superior performance in cleaner applications without heavy metals.
Maximum effectiveness of application techniques
Use the appropriate tools to ensure even distribution. Controlled dispensing can be used in critical applications to ensure consistency of application across all components and reliable protection against galling.
Routines for maintenance and inspection
Regular inspection schedules
Implementing a regular inspection program is crucial to detecting galling early and preventing its progression. Focus inspections on identifying signs of wear, material transfers, and changes in assembly torque. Prioritizing and monitoring high-risk components is essential to address any potential issues.
Maintenance practices that reduce galling
Galling can be significantly reduced by following preventive maintenance practices such as cleaning the threads before assembly, checking for damaged components and replacing them, and using correct assembly procedures.
Using torque wrenches and tightening fasteners according to the manufacturer’s specifications can maintain the optimal pressure on the thread and reduce the risk.
Assessment of Damage
Repair vs Replace: When should you repair or replace?
It may be possible and cost-effective to repair threads that are lightly galled. When galling is severe or threatens structural integrity, it’s often safer to replace the part.
Methods for evaluating galling damage
Evaluation of galling damage includes visual inspection, thread measurement, and assessment of the part’s function. Thread gauges and micrometers are valuable tools for determining wear and deformation. Ultrasonic testing is a non-destructive method that can provide insight into internal damage.
Repair Techniques
Mechanical Repair Methods
The mechanical repair of galled threads includes re-tapping, re-threading, thread repair inserts, or machining to accept a more oversized fastener.
These techniques can restore the threaded connections. However, evaluating the area’s mechanical properties and stress resistance is essential to ensure that the repair meets the operational requirements.
Welding & Filling
Repairing severely damaged components with welding and filling is a more invasive method. This process involves adding filler to the damaged area before machining the component to its original specifications.
These methods can save components that might otherwise be scrapped. However, you must control them carefully to prevent further heat damage and ensure the added metal properly bonds with the base metal.
Conclusion
While thread galling is challenging for manufacturing and machine maintenance, it’s manageable. Understanding the causes of thread galling, recognizing early signs, and implementing prevention strategies are crucial to managing and mitigating its impact. It is important to consider material selection, design optimization, and proper lubrication to prevent galling.
Do you need a reliable sheet metal parts manufacturer? Shengen is the place to go. We specialize in sheet metal laser cutting, bending, surface finish, and sheet metal welding. Reach out to Shengen Today and seek help from professionals!
FAQs:
What is the most common reason for thread galling?
Thread galling can be caused by excessive friction between threads, mainly when they are of the same or similar material.
Can galling be prevented completely?
Eliminating the risk of thread galling is challenging. However, you can reduce the risk by using suitable materials, lubrication, and optimizing design.
How often should threads be checked to prevent galling?
Inspections can be more frequent in high-risk or critical applications. As a rule, you should inspect threads during maintenance intervals and whenever components are disassembled or reassembled.
What should be done immediately if galling is suspected?
To prevent further damage, stop all installation and tightening on the components suspected to be galling. If possible, carefully disassemble the components and check for signs of damage, welding, or material transfer. Depending on the severity of the corrosion, it may be necessary to replace the galled parts.
More Resources:
Anti-Galling Materials – Source: Micro Surfacecorp
Thread Repair Kits – Source: Ezlok
Mechanical Fastening and Design – Source: ASME
Temperature Effects on Metal Fasteners – Source:
Hey, I'm Kevin Lee
For the past 10 years, I’ve been immersed in various forms of sheet metal fabrication, sharing cool insights here from my experiences across diverse workshops.
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Kevin Lee
I have over ten years of professional experience in sheet metal fabrication, specializing in laser cutting, bending, welding, and surface treatment techniques. As the Technical Director at Shengen, I am committed to solving complex manufacturing challenges and driving innovation and quality in each project.
