Why Does Locking Happen? Key Causes of Stainless Steel Lockup

Several factors contribute to the locking of stainless steel screws:

1. Thread Fit Angle and the Softness of Stainless Steel

• Stainless steel is softer compared to other metals. During the tightening process, if the force is uneven or the bolt is misaligned (not perfectly centered), it can cause the threads to tilt or bind.
• Misalignment or improper thread engagement increases friction, which can damage the thread pattern. As the screw and nut grind against each other, metal filings (small particles) are created. These filings do not easily detach and can lodge between the threads, further increasing friction and leading to a lockup.

2. Excessive Force and Stainless Steel’s Low Thermal Conductivity

• During assembly, the friction between the stainless steel bolt and nut generates heat. However, stainless steel has low thermal conductivity¹—about 1/3 that of carbon steel (typically between 10-30 W/m°C).

• Because heat dissipates slowly, it builds up in the threads, especially when excessive force is applied. This heat can damage the chromium oxide layer that gives stainless steel its corrosion resistance.

• Once the oxide layer is damaged, the exposed metal surfaces can adhere to one another, creating a condition where the threads become “welded” together.

• The softer nature of stainless steel means it is more susceptible to this kind of thread adhesion under pressure and heat, further increasing the likelihood of locking.

Common Scenarios That Lead to Locking

To better visualize the problem, here are typical situations where stainless steel locking occurs:

1. High Friction Under Heavy Load: When tightening with excessive torque, the pressure between the bolt and nut generates significant friction and heat, leading to seizing.
2. Improper Alignment: If the bolt and nut threads are not properly aligned during installation, the contact points are uneven, causing premature wear and metal filings.
3. Rapid Tightening: When tightening occurs too quickly, heat accumulates faster than it can dissipate, increasing the risk of adhesion and locking.

How to Prevent Stainless Steel Screw Locking

While locking can be frustrating, there are effective ways to prevent it:

1. Use Anti-Seize Lubricants

   • Apply a specialized anti-seize compound² to the screw and nut threads before assembly. These lubricants reduce friction, minimize heat generation, and prevent metal-to-metal adhesion.

2. Tighten Slowly and Gradually

   • Avoid rapid tightening, as this generates excessive heat. Use controlled, gradual force when assembling the bolt and nut to reduce friction and temperature buildup.

3. Use Proper Tools and Techniques

   • Ensure the tools you use allow for even pressure during tightening. Use a torque wrench³ to apply the correct force without overloading the threads.

4. Choose High-Quality Stainless Steel Fasteners

   • Opt for precision-manufactured stainless steel screws and nuts to ensure better thread alignment and engagement. High-quality components reduce the risk of thread damage and lockup.

5. Opt for Coated or Treated Fasteners

   • Consider using fasteners with coatings⁴ such as PTFE or other lubricating finishes, which reduce friction and make locking less likely.

Conclusion: Understanding and Solving Stainless Steel Locking

Stainless steel screw locking occurs due to a combination of factors, including the softness of the material, friction, and its low thermal conductivity. Excessive force, improper alignment, and rapid tightening further exacerbate the issue.

By applying anti-seize lubricants, tightening gradually, and using high-quality fasteners, you can effectively prevent lockup and ensure a smoother assembly process.

At FHT, we specialize in providing precision-engineered stainless steel screws that are designed to meet your performance needs. Our products are crafted to minimize common issues like locking, ensuring reliable performance in your projects.

Looking for high-quality stainless steel fasteners?
[Contact us today to discuss your requirements and request samples!]

Footnotes

1. Seizing: The adhesion or locking that occurs between stainless steel threads during assembly.
2. Thermal conductivity of stainless steel: Explains why heat buildup occurs during tightening.
3. Anti-seize lubricants: These compounds reduce friction and prevent metal-to-metal welding.
4. Torque wrench: A tool used to apply precise force, reducing the risk of thread overload.
5. Coated fasteners: PTFE or other coatings that minimize friction and prevent locking.