Factors Affecting the Loosening Prevention Performance of Thread Locking Adhesive on Fasteners

Factors Affecting the Loosening Prevention Performance of Thread Locking Adhesive on Fasteners

Thread-locking adhesives are widely used in industries such as automotive and electronics to prevent fasteners from loosening. These adhesives are a significant cost component in finished fasteners. But how effective are they at preventing loosening? What factors could reduce the effectiveness of thread-locking adhesives, and how should we assess and address these issues?

What Is Thread Locking Adhesive?

Thread-locking adhesives, such as microencapsulated adhesive layer-locking adhesives, work by chemically reacting and curing after the fastener is tightened. This curing process helps prevent loosening by increasing the bond strength of the fastener's threads.

Performance Requirements and Testing: According to industry standards, the adhesive should be applied to the fastener as follows:

Lb represents the length of the adhesive coating.

A is the distance from the adhesive zone to the end of the threads, typically 2-3 thread pitches.

d is the outer diameter of the bolt.

p is the thread pitch.

For example:

Bolt size M12x80, grade 8.8, with a friction coefficient requirement of 0.10-0.16. The adhesive length lb should be 30mm, and the distance should be 10mm.

How Does Thread Locking Adhesive Improve Loosening Prevention?

Using a thread-locking adhesive can significantly increase the loosening torque of bolts. Let's take an M10 thread as an example:

Tighten the bolt to a specific torque (MA).

After some time, the loosening torque (MLB) is measured by loosening the bolt in the opposite direction.

The ratio of MLB/MA is used to assess the effectiveness of the adhesive in preventing loosening.

For example, when the friction coefficient is μ = 0.15 (a familiar setting for bolts), the ratio MLB/MA increases to approximately 0.77, indicating good performance.

How Much Does the Adhesive Improve the Loosening Torque? According to DIN267-27, MLB/MA should exceed 0.9, meaning the loosening torque should be at least 90% of the tightening torque. For an M10 bolt with a friction coefficient of 0.15, applying adhesive can increase MLB/MA by more than 13%.

Factors Reducing the Loosening Prevention Performance of Thread Locking Adhesives

While thread-locking adhesives improve the performance of fasteners, certain factors can reduce their effectiveness.

1. High Temperature

Fasteners used in high-temperature environments are more likely to experience a reduction in the effectiveness of thread-locking adhesives. For instance, adhesive curing tests at 150°C may show degraded performance.

How to Handle High-Temperature Testing:

The tightening torque MA is determined based on the bolt size and grade.

After a 24-hour resting period, place the untreated bolt in a 150°C oven for 3 hours, then test the loosening torque ML.

For adhesive-treated bolts, repeat the process and test the loosening torque MLB.

The adhesive effectively prevents loosening if the ratio MLB/MA exceeds ML/MA. The standard recommends that MLB/MA exceed ML/MA by 10-20% for a successful test.

2. Surface Lubrication

Lubrication on the fastener surface can reduce the adhesive's bonding strength, decreasing the MLB/MA ratio. This is because lubricants interfere with the adhesive's ability to bond effectively to the surface.

How to Solve the Surface Lubrication Issue: To avoid this problem, it is common practice to apply thread-locking adhesive first, followed by lubrication or surface coatings after electroplating and priming. This method ensures strong adhesive adhesion while maintaining the fastener's desired friction coefficient.

3. Short Adhesive Length

The adhesive should cover at least 0.8d of the bolt to ensure adequate bonding. If the adhesive length is shorter, fewer threads will be bonded, which may result in a loosening torque below the standard requirement.

How to Address Short Adhesive Length: Designs should avoid short thread engagement lengths. Alternatively, the adhesive can be applied to bolts with the same surface treatment but longer adhesive lengths. The adhesive performance is considered acceptable as long as the MLB/MA ratio meets the required standards.

Conclusion: Key Takeaways

Microencapsulated adhesive layer locking adhesives significantly improve the performance of threaded connections in preventing loosening. For example, applying adhesive to an M10 coarse-threaded bolt with a friction coefficient of 0.15 can increase the loosening torque by over 13%.

Factors that reduce the effectiveness of thread-locking adhesives include:

High-temperature environments, where adhesive performance may degrade.

Lubricated fasteners, which interfere with the adhesive bond.

Short adhesive lengths result in fewer bonded threads and reduced loosening torque.

Improve Fastener Longevity with Thread Locking Adhesive

Understanding these key factors and applying best practices during manufacturing and testing, fasteners with thread-locking adhesives can provide long-lasting performance, even in challenging environments.

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