How to Test Static Torque for Threaded Bolts with Pre-Applied Thread Locker Adhesive and Cure Confirmation

How to Test Static Torque for Threaded Bolts with Pre-Applied Thread Locker Adhesive and Cure Confirmation

How can I confirm the Proper Curing of Threaded Bolts with a Pre-Applied Thread Locker, and how can I check the Static Torque for Threaded Bolts with Adhesive?

1. Why Choose Threaded Bolts with Pre-Applied Thread Locker?

The primary causes of bolt loosening include part movement, vibration, impact, and other external forces. These factors lead to the loss of the pre-tightening force between the connected parts, significantly when lateral loads exceed the friction force generated by the bolt's pre-tightening force. This can cause relative movement between the parts and result in loosening.

Several factors influence bolt loosening, including hardness, material, surface treatment, lubrication, and the fit between the bolt threads.

Various anti-loosening methods are used in automotive applications, with thread-locking adhesives being one of the most common. However, other anti-loosening mechanisms, such as toothed washers (e.g., RIPP LOCKING washers), are also frequently used. These washers have a unique curved, radial tooth design, differing from standard washers, and have been shown to resist loosening even under lateral vibrations effectively.

Automakers like General Motors prefer thread-locking adhesives for steering knuckles and bearing connection applications. The anti-loosening method often depends on the length of the bolt: for long bolts, sufficient pre-tightening force may be enough without additional anti-loosening measures. However, even with adequate pre-tightening force, extra precautions are necessary for short bolts to prevent loosening, as short bolts are more prone to torque decay and loosening under vibration.

2. Static Torque and Curing Tests After Adhesive Application

International standards, such as the DIN 267 series, provide guidelines for the performance of bolts with thread-locking adhesives. These standards help define terms like "loosening torque" (TLB) and "unthreading torque" (Tout), which are essential when evaluating the effectiveness of the adhesive and curing process.

Loosening Torque (TLB): This is the maximum torque required to unfasten the bolt, regardless of whether the adhesive has fully cured.

Unthreading Torque (Tout): This is the maximum torque measured after the pre-tightening force has been entirely removed, and the bolt continues to be unscrewed until the adhesive is reached.

The required torque values vary depending on whether the bolt has a pre-tightening force applied. The following table illustrates the torque performance requirements for bolts with a pre-applied thread locker.

The performance requirements specify a minimum loosening torque and a maximum unthreading torque for bolts without pre-tightening force, ensuring that the bolt remains secure under dynamic conditions.

3. Testing Static Torque for Bolts with Pre-Applied Thread Locker

When testing static torque for bolts with a pre-applied thread locker, measurements should be taken within 5 minutes. However, testing static torque beyond this window is not recommended, as the adhesive may begin to cure, potentially affecting the results.

Based on the standard performance requirements, the loosening torque of a pre-applied adhesive bolt should be at least 90% of the tightening torque. Static torque tests should ideally be conducted 24 hours after application to ensure that the adhesive has cured properly. During this period, the adhesive will have sufficient time to heal, and the bolt should exhibit a static torque of 90% of its initial tightening torque.

If the bolt does not rotate when tightened to this value, the static torque is considered to meet the requirements, and it can be assumed that the adhesive has fully cured. Additionally, it is recommended to conduct real-world tests on a batch of bolts to confirm the adhesive's bonding strength and effectiveness.

The performance of adhesive-treated bolts shows that they provide an additional 10% resistance compared to untreated bolts, ensuring a more secure connection under vibration and dynamic loads. Testing static torque on adhesive-treated bolts can help confirm their reliability and effectiveness.

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