Helicoil Screw Lock Torque: Precise Fastening For Secure Assemblies

Helicoil screw lock torque is the force required to tighten or loosen a helical coil insert, which is a type of screw thread insert used in manufacturing. The locking torque of a helicoil is the measure of how much force is needed to prevent a screw from vibrating loose. This is an important consideration in product design, as it can impact the safety and performance of the final product. The locking torque for helicoil screw locks is typically provided in industry standards, such as the National Aerospace Standard (NASM8846), which specifies torque limits for different types of inserts. However, the actual locking torque measured in specific applications may vary due to factors such as the materials used, the hardness of bolts, and the presence of lubricants or coatings. Understanding the factors that influence helicoil screw lock torque is crucial for engineers and designers to ensure the proper installation and functionality of their products.

HeliCoil Screw Lock Torque Characteristics

Locking torque and the factors that affect it

Locking torque is the torque required to lock a helical coil insert in place. The locking torque values specified in the National Aerospace Standard (NASM8846) are often different from those obtained in practical applications. This discrepancy can be attributed to several factors that influence locking torque.

One key factor is the choice of test material. NASM8846 mandates the use of 2024-T4 or T351 Aluminium for Test Nuts or Test Plates. However, if a softer or harder material is used in practice, the locking torque can be affected. For instance, with a very soft parent material, the locking coil may expand further, resulting in lower measured insert locking torques.

The tapped hole thread tolerances also play a significant role in locking torque. Deviations from the NASM8846 specification, which outlines the requirements for tapped holes and inserts, can lead to variations in locking torque. An oversized tapped hole will generally result in lower locking torques, while an undersized tapped hole will tend to produce higher locking torques.

Additionally, the minor diameter of the tapped hole is important. If the minor diameter is undersized, it restricts the outward expansion of the locking coils, potentially resulting in higher locking torques.

Another critical factor is the bolt material and hardness. NASM8846 specifies the use of MS16997 bolts or equivalent Class 3A cadmium-plated socket head cap screws. The bolts used in the test have a hardness range of 38-42 Rc, which is similar to the insert hardness of 43-50 Rc. If bolts with different material properties are used, it can lead to variations in locking torque. For example, using unplated, 300 series stainless steel bolts with locking inserts may yield different results due to the softness of the bolt material and the absence of plating or lubricant.

In conclusion, locking torque is influenced by various factors, including test materials, tapped hole thread tolerances, minor diameter, and bolt material and hardness. Understanding these factors is essential to achieving the desired locking torque and ensuring the proper functioning of helical coil inserts.

Differences between actual and published torque values

The KATO Tech Group often receives inquiries from customers about why the locking torque they are measuring differs from the values published in the National Aerospace Standard (NASM8846) and the KATO catalog. NASM8846 specifies torque limits for helical coil inserts, and these values can be found in the KATO catalog and on their website. Despite this, varying results are often observed.

The test criteria specified in NASM8846 are very specific, and deviating from these specifications will likely lead to measured torque values that differ from the published values. One key factor is the material used for the test. NASM8846 specifies the use of 2024-T4 or T351 Aluminum for Test Nuts or Test Plates. If a softer or harder material is used, the locking torque can be affected by the outward expansion of the locking coils. For example, if a very soft parent material is used, the locking coil may expand further, resulting in lower measured insert locking torques.

Variations in the tapped hole thread tolerances will also impact the insert locking torque. An oversized tapped hole will result in lower locking torques, while an undersized tapped hole will typically lead to higher locking torques. Additionally, the minor diameter plays a role in determining the percentage of full thread. If the minor diameter is undersized, the locking coils may have difficulty expanding outward, resulting in higher locking torques.

Another critical factor is the bolt material and hardness. NASM8846 specifies the use of MS16997 test bolts or equivalent Class 3A cadmium-plated socket head cap screws. These bolts have a hardness range of 38-42 Rc, similar to the insert hardness of 43-50 Rc. The cadmium plating provides lubricity to prevent galling during the tests. However, if unplated, softer bolts are used, such as 300 series stainless steel bolts, the results may vary significantly from the published torque values due to galling and seizing issues.

In conclusion, several factors, including test criteria, materials, hole thread tolerances, and bolt hardness, can contribute to differences between actual and published torque values. It is important to refer to the specific standards and recommendations provided by organizations like NASM8846 and KATO to ensure accurate torque measurements and applications.

The importance of test criteria and specifications

When it comes to helicoil screw lock torque, adhering to test criteria and specifications is of utmost importance. The locking torque of a helicoil determines its ability to resist the screw vibrating loose. Understanding and controlling this torque is crucial for the successful application and long-term performance of the fastening system.

The National Aerospace Standard (NASM8846) provides specific guidelines for torque limits of helical coil inserts, ensuring consistency and safety in their usage. However, it is not uncommon for users to encounter discrepancies between the published values and their measured results. These variations highlight the significance of test criteria and specifications.

The test criteria outlined in NASM8846, such as the use of specific materials for test nuts or test plates, directly impact the locking torque. Deviations from these specifications, such as using softer or harder materials, can lead to differences in measured torque values. Therefore, it is essential to follow the prescribed test parameters to ensure accurate and reliable results.

Moreover, the tapped hole thread tolerances, bolt material, and hardness also play a significant role in determining the locking torque. Variations in these factors can lead to either lower or higher locking torques. For instance, an oversized tapped hole will result in lower locking torques, while an undersized hole will typically yield higher torques. Similarly, the bolt material's hardness and lubricity can affect the measured torque values, with softer bolts tending to deviate from the published values.

In conclusion, the test criteria and specifications outlined in standards like NASM8846 are crucial for achieving consistent and accurate helicoil screw lock torque values. Deviations from these specifications can lead to variations in measured torque, impacting the overall performance and reliability of the fastening system. Therefore, it is essential to understand and adhere to these test criteria to ensure the optimal performance of helicoil screw lock applications.

The impact of tapped hole thread tolerances

Thread tolerances refer to the acceptable range of variation in the dimensions of threads, ensuring a correct fit between a screw and a tapped hole. This range, known as the tolerance field, is determined by the thread tolerance class, which consists of a number and a letter. The number indicates the degree of tolerance or precision, while the letter denotes the tolerance field's position relative to the nominal size.

For tapped holes, maintaining the appropriate thread tolerances is crucial to achieving a strong connection with the screw. Basic design rules dictate that bolted joints should be designed so that the screw breaks before the threads strip. This criterion is ensured by adjusting the length engagement, which is the number of engaged threads between the screw and the tapped hole.

The minimum recommended thread engagement for a strong connection is approximately one times the nominal diameter in steel and two times the nominal diameter in aluminum. However, in certain cases, such as tapped holes in softer materials or special alloys, these values may not be sufficient, requiring the use of more complex formulae.

By adhering to the recommended thread tolerances and considering the specific materials and coatings involved, one can ensure a secure and functional connection between the screw and the tapped hole. It is also important to note that thread tolerances may vary depending on the surface condition, with some coatings requiring more leeway than others.

The role of bolt material and hardness

The bolt material and hardness play a crucial role in determining the torque values of helical coil inserts. The National Aerospace Standard (NASM8846) specifies the use of bolts with a hardness range of 38-42 Rc for testing. This hardness range is very close to the insert hardness, which falls between 43-50 Rc.

The choice of bolt material can significantly impact the performance of locking inserts. For example, using unplated, 300-series stainless steel bolts with locking inserts can lead to varying torque results above or below the published values. The softness of the bolt material can cause galling and seizing issues, affecting the overall performance of the locking mechanism.

To mitigate these issues, it is essential to select an appropriate bolt material that matches the hardness requirements of the application. Additionally, lubricants or plating can be used to minimize galling and seizing. For instance, cadmium plating provides lubricity, preventing galling during tests.

However, it is worth noting that lubricants and plating might not always solve the issue. In such cases, dry film lubricant coating on the inserts can be considered, although it may not always provide a permanent solution. Therefore, it is crucial to carefully select the bolt material and hardness, taking into account the specific requirements of the application, to ensure optimal performance and torque values of the helical coil inserts.

By understanding the relationship between bolt material, hardness, and torque values, engineers can make informed decisions when designing and selecting appropriate fasteners for their applications. This knowledge helps ensure the reliability and longevity of the locking mechanisms in their products.

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