Torque Load Retention

Torque load retention cannot be measured and published as a property of gasket material. However, compressibility, recovery, and creep relaxation help stabilize an assembly and reduce the need for future maintenance. Proper torque application secures joints and minimizes the risk of failures caused by fatigue, vibration, and environmental factors.

What is Torque Load Retention?

The measure of torque retention indicates how effectively a bolted joint can withstand the force applied to each bolt. When tightening a bolt, the initial step is to overcome macro effects, such as bringing the bolt head and surface into contact, closing the gap, or aligning the bolt with the surface. After addressing these macro effects, the micro effects driven by friction come into play. When applying torque to fasteners, approximately 90% is used to overcome thread friction, with only 10% of the torque tightening the bolt.

The process of torque load retention starts by creating a seal through the tensioning of a fastener within the elastic range of its stress-strain curve. While it may seem counterintuitive, bolt stretch is crucial for an effective sealing system as it helps maintain the integrity of the seal. In this context, the flange, gasket, and fastener form a spring system, which is essential for maximizing torque load retention. Bolt stretch plays a vital role in this retention by acting like a spring that keeps the flange interface sealed against dynamic loading, vibration, and reduction of joint thickness. This behavior can be observed when bolt stress remains within the elastic range of the stress-strain curve and must remain in this region throughout its operational life to preserve these spring-like properties.

Research shows that the force applied to bolts to achieve elastic conditions usually ranges between 50% and 80% of the allowed bolt stress. Other factors should be considered, including the fastener material composition, physical constraints, internal pressures, and chemical requirements. Since each situation is unique, we recommend calculating and testing your assembly to find the appropriate torque load for your application.

Fatigue Failure

Understanding how much force a fastener can handle at a joint is crucial to preventing fatigue failure. Various factors influence the stiffness of a spring, including materials, temperature ranges, thicknesses, and number of bolts used. It's important to note that stiffer bolts are not always better. Stiffer bolts experience higher fatigue stress than more flexible alternatives under the same load. Additionally, relative to the joint, the loading position on the flange can significantly affect the forces the bolt must endure. The consequences of these failures highlight the importance of design and analysis to ensure the fastener protects against fatigue over time.      

A material's failure typically progresses through three distinct stages: initiation, propagation, and final rupture. Initiation occurs when repetitive stress is applied to the material, forming cracks or defects. This stage is often subtle and may not be immediately apparent, but it sets the groundwork for fatigue failure. As the material begins to endure this repetitive stress, the cracks and defects grow, leading to the propagation phase. Here, a change in the direction of the stress points causes the cracks to extend further into the material. Environmental factors, such as corrosion or fatigue, can accelerate this phase, which may alter the distribution of stresses. The final phase of this process is the final rupture. The material can no longer handle the repetitive stress, breaking the part. Understanding these stages is essential for predicting failure and effectively maintaining the durability of a part.

Torque Loss

Proper gasket installation is crucial to minimize torque loss. Torque loss can happen in any bolted joint and changes due to bolt relaxation, system vibrations, thermal expansion, and the thickness of the gasket. Other factors affecting torque loss include the heat-up rate, the condition of bolts or studs, hardened steel washers, and the lubrication of bolts, nuts, and their facings. Bringing flanges together slowly and parallelly with a minimum of four tightening passes and correct sequence will significantly help reduce future maintenance. Even when following all these guidelines perfectly, there’s a chance that bolts may still loosen over time.

Selecting Fasteners

When choosing fasteners for a specific application, it's essential to consider several key factors to ensure reliability. First, it's crucial to understand the minimum and maximum flange pressures the gasket requires to prevent under- or overloading. Using longer bolts for gasket thickness and flange contraction loss is also advisable, as it allows for a greater strain allowance. In addition, the bolt diameter is a factor to consider, as larger diameters can lead to excessive loads on the flanges. Finally, it's necessary to be mindful of temperature ranges and thermal expansion and to ensure accurate torque measurement during installation to maintain optimal performance.