Torque relaxation is the gradual loss of preload or clamping force within a bolted joint over time. It’s also known as bolt relaxation or joint relaxation. In simple terms, the tension originally applied to secure the joint decreases, which can affect its stability and performance.
This issue is particularly important in industries where bolted joints play a critical role, including:
- Aerospace
- Automotive
- Construction
- Process and manufacturing environments
In this article, we examine the main causes of torque relaxation, assess the risks it poses in mechanical and engineering applications, and outline steps to minimise bolt relaxation and protect long-term joint integrity.
Torque relaxation explained
When a bolt is first tightened, it creates tension that pulls the joint components together. This tension produces the clamping force that keeps everything securely in place. Over time, however, that force can gradually reduce.
Changes in temperature, vibration, dynamic loads and even the natural behaviour of materials can all play a part. This gradual loss of clamping force is known as torque relaxation, or bolt relaxation.
Torque relaxation matters because it directly affects the strength and reliability of a mechanical assembly. If not properly controlled, joints can loosen, allowing movement between parts.
This can lead to increased wear, fatigue and reduced performance. In more serious situations, it may result in joint failure, raising safety concerns, causing unexpected downtime, and leading to expensive repairs.
Common causes of torque relaxation
Recognising the underlying causes of torque relaxation is key to protecting joint performance and long-term reliability. In most cases, preload loss develops gradually as materials and components respond to load, temperature and external forces.
Common causes include:
- Material Creep: When materials are subjected to a constant load, they can slowly and permanently deform. This gradual movement in the bolt or jointed components reduces preload and weakens the joint over time.
- Stress relaxation: This occurs when a material decreases the stress at a constant strain. This means the stress in the bolt can be reduced while the elongation remains constant.
- High operating temperatures: Large temperature fluctuations can significantly alter the clamping force of bolted joints. These thermal changes cause the materials to expand and contract, compromising the joint’s overall integrity.
- Thermal cycling: Any fluctuation of temperature in any material causes expansion or contraction. As materials expand or contract with temperature changes, this may alter clamping force and contribute to bolt relaxation at elevated temperatures.
- Vibration: External influences, such as vibrations and dynamic loads, can impact the clamping forces of bolted joints. These are the main reasons for the bolts’ loosening over time and torque relaxation.
- Dynamic loads: When variable forces are applied at the connection, the clamping force will decrease over time because the tensioner can move or flex under dynamic loading.
By understanding these contributing factors, engineers can make more informed decisions around design, material selection and maintenance planning to reduce the risk of torque relaxation.
The impact of torque relaxation
Torque relaxation reduces clamping force, which can directly and sometimes significantly affect joint performance. As preload is lost, the joint’s ability to securely hold components in place decreases. This increases the likelihood of mechanical issues and long-term failure.
The consequences of torque relaxation include:
- Compromised joint integrity: Reduced clamping force weakens the joint, increasing susceptibility to loosening under load.
- Increased wear and fatigue: As preload drops, relative movement between joint components increases, accelerating surface wear and fatigue damage over time.
- Higher risk of failure in safety-critical applications: In sectors such as aerospace, automotive and construction, bolt relaxation can pose serious safety risks if not properly managed.
- Performance and reliability issues: In engine assemblies, for example, torque relaxation can contribute to engine misfires, loss of efficiency or complete mechanical failure.
Understanding the impact of torque relaxation allows engineers to take proactive steps, such as improving joint design, applying torque correctly and conducting regular inspections. These measures help maintain joint integrity, extend component life and ensure the ongoing safety and reliability of mechanical assemblies.
Preventing torque relaxation
Reducing torque relaxation starts with a proactive approach to joint design, material choice and assembly practices. Rather than treating bolt relaxation as an unavoidable issue, it should be considered early in the design and installation process.
By understanding how and why clamping force is lost over time, engineers can apply targeted measures to improve durability, safety and long-term reliability, even in demanding operating conditions.
Key strategies to help prevent torque relaxation include:
Selecting the right materials
To minimise bolt relaxation, materials with low creep rates and good stress relaxation properties should be considered. Materials that deform less under constant load and maintain their stress over time are essential for ensuring the long-term stability of bolted joints.
Using controlled and repeatable assembly processes
Applying the correct tightening method and using calibrated torque tools ensures the correct initial clamping force is achieved. Consistent torque application reduces early settling and minimises the risk of uneven loading across the joint.
Incorporating locking solutions
Lock nuts, spring washers and thread-locking compounds help resist loosening caused by vibration or dynamic loads. These solutions provide additional security and help maintain preload where movement or shock is unavoidable.
Planning regular inspection and re-torquing
Routine maintenance allows any loss of clamping force to be identified and corrected before it becomes a problem. Scheduled checks help extend joint life and prevent unexpected failures.
Designing with relaxation in mind
Well-designed joints account for some level of preload loss. Spreading loads evenly and avoiding stress concentrations reduces the impact of relaxation and improves overall joint resilience.
When applied together, these measures create a more robust approach to managing torque relaxation. The result is improved joint integrity, reduced maintenance demands and greater confidence in the long-term performance of mechanical assemblies across a wide range of industries.
Why choose Crane Electronics?
With more than five decades of experience in torque measurement, Crane Electronics have been helping ensure accuracy, reliability and traceability in manufacturing environments. Our team of torque management experts are dedicated to enhancing quality, safety and efficiency across manufacturing industries.
Browse our range of torque measurement products. If you need assistance or have questions, please contact a member of our team.