Common fasteners - bolts and nuts. How does the torque of bolts decay?
1. Why use bolts and nuts?
2. Basic terms and principles of tightening
3. Dynamic torque and static torque
4. Torque attenuation
I. Why use bolts and nuts?
• Simple assembly
• Easy disassembly
• High efficiency
• Low cost
II. Basic terms and principles of tightening
1. Tightening and its principle
Tightening principle
The bolt is inserted into the connected part and tightened with a nut or internal thread to stretch and deform the bolt. This elastic deformation generates axial tension, which presses the clamped parts together, known as pre tightening force. In theory, as long as sufficient clamping force is generated, it is completely possible to ensure the safe operation of the clamped parts in harsh environments such as vibration, high and low temperatures, without the need for auxiliary methods such as coating.
2. How to tighten bolts and nuts - torque
To tighten the bolts and nuts, force must be applied to tighten the nut/screw.
Torque (T)= Force (F) x Arm (L)
3. Forces in bolted connections
Rotate the bolts and nuts to elongate the screw under force. The clamping force generated by the elongation of the screw clamps the connecting piece. What we need is the clamping force in the connectors.
90% of torque is consumed by friction.
Only 10% of the torque is converted into clamping force.
5-4-1 principle
90% of torque is consumed by friction. Only 10% of the torque is converted into clamping force.
The Relationship and Influence of Clamping Force and Friction Force
Classification of threaded connection status
Definition source: ISO5393 "Performance test method for rotary pneumatic assembly tools for threaded fasteners". (The corresponding version of the national standard is GB/T26547-2011)
A. Hard connection: After reaching the fitting point, rotate within 30 degrees to reach the target torque.
B. Soft connection: After reaching the fitting point, rotate 720 degrees (2 turns) or more to achieve the target torque.
C. Neutral connection: After reaching the fitting point, rotate within 30 degrees -720 degrees (2 turns) to reach the target torque.
The degree of excessive torque is affected by the hardness of the connecting components and the speed of the tool.
Factors affecting clamping force
III. Dynamic torque and static torque
IV. Torque attenuation
1. Definition of torque attenuation
Torque attenuation: The phenomenon of torque reduction occurring on fasteners after tightening is called torque attenuation. The attenuated torque is lower than the target value but relatively stable. Generally, more than 60% of the torque attenuation will be completed within 30ms after the tightening operation is completed.
For any connection, there will be a certain degree of torque decay over time, which generally occurs in the following two situations.
(1) The attenuation caused by rough surface matching.
(2) Torque attenuation in soft connections.
2. Measurement process of torque attenuation
Static torque will decay over time (i.e. clamping force decay), especially when the fastener is non-metallic. However, there are many factors that affect static torque, and the linear relationship between static torque and clamping force is not obvious. Therefore, the attenuated clamping force cannot be calculated based on the value of static torque. Only professional experimental equipment can be used to determine the attenuated clamping force and find the corresponding relationship between clamping force and static torque under specific product states. Static torque can then be used to monitor the stability of the production process.
3. Measurement method of static torque
Method 1: Click and Pull Technique (can only be used as a product re inspection method)
Method 2: Loosening method
Method 3 Marking Method
Method 4 Tightening Method (T)
Method 5: Move on Method (Use a small angle (2-4 degrees) to deduce the required torque)
Method 6: Instantaneous Break Away Method (Atlas Patent)
Method 1: Click and pull technique (can only be used as a product re inspection method)
Click wrench: can only detect low torque (usually set at 90% of the lower limit of torque) and cannot accurately detect static torque.
Due to its simple operation, many methods are currently used in the production process.
Method 3 Marking method
Used for: After tightening for a period of time, corrosion or other reasons on the bolt may cause an increase in static torque (less commonly used in the automotive industry)
Method 4 Tightening method (T)
Method 5: Move on method
Method 6: Instantaneous loosening method
Method 6: Break away procedure
4. Factors affecting torque attenuation
There are many factors that can affect torque attenuation, such as connection failure caused by torque attenuation. When the product requirements are not met, analysis and improvement should be carried out from the perspectives of design and process.
Examples of influencing factors
1. Surface roughness of assembled parts: material deformation - local embedding
Response strategy: Try to avoid excessive surface roughness of components as much as possible
2. Elastic connection material: especially plastic or sealing components
Response strategy: Reduce the final tightening speed
Step by step tightening - such as setting the target step by step
Standard torque 60% -80% -100%
Use the method of tightening (e.g. to 80% of the target torque)+ loosening + final tightening
3. Excessive assembly speed and unreasonable assembly actions
Response strategy
Reduce the final tightening speed step by step - such as setting the target torque in steps of 60% -80% -100%
Use the method of tightening (e.g. to 80% of the target torque)+ loosening + final tightening
① Choose appropriate tools
② Multi axis synchronous tightening
③ Tightening sequence
The tightening force and tightening sequence are quite important when connecting threads. If the tightening force and tightening sequence are not properly matched, the threads may appear to be tightened on the surface, but in reality, the threads will quickly loosen after vibration, impact, and alternating motion. So when tightening bolts and nuts in groups, be sure to tighten them in the correct tightening sequence one by one (usually two or three times). Generally, the first tightening force is 25%, the second tightening force is 50%, and the third tightening force is 100%.
The tightening sequence of various connectors
Long strip parts: Tighten from the middle to both sides to prevent deformation of the parts.
Symmetrical parts: Tighten starting from the diagonal, such as square or circular parts.
Tightening of porous parts: symmetrically diverging from the center to the periphery.
4. Other: such as temperature during assembly (complex)
Response strategy:
• Avoid unreasonable friction
• Avoid significant differences in thermal expansion coefficients
5 Improvement measures for torque attenuation
There are many factors that affect torque attenuation, and improvement measures vary for different forms of torque attenuation. Based on the above content, common improvement measures for torque attenuation from the perspectives of process and design are summarized as follows. Of course, improvement measures are not limited to the following content.
Design perspective
1. Surface roughness
The smaller the surface roughness, the smoother the material surface, and the smaller the torque attenuation after tightening.
2. Material hardness
The harder it is to embed the material surface into each other and the smaller the torque attenuation, the higher the material hardness.
3. Elastic material
Plastic or rubber should be used as little as possible. If necessary, a comprehensive tightening strategy should be developed to ensure that the clamping force after attenuation meets the product requirements.
4. Selection of bolts and nuts
Compared to coarse threaded bolts and nuts, fine threaded bolts have a smaller pitch and a smaller thread angle, making them less prone to loosening during use. Therefore, using fine threaded bolts will result in lower torque attenuation than coarse threaded bolts and nuts.
Process perspective
1. Tightening strategy
Change the tightening strategy to two-step or multi-step tightening. Pause for 50ms during the tightening process to release elastic strain and reduce attenuation.
2. Tightening speed
After the workpiece is compressed, the burrs deform under a larger clamping force, causing a decrease in clamping force and a synchronous decrease in residual torque. The faster the tightening speed, the smaller the initial deformation of the burrs, and the more the residual torque decreases. Therefore, reducing the tightening speed can reduce torque attenuation.
3. Tightening sequence
Changing single axis tightening to simultaneous tightening of multiple axes can reduce torque attenuation. Alternatively, adopting a single axis multi-step gradual tightening to the target torque can also reduce torque attenuation.
