Automotive industry bolt tightening quality monitoring

Bolt tightening is one of the most important component assembly methods in the automotive industry, and the quality of bolt tightening is closely related to the core quality of the whole vehicle such as vehicle safety, reliability and NVH. The quality of bolt tightening involves many variables and difficult to analyze, and monitoring it is an extremely important quality assurance technology. This paper summarizes the quality monitoring of bolt tightening from four dimensions: monitoring object, monitoring method, mathematical model and monitoring application.

Bolt tightening quality monitoring objects

The principle of bolt tightening is to apply a torque parallel to the connecting surface of the part by rotating the bolt and nut relative to each other, so as to obtain a vertical clamping force and tighten the part. Therefore, the clamping force is a direct factor in the tightening of the part. The modes of bolt tightening failure are: bolt rotation loosening due to insufficient clamping force; Sliding of parts and bolt shearing due to insufficient clamping force; Bolts, parts pulled off and extruded due to excessive clamping force.

However, since the clamping force cannot be measured during bolt tightening, the object of bolt tightening quality monitoring is currently mainly torque, angle or bolt elongation. Therefore, the realization process of bolt tightening quality monitoring is: using tensile testing machine or thread tightening analyzer and other test equipment, refer to GB/T16823-2010 "Threaded Fastener Torsion-Tensile Relationship Test Method" or other appropriate methods to measure and establish a relationship model of clamping force and torque, angle or bolt elongation.

During or after tightening, the torque, angle or elongation are monitored, and the clamping force and bolt tightening quality are judged according to the correlation model.

Bolt tightening quality monitoring method

The quality monitoring methods of bolt tightening in the automotive industry are divided into dynamic monitoring and static monitoring. Among them, dynamic monitoring is the quality monitoring in the bolt tightening process, which is the monitoring in the event. Static monitoring is the quality monitoring for a period of time after the bolt tightening is completed, which is post-event monitoring.

Dynamic monitoring is mainly realized on electric tightening machines. Electric tighteners are typical closed-loop feedback control industrial systems with measurement, control and actuation functions at the same time. The electric tightening machine realizes real-time measurement of torque and angle by integrating high-precision torque sensors and angle sensors, can store data and output tightening curves, and realize monitoring and error proofing according to set alarm parameters. Dynamic monitoring is used in conjunction with the tightening process.

When tightening by the "torque method", the real-time torque measurement data is used as process parameter feedback control for electric tightening machine tightening, and the real-time measurement data of the rotation angle is used as dynamic monitoring. When tightening using the "torque-turn method" or "yield point method", the real-time measurement data of the rotation angle is used as the process parameter feedback control electric tightening machine, and the real-time measurement data of torque is used as dynamic monitoring.

Static monitoring mainly occurs in the experimental development, manufacturing or after-sales stage, and the sampling monitoring of bolt tightening quality can be for torque or bolt elongation. Static torque monitors the effects of torque attenuation as opposed to dynamic monitoring.

Torque measurement is the most commonly used form of static monitoring, and torque sampling in factories is typically completed within half an hour of bolt tightening, when most of the bolt decay has already occurred. Torque measurement methods are divided into fastening method, loosening method, and marking method. A commonly used fastening method is to further tighten the tightened thread pair with a test wrench, which reads the torque value displayed by the test wrench when a tiny corner (e.g. less than 5°) is generated. Bolt elongation measurement is relatively more accurate, but it is difficult, costly, and time-consuming, so it is less used. Bolt elongation measurement can be performed using infrared equipment, micrometry tools, special strain bolts, etc. When measuring with infrared equipment, the machined bolt is used to attach the attachment, and then the reflection time is measured with infrared rays to obtain the bolt elongation and calculate the clamping force.

When measured by the fastening method, there is a systematic error in the measured torque relative to dynamic monitoring because of the static friction. At the same time, the "soft, medium and hard" connection methods of different parts also have an impact on static monitoring and dynamic monitoring of the difference in measured values.

Mathematical model of bolt tightening quality monitoring

The mathematical model for dynamic monitoring of bolt tightening quality changes with different stages of the tightening process. The complete effective tightening process is divided into four stages: bonding, elastic deformation, yield and plastic deformation.

The fitting between torque, angle and clamping force in the elastic deformation stage is linear, and the mathematical model can be referred to:

F clamping force = M torque ÷ (0.16×P pitch + 0.58×d2 medium diameter×μ G thread friction coefficient + DKm effective friction radius of the support surface ×μ K bearing surface friction coefficient ÷2) = θ Turning angle×P pitch×C stiffness. Accurate mathematical models, including yield and plastic deformation phases, can fit tightening curves using experimental data.

From the mathematical model, it can be seen that when torque is used for dynamic monitoring, friction also has an impact on the clamping force. Studies have shown that 90% of the torque is used to overcome friction and 10% to generate clamping force. Therefore, the friction coefficient, friction radius and other parameters change too much, which will make the dynamic monitoring produce false alarms or leakage alarms.

From the mathematical model, it can be seen that the use of rotation angle for dynamic monitoring, bolt pitch and stiffness consistency is a necessary prerequisite. At the same time, due to the existence of the bonding process, a starting torque is also required as an input value to determine that the plastic deformation has begun.

The mathematical model of static monitoring of bolt tightening quality is essentially different according to different monitoring objects. When monitoring bolt elongation, the mathematical model also distinguishes between elastic and plastic deformation. The fitting between bolt elongation and clamping force in the elastic deformation stage is linear, and the mathematical model can refer to: F clamping force = ΔL Elongation ×C Stiffness. Accurate mathematical models, including yield and plastic deformation stages, can fit the tightening curve with experimental data.

From the mathematical model, it can be seen that static monitoring using elongation mainly considers the impact of bolt stiffness on the clamping force. Therefore, the measurement accuracy is relatively the highest.

When monitoring the torque, the mathematical model must consider the measurement error of the tightening method, the mathematical model is an empirical model, which can be referred to: M2 static torque = M1 dynamic torque × (1 + ΔX connection method correction experience coefficient), T2 static torque monitoring tolerance 2 = T2 dynamic torque process tolerance 2 + (T1 dynamic torque × ΔY connection method correction experience coefficient)2 . The ΔX connection correction factor and the ΔY connection correction factor are related to which of the three connection methods of "soft, medium and hard" is used. Another empirical model commonly used in the automotive industry is M2 static torque = M1 dynamic torque × (90%~110%).

It can be seen from the mathematical model that the use of torque for static monitoring has many empirical coefficients in the mathematical model, and the mathematical model of dynamic monitoring is also required to correlate the clamping force.

Bolt tightening quality monitoring applications

Bolt tightening quality monitoring applications are a systematic project. Bolt tightening quality monitoring applications must have systematic technical standards and management processes. Documented, documented and traceable is also the guiding principle for bolt tightening quality monitoring applications. The frequency, scope, method and resources of bolt tightening quality monitoring must have complete planning and regulations, and this planning and regulation itself also needs to comply with the principle of hierarchical and hierarchical quality risk management and control.

Whether it is dynamic monitoring or static monitoring, in practical applications, in addition to calculating the clamping force when monitoring the bolt elongation, the clamping force is generally not calculated when monitoring the torque and rotation angle. Therefore, it is very important to output complete and reasonable torque and angle monitoring parameters in the early stage of product and process development.

Bolt tightening quality monitoring applications also require surveyors and analysts to have the necessary knowledge and skills, and measuring tools and tightening machines must not only be properly equipped, but must also be calibrated regularly. New electric tighteners are also often networked, with access to the factory network for network error proofing, historical traceability, and interconnection with ANDON, QMS and MES systems. In addition to real-time error proofing, professional bolt tightening quality monitoring software can explore the trends, causes and effects of tightening quality changes through integrated statistical analysis functions, and vector-based tightening curve slope analysis opens up a whole new field.

Since factors such as friction coefficient, friction surface size, and temperature directly affect the clamping force, the control of incoming parts and the environment is also an important part of tightening quality monitoring applications. Because the oil causes the bolt to be screwed off, because the welding slag causes the tightening machine to alarm, because the part size causes false torque and other problems, which are common problems in practical applications.

Conclusion

In the era of Industry 4.0, with the popularization and development of smart factories, bolt tightening monitoring, the core quality assurance technology of the automotive industry, will inevitably further mature, so as to provide users with safer, more reliable and leaner automotive products.