According to the tightening problem of automobile assembly fasteners, the dynamic and static torques are explained according to the thread connection characteristics, and the static torque control is studied by the SPC principle, and the process monitoring of the automobile assembly fasteners is carried out to ensure that the automobile parts are in the process. The stability of the mounting connection on the vehicle is guaranteed.

In recent years, SAIC-GM-Wuling (SGMW) has achieved continuous and rapid development in new product development and capacity building. The new products are also launched Wuling Hongguang, Baojun 730 series and Baojun 650 series occupying the top of the MPV and SUV sales charts. Product positioning is gradually increasing.

At the same time, with the construction of Baojun Base Phase I, Chongqing Base and new energy plants, the company has formed an annual production capacity of over 2.5 million. While product R&D and market share continue to grow bigger and stronger, how to improve the manufacturing quality of the whole vehicle and provide high-quality complete vehicle products to downstream customers is a subject that the final assembly manufacturing engineer (ME) needs to study.

Threaded connections are the most extensive form of fastening in automotive assembly, and the control of torque in threaded connections directly affects assembly quality. To this end, this paper describes the application of the static torque control range calculation method in the automotive assembly process.

Threaded connection

1.1 Threaded connection principle

Automotive assembly is a key part of the automotive manufacturing process, while threaded connections are the most widely used connection method in the assembly process. By applying a certain amount of torque on the thread pair, the connecting members are stably fixed to each other to meet the design performance requirements.

The torque control methods mainly include torque control method, torque/rotation angle control method, yield point control method, quality assurance method and torque slope method. Among them, the torque control method is the most assembled in the automobile because of its simple control method, low control cost and easy monitoring. Commonly used controls.

During assembly, the set torque is applied to the nut (bolt), and the tension generated by the elastic deformation of the bolt creates sufficient clamping force between the connected workpieces to meet the design tightening requirements (as shown in Figure 1).

Figure 1 Schematic diagram of the force of the threaded connection

1.2 Threaded connection characteristics

The control of the thread secondary torque directly relates to the quality and operational reliability of the entire vehicle. There are many factors that affect the assembly torque: the material and diameter of the threaded part, the surface roughness of the thread, the friction coefficient of the bolt (mother) and the contact surface of the joint, the accuracy of the tightening tool, the rotational speed, the tightening process sequence, etc. Has an important impact. In addition to this, the state of the threaded secondary coupling also plays a key role in the formation of the final torque.

In GB/T 26547-2011 "Test method for performance of rotary tools for threaded fasteners", the state of the threaded secondary joints is divided into three categories according to the influence on the torque: soft connection, hard connection and neutral connection ( Transitional connection).

Soft connection means that the material of the threaded sub-connector is soft or the elastic material such as rubber piece is sandwiched between the connecting parts. When tightening, after the thread pair reaches the bonding point, it needs to continue to rotate more than 720° to reach the target torque, and the torque exists after tightening. attenuation.

Hard connection means that the joint has high hardness, rigidity, smooth joint surface and high degree of fit. When tightening, the thread pair needs to continue to rotate below 30° after reaching the fit point to reach the target torque. After tightening, the torque may be reversed. Punching (over tightening) phenomenon.

A neutral connection (transitional connection) is a connection between a soft connection and a hard connection that does not typically exhibit torque decay and kickback after tightening.

2. Dynamic and static torque definitions and characteristics

The SGMW assembly threaded pair connection was originally designed with the design torque issued by the design engineer, which is the control torque range for the tightening process and also used as the tightening result to detect the torque range.

However, due to the soft connection and hard-wired connection characteristics, the fasteners that have been tightened according to the design specifications may also exhibit torque decay (backlash) or exceed the torque design range when the tightening result is detected. .

In order to effectively solve this problem and truly reflect the quality of the torque process, the thread torque should be differentiated between dynamic torque and static torque. That is, the tightening process is controlled by dynamic torque, and the torque result is monitored using static torque.

2.1 Dynamic torque

Dynamic torque is the torque control range that the design engineer calculates based on the axial preload required for the part to tighten and requires the tightening process to be performed.

At the assembly site, the tightening tool with the median value of dynamic torque is used for tightening. The peak torque obtained during the final or torsion process of the tightening process is the dynamic torque measurement value. (The tightening gun with sensor and display device can directly show the final implementation. Dynamic torque).

2.2 Static torque

The static torque is the torque at which the threaded fastener in the tightened state continues to be tightened and the thread pair is relatively rotated.

When measuring the static torque, the torque measuring tool can be used to gradually increase the torque of the tightened bolt (nut) in the tightening direction until the upper bolt (nut) once again produces the tightening motion, and the electronic instrument or dial pointer records just after the generation. The torque value of the motion, which is the static torque measurement.

Depending on the threaded connection characteristics, there are three regularities between the static torque and the dynamic torque as shown in Table 1:

Table 1 Effect of thread characteristics on torque

The correlation law between the dynamic torque and the static torque described above can be clearly demonstrated by the data collected in the field.

As shown in the data in Table 2, it is the comparison of the dynamic torque and the static torque measurement value of a soft connection connection point. Due to the influence of the elastic material on the threaded connection piece, the measured static torque value is attenuated, and the overall situation is obvious. Below the dynamic torque measurement during tightening.

Table 2 Soft connection fastening point dynamic static torque comparison table

The performance in Table 3 is a static torque that is biased toward the hard joint. Due to the static friction, a certain degree of torque kickback occurs, resulting in a measured value that is significantly higher than the dynamic torque.

Table 3 Hard connection fastening point static torque comparison table

3. Static torque control range calculation

3.1 Calculation steps

Due to the different three thread connection characteristics, for the dynamic torque used in the tightening process, we need to calculate a corresponding static torque upper and lower limits to monitor and test the tightening process in order to truly judge and reflect the tightening process and parts. Whether the status is within the quality control range.

For the calculation of the static torque range, we have developed a set of processes, consisting mainly of two steps:

Step 1: Torque data collection

In the new product development phase, starting from the vehicle production phase of the project trial production line, after ensuring that the man-machine material method ring is in a stable and correct state, the torque tool pair set by the dynamic torque median (nominal value) issued by the design engineer is adopted. Torque is implemented at the corresponding fastening points, and 30 sets of 30 (3*10) continuous dynamic torques and corresponding static torque values ​​are collected for each of the three groups.

When torque is applied, if an electric tightening gun with a sensor is used, the dynamic torque value can be read directly from its system display. If you are using a sensorless pre-set torque tool (clutch shut-off, Click wrench, etc.), dynamic torque collection can be performed by connecting an on-line dynamic torque harvesting sensor to the output of the torque tool.

Static torque collection is required within 5 minutes of the fastener tightening operation. Static torque collection can use conventional torque detection tools, such as dial torque wrenches and digital torque wrenches. However, in order to ensure that the static torque release range is consistent with the subsequent numerical values ​​obtained from torque detection, it is recommended to use static before and after each stage. The torque collection tools are consistent.

In our company, the ASI DM600 handheld data acquisition instrument is used for torque collection (as shown in Figure 2). When in use, put on the corresponding size of the sleeve and set it to the corresponding data collection mode. According to the operating specifications for torque collection, the static torque corresponding to the fastening point can be measured and stored.

Figure 2 ASI DM600 handheld data acquisition instrument

Step 2: Static Torque Range Calculation

After the 3*10 sets of dynamic torque and static torque data are collected separately, the process engineer first needs to use the dynamic torque data to check the process capability of the torque implementation: calculate the 30 dynamic torque values ​​and set values ​​(dynamic torque nominal value) The average value of the difference, if the average value is within 5% of the nominal value of the dynamic torque, the verification is passed, the process is within the acceptable range, and the corresponding static data can be used for the static torque of the fastening point. The release calculation of the range.

For the release calculation of the static torque range, our company uses a special Excel spreadsheet, the relevant static torque algorithm, various factors and judgment principles are preset in the table.

As long as the engineer inputs the collected dynamic and static torque information in sequence, the Excel table can automatically calculate, analyze and judge the static torque. To facilitate the elaboration of the entire process, the following is an example of a computational process analysis.

3.2 Static Torque Calculation Case

For example, in a new model developed by our company, the design engineer used four M14 mounting bolts for a certain part, and the dynamic torque of the threaded fastener was (108±12) N·m.

During the trial production phase, the process engineer sets the output torque of the electric tightening gun corresponding to the fastening point to 108 N·m, tightens the fastening point with a pre-set tightening procedure, and records the display on the tightening system control cabinet. Dynamic torque value.

At the same time, within 5 minutes of the dynamic torque implementation, the quality engineer used the DM600 handheld data acquisition instrument for static torque acquisition. The dynamic and static torques obtained are shown in Table 4.

Table 4 Torque data collection table

The dynamic and static torque data collected in this case are respectively input into an Excel table for calculating the static torque, as shown in the screenshot (Fig. 3), which automatically derives the mean (MEAN) of 108.38 N·m and 94.27 N·m, respectively.

Figure 3 screenshot of dynamic and static torque value filling interface

For dynamic torque, CPK ≥ 1.33 is required to meet the static torque calculation requirements. In this example, the electric tightening gun is used for torque calculation, based on the measured values ​​of dynamic torque and the upper limit of dynamic torque (USL) and lower limit (LSL).

The table actively calculates the mean (X-bar) as 108.38 N·m, the mean range (R-bar) is 0.24, and the Sigma(σ) is 0.14, so the dynamic torque CPK=27.32>1.33 is obtained, which meets the requirements, as shown in Fig. 4. Show.

Figure 4 screenshot of the dynamic CPK calculation interface

At the same time, the table calculates the mean value of the static torque (X-bar) as 94.27, the mean range (R-bar) is 5.73, and the Sigma(σ) is 3.38 according to the preset formula and coefficient, and is calculated according to the ±3σ principle. The theoretical upper limit (USL) of static torque is 107.78 N·m, and the lower limit (LSL) is 80.77 N·m. After the average rounding, the median static torque (average Mean) of this control point is 94 N·m. Tolerance is ±14 (ie static torque control circumference: 80~108 N·m), as shown in Figure 5.

Figure 5 Static torque control range calculation interface screenshot

For the collected dynamic and static torque data, it is also necessary to use the XR map in the SPC statistical mass analysis to analyze the stability of the tightening process.

Figure 6 shows the dynamic torque XR diagram of this case. It can be seen from the SPC principle that the process is stable and within specifications.

Figure 6 Dynamic Torque XR Diagram

After the calculation is completed, the static static torque range (New Static Torque Spec) also needs to meet two conditions in order to pass the test: First, the ratio of the static torque tolerance to the static nominal value is less than 35% (Is the range less than 35%); It is also required that the ratio of the nominal value of the static torque to the nominal value of the dynamic torque and the dynamic nominal value is less than 15% (Is the mean shift less than 15%).

In this case, the ratio of static torque tolerance to static nominal value (Range): 14÷94*100%=14%<35%; the ratio of the nominal value of static torque to the nominal value of dynamic torque and the dynamic nominal value: (108-94) ÷108*100%=13%<15%, both of which meet the requirements, so they are judged as “Yes”.

Since all conditions of static torque release are met, the system automatically determines the Static Static Range (Approve). As shown in the Excel calculation table screenshot (Figure 7).

　　 Figure 7 Static torque control range pass or fail judgment interface screenshot

After the calculation of the static torque range is completed, the process engineer needs to feed back the data to the product engineer, who will confirm the torque sampling data or historical experience values ​​for each stage of the new product trial and road test. For the characteristics of the soft connection of the fastening point, it is finally determined that the static torque range is 80~108 N·m.

Before the formal production, the process engineer needs to organize all the dynamic and static torque ranges of the whole vehicle into a table, and release the torque meter to the manufacturing system and the quality area. The latter two perform daily static torque detection and monitoring according to the static torque range.

3.3 Problem solving

According to past experience, the static torque range calculated from the static torque measurement value collected for the first time has a tightening point of about 20%, and the calculation result cannot directly satisfy the above judgment conditions.

For these points, process engineers need to organize the workshop, design engineer, and quality engineer to analyze the cause. There are three main aspects of unqualified content:

(1) Static torque attenuation is large, less than 15% of the nominal value of dynamic torque. Through analysis, the main reasons are (decreasing by the frequency of occurrence):

1) The fastening connector is soft-connected (such as elastic structure, rubber material, spring washer, etc.);

2) The quality of the parts manufacturing (such as the joints do not fit, interference, gaps between the plates of the welded parts, etc.);

3) Defects in part design (such as too small contact area between bolt and workpiece, insufficient strength of workpiece, etc.);

4) After five minutes of measurement (if the start-up, the difference in thermal expansion coefficient between the screw and the connector);

5) Other.

(2) Static torque kickback is large, which is greater than the nominal value of dynamic torque of 15%. Through analysis, the main reasons are (decreasing by the frequency of occurrence):

1) The fastening connector is a hard-wired feature;

2) The operation is not standardized (such as Click over);

3) The speed of the pre-tightening power tool is too high;

4) The tightening procedure of the electric tightening gun is unreasonable (such as the speed of the tightening phase is too high);

5) Other.

(3) The static torque tolerance range is large, exceeding the range of 35%. Through analysis, the main reasons are:

1) The parts are of poor manufacturing quality and low processing consistency (such as workpiece thickness, coating thickness, and flatness);

2) The same connecting piece, multiple fastening bolts (mother) data mixed calculation;

3) Pre-tightening tools have poor precision and operational differences (such as pre-tightening with an impact wrench);

4) The operation is not standardized (such as Click over twisting, not tightened in order);

5) Other.

Through detailed analysis of the man-machine method loop of the unqualified fastening points, after eliminating the unqualified factors one by one, collect a batch of stable data, and perform static torque calculation and release according to the above procedure.

It should be specially stated that even if all the unqualified factors are eliminated, there will still be about 5% of the fastening points and the static torque nominal value will exceed the nominal value of the dynamic torque due to the soft connection and hard connection characteristics of the threaded connection. For the 15% range, for this part, it will be released after comparing and comparing the road test torque and the experience data of the production car.

3.4 Other considerations

(1) Dynamic torque can be used as the initial static torque application before the static torque range value is calculated.

(2) When using multiple fasteners for a part: If there is no significant difference in static torque measurement, the static torque range can be shared under the premise of designing dynamic torque and fasteners; for example, static torque at each fastening point The measured values ​​are significantly different, and the static torque needs to be calculated and released separately.

(3) After the static torque range is released, the process engineer needs to re-collect the data to release the static torque range after design parameters (such as dynamic torque, part structure, fastener changes, etc.).

(4) After the manufacturing conditions have changed (such as tool or equipment tightening program changes, production line changes, process sequence changes, etc.), if the detected static torque is offset from the current static torque range, the feedback process engineer needs to re-release the static torque range. .

(5) Threaded fasteners with glued, loosened threaded fasteners and corner and yield control methods are not suitable for this method.

3. Process torque monitoring

After the production and quality areas receive the static torque range issued by the process engineer, the relevant torque control and quality control work is carried out according to the QCOS (Quality Control Operati System) control program requirements. The main processes are:

(1) Quality area The QCOS account is prepared and released to the manufacturing area according to the static torque list and fastener control level issued by the process engineer. In the manufacturing area, the torque control trend table is compiled according to the QCOS account and the corresponding control resources of the organization (see Table 5).

Table 5 Torque Control Trend Table

(2) After the production station performs the torque operation according to the operation instruction, the static torque is sampled once per shift for the torque control point in the QCOS account according to the requirements, and the relevant information is filled in the control operation trend table. in.

If there are 4 consecutive points in Zone A, the team needs to investigate the root cause of the problem and make corrective measures; if the torque is out of tolerance, it is necessary to go back to the 100% inspection until the torque of 5 consecutive vehicles is qualified, and record the countermeasures and Frame number; for unqualified torque, analyze the cause and take corresponding measures according to relevant specifications.

(3) The quality engineer analyzes the contents of the torque control trend table on time, grasps the stability of all torque control points and formulates corresponding lifting plans to ensure the dynamic torque process capability (CPK) and static torque of each level of fastening points. The pass rate meets the established torque quality goal.

5. Conclusion

The threaded connection of the fastener is the most widely used connection method in the assembly process of the automobile. The torque control process is of vital importance to the quality assurance of the whole vehicle. At present, many automobile factories in China have introduced the dynamic and static torque concept very early and are maturely used in actual production.

Our company's final assembly ME (Manufacturing Engineering) has also successfully calculated the static torque range in the newly developed products in recent years, and applied the actual production quality control process. Currently, it is combining the design, quality, workshop and other related areas to improve the static torque control range. The calculation process, the static torque is applied to other production models as planned.

With the change of torque control concept and the application of static torque control, the various unreasonable phenomena brought about by the unique torque and process and result detection will be completely eliminated, and the manufacturing quality of the whole vehicle will be further stabilized and improved.

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