1. Introduction
In the manufacturing process of engines, the reliable connection of components is of utmost importance. Threaded fasteners, especially connecting rod bolts, play a crucial role in ensuring the normal operation of the engine. The torque applied during the tightening process of connecting rod bolts directly affects the clamping force, which in turn influences the reliability and performance of the engine. However, in batch production, it is often difficult to directly monitor the clamping force. Therefore, setting a reasonable lower limit value of dynamic torque becomes a key issue to ensure product quality and the First Time Quality (FTQ) of the manufacturing process. This research focuses on solving the problem of low FTQ caused by the dynamic torque of connecting rod bolts being below the lower tolerance limit, aiming to provide a scientific and reasonable method for determining the lower limit value of dynamic torque.
2. Significance of Connecting Rod Bolts in Engine Assembly
Connecting rod bolts are essential components in the engine. They are responsible for firmly connecting the connecting rod and the crankshaft, withstanding complex forces during the engine’s operation, such as high – pressure gas forces, inertial forces, and frictional forces. A properly tightened connecting rod bolt can ensure the stable operation of the engine, while an inadequately tightened bolt may lead to loosening, which can cause serious consequences like engine failure, reduced power output, and even engine damage. Table 1 summarizes the main functions and potential problems of connecting rod bolts.
| Functions of Connecting Rod Bolts | Potential Problems due to Improper Tightening |
|---|---|
| Transmit forces between the connecting rod and the crankshaft | Bolt loosening, resulting in abnormal engine vibration |
| Maintain the relative position of components | Uneven wear of components, reduced engine efficiency |
| Ensure the sealing performance of the connecting rod – crankshaft connection | Oil leakage, affecting lubrication and engine performance |
3. Current Situation of Connecting Rod Bolt Tightening Technology
3.1 Tightening Tools
In modern engine manufacturing, automatic tightening equipment is widely used for connecting rod bolts. For example, in a certain engine production line, an automatic tightening device with multiple tightening shafts is employed. As shown in Figure 1, this equipment can automatically tighten 8 connecting rod bolts in two steps. The first step tightens 4 bolts (#3 – #6), and the second step tightens the remaining 4 bolts (#1, #2, #7, #8). This type of equipment can improve the tightening efficiency and ensure a certain degree of consistency in the tightening process.
[Insert Figure 1: Schematic diagram of the automatic tightening equipment for connecting rod bolts]
3.2 Tightening Strategy
The torque + angle control method is commonly used for connecting rod bolts. In a specific engine production, the process requires a torque of 20 N·m + (90°±4°), and the final torque is set within the range of 38 – 60 N·m. The tightening process is controlled by a detailed program, as shown in Table 2. Taking Step 5 as an example, it requires tightening at a speed of 10 rpm for 90°, with a maximum limit torque not exceeding 65 N·m and a time limit of 5 s. After tightening, the peak torque (the maximum torque during the tightening process) is monitored within the range of 38 – 60 N·m, the angle within 86° – 94°, and the cut – off torque (the torque when the tightening stops) within 38 – 60 N·m.
| Step | Step Name | Control | | | Limit | | | Check | | |
|—|—|—|—|—|—|—|—|—|—|—|—|
| | | Torque/Angle | Rotation Speed | Time | Torque/Angle | Rotation Speed | Time | Peak Torque | Peak Angle | Cut – off Torque |
| Stp1 | D→Tightening Shaft Self – check | 1 | | 1 | 1 | 1 | 1 | | | |
| Stp2 | E1→Sleeve Alignment | 4N·m | 10rpm | 1s | 7N·m | 1 | | 5s, 6N·m | 1 | 1 |
| Stp3 | T→Torque | 10N·m, 130rpm | | | 15N·m | 1 | | 5s, 14N·m | 1 | 1 |
| Stp4 | T→Torque | 20N – m, 20rpm | | | 25N·m | | | 2s, 18 – 22N·m, 10 – 50° | | |
| Stp5 | A→Angle | 90°, 10rpm | | | 65N·m | | 5s | 38 – 60N·m | 86 – 94° | 38 – 60N·m |
| Stp6 | CE→Cycle End | | 11 | | 38 – 60Nm, 186 – 94° | | 11 | | | |
3.3 Tightening Curve
The tightening curve of connecting rod bolts provides important information about the tightening process. As shown in Figure 2, when the bolt torque reaches 20 N·m, the angle begins to be calculated. When the bolt is tightened by 90° (with an angle range of 86° – 94°), the bolt torque should reach the range of 38 – 60 N·m after the tightening machine stops. This range allows for real – time monitoring of the bolt torque, which is crucial for ensuring the quality of the tightening process.
[Insert Figure 2: Angle – torque curve of connecting rod bolts]
4. Production Problems and Challenges
4.1 Problem Description
In a certain engine production line, data from four consecutive months showed that among a total of 38,000 engines produced, 1,520 engines had a torque less than 38 N·m, accounting for 96% of the total number of unqualified products. This situation deviated significantly from the target FTQ of 99%, having a negative impact on the assembly line operation efficiency, production rhythm improvement, and production capacity output. Figure 3 shows the production process data, where the lower tolerance limit of torque is clearly below the target value in many cases.
[Insert Figure 3: Production process data of engine torque]
4.2 Analysis of the Impact on Production
The low torque of connecting rod bolts not only affects the quality of engine assembly but also has a series of negative impacts on the production process. In terms of assembly line operation, unqualified torque requires additional inspection and re – tightening steps, which increases the workload of workers and extends the assembly cycle. From the perspective of production rhythm, the need for rework disrupts the original production plan, reducing the overall production efficiency. In terms of production capacity, the time and resources spent on dealing with unqualified products limit the number of engines that can be produced within a certain period, resulting in a decrease in production capacity. Table 3 details the specific impacts on production.
| Impact Aspect | Specific Impact |
|---|---|
| Assembly Line Operation | Increased inspection and re – tightening workload, extended assembly cycle |
| Production Rhythm | Disrupted production plan, reduced production efficiency |
| Production Capacity | Limited production quantity within a given time, decreased production capacity |
5. Calculation of Dynamic Torque Upper and Lower Limits for Connecting Rod Bolts
5.1 Theoretical Calculation of Torque T Upper and Lower Limits
The calculation of torque limits is based on the relevant parameters of the connecting rod bolt. For a connecting rod bolt with specifications M8×1×45 and a grade of 10.9, the main parameters are as follows: bolt major diameter , bolt pitch diameter , support surface diameter , threaded through – hole diameter , pitch , and thread friction coefficient . The required clamping force is 31 – 43 kN according to the design drawing.
First, calculate the friction torque diameter of the support surface:
Then, calculate the torque coefficient :
When takes the maximum value of 0.12:
When takes the minimum value of 0.09:
Finally, calculate the torque values:
Based on the above calculations, the theoretical dynamic torque upper and lower limits are 34 – 60 N·m. Table 4 summarizes the calculation process and results.
| Parameter | Symbol | Value | Calculation Formula |
|---|---|---|---|
| Bolt major diameter | 8 | – | |
| Bolt pitch diameter | 7.35 | – | |
| Support surface diameter | 14 | – | |
| Threaded through – hole diameter | 9.4 | – | |
| Pitch | 1 | – | |
| Thread friction coefficient (max) | 0.12 | – | |
| Thread friction coefficient (min) | 0.09 | – | |
| Clamping force (max) | 43 kN | – | |
| Clamping force (min) | 31 kN | – | |
| Friction torque diameter of support surface | 11.9 | ||
| Torque coefficient (max) | 0.173 | ||
| Torque coefficient (min) | 0.135 | ||
| Torque (max) | 60 N·m | ||
| Torque (min) | 34 N·m |
5.2 Clamping Force Test
To verify the calculated torque values, a clamping force test was carried out on 24 connecting rod bolts. The test results are shown in Table 5. All the clamping forces of the tested bolts meet the requirement of 31 – 43 kN. The relationship between the axial force and the tightening time is shown in Figure 4. It can be seen that the clamping force changes linearly with time before reaching 32.5 kN. The bolt does not break until , while the normal tightening process of the connecting rod only takes 4.5 s. This indicates that the bolt can meet the clamping force requirements without fracture failure. Figure 5 shows the relationship between the bolt clamping force and torque. When the torque is 34 N·m, the axial clamping force is 37 kN, which meets the product requirements.
| Connecting Rod Number | Dynamic Torque (N·m) | Static Torque (N·m) | Axial Force (kN) | Residual Elongation |
|---|---|---|---|---|
| 1 | 44.6 | 43.6 | 37 | 0.035 |
| 2 | 41.8 | 40 | 37.1 | 0.037 |
| 3 | 40.2 | 42.8 | 37.2 | 0.039 |
| … | … | … | … | … |
| 24 | 45.8 | 43.2 | 37.9 | 0.039 |
| Mean | 43.5 | 42.5 | 37.5 | 0.04 |
| [Insert Figure 4: Time – torque change curve] | ||||
| [Insert Figure 5: Bolt relationship] |
6. Verification and Optimization
6.1 Verification Process
After determining the theoretical dynamic torque lower limit value, a two – month production data tracking verification was carried out. The results are shown in Figure 6. Among the 118 unqualified products, 108 bolts had a torque between 36 – 38 N·m, accounting for 93% of the unqualified products. This shows that adjusting the dynamic torque lower limit value from 38 N·m to 36 N·m can effectively solve the FTQ problem in batch production.
[Insert Figure 6: Production unqualified data after adjustment]
6.2 Optimization Measures and Their Effects
Based on the verification results, in addition to adjusting the dynamic torque lower limit value, some optimization measures can be further taken in the production process. For example, optimizing the calibration process of the tightening equipment can improve the accuracy of torque control. Regularly maintaining the equipment can ensure its stable operation. These measures can further improve the FTQ of the connecting rod bolt tightening process. Table 6 shows the possible optimization measures and their expected effects.
| Optimization Measure | Expected Effect |
|---|---|
| Optimize the calibration process of tightening equipment | Improve torque control accuracy, reduce torque deviation |
| Regularly maintain tightening equipment | Ensure stable operation of equipment, reduce the occurrence of equipment – related failures |
| Strengthen operator training | Improve operator’s operation skills, ensure the standardization of the tightening process |
7. Conclusion
This research on the lower limit value of dynamic torque for engine connecting rod bolts has achieved the following results:
- Through the analysis of the tightening process and strategy of connecting rod bolts, combined with theoretical calculations and clamping force tests, the correct lower limit value of dynamic torque has been determined. Adjusting the dynamic torque lower limit value from 38 N·m to 36 N·m can effectively solve the FTQ problem in batch production.
- The research process provides a useful analytical thinking for optimizing the dynamic torque limit in the manufacturing field. The methods of theoretical calculation, experimental verification, and production data tracking can be applied to other similar manufacturing processes.
- Although the problem of the lower limit value of dynamic torque for connecting rod bolts has been solved to a certain extent, there is still room for further improvement. Future research can focus on more accurate torque control methods, the influence of different materials and surface treatments on torque – clamping force relationships, and the optimization of the entire production process to further improve the quality and reliability of engine assembly.