Introduction
With the acceleration of locomotive running speed and the improvement of traction capacity, the maintenance cycle of locomotives is shortened accordingly, which puts forward higher requirements for locomotive maintenance quality and maintenance efficiency. In the disassembly and assembly of diesel engines, many processes are carried out around the disassembly and assembly of high-torque threads. The torque value needs to be precisely controlled in the assembly process of the thread pair, and the disassembly process does not need precise control. Urumqi Railway Bureau took the lead in developing a series of high-torque thread hydraulic torque wrenches (hereinafter referred to as "hydraulic torque wrench"), involving 4 types of disassembly and assembly equipment such as main bearing nuts, cylinder head nuts and connecting rod screws. Each type is compatible with Type design to meet the needs of disassembly and assembly of diesel engines of different models. The design of the hydraulic torque wrench adopts the hydraulic principle, relies on the electric motor-hydraulic pump-hydraulic motor transmission mode, adopts the relay logic secondary circuit control, realizes the mechanization of the high-torque thread disassembly and assembly operation, and improves the maintenance quality and efficiency of the diesel engine. It was welcomed by the production line workers. But there is still a key problem, that is, the problem of low constant torque accuracy during the assembly process of the diesel engine thread pair has yet to be solved.
1 Traditional Torque Value Control Method
The traditional torque value control method is based on the characteristic principle that the output torque of the hydraulic motor is proportional to the pressure difference between the inlet and outlet of the motor, and the tightening torque is indirectly controlled by the oil pressure value of the system. That is to use the set pressure value of the relief valve in the hydraulic system to control the output torque of the hydraulic motor. The value controls the hydraulic motor output torque. Therefore, in this way, the operator uses the reading of the mechanical pressure gauge to indirectly judge the output torque value of the motor, and the output torque of different pressure values is measured on the test bench through the mechanical torque force measuring rod in advance. This indirect control mode causes the tightening torque to fluctuate greatly with the system pressure, and the data repeatability is poor.
Taking the main bearing nut hydraulic dismantling machine as an example, according to the technical regulations for the maintenance of the locomotive depot, the tightening torque value of the main bearing nut assembly is required to be (2 975±25) Nm. When tightening is automatic, the output torque value of the motor is controlled by the relief valve, and the operator indirectly judges the tightening torque according to the reading of the high-pressure pressure meter. As measured on the test bench, when the high-pressure pressure gauge reads 9.7MPa, the tightening torque value is approximately 2975 Nm, but due to the volatility of the data, there is a certain rework rate. The repeatability test data of the tightening torque value of the main bearing nut hydraulic torque wrench are shown in Table 1.
| Test number | High pressure gauge reading/MPa | Tightening torque theoretical value/Nm | Actual measuring value of torque rod |
| 1 | 9.7 | 2975 | 3025 |
| 2 | 9.7 | 2975 | 2975 |
| 3 | 9.7 | 2975 | 3000 |
| 4 | 9.7 | 2975 | 2975 |
2 The New Control Method of Torque Wrench
2.1 Realistic requirements of maintenance work
As the management modernization of railway locomotive depots is gradually put on the agenda, data detection in maintenance operations is required to be intuitive, automated, and informative, and various key data in maintenance operations can not only be displayed on site in real time and monitored at any time, but also the final monitoring data. It is transmitted to the database to replace the previous manual detection and manual recording for information management. Under this requirement, a comprehensive technical upgrade of mechatronics was carried out on the hydraulic torque wrench.
2.2 Control principle
Using HMI-PLC technology, the corresponding relationship between pressure and torque value is converted and stored in the PLC through the reaction torque sensor in advance, and then the pressure sensor with temperature compensation function installed at the motor inlet and outlet in the hydraulic torque wrench is installed. The collected data is input to the PLC, and after logical operation, the dynamic torque value is displayed through the HMI man-machine interface. The operator can correct the data at any time according to the maintenance process, and realize the direct control target of the tightening torque value in the high-torque thread assembly operation. After the improvement, the working principle of the torque wrench is shown in Figure 1.
The "torque setting" displayed on the torque value control panel is the maximum output torque value set to protect the diesel engine body from damage, and it is the final control for the abnormal situation of the hydraulic system. "Current torque" is the actual output torque value, "Working oil pressure" is the oil pressure difference between the inlet and outlet of the hydraulic motor.
2.3 The way of implementation
2.3.1 Establish the corresponding relationship between pressure and torque values through pressure and torque sensors
Each type of hydraulic torque wrench completes the calibration of the corresponding relationship between pressure value and torque on a special test bench by installing pressure sensor and reaction torque sensor. After multi-point calibration, multiple sets of data are obtained. Solve the linear regression equation by multiplication, and finally obtain the corresponding relationship between the torque Y and the pressure X of different types of torque wrenches. Taking the connecting rod hydraulic torque wrench as an example, the relationship between the pressure value and the torque of the connecting rod hydraulic torque wrench is shown in Table 2.
| Number | Pressure sensor/MPa | Torque sensor/Nm | Number | Pressure sensor/MPa | Torque sensor/Nm |
| 1 | 5.0 | 1236 | 11 | 10.0 | 2462 |
| 2 | 5.5 | 1355 | 12 | 10.5 | 2577 |
| 3 | 6.0 | 1477 | 13 | 11.0 | 2699 |
| 4 | 6.5 | 1599 | 14 | 11.5 | 2821 |
| 5 | 7.0 | 1721 | 15 | 12.0 | 2947 |
| 6 | 7.5 | 1848 | 16 | 12.5 | 3065 |
| 7 | 8.0 | 1968 | 17 | 13.0 | 3193 |
| 8 | 8.5 | 2.89 | 18 | 13.5 | 3310 |
| 9 | 9.0 | 2217 | 19 | 14.0 | 3436 |
| 10 | 9.5 | 2340 |
According to the test data, the linear regression equation of the relationship between the pressure value and the torque of the connecting rod hydraulic torque wrench is obtained as Y= 244.5X+11.93.
After calculation, the correlation coefficient between Y and X is 0.999 991, the total deviation sum S is 8, 519, 198, the regression sum of squares S is 8, 519, 052, the residual sum of squares S is 145, and the residual standard deviation of the regression equation is the regression The error s of the equation is 2.92 Nm. The value of S remaining reflects the influence of experimental error and other random factors on the output torque value. The smaller the error S value of the regression equation, the better the regression equation effect. Therefore, the linear regression equation is particularly significant.
2.3.2 Closed-loop control
After a comprehensive technical upgrade of mechatronics, the high-torque thread hydraulic torque wrench consists of four parts: hydraulic system, transmission system, electrical control system and auxiliary devices. Closed-loop control is adopted between each system, which reduces the influence of oil pressure fluctuations and improves the accuracy and stability of the output torque value. The closed-loop control principle is shown in Figure 2.
2.3.3 The torque value error rate meets the maintenance process requirements
In order to verify the actual error of the displayed torque value, the verification test was carried out with the connecting rod hydraulic torque wrench as a model. According to the maintenance process requirements, the tightening elongation of the connecting rod screw should reach 0.54 mm~0.58 mm after tightening, and the required tightening torque under normal lubrication conditions is 2, 000 Nm~2, 060 Nm. The error test of the displayed torque value of the connecting rod screw hydraulic torque wrench under different working conditions is shown in Table 3.
| Number | Working conditions | Rated pressure/MPa | Display torque/Nm | Torque sensor/Nm | Error value/Nm |
| 1 | automatic tightening | 8.38 | 2055 | 2047 | 8 |
| 2 | automatic tightening | 12.82 | 3026 | 3047 | 19 |
| 3 | automatic unscrew | 12.78 | 3017 | 3047 | 20 |
| 4 | automatic unscrew | 12.69 | 3058 | 3110 | 52 |
See Table 4 for the error test of the displayed torque value under the "automatic tightening" working condition of the connecting rod screw hydraulic torque wrench.
Tests have proved that under different working conditions, the torque output display is stable, and the maximum error occurs in the jogging link of twist loosening, with an error rate of 1.7%; under automatic tightening conditions, the torque output display is stable, and the torque value and torque displayed on the HMI interface The value detected by the 5000 Nm reaction torque sensor is basically the same, the maximum error is 8 Nm. The error rate is 0.4%, and the rated pressure is within the range of 8.27MPa ~ 8.38MPa. The output torque value is within the allowable range. In actual use, the actual tightening torque is generally controlled at 2030 Nm, or about 8.27MPa.
| Number | Working conditions | Rated pressure/MPa | Display torque/Nm | Torque sensor/Nm | Error value/Nm |
| 1 | automatic tightening | 8.38 | 2060 | 2060 | 0 |
| 2 | automatic tightening | 8.30 | 2042 | 2038 | 4 |
| 3 | automatic tightening | 8.35 | 2055 | 2047 | 8 |
| 4 | automatic tightening | 8.28 | 2038 | 2038 | 0 |
| 5 | automatic tightening | 8.32 | 2048 | 2045 | 3 |
| 6 | automatic tightening | 8.30 | 2046 | 2040 | 6 |
| 7 | automatic tightening | 8.30 | 2043 | 2038 | 5 |
| 8 | automatic tightening | 8.27 | 2032 | 2028 | 4 |
The diesel engine maintenance process only has fixed torque requirements for the assembly link of the thread pair, that is, the fixed torque requirements are mainly aimed at the operation link of "automatic tightening".
3 Conclusion
After several technical upgrades, the high torque series hydraulic torque wrench integrates machinery, hydraulics, sensors, HMI-PLC technology, and interface technology to achieve the goal of mechatronics upgrading. The HMI-PLC technology enables the operator to set the torque target value according to the maintenance process. During the operation, the pressure and torque values can be dynamically displayed. After reaching the set torque, it will automatically stop and alarm, and at the same time, it will automatically record and store the torque changes under various working conditions curve, and provides an external interface to facilitate the transmission and verification of job data. Operators can transmit key data such as tightening torque to the key database of diesel engine maintenance through the data interface for future reference. Quality inspectors and managers can dynamically grasp the quality of locomotive maintenance and assess the work of operators through the network at any time. At present, the improved dismantling machine has been popularized and applied in the locomotive depots on and off the road, which not only effectively improves the technical content of diesel engine maintenance tooling, realizes mechanized operation, automatic detection, and information management, it also promotes the improvement of the management level of the maintenance operation of the railway locomotive depot.