Dynamic Behavior Study of Large-Scale Boring Bar Extension Mechanisms
Heavy-duty, extra-long boring bars are essential components of precision boring machines, used to process large cylinder liners for low-speed diesel engines. To address the issue of insufficient processing length, a new extension device structure has been proposed.
The static and dynamic characteristics of the two most unfavorable working conditions were analyzed using the NX Nastran function. The static analysis indicates that the extension device meets the necessary safety factor requirements. The dynamic characteristics analysis reveals that if the actual speed of the extension device is significantly different from the first-order natural frequency, resonance will not occur.
Furthermore, through a direct frequency response analysis, it was determined that the vibration displacement at the tool tip meets the requirements for finishing cylinder liners. This analysis can serve as a reference for the design and optimization of similar heavy-duty extra-long boring bars, and it offers new insights for expanding the processing range of precision boring machines.
01 Introduction
Precision boring machines are specialized heavy-duty equipment designed for processing cylinder liners in low-speed, high-horsepower diesel engines. The diameter specifications for boring bars range from 200 to 400 mm, with lengths between 6 and 12 m. As diesel engine technology has advanced, the length of cylinder liners has increased, which often necessitates the creation of longer boring bars to meet processing requirements. However, manufacturing these longer boring bars presents several challenges, including long production cycles, high costs, strict precision requirements for reinstallation and debugging, and significant time and labor investment.
To address these issues, a lengthening device was designed, and finite element analysis was conducted on the boring bar of a medium-sized precision boring cylinder machine. Tests have demonstrated that effectively lengthening the original boring bar can maintain structural strength and processing accuracy, thereby facilitating the mass production of cylinder liners.
02 Structural design of heavy-duty ultra-long boring bar lengthening device
2.1 Structure of precision boring cylinder machine and working condition analysis of boring bar
The precision boring cylinder machine is made up of several key components: a main transmission mechanism, a feed mechanism, a clamping device, a cutter body, an auxiliary device, and an electrical system. The structure of the precision boring cylinder machine is illustrated in Figure 1.
Figure 2 illustrates a three-dimensional schematic diagram of the original boring bar. It has a diameter of 300 mm, a length of 7 m, an aspect ratio of 23.3, and a mass of 3.4 tons, making it a typical heavy-duty extra-long boring bar.
The boring bar operates under four specific conditions:
- Working Condition 1: The boring bar is supported at two points by the front auxiliary support and the front support. This condition is used for loading and unloading workpieces, installing tools, and measuring.
- Working Condition 2: The boring bar moves to the initial processing position, with the tool at the entrance of the cylinder sleeve. It is supported at three points by the front auxiliary support, the front support, and the rear support.
- Working Condition 3: The boring bar continues to advance, and the tool is positioned between the front and rear supports. In this state, the boring bar is supported at four points: the front auxiliary support, the front support, the rear support, and the rear auxiliary support.
- Working Condition 4: The tool moves to the exit of the cylinder sleeve. The support configuration remains the same as in working condition 3, but a small portion of the front end of the boring bar extends beyond the rear auxiliary support.
The three-dimensional structure of the cylinder sleeve is illustrated in Figure 3. Its length is 2695 mm, with an inner diameter of f500H8. The roundness requirement is ≤0.05 mm, and the surface roughness value (Ra) is 1.6 μm. The material used for the cylinder sleeve is vermicular ductile iron. Since the length of the cylinder sleeve exceeds the maximum processing range of the original boring bar design, a lengthening device must be designed to meet the processing requirements.
2.2 Structural design of the extension device
The assembly of the extension device is illustrated in Figure 4. The extended boring bar is aligned with the original boring bar through the step hole. Once assembled, the two shoulder surfaces are pressed together securely. The end key is responsible for transmitting power between the extended boring bar and the original boring bar. A hexagon socket screw is tightened into the keyway on the end face of the original boring bar. The extended boring bar is secured to the original boring bar using stud bolts, thick hexagon nuts, and elastic washers. The left end features a positioning stop and an end face keyway. This design allows for connection to the transmission mechanism of the original equipment and also enables assembly with another similar extension device, making it easy to further extend the processing length of the boring bar.
03 Force analysis of heavy-duty super-long boring bar extension device
Figure 4 shows that the total length of the boring bar after assembly is 7.7 meters, with an aspect ratio of 25.67 and a mass of 3.7 tons. To ensure the boring bar has sufficient strength and high processing accuracy when metal parts machining the cylinder sleeve, both static and dynamic analyses of the extension device are required. Material mechanics indicates that the boring bar is most affected by gravity when it is supported at two points. Therefore, a static analysis should be conducted for working condition 1. The largest deformation of the tool occurs in working condition 3, when it is positioned between the two supports, necessitating a dynamic characteristic analysis as well.
After modeling in UG, the next step is to enter NX Nastran, where the main parts of the extension device will be idealized. Features that do not impact the analysis, such as chamfers, fillets, screw holes, and empty tool grooves, will be omitted. To enhance mesh quality and reduce computational demands, the original boring bar is divided into two sections. The portion near the connection end, which contains the screw hole, is 100 mm long and is meshed using 3D tetrahedron CTERA (4). The remainder is primarily meshed with 3D swept CHEXA (8), which is automatically generated through mesh pairing. The extension device and flat key utilize a 3D tetrahedron CTREA (4) mesh, while the bolt is composed of RBE3 and 1D units. It is essential to check unit quality after meshing.
The material of the original boring bar and the extension boring bar is QT600-3, the flat key is made from 45 steel, and the bolt is composed of 40Cr. The characteristic parameters of each material are shown in Table 1.
After entering the simulation environment, the key and the original boring bar are set for face-to-face bonding. The key is also positioned to make contact with the extended boring bar, while the original boring bar is placed in face-to-face contact with the extended boring bar. The static friction coefficient is set to 0.2, and both ends of the assembly are fixed in place. A gravitational load is applied, and the bolt preload is specified as 50240 N. Following these parameters, the simulation solution is performed.
The deformation cloud map of the extension device is displayed in Figure 5, the deformation cloud map of the extended boring bar is shown in Figure 6, and the von Mises stress deformation cloud map is presented in Figure 7. Additionally, the stress analysis results for each section of the extension device are summarized in Table 2.
It can be seen from the ratios in Table 2 that each part has sufficient safety factor to meet the static requirements.
04 Dynamic characteristics analysis of the extension device
4.1 Real eigenvalue analysis
Real eigenvalue analysis serves as the foundation for dynamic characteristics analysis. In this process, the Lanczos method is utilized within the NX Nastran function for solving the equations. The meshing and contact settings are aligned with those used in the static analysis. A cylindrical coordinate system is established for this analysis. The boring bar is supported at four points, allowing it to rotate freely along the axial direction, while the remaining five degrees of freedom are fixed. Since the extension device operates at a low speed, only the first four modes are extracted for analysis. The vibration modes of the extension device are illustrated in Figure 8.
The first-order natural frequency is measured at 87.02 Hz, as shown in Figure 8. Given that the actual machining speed of the heavy-duty boring bar is 30 rpm, which is significantly lower than this frequency, we can confirm that the boring bar will not experience resonance during high precision machining. Additionally, the third and fourth-order vibration modes, which are closely associated with the extension device, have even higher frequencies and, therefore, will also not resonate.
4.2 Direct frequency response analysis
The parameters from the finite element method in the real eigenvalue analysis are imported into the direct frequency response analysis of NX Nastran for plate number 1. The original boring bar, the extended boring bar, and the key joint surface are configured to be bonded. A load is applied at the tool tip, with the main cutting force set at 1730 N, the cutting resistance at 923 N, and an additional cutting resistance at 432 N. The structural damping is 0.06.
For the frequency sweep settings, the starting frequency is 0.5 Hz, the frequency increment is 3 Hz, and the total number of frequency increments is 40. The solution is then performed. The frequency displacement response curves for the X, Y, and Z directions, as well as for the tool tip, are illustrated in Figure 9.
Graphical analysis and discussion are as follows.
1) Under the influence of the cutting force, the displacement frequency is greatest in the Z direction, followed by the X direction, while the Y direction experiences the smallest displacement. Both the Z and X directions resonate at a frequency of 87 Hz, exhibiting an amplitude of 0.13 mm in the Z direction and 0.068 mm in the X direction. In contrast, the displacement in the Y direction shows a nonlinear increase with rising frequency, although the overall magnitude remains relatively small.
2) The overall displacement of the tool tip is minimal at frequencies between 0.5 and 20 Hz, only increasing by 0.58 mm. This is advantageous for enhancing the speed of the boring bar and improving processing efficiency.
3) The boring bar operates at a processing speed of 30 revolutions per minute (r/min) and has a frequency of 0.5 Hz. The comprehensive frequency displacement at the tool tip is measured at 0.0106 mm. The roundness requirement for processing the cylinder sleeve is 0.05 mm over the total length. The vibration displacement at the tool tip remains within an acceptable range, and the extension device is capable of ensuring that the processing accuracy meets the specified requirements.
05 Conclusion
Based on an analysis of the working conditions of the boring bar in the fine boring cylinder machine, a lengthening device has been proposed. The NX Nastran function in UG modeling was utilized to address the issue, and the static and dynamic characteristics were analyzed for the two most unfavorable working conditions. The research results indicate that the lengthening device has a high safety factor when subjected to static forces, does not experience resonance during low-speed cutting, and the vibration displacement of the tool tip meets the requirements for processing cylinder liners. The rationality and reliability of the lengthening device were further validated through experiments, demonstrating that this approach can be applied to the lengthening design of other similar heavy-duty ultra-long boring bars.
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