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Ultra-high-speed cutting is high quality, efficient, characterized by advanced manufacturing technology, it can bring high-speed cutting mechanism, high-speed spindle unit, high acceleration and deceleration linear feed motor, control system and a series of high-performance technology development related units. High-speed spindle unit is the core of bearing high speed spindle bearings. Ceramic bearings for its resistance to high-speed, light weight, long life and excellent performance, high-speed spindle unit in CNC machine tools are widely used.
A high-speed spindle unit with spindle bearings
High-speed spindle unit is a key technology to achieve high-speed processing is one of the key components of high-speed machine tools. High-speed spindle unit is the core of high-speed precision bearings, and its performance directly affects the performance of the work spindle unit. As speeds increase, the bearing temperature, vibration and noise increases, reduced life. Therefore, improving the spindle speed on the premise that research and development of superior performance high speed spindle bearings. At present, the high-speed spindle unit, spindle bearing mainly uses magnetic bearings, fluid hydrodynamic bearings, ceramic ball bearings in three forms. Magnetic Bearings and high-speed performance, high precision, easy to implement diagnostics and online monitoring. However, practice shows that the bearing due to electromagnetic control system is too complex and has not yet been widely used. Liquid Hybrid Bearing combines hydrostatic bearings and hydrodynamic bearings advantages, but the bearings must be specially designed according to the specific machine, a separate production, low degree of standardization, maintenance is also difficult.
Currently, the most widely used high-speed spindle bearings or hybrid ceramic ball bearings, rolling elements that use hot isostatic pressing or Si3N4 ceramic balls, bearing rings remains rims. This bearing high degree of standardization, low price, small changes to the machine, easy maintenance, especially for high-speed operation occasion. It d • n value has exceeded 2.7 × 106. To increase bearing life, increase the wear resistance of the raceway, the roller coating or other surface for treatment.
2 Study
Test conditions
The ceramic ball bearing used in the test parameters are shown in Table 1. The material used for the ceramic ball HIPSN (hot isostatic pressing Si3N4), accuracy class is the G3 level, ball and ring assembly according to the specified value fine matching. The bearings are made of "small pearl beads secret" structure, and use the thin outer cage. Used in the test steel bearings and ceramic ball bearings with the same structural parameters.
Spindle used in the tests, there are two models, namely A-type and B-type. Structure indicated in Figure 1, the bearing is installed in the form of DBB. Table 2 are the two main performance parameters of the spindle.
Table 1 Test parameters with ceramic ball bearing structure
Model Dimensions (mm) ball diameter (mm) balls raw material contact angle ceramic ball cage material Ferrule Material
B7007CY35 × 62 × 147.1441615HIPSN resin GCr15
Table 2 spindle main performance parameters
Model Limiting speed (r / min) spindle diameter (mm) Output Power (kW) Lubrication
A type 30000354.5 mist
Type B 24000354.5 fat
Figure 1 High-speed spindle unit structure diagram
This test uses thermocouple measurement spindle front bearing outer ring temperature rise. Also the use of high-speed spindle PDB measuring front and rear ends of the vibration spectrum. Analysis of two bearings on the spindle running accuracy.
Experimental analysis of test results
1) spindle bearing temperature rise
According to two high-speed spindle experimental data plotted temperature characteristic curve visible, A-type spindle speed is lower than 15000r/min, the two bearing temperature rise is basically the same. When the speed is higher than 15000r/min, the ceramic ball bearing temperature rise significantly lower than steel bearings. Steel bearing temperature rise rate faster than ceramic ball bearings.
A type Spindle speed up to the speed limit 30000r/min 2000r/min, the steel bearing temperature from 4 ℃ up to 35 ℃; spindle speed up to 40000r/min by the 35000r/min, the ceramic ball bearing temperature from 35 ℃ up to 43 ℃, the temperature is too high in order to prevent damage to the ceramic ball bearings, to stop increasing speed. Temperature rise at the same level, ie, when the temperature is 35 ℃, with ceramic ball bearing steel bearing spindle motor spindle speed ratio increased by about 17%.
B-type spindle bearing temperature rise of the overall trend similar to A-type spindle. However, the spindle speed is low, slightly higher than the temperature rise of the ceramic ball bearing steel bearings, steel bearings smaller than the growth rate of temperature rise. When the speed n> 17000r/min time to show ceramic ball bearing temperature rise characteristics. Grease lubrication conditions, the operating speed of the ceramic ball bearings and oil mist lubrication steel bearing running speed considerably. It was found that, B-type ceramic ball bearings to reach thermal equilibrium temperature and time required, and the A steel ball bearings to reach thermal equilibrium temperature and time required to close.
From the foregoing, whether or grease lubrication with oil mist in high speed or insufficient lubrication, the ceramic ball bearings are less than steel bearing temperature. Analysis: (1) HIPSN a density of only 40% of bearing steel. Centrifugal force generated by the ceramic balls and gyroscopic torque is small, so that less heat ceramic ball bearings. (2) consisting of ceramic and steel friction coefficient of friction than steel and the steel composition of the friction coefficient of friction, generate less heat, the temperature is low. (3) required for assembly bearing preload preload greater deformation and more heat, the faster the bearing temperature rise. Bearing high-speed operation, the bearing to withstand a total load includes the initial preload and bearing internal load. Internal load by the centrifugal force and the difference in thermal expansion caused. The bearing outer ring of the bearing is preferably heat, followed by the inner, heat rolling the worst conditions, the inner ring and the rolling elements due to temperature rise caused by thermal expansion greater than the thermal expansion of the outer ring, the bearing? For the preload is greater than the original assembly preload force, thereby increasing the friction heat, bearing temperature increases. As HIPSN coefficient of thermal expansion ceramic material is only 25% of bearing steel, so when the speed is increased, the temperature rise of the ceramic ball bearing is much smaller than the steel bearing. Data show that the ceramic ball bearing inner material thermal expansion coefficient of 20% smaller than the bearing steel, stainless steel, carburized steel and other materials, can reduce the temperature rise of the bearing.
From the above test results and analysis, ceramic ball bearing steel bearing is more than suitable for high speed operating conditions.
Spindle vibration spectrum analysis
Using highly sensitive piezoelectric crystal sensor signals using the discrete Fourier transform calculation principle, the use of type A PDB measured spindle vibration spectrum visible, electric front spindle vibration acceleration fluctuations, resulting in lower electric spindle running accuracy, stiffness degradation . Ceramic ball bearings with electric front spindle acceleration change little, spindle running dynamic and high accuracy. Comparison of the two types of electric spindle that the use of ceramic ball bearings, can effectively reduce the vibration of the spindle, spindle improved running accuracy and stiffness.
Problems and Countermeasures
Experiments, the ceramic ball bearings with two types of electric spindle, when the speed is low, there is a beginning of the operation (low speed) poor rigidity, low accuracy.
Analysis, mainly by the bearing clearance and work preload change impacts. Low speed, the preload force, bearing clearance is small, high stiffness; high speed, high speed inside the bearing due to a greater load, the two superimposed, the actual speed of the bearing preload is far more than the initial preload, resulting in bearing temperature rise, low-life, prone to injury early sintering. Therefore, in order to extend bearing life, requiring early ceramic ball bearings preload smaller. But the initial preload is too small, the spindle starts, ceramic ball bearing gap, running deformation, stiffness difference. The spindle vibrations increase, seriously affecting spindle machining accuracy.
The solution is research and development of bearing preload mechanism can be transformed. Low speed, the ceramic ball bearing preload force, as the speed increases, the bearing preload force becomes smaller, the ceramic ball bearings are always in a good state of motion. There are two main measures: (a) implement given position preload transformation. (2) emphasis on running accuracy, low speed, the implementation of fixed position preload; high speed, the use of preload can transform organizations.
3 Conclusions
Experimental results show that the ceramic ball bearings used in high speed spindle unit is designed and manufactured, can raise the spindle speed limit, reducing the vibration of high-speed spindle, the spindle operation to improve accuracy and rigidity; can extend the life of the spindle, simplification and The supporting lubrication system. And to solve the low speed condition, the ceramic ball bearing stiffness is poor, low accuracy.
| Product Model | Inside Diameter | Outside Diameter | Thickness |
| 32232U NTN | 160 | 290 | 80 |
| 30232U NTN | 160 | 290 | 48 |