Abstract
Keywords
Introduction
With the development of computer numerical control (CNC) machine tools to high-speed, high-precision, high-efficiency, and environmental-friendly, the requirements in spindle speed and reliability are also increasing.1,2 High-speed bearing is the key component of motorized spindle, whose performance will directly affect the stand or fall of motorized spindle performance, that is, tool’s accuracy, part surface quality, production efficiency, vibration, and noise of the machine. Thus, higher requirements for the performance and operating life of high-speed spindle are put forward for designing high-speed bearing.
The main ways of bearing lubrication are grease lubrication, oil mist lubrication, and oil air lubrication; the bearings of high-speed motorized spindle are used for the oil mist lubrication which is preferred for bearing to high speed, and it is simple to replace the lubricating oil, but it is complex to design the channels of oil lubrication. Usually, the oil mist will direct discharge to the air; the environmental pollution is serious and it damages operating workers’ health. The bearings with grease lubrication can save oil mist lubrication system’s required tank, pump, tubing, and transmission parts; the lubricating system is greatly simplified, and it has the advantages of small volume, light weight, and little pollution. However, usually the bearing uses grease lubrication only in low speed. It needs a experimental platform to test and verify the speed limit of the grease lubricated bearing.
Currently, although lots of researches have investigated on the high-speed bearing, it is mainly about oil mist lubrication for the high-speed bearing on the experimental platform without radial loading. Min-Chun Pan and Wen-Chang Tsao 3 used a test rig to acquire data for envelope analysis in multiple fault diagnosis of ball bearings. Young Kug Hwang and Choon Man Lee 4 introduced a test bench to measure the magnetic attraction force. Said Lahriri and Ilmar F. Santos 5 studied dynamic forces and shaft motion in two different types of backup bearings under several contact conditions. R. Tiwari and V. Chakravarthy 6 used the experimental data in a rotor-bearing system to estimate the residual unbalance and bearing dynamic parameter. M.M. Gatzen 7 investigated the effect of greases enhanced with polymer additives on the tribological behavior of high-speed angular contact bearings. The bearing test results indicate lower operating temperatures for grease enhanced with the reference grease. M.B. Dobrica 8 evaluated the bearing performance degradation; several operating parameters are computed, including minimum film thickness, average oil temperature, and maximum hydrodynamic pressure. F.P. Brito 9 compared with the single groove arrangement, namely, due to uneven lubricant feed through each groove. Jafar Takabi 10 studied the evolution of the temperature with time in a deep-groove ball bearing in an oil-bath lubrication system; the simulation results indicate that higher rotational speed, oil viscosity, and housing cooling rate lead to the larger temperature gradient and thermally induced preload in ball bearings. D. Koulocheris 11 tested greases contaminated with particles of different sizes and hardness by a test rig. Takayoshi Itagaki 12 studied the abnormal vibration of ball bearings lubricated with grease; the axial-loaded ball bearings were operated at a constant rotational speed, and the vibration and the outer ring temperatures of the test bearings were measured. Jan Lundberg 13 examined nine different commercial greases in the wheel bearings of five ore wagons; the damage on the bearings was also studied after the end of the test period.
However, these methods cannot completely simulate the actual working condition of the bearing, because the bearing is likely to bear radial load during operation.14–18 There are few reports about the high-speed bearing experimental platform under the complicated condition with grease lubrication, oil mist lubrication, axial loading, and radial loading.19,20
In this article, the author designs an experimental platform which can exert axial, radial load, and intelligent control of the system. The experimental platform can simulate various work conditions of the high-speed bearing with oil mist lubrication or grease lubrication; it is the most close to the actual working condition of high-speed bearing and is easy to operate for laboratory assistant. Meanwhile, high-speed performance tests of angular contact ceramic ball bearings were carried out under grease lubrication and oil mist lubrication, respectively; the relationships among speed, load, and temperature were established. The experimental platform will redound to bearing industry as well as the grease industry.
Design of experimental platform
Key parameter of experimental platform
According to the actual working condition, extreme working condition, and operating characteristics of high-speed bearing, the experimental platform should meet the main technical parameters:
The bearing bore diameter of the test bearing: Φ15–Φ30 mm.
Maximum rotation speed: 60,000 r/min.
Radial loading range: 0.1–10 kN, which can be adjusted continuously.
The axial loading range: 0.1–5 kN, which can be adjusted continuously.
Lubrication form: oil mist lubrication and grease lubrication.
Cooling method: circulation water cooling.
Control method: manual and automatic control system.
Measurement: step-less adjustable, real-time monitoring of bearing working condition; display, processing, and storage of the parameters; proper treatment measures can be taken automatically in system failure state.
Technical scheme of experimental platform
The experimental platform system consists of the drive system, the test portion, the loading system, the cooling system, the lubrication system, the control system, the data acquisition system, and so on. The diagram of the experimental platform system is shown in Figure 1.
Experimental platform system diagram.
Main body of the experimental platform
The main components of the experimental platform are shown in Figure 2, which consist of the test spindle, the test bearing, the load bearing, the load components, the bearing bush, the cover, and the base. Also can be seen from the figure are the temperature sensors measuring the outer surface temperature of test bearings. The geometrical structure of the whole experimental platform is shown in Figure 3, which adopts the split structure to facilitate disassembly; in order to enable high rotation speed and stability of the test spindle, the bearings for test are installed at both ends of the test spindle with the load bearings in the middle; the test bearings and load bearings are surrounded by circulating cooling water. In addition, the experimental platform is expected to grease lubrication way and oil mist lubrication way, so an oil inlet hole and an oil return hole in the bearing bush and the base were designed; a mounted gun was installed at the oil inlet base, whose nozzle orientation is designed at an angle 15° from the horizontal, so that the oil mist can directly be injected into the rolling element of the bearing.
Blueprint of the main body.
Main body of the experimental platform.
Structure design of test head
The structure of the test head has direct effect on reliability and operating characteristics of the test machine, which consists of the spindle, the testing bearings, the loading bearings, the bearing bushes, the sleeve, the spray gun, and so on. As shown in Figure 4, the test head adopts symmetric structure, which is easy to achieve a high speed and stability. A group of annular springs were arranged to preload the outer ring of the two sets of load bearing; it is beneficial to improve the stiffness. The outer ring of the two sets of load bearing is arranged on the radial loading sleeve with a circulating water channel, to play a role in extending the life of the bearing. The test bearings adopting back-to-back mounting are located at both ends of the test spindle, while the load bearings adopting face-to-face mounting are located in the middle. As shown in Figure 5, the radial load sleeves with water channel are designed to connect the circulate water cooling device on the outer ring of the load bearings. On the right side of the two pores of the sleeves, the orifices facing the inner ring raceway of the test bearing are symmetrically processed. In addition, the spray guns are installed at the left side of the base to achieve oil mist lubrication. In view of the high working temperature, the material of the test spindle is processed by 38CrMoAlA nitriding, while the bushes, the spacers, and other small linear expansion coefficient parts are processed by 9Cr18 stainless steel.
Structure design of test head.
A disassembled test head.
Drive method
Usually, the drive mode uses the prime electromotor, such as alternating current (AC) motor and direct current (DC) motor, to produce rotational movement and transmit it to the test bearing through the belt, the gear, or the speed change mechanism. But these kinds of systems have poor reliability, large noise, and vibration; the testing speed is also restricted. However, the motorized spindle has advantages of light weight, small vibration, low noise, fast response, big power, and so on. Therefore, the experimental platform adopts motorized spindle with the power of 20 kW and the maximum speed of 60,000 r/min; also, the step-less speed regulation is realized by the frequency converter.
In addition, in order to avoid the vibration generated during the test to pass onto the spindle, flexible joints are used to connect the motorized spindle with the test spindle, that is, nylon rope is used to connect the flange on the motorized spindle with the corresponding holes on the test spindle. This connection also reduces the additional bending moment produced by bad concentricity of the test spindle and the motorized spindle, which can improve the stability of the test spindle.
Loading method
Common loading methods of bearings are electric loading, mechanical loading, and hydraulic loading. Because the operative time of bearing is long and bearing loads change frequently during operation, it is difficult for mechanical loading to achieve continuous and automatic loading. Besides, the electric loading device is large and it should not be used for a long time. Both mechanical loading and electric loading are easy to produce the impact loads in loading process. On the contrary, hydraulic loadings have some characteristics of light weight, good stability, good adjustability, easy control, and so on. Therefore, the servo-hydraulic loading is adopted for designing the experimental platform. The motor drives hydraulic pump to work, and the pressure oil is separated into two proportional pressure reducing valves, which output the given pressure oil under the control of the controller. In view of the effect of oil pressure, the axial piston puts the axial load on the outer ring of the bearing through the load-adding sheath, and the radial piston puts the radial load on the inner race of the bearing through the loading bearing in the middle. There are coolers on the outlet to control oil temperature and ensure oil pressure stable. The size of the load is controlled by proportional pressure reducing valve which can be set by computer.
Data acquisition and control system
The test parameters include the speed, the load, the host current, the voltage, the bearing temperature, the vibration, and so on. To make it possible to accurately measure the bearing temperature and the shafting vibration, both the platinum resistance temperature sensors and the accelerometers are chosen and installed on the bearing seat, 30° from the horizontal direction and close to the outer ring of the bearing. The control system of the experimental platform consists of the electrical control system, the frequency conversion and the speed governing systems, the load control systems, and the computer measurement and control system. In order to achieve precise control of the speed and the load of bearing, the speed governing system and the loading control system adopt the closed-loop control. The voltage signal is given by the computer digital-to-analog (D/A) output or manual adjustment instruction potentiometer. The actual output value measured by the corresponding sensor is sent back to the computer through the transmitter. Then, after the proportional–integral–derivative (PID) to control the speed and load by the computer, the D/A output forms a closed-loop control system. The block diagram of load closed-loop control system is shown in Figure 6.
Load closed-loop control system block diagram.
Electrical control mode can be divided into manual control and automatic control. Automatic control mode is determined by switch value outputted from computer; on/off of all electrical equipments is controlled through the power board. The signals of the temperature, the pressure, the speed, and the vibration are received and transformed by the analog-to-digital (A/D) conversion board and can be read through the display interface. To ensure the reliability and accuracy of the critical data extracted during the test, a dual display mode (numeration table mode and computer screen display mode) is designed to display the main parameters in the test. In addition, computer-aided monitoring system is used to judge, compute, dispose, and store the real-time signal. The D/A conversion board transforms and outputs analog voltage signals to control the controlled portions, while the input/output (I/O) conversion board outputs electrical equipment working state of the control system. During the experiment, the I/O board can detect abnormal situations (water or air pressure under voltage, overload, etc.) and output an alarming signal or automatically stop the platform and then store the cause of the malfunction for later reviewing.
Performance of experimental platform
Bearing for test
The test bearings for high-speed grinding spindle are angular contact ceramic ball bearings B7005C/HQ1P4, and the structural parameters are shown in Table 1.
Structure parameters of angular contact ceramic ball bearing.
Test process
Contrast tests of the ceramic ball bearing are conducted under conditions of oil mist lubrication and grease lubrication. Eight sets of bearings are selected under each condition and two sets of bearings are used for each test. Both test bearings and load bearings are cooled by circulating water at temperature of 15°C. The grease is Kluber L252; the oil of oil mist lubrication is 32 # turbine oil, whose pressure is 0.4 MPa; and the amount of droplets is 30 drops/min. Under different speeds and load conditions, the relationships among the bearing temperature, the speed and the load can be obtained by the experimental platform, and the temperature rise in the bearing outer ring is used as the parameter in the test. The test is divided into two steps: first, the impact of the speed on bearing performance is examined; when the axial load is Fa = 100 N and the radial load is Fr = 50 N, the speed gradually increased from 10,000 to 500,00 r/min ( accelerates 5000 revolutions per minute every another hour from the start speed to ultimate speed); then the bearing performance at 10,000, 40,000, and 50,000 r/min under different loads is investigated. The test is, respectively, carried out under two conditions: one is about the fixed axial load (Fa = 250 N) and the variable radial load, and the other is about the fixed radial load (Fr = 150 N) and the variable axial load.
Results and discussions
Variable rotating speed test
Stable temperature value at each rotation speed of each set of bearing is obtained through the test; the temperature values of eight sets of bearings are averaged and contrastively analyzed at different speeds. The relationships between the temperature rise and the rotation speed are shown in Figure 7. It is easy to see that the bearing temperature increases rapidly along with the increase in the speed. Meanwhile, the load, the speed, and the time are the same; the temperature rise in the outer ring of the bearing under the condition of grease lubrication is higher than that under the condition of oil mist lubrication, but when the speed is higher than 25,000 r/min or DN value is above 0.9 × 106 mm r/min, two kinds of temperature curves nearly remain parallel and the temperature difference is not more than 3°C.
Bearing temperature versus the speed change.
Variable load test
The average values of the temperatures of eight sets of bearings are analyzed. The relationship between the temperature rise and the axial load at constant speed is denoted by the curves in Figure 8. The relationship between the temperature rise and the radial load at constant speed is denoted by the curves in Figure 9.
Curve of bearing temperature rise with axial load at different speeds.
Curve of bearing temperature rise with radial load at different speeds.
Figure 8 shows that the bearings’ temperature increased with the increase in axial load at constant speed; when the axial load increased from 150 to 400 N, at the speed of 10,000 r/min, the variation in the bearing temperature under the condition of grease lubrication is about 1.9°C which is less than that under the condition of oil mist lubrication; when the speed reaches 40,000 r/min, two curves basically overlap and the variation under the condition of grease lubrication is about 0.2°C which is more than that under the condition of oil mist lubrication; when the speed is 50,000 r/min, oil mist lubrication is slightly better than that for grease lubrication, and the temperature difference is about 1°C. The curvilinear trends are almost the same in Figures 8 and 9. Under two kinds of lubrications, the temperature difference is 1.5°C, 0.8°C, and 2°C at the speeds of 10,000, 40,000, and 50000 r/min, respectively.
When the speed is lower than 40,000 r/min or the DN value is smaller than 1.44 × 106 mm r/min, whether the bearing is under the radial or axial variable load, the temperature rise under the condition of grease lubrication is lower than that under the condition of oil mist lubrication. The reason is that the viscosity of grease lubricant is higher than that of oil mist lubrication, and the grease lubricant easily forms the lubricant film. However, when the speed is higher than 40,000 r/min or the DN value is more than 1.44 × 106 mm r/min, the temperature rise under the condition of oil mist lubrication is higher than that under the condition of grease lubrication.
Conclusion
In this article, design and application on experimental platform for the high-speed motorized spindle bearing are studied and investigated. Some important conclusions are summarized as follows:
A experimental platform for high-speed grease has been designed; it can imitate the actual working state of the bearing by axial load and radial load and test the functioning of the bearings under different conditions. All the test data can be automatically acquired and analyzed.
The performance of high-speed angular contact ceramic ball bearings B7005C/HQ1P4 with grease lubrication and oil mist lubrication is tested and contrasted by the experimental platform. Under the circulating water cooling conditions, the speed is lower than 40,000 r/min or the DN value is below 1.44 × 106 mm r/min; grease can be considered for the high-speed spindle with ceramic ball bearings, so that the lubrication system is greatly simplified for reducing pollution.