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Bearing Test Technology and Testing Machine

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Bearing is an important supporting basic part of mechanical transmission mechanism, which directly affects the technical level and quality performance of equipment, and is known as "industrial joint". At present, high-end bearings such as wind turbine bearings and high-speed rail bearings are monopolized and controlled by foreign companies. High-end bearings have become important strategic resources and important materials for national defense security and people's livelihood. There is still a big gap between the product research and development capabilities of domestic bearing enterprises and foreign advanced enterprises, especially in the comprehensive performance test, inspection and testing, product research and development, engineering and technical services, etc. These gaps have become the bottleneck restricting the innovation and upgrading of domestic bearing products and technological progress. Bearing enterprises to strengthen the new product trial production means, comprehensively enhance the bearing test ability, improve the bearing comprehensive inspection and testing means, can enhance the enterprise's independent research and development innovation ability, enhance the enterprise's market competitiveness.

With the continuous research and development of high-speed, high-precision, high-reliability, low-friction, low-vibration, low-noise bearings and special bearing products used under special conditions, higher requirements have been put forward for bearing detection technology and testing. It also provides a broad market and opportunities for the vigorous development of bearing test equipment. At present, China Aircraft Test has provided various bearing test equipment for many industries including aviation, aerospace, rail transit, scientific research institutes and universities, including joint bearings, rolling bearings, automobile and train hub bearings, Aircraft landing gear bearings, helicopter main rotor bearings and other fatigue life test equipment.

There are many types of bearing testing machines, and their functions and uses are also different, including bearing performance testing machines, fatigue life testing machines, strengthening testing machines, design verification testing machines, etc., but the main structure principles of these testing equipment, load application technology, Drive technology, test technology, and control technology are basically the same. The common technology of bearing testing machine is introduced in detail below.

2 host structure principle

The host frame of the testing machine is mainly used for the connection of bearing and bearing special tooling, the installation layout of measurement and control components, etc. The special tooling includes bearing spindle, test bearing and bearing lubrication seal. Among them, the test bearing is the key component of the testing machine, and its performance determines the limit speed and maximum bearing capacity of the testing machine, and also determines the rotation accuracy of the testing machine, which affects the spindle runout and the accuracy of the test data. According to the host structure classification, generally divided into bridge structure, cantilever structure, combined structure.

2.1 bridge structure

The bridge structure is divided into three-point structure and four-point structure. In the three-point structure, the middle is a test bearing, and two test bearings are installed at both ends to play a supporting role. In the four-point structure, there are two test bearings in the middle, and two test bearings are installed at both ends to play a supporting role. Among them, the test bearing can also use the same specifications of the test bearing, can be tested at the same time 4 bearings. Bridge structure testing machine is used for fatigue life test.

2.2 cantilever structure

The test bearing installation shaft adopts cantilever beam structure, one side is supported, and the test bearing is installed on one side. Only one set of bearings can be tested at a time, which is convenient for disassembly and testing, and easy to expand functions, such as adding high and low temperature environment, water spraying and powder spraying environment, mud environment, etc. The cantilever structure testing machine is mostly used for performance test and research.

2.3 combined structure

The installation structure under the condition of similar test bearing is adopted, or the main structure of the axle box and spindle when the test bearing is used directly is adopted. This kind of structure is complex, but the test method is closer to the actual working condition, which is more instructive to the application of the bearing, and is mainly used for the working condition simulation test machine.

3 Load application technique

The test load is a key indicator of the bearing testing machine, its load size determines the size range of the test bearing, and its loading accuracy determines the accuracy of the test results. Bearing testing machines with different functional requirements have different requirements for accuracy and speed. For example, bearing strengthening testing machines have higher requirements for loading accuracy, generally reaching 0.5%FS, but the loading speed requires slow loading. Commonly used load applying methods include lever weight loading, spring loading, electric loading, hydraulic loading, etc.

3.1 lever weight loading

Lever Weight Loading The magnitude of the load is determined by the mass of the loaded weight and the lever ratio. This loading method is simple in structure, does not require load sensor testing, and has low cost. It is suitable for life test, but the test load cannot be changed during the test, and it is not suitable for high speed. Too high speed will cause weight vibration and lead to load instability.

3.2 spring loaded

Loading is performed by a compression or tension spring. The stiffness of the spring shall be calibrated before loading, and the test load shall be adjusted by adjusting the compression or elongation of the spring during the test. The adjustment range of spring loading is small, the cost is low, and it is generally used for constant load fatigue test.

3.3 electric loading

The worm gear lift is driven by a stepping motor to load, or the ball screw is driven by a servo motor to load. The size of the load is measured by a load sensor and a closed-loop control is formed to achieve accurate loading of the load. The loading frequency of electric loading is generally low, not more than 5Hz. In order to prevent overshoot of the loading force, an elastic device is generally installed in the loading head to reduce the impact.

3.4 hydraulic loading

The size of the load is controlled by adjusting the pressure of the hydraulic cylinder loading chamber. According to the different adjustment methods, it is divided into manual hydraulic loading, proportional valve automatic loading, electro-hydraulic servo valve loading.

(1) Manual hydraulic loading: Adjust the pressure of the hydraulic cylinder through the manual knob to achieve the purpose of controlling the test load. This loading method is simple, no noise, but the accuracy is low, the need for artificial adjustment, and by the loading cylinder friction and internal leakage of greater impact, generally used for life test.

(2) Automatic loading of proportional valve: a closed-loop system composed of hydraulic station, proportional valve control system and pressure sensor or load sensor. The opening size and direction of the proportional valve are controlled by the digital regulator or function generator of the proportional valve control system, and then the load applied to the test bearing is controlled. This hydraulic system is more complex, higher cost, but the control accuracy is good, through the PID adjustment can achieve 1% control accuracy. The biggest feature of the system is that the load can be customized according to the test requirements, especially suitable for simulation test.

(3) electro-hydraulic servo valve loading: the response of electro-hydraulic servo valve is much higher than that of proportional valve, when the test frequency is higher than 5Hz or the load curve changes more intense, the proportional valve automatic loading will be serious lag and can not reach the test peak and valley value, at this time need to use electro-hydraulic servo valve loading system. The loading principle of electro-hydraulic servo valve is the same as that of proportional valve, but the loading speed of the system is faster and the frequency response is higher. The friction pair actuator can reach 50Hz, and the static pressure actuator can reach 300Hz. Such test equipment is expensive and is often used in military tests and special tests.

4 Drive technology

When carrying out the bearing test, the test spindle needs to rotate at a certain speed or swing back and forth at a certain frequency and swing angle. There are ways to drive the spindle: variable frequency motor drive, servo motor drive, electric spindle drive, hydraulic drive, etc.

4.1 variable frequency motor drive

Variable frequency motor drive consists of frequency converter, variable frequency motor and transmission components. By adjusting the frequency of the motor power supply through the frequency converter, the rotation speed of the motor can be freely controlled. Generally, the motor and the spindle are connected by a belt or a coupling to drive the inner ring or outer ring of the test bearing to rotate.

4.2 servo motor drive

The servo motor drive is composed of a servo motor, a servo driver, an encoder, etc. It is mainly used when the speed regulation range of the test bearing is very large, such as when the minimum speed of the test is 2% of the maximum speed, it is more suitable for use. Its speed control accuracy is high, can reach 0.1 or higher, but the test power is small, more than 30kW servo motor needs special customization.

4.3 electric spindle drive

When the speed of the bearing test exceeds 8000r/min, the electric spindle direct drive can reduce the moment of inertia of the transmission components and improve the response speed of the system. The system is composed of electric spindle, speed sensor and frequency converter. Considering the heating of high-speed bearings, it is necessary to lubricate and cool the electric spindle bearings, support bearings, and test bearings (if necessary). The system is generally equipped with a cooling and lubricating device.

4.4 hydraulic drive

Hydraulic drive is mainly used for joint bearings, bearing bushes and other friction pair bearing testing machines. It is mainly driven by a swing actuator or a hydraulic motor to drive the test spindle, and the motion form is generally reciprocating swing. The main advantage of hydraulic drive is the large driving torque, up to 10000N · m or even higher. For actuators with small torque, the oscillation frequency can be controlled to 100Hz. Hydraulic drive is mainly used for aerospace self-lubricating bearing fatigue testing machine.

5 Testing techniques

Testing technology is the key technology of testing machine. The control parameters and measurement feedback parameters of the testing machine need to be collected, stored and processed, mainly through the microcontroller, PLC, data acquisition card, etc., and through the industrial computer or industrial display to display and operation, the main test parameters are load, speed, temperature, vibration, wear and so on.

5.1 load test

The test load is a key indicator of the bearing testing machine, the size of the load determines the size range of the test bearing, and its loading accuracy determines the accuracy of the test results. Generally, the static accuracy shall not exceed 1% of the indication value and the dynamic accuracy shall not exceed 3% of the indication value. It can be measured directly by a load sensor, or it can be converted by measuring the oil pressure of the loading chamber with a pressure sensor. When the accuracy requirement is high, the load sensor is used for direct measurement.

5.2 speed test

The speed of the test bearing is the main control parameter of the testing machine. The open-loop speed control error will exceed 2%, which is not recommended for the bearing testing machine. At present, the testing machine uses photoelectric encoder or other forms of speed measurement sensor and motor inverter or controller to form a closed-loop control of speed, and the speed control accuracy can reach 0.5%.

The temperature rise of the test bearing is a comprehensive reflection of the quality of the bearing, and the temperature rise of the bearing needs to be tested when the new product is developed. When testing, it should be as close to the inner ring or outer ring of the bearing as possible. When the test is running, which ring is fixed, which ring is measured. In special cases, it is necessary to measure the inner and outer rings at the same time, and the temperature signal on the rotating side needs to be exported to the measurement and control signal through the electric slip ring, but the reliability deviation and short life of the electric slip ring are not recommended. For the test that requires lubricating oil during use, the oil supply temperature and oil return temperature of the test bearing also need to be measured. For more demanding tests, the temperature of the lubricating oil supply must also be strictly controlled. The temperature sensor can use K-type and J-type thermocouples, with a temperature range of -60 ℃ ~ 350 ℃ and an accuracy of 1 ℃.

5.3 vibration test

Vibration parameters are often used to determine whether a bearing is failing. For different test bearings and different testing machines, the vibration value of bearing fatigue failure is not a fixed value, which needs to be determined by the test personnel according to experience. In order to analyze the vibration signal, it is necessary to collect the signal at high speed, and the sampling frequency is generally required to be greater than 10kHz. However, in order to avoid false shutdown due to vibration overrun caused by interference of the control system or loading system, data processing of the overrun value is required, and multiple triggers or a period of time can be used to consider the vibration overrun effective.

5.4 wear test

The wear test is mainly used to measure the wear thickness of friction pair bearings such as joint bearings and bearing bushes during long-term tests, divided into online measurement and offline measurement. On-line measurement can be used grating ruler, differential transformer, laser displacement sensor, etc., the sensor itself is high precision, but due to the operation of more interference factors, can only be used as a reference. The value of the specific wear amount needs to be measured offline.

6 Control technology

At present, the core of the control technology of the bearing testing machine is the controller, mainly including special dynamic and static controllers, industrial computers, NI or PLC, etc. Basically, long-term tests can be realized unattended. The testing machine can run automatically according to the set program. All test parameters and feedback measurement parameters can be set with upper and lower alarm values. When the tested parameter exceeds the alarm value, the system will automatically stop. Configuration of PC software, easy to operate, flexible to compile the test process.

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