[Abstract] Through the fatigue test of composite rolling bearing, the fatigue life and failure form of glass fiber/nylon 66 rolling bearing developed by the short fiber injection molding process are studied, which provides a valuable reference for the design and application of composite rolling bearing.
Key words composite rolling bearing fatigue test failure analysis
Test Research on the Fatigue of the Composite
Material for Rolling Bearings
Zhang Li Yang yong Zhang Heng
(Loyang Institute of Technology)
[Abstract] This paper deals with a fatigue test of the composite material for rolling bearings, and analyses the fatigue life and fatigue failure of glass-fiber/nylon 66 mosaic rolling bearings formed by injection.The valuable testing method is obtained for the design and Application of composite rolling bearings.
Keywords composite rolling bearings fatigue test fatigue failure
1 Introduction
The composite rolling bearing has good wear resistance, corrosion resistance, heat resistance and dimensional stability, and can reduce vibration and noise, and has low cost, so it has been increasingly widely used in many industrial fields. However, the fatigue properties of composite materials are significantly different from those of metal materials [1]. Composite materials are sensitive to loading frequency and test temperature, and fatigue life measured data is more discrete, so it is obviously not suitable to use metal fatigue test methods for composite materials. In this paper, the fatigue test of composite rolling bearing is studied, and a simple and convenient test method is provided to ensure the safety and reliability of composite rolling bearing, and promote the application and development of composite rolling bearing in industry.
2 Experimental studies
2.1 Test Pieces and Test Equipment <br> 25 sets of 204 composite rolling bearings were used in the test, and the composite rolling bearing was made of nylon 66 with short glass fiber reinforcement. The production process used meltable alloy core injection molding [2].
The test equipment used JB-30 type rolling bearing fatigue tester.
2.2 Test Method <br> Bearing fatigue life is very discrete and mathematical statistics are used to process the data. This test uses a simple and convenient truncation test method.
Assume that the set of test bearings is n sets, in which the r sets of bearings have been destroyed and their life spans are L1, L2,..., Lr, respectively. The remaining (nr) sets of bearings have been tested for Lr+1, Lr+2,...Ln times, respectively, and have not been destroyed. At this point, no test can be performed and the estimated parameters of the original life distribution can be calculated using the experimental data obtained. Censored test method has two kinds of timing and fixed number, this test uses the method of censoring. The number of bearing sets r, also known as the number of truncations, should be given in advance to obtain the number of fatigue failures. After the number of truncations is tested, the test shall be stopped and the life of the non-fatigue-damaged bearing shall be greater than the value of the longest life of the fatigue-failed bearing. .
A radial load of 588N was applied to the 204-type composite rolling bearing and the oil was lubricated with a truncated number of 15 given. The speed of the testing machine is 12800r/min, and the test frequency and temperature environment of all test pieces are the same. In addition, for comparison purposes, 25 sets of 204 plastic bearings made of Nylon 66 were tested at the same time. The radial load was 392N and the oil was lubricated at 20#. The truncated number and tester rotation speed were the same as those of the composite bearings.
2.3 Processing test data <br> The study of fatigue life found that Weibull distribution is closer to fatigue failure than normal distribution. A large number of test results show that the life of the rolling bearing meets the two-parameter Weibull distribution. The failure probability of the bearing life L0 is expressed as a Weibull function [4]:
F(L0)=1-exp[-(L0/β)e]
Where L0 = 106 revolutions, e is the slope parameter, and β is the characteristic lifetime parameter. This paper uses the best linear invariant estimation method to estimate the parameters e and β [3], for composite bearings, e=5.756, β=4.467×106; for plastic bearings, e=3.02, β=7.104×106.
2.4 Monitoring of Fatigue Failure <br> This test uses a surface thermometer and uses temperature rise monitoring method and sound judging method to judge the fatigue failure of rolling bearing.
After the bearing operates, frictional heat is generated between the respective moving surfaces, and the temperature of the bearing gradually rises from the original temperature. The heat generated after the bearing operates for a certain period of time and the heat released are balanced, and the temperature remains unchanged. When the bearing experiences fatigue failure, the friction increases, the heat generated increases, and the temperature gradually increases. The temperature change during bearing operation is shown in Figure 1. T1 is room temperature, about 18°C; T2 is the temperature when the bearing runs normally, about 38°C; t1 is the initial running time, about 50min; t2 is the normal running time, about 150min.
Figure 1 Bearing operation and temperature changes
Fig.1 The effects of the time and the temperature on the bearing
In order to ensure the accuracy of the fatigue damage judgment, sound judging method is adopted at the same time. At the time of the test, the parts bearing the bearing were contacted with a sound-transmitting tool, and the sound change was observed when the bearing was rotated, and the bearing was judged whether or not to be destroyed based on the sound change before and after the bearing fatigue failure.
3 test results and analysis
3.1 Fatigue Life <br> Take the natural logarithm twice for equation (1) and, on double logarithmic coordinates, (1) becomes a linear equation. According to the test results, the fatigue failure probability diagram of Type 204 composite materials and engineering plastics rolling bearings is shown in Figure 2.
According to the international standard for fatigue life of bearings [5], 90% of bearings in a batch of bearings can reach and exceed the total number of revolutions (106 revolutions) before fatigue exfoliation.
It can be seen from Fig. 2 that for a failure probability of 10%, the rated fatigue life of a 204-type composite rolling bearing under a load of 588N is 3.1 × 106 rpm. The 204-type engineering plastic rolling bearing under the application of the load of 392N rated fatigue life of 2.2 × 106 rpm [6]. Obviously, the load bearing capacity of composite rolling bearings is increased by 50% compared to plastic bearings, and the fatigue life is still greatly improved.
Fig.2 Fatigue failure probability diagram of 204 type composite rolling bearing
Fig.2 The probability of fatigue failure of model
204 composite material rolling bearings
3.2 Fatigue Fracture Characteristics <br> According to the fatigue failure test of composite rolling bearing, the fatigue fracture was observed microscopically and four failure modes of composite rolling bearing were proposed.
3.2.1 Surface Fatigue Surface fatigue includes rolling contact fatigue, pitting, chipping, and exfoliation.
The rolling contact fatigue failure of rolling bearings is caused by repeated stress that exceeds the endurance strength of the material surface or subsurface. These repetitive stresses cause cracks near the raceway surface or subsurface of the outer ring of the composite material. The extended extension of these cracks causes the debris of the raceway material to peel off leaving pitting corrosion. The pitting phenomenon is determined by the surface contact stress and the number of cycles. As the stress or number of cycles increases, a phenomenon similar to that of pitting occurs. As shown in FIG. 3, the spalling and breaking has irregularities such as a large pit like a crater.
Figure 3 Flaking 10×
Fig.3 The sheet breaking-off
3.2.2 Plastic flow Plastic flow occurs when the contact stress between the ball and the raceway exceeds the endurance strength of the surface and subsurface and causes geometrical deformation of the bearing. Surface deformation is a form of plastic flow. Other plastic flows include cavities and smearing. Under heavy load and high speed conditions, the temperature of the bearing will become very high, causing overheating of the inner and outer rings, causing the plastic melting of the raceways and shoulders, as shown in Figure 4.
Figure 4 Plastic Melting of Raceways and Shoulders × 10
Fig.4 Plastic melting of roll way nest and shaft shoulders
3.2.3 Abrasion wear is the fact that a large number of bearing materials are ground uniformly or not uniformly from the contact surface. This type of failure is characterized by the presence of worn debris in the lubricating fluid and on the contact surfaces. It can cause ball and roll. The gap increases.
3.2.4 The associated failure wear may also occur between the bearing outer ring and the bearing fixture due to the relative movement between them.
For a set of bearings, the failure mode can be more than one, that is, two or more failures may occur at the same time.
4 Conclusions (1) In this paper, 25 sets of composite rolling bearings were selected for fatigue testing. Using the fixed-number censored test method, the linear invariant estimation method was used for data processing to estimate the Weibull distribution of the bearing life. The two parameters were simple and easy to implement.
(2) The 204-type composite rolling bearing with glass-reinforced nylon 66 has a rated fatigue life of 3.1×10 6 revolutions under a load of 588 N, a rotation speed of 12800 r/min, and oil lubrication, which proves the feasibility of the composite material to manufacture a rolling bearing.
(3) Fatigue failure of composite rolling bearing was observed, and four failure modes of surface fatigue, plastic flow, wear and related failure were proposed. It provides a valuable reference for the design, performance, service life and reliability of composite rolling bearings.
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