Abstract: using the self-designed radial fretting wear test system for car hub bearings, three groups of hub bearings were tested. Macroscopic strip wear traces appeared on the test bearing raceway, continuous micro holes appeared at about 20m of the bearing sub surface, and micro cracks approximately parallel to the raceway appeared on the bearing sub surface with severe fretting wear. At the same time, the internal 3D stress distribution of hub bearing under radial load is simulated. The results show that the amplitude fluctuation of strip stress / strain peak appears in the load area of hub bearing. On this basis, a generalized composite fretting wear mechanism model of wheel hub bearing is proposed based on the load-bearing characteristics of wheel hub bearing when the car is running. The model provides a new way to reduce the fretting wear of hub bearings and improve the reliability of bearings.
Key words: hub bearing, fretting wear, stress distribution, automotive, mechanism model
0. Preface
Fretting wear is a wear phenomenon caused by small amplitude relative motion between components which are closely fixed and matched with each other under the action of structural vibration or alternating stress. According to different motion modes, fretting wear can be divided into translational fretting, rolling fretting, radial fretting and torsional fretting. At present, most of the research work reported in the literature focuses on translational fretting. However, there are few reports on the research work related to the last three fretting forms, and the Composite Fretting formed by more than two fretting forms is more complex, so there are fewer research work in this field. Fretting wear is one of the main failure forms of car hub bearings. In this paper, a radial fretting wear test system of hub bearings is designed to carry out fretting wear tests of three pairs of hub bearings. At the same time, the FEM mechanical model of hub bearing system is applied with radial load under test conditions to simulate the 3D stress distribution of hub bearings under radial load, and the mechanical relationship between the variation of radial load amplitude and fretting wear is discussed. On this basis, combined with the load-bearing characteristics of the wheel hub bearing under typical driving conditions of the car, taking the rolling element in the center of the wheel hub bearing load-bearing area as an example, the mechanism models of the radial, translational and rolling Generalized Composite Fretting Wear of the wheel hub bearing are established. The model will provide a new idea and approach for the further study of fretting wear resistance.
1. Wheel hub bearing radial fretting wear test
1.1 Test system
The principle of the hub bearing radial fretting wear test system is shown in Figure 1: the front hub bearing suspension is connected with the fixed support, the motor drives the hub bearing to rotate, the vertical loading actuator moves up and down under the control of the hydraulic system, and drives two backup rollers to apply radial static load and dynamic load to the hub bearing at the same time. Figure 2 shows the fretting wear test system device.
Fig. 1 Schematic diagram of fretting wear test
Fig. 2 fretting wear test system device
1.2 Test conditions
According to the actual load-bearing characteristics of the car hub bearing, the radial fretting wear test is divided into two stages: ① simulating the radial load only borne by the hub bearing in the passive transportation process of the car. At this stage, the hub bearing does not rotate. The hydraulic system of the actuator applies a vertical static load of 5kN (simulating the vehicle weight, etc.) and a dynamic load of 1kN (simulating the vibration load of the hub bearing during the transportation of the car). The loading frequency of the actuator is 5Hz and the actuator vibrates continuously for 1.5 million times. ② Simulate the actual radial load on the hub bearing during the operation of the car. The motor drives three pairs of hub bearings to rotate 600000 rpm, 800000 rpm and 1million rpm at the working speed of 600r / min. while the hub bearing rotates, the actuator applies a vertical static load of 5kN and a dynamic load of 1kn to the hub bearing at the loading frequency of 5Hz.
The dynamic load of the test system is controlled by RS system. The dynamic load part is sinusoidal. The loading frequency of the actuator avoids the natural frequency of the front axle assembly of 15 ~ 20Hz. The fretting wear failure of hub bearing is detected by measuring the outer ring temperature of bearing the outer ring temperature measured by thermocouple exceeds 60. C, the test will stop automatically) and assist in analyzing the power spectrum of wheel hub vibration signal for discrimination.
1.3 Test results
The bearing area of the disassembled test hub bearing showed slight strip wear marks. The macroscopic failure mode is shown as a slight indentation with equal spacing on the local raceway, and the indentation spacing is consistent with the rolling element spacing along the bearing circumference.
The samples of micro failure form are sampled from the inner and outer race raceways of the hub bearing with fretting wear. The surface morphology and cross-section SEM of fretting wear are shown in Fig.3. Fig. 3a shows the initial stage of fretting wear of hub bearing. Adhesion, tearing and wear debris with a length of less than 20GM appeared on the fretting surface; Fig. 3b shows the microscopic characteristics of the cross section of the wheel hub bearing at the initial stage of fretting wear. There are continuous holes at about 20 ~ 30um of the sub surface layer of the bearing, which is related to the dislocation of the sub surface layer; Fig. 3C shows the development stage of fretting wear of hub bearing. At about 20 ~ 30p.m of the sub surface layer of the bearing, cracks with main directions roughly parallel to the bearing surface appear. Fig. 3D shows the serious stage of fretting wear of hub bearing. Cracks approximately parallel to the bearing surface appear micro-cracks extending to the bearing surface. Micro-cracks cause the bearing to produce falling bodies, which will further promote the secondary surface micro-cracks to expand to the bearing surface. Eventually, severe fretting wear, i.e. indentation, occurs.
Fig. 3 SEM of fretting wear
2. 3D stress distribution of hub bearing under radial load
The FEM mechanical model of the front wheel hub bearing system of the test car established in reference is used to simulate the mechanical conditions of the radial fretting wear test (the upper end of the front suspension is subject to rigid constraints, the inner, outer ring and rolling element of the bearing adopt relative constraints and contact elements, and the radial outward load is 6kN). The calculated 3D stress distribution in the bearing area of the inner and outer ring of the hub bearing is shown in Figure 4. The abscissa and ordinate in the figure respectively represent the bearing area of the inner (outer) ring of the hub bearing (about 150% of the bearing envelope angle) Circumferential and axial length of (mm); The stress distribution coordinate axis represents the stress amplitude (MPA) per unit area of the inner (outer) circle load area.
Fig. 4 three dimensional stress distribution of inner and outer ring of front hub bearing
It can be seen from the figure that the 3D stress distribution in the bearing area of the inner (outer) ring of the hub bearing appears a strip-shaped stress peak, and the position corresponds to the rolling element. The amplitude of the stress peak in the contact area between the rolling element and the inner and outer ring is different, and the amount of elastic deformation is also different. In the process of car driving, the hub bearing will bear both vertical static load and dynamic load due to the influence of ground roughness. Under the action of vertical dynamic load, the radial elastic deformation of the rolling element and the raceway of the inner (outer) ring will change repeatedly, resulting in micro radial fretting between the rolling element and the raceway. This is the internal reason for the inevitable radial fretting wear of car hub bearings.
3. Generalized Composite Fretting Wear Mechanism of hub bearing
3.1 Analysis of Composite Fretting Wear of hub bearing
In the process of passive transportation, the wheel hub bearing bears vertical load, and the rolling element of the wheel hub bearing will have radial fretting; When turning or swinging left and right, the hub bearing will bear the lateral load / bending moment, and the rolling element will have translational fretting relative to the inner (outer) ring; When accelerating or decelerating, the hub bearing also bears the longitudinal load at the same time. The rolling element has a rotation trend relative to the inner (outer) ring raceway, and rolling fretting will occur between the rolling element and raceway. The Composite Fretting during car transportation will first cause fretting wear marks or pre cracks of bearings.
When it is transported to the place of sale, when the car is running, each time the rolling element passes through a fretting stripe or pre crack area, it will produce an impact or rolling on the fretting wear area. After several times of impact or rolling, the micro cracks on the sub surface of the bearing will extend to the surface, causing local particles to fall off, which will induce more serious fretting wear and eventually lead to fatigue failure. It can be seen that avoiding fretting or reducing fretting wear amplitude during car transportation is the premise to avoid serious failure of hub bearing.
3.2 Generalized Composite Fretting Wear Mechanism Model
Based on the radial fretting wear test of hub bearing and the 3D stress simulation results of the bearing under the test conditions, the radial fretting wear mechanism model of hub bearing is established. The mechanism of axial fretting wear of hub bearings is discussed in detail in reference. Combined with the fretting type of the wheel hub bearing under the actual driving conditions of the car, this paper takes the rolling element in the bearing center of the wheel hub bearing as an example, and puts forward the Generalized Composite Fretting Wear Mechanism Model of the wheel hub bearing, as shown in Figure 5. The mechanism model provides a basis for further discussion on the quantitative influence of the frequency, amplitude and various load ratios of the composite load on the fretting wear degree of the hub bearing.
Fig. 5 fretting wear mechanism model of hub bearing
3.3 Approach to anti fretting wear of hub bearing
It is an important factor to improve the reliability of hub bearing to restrain the compound fretting or reduce the Fretting Amplitude of hub bearing in the process of car transportation. Therefore, the way of anti-fretting wear of hub bearing is to quantitatively study the contribution of composite load amplitude and frequency to the degree of fretting wear, and finally put forward the anti-fretting wear measures of hub bearing from the specific links of design, manufacturing and assembly.
4. Conclusion
The radial fretting wear test shows that there are strip-shaped slight wear marks on the local raceway of the hub bearing, and there are continuous micro holes or pre cracks on the sub surface of the worn bearing, which are approximately parallel to the bearing surface. Combined with the simulation results of 3D stress distribution of hub bearing under experimental stress conditions, the radial fretting wear mechanism of hub bearing is proposed. On this basis, the actual stress condition of the hub bearing in the transportation and driving stages of the car is analyzed, and the Generalized Composite Fretting Wear Mechanism Model of the hub bearing is proposed. The model will provide a new idea and approach for further study on the anti-fretting wear mechanism of hub bearings.
More about UNIM Wheel Hub Bearing：
The wheel bearing is a round metal part found in the center of the hub that connects the axles to the wheels and helps them turn smoothly. They usually have greased metal balls encased between two rings called races.
Wheel Hub Bearings Also known as 2nd or 3rd generation wheel bearing, it’s designed to allow rear wheels, especially in passenger cars, to turn on an axle even under heavy loads. Apart from having components such as the wheel bearing, flange, and anti-lock braking system (ABS), it has built-in sensors that allow location tracking as well as speed monitoring via global positioning system (GPS).
2022-07-04
