Abstract: The finite element model of bearing system was established for the fracture accident of four-row tapered roller bearing cage of vertical roll mill in a plate rolling plant. A surface to surface contact pair is established by using a contact element at the important contact position between bearing elements. The deformation and stress distribution of the cage are obtained by simulation, the reason of the fracture of the bearing cage is found out, and the improvement suggestions for the design of this kind of bearing are put forward.

Key words: four-row tapered roller bearing; Vertical rolling mill; Cage; Finite element; Pulsating stress

Bearings are important supporting parts that bear loads in various machinery, often referred to as the "joint" of machinery. Rolling mill bearings must meet many requirements such as high speed, heavy load, large impact and harsh environment.

The four row tapered roller bearings used in the roll neck bearings of a cantilever vertical roll mill in a plate rolling plant stopped production for several consecutive times due to the fracture of the cage (see Figure 1), resulting in serious losses. In this paper, the finite element calculation of the bearing system is carried out, and the deformation and stress of the cage are emphatically analyzed.

The bearings used in the vertical rolling mill are non-standard four-row tapered roller bearings, which are composed of two double-row tapered roller bearings. Its basic structure is shown in Figure 2: it consists of two double raceway outer rings, four single raceway inner rings, rollers, isolation rings, pins and cages.

Four row tapered roller bearings can withstand combined radial and axial loads. Considering the symmetry of the bearing structure and the distribution of the load borne by the bearing (the radial load is distributed as cosine in the circumferential direction of 120° [1] and uniformly along the axial direction). In this paper, half of the bearing structure is analyzed, as shown in Figure 3.

2 Mesh division and connection mode between bearing elements

2.1 Grid Division

Three-dimensional solid units are selected and the grid is divided by the form of volume Sweep. Volume scanning can not only obtain regular hexahedral elements, but also control the node distribution of different parts' contact positions. This prepares the nodes for the next step of incorporating certain bearing elements.

2.2 Connection mode between bearing elements

A face-surface contact pair is established between the rolling element and the raceway, between the big end of the roller and the inner rim, and between the pin and the cage in the radial load area (60°). The connection between the pin and the rolling element and between the pin and the cage outside the radial load area is treated by the method of node merging.

3 Load and constraint

3.1 Load

FIG. 4 shows the force diagram of the axis. Where Fr is the radial force at the four-row tapered roller bearing studied, Fr = 2000kN; The tangent value of the Angle between the roll surface and the axis tanα = 0.0417; The diameter of the roll at the center of the slab D = 860.416mm; Distance L1 = 1052.5mm, L2 = 1697.5mm.

The moment of the left end is:

The axial load Fa = 2.73kN is obtained by combining the above three types.

In finite element analysis, the axial load is applied to the position where the bearing and the shoulder contact. The radial load is added to within 60° of the bearing inner ring surface, and is distributed as cosine in the circumferential direction ‚ and uniformly along the axial direction.

3.2 Constraints

According to the installation form and load of the bearing ‚ take the boundary conditions as follows:

Nodes on the outer surface of the outer ring: radial displacement constraint Ux = 0 and circumferential displacement constraint Uy = 0;

The node on the right side of the outer ring: axial displacement is about Uz = 0;

Symmetry constraint: node SYMM = 0 on symmetric surface [2].

The effect of bearing loading and restraint is shown in Figure 5.

4 Analysis of finite element calculation results

4.1 Deformation of cage

Affected by the axial bending deformation, the axial displacement of the cage should be opposite up and down. As shown in Figure 6, section Ⅰ parallel to the bearing end face is taken at half the width of the bearing. From the positive and negative displacement values of the cage, it can be seen that the upper end of all cages (bearing load area) is far away from section Ⅰ, while the lower end of the cage (bearing non-load area) is close to section Ⅰ. This indicates that the cage is tilted relative to section I.

Similarly, the radial displacement of the cage is not the same up and down. Therefore, especially under high speed and heavy load conditions, the deformation of the cage has a great impact on the stability of the entire bearing.

4.2 Stress distribution of cage

FIG. 7-9 shows the first principal stress cloud diagram, the third principal stress cloud diagram and the equivalent stress cloud diagram of the cage, respectively.

It can be seen from the above figures that the area around the pocket hole of cage is a stress concentration area, where the first principal stress S1 = 36.968MPa and the third principal stress S3 = -58.59MPa at the position of maximum stress. It can be seen that the stress state there is both tension and pressure, and the equivalent stress should be taken to check its strength. The equivalent stress Seqv = 84.6MPa. When the bearing is rotated 180°, the equivalent stress there is basically 0, so the cage is subject to the fluctuating cyclic stress of the cyclic characteristic r = 0 [3]. Due to the great impact load of the vertical rolling mill, the cage is subjected to great pulse cyclic stress around the hole, and the cage breaks at the hole due to fatigue.

5 Conclusions

1) In the past, it was considered that the cage was a non-stressed part that only rotated around the bearing axis. But through the calculation and analysis of this paper, it can be seen that the cage will be deformed and subjected to cyclic stress under the influence of shaft deformation and roller impact.

2) In the future, when designing bearings, it should be ensured that the bearing cage has sufficient strength.

【 Reference 】

[1] Wang Zhenhua. Practical bearing manual [M]. Beijing: Shanghai Scientific and Technical Literature Press ‚1996

[2] Zeng Pan. Finite element analysis and application [M]. Beijing: Tsinghua University Press, 2004

[3] Xu Hao et al. Mechanical Design Manual [M]. Beijing: China Machine Press, 1991