Abstract: In this paper, the axial hydrostatic bearing, the key supporting part of the permanent magnet direct drive motor for the raw material vertical mill, is optimized. Through the analysis of the actual force on the thrust pad of the mill, the targeted design optimization is carried out, and the lubrication system is also improved accordingly. Through reasonable test verification, reliable data is obtained. The innovative design of the thrust bearing enhances the bearing capacity and reduces the product cost, It is beneficial to the cost reduction and promotion of permanent magnet direct drive motor.

The actual transmission power of the traditional asynchronous drive system of the vertical mill is low, and the gear of the reducer is easy to break under the actual bad working conditions. Especially in the peak period of cement production, once the reducer is damaged, it will have a huge impact on the production line, and the equipment maintenance cycle is long. For this reason, each manufacturer has a reducer, which also causes the increase of the production cost of the cement plant.

In view of this situation, our company has developed a high-power direct drive motor for vertical mill, which is driven by the perfect harmonic free frequency converter from foreign manufacturers. It has the characteristics of low speed and large torque. Since there is no multi-level gear reducer, the transmission efficiency is improved. In addition, due to the existence of the frequency converter, the system can adjust the speed by floating 10% of the rated speed up and down according to the material and grinding state, so as to obtain the best operating speed. In fact, a lot of speed adjustment tests have been done on the applied equipment in operation, which has achieved an increase of about 8% in production, with obvious results. The maximum applied power of vertical permanent magnet direct drive motor is 2600kW. In actual production, there is a problem of high cost. Therefore, the product has been optimized and upgraded. This paper mainly introduces the optimization and improvement of hydrostatic thrust bearing.

1 Stress analysis

The grinding power of the vertical mill comes from the press roll. The force is transferred to the material through the press roll, and then the material is transferred to the grinding plate. Finally, it is transferred to the vertical mill base and the ground through the direct drive motor. As one of the supporting links, the direct drive motor bears great static and dynamic pressure. The force on the direct drive motor is transmitted from the top mirror plate to the support shell through the hydrostatic thrust pad, as shown in Figure 1. The existing hydrostatic thrust bearing uses the structure of high rigidity mirror plate and 12~16 uniformly distributed Wujin thrust pads, which is the same as the conventional vertical reducer. However, considering that the force transmission of the roller of the vertical mill follows the principle of the shortest path, most of the force is often directly transferred to the ground from several points near the roller, and the actual effect of some thrust pads is not obvious.

Fig. 1 Schematic Diagram of Raw Mill and Direct Drive Motor

2 Optimization scheme

Based on this, the force situation and distribution of the whole bearing structure are analyzed. Through calculation, the direct drive motor with large and small thrust pads distribution structure is used for force transmission on the raw meal vertical mill, and the entire oil supply system is improved. First of all, the raw meal vertical mill is a two roll structure, and the pressure of a single roller is 1200kN. Therefore, according to the stress situation, three large thrust pads are set directly below the roller, and a total of 12 small thrust pads are used in other areas. See Figure 2 for the specific layout. The upper part of the six large pads in the dotted line box is two pressure rollers. At the same time, according to the change of the thrust pad area, adjust the lubricating oil flow of each circuit to ensure the floating distance of the mirror plate, so as to form an effective oil film between the thrust pad and the mirror plate.

3 Effect

The optimized permanent magnet direct drive motor is used in carbide slag vertical mill. The rated power of the whole equipment is 800kW, the rotating speed is 33.5r/min, the static load carried is 2400kN, and the dynamic pressure is between 2 and 3 times. As it is the innovation and improvement of key functional parts of key equipment, a series of tests were carried out. After the direct drive motor is installed at the site, the floating test of the mirror plate under the static state of the whole equipment (the grinding roller is not pressurized or pressurized) was conducted. Starting from the lower thrust pad of the roller, a total of four evenly distributed points were tested. The floating data are shown in Table 1.

Fig. 2 Layout of thrust pad

It can be seen from Table 1 that the buoyancy value of thrust bearing is reliable and can meet the load operation of the whole system. The vertical mill starts no-load and gradually loaded commissioning, during which the temperature of thrust pad and the oil supply pressure change of the entire oil circuit system shall be mainly concerned. See Table 2 for specific values.

Table 2 Temperature and Oil Pressure Values of Bearing under Load

It can be seen from Table 2 that the temperature of the thrust pad is basically maintained below 50 ℃, the oil pressure is also stable at about 9MPa, and the temperature rise and oil pressure between the large and small thrust pads change little, so the application data is reliable.

4 Conclusion

The optimization and improvement reduced the cost of the thrust bearing. Compared with the original direct drive motor thrust bearing, the weight of the thrust pad decreased by about 24%, which not only guaranteed the reliability, but also effectively reduced the cost of maintenance and replacement of the thrust pad. In terms of energy consumption, as the sliding characteristics of the entire bearing system have not changed, the energy consumption has not changed much. However, due to the reduction of the total area of the thrust pad, the oil consumption of the hydrostatic oil circuit system has decreased, and the single oil supply flow of the small pad has decreased by up to 4%. The improved direct drive motor has been applied to carbide slag grinding since 2016. Up to now, the equipment is in good condition and has achieved the expected effect.

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