0 Introduction
The raw material grinding system put into use adopts two Lecher vertical mills, model LM35.4, equipped with a WPU-78E vertical reducer, with a power of 1600kW and a speed ratio of 32.04. The reducer adopts two-stage reduction, with the first stage being a spiral bevel and the second stage being a planetary gear transmission. The positioning bearing of the planetary carrier is a sliding bearing, and the planetary carrier shaft is connected to the grinding disc through a cylindrical pin to transmit power. The huge pressure and impact of the mill are borne by the sliding bearing thrust pad. The thrust pad consists of a total of 13 pieces, each with a high-pressure oil chamber. The pressure in the oil chamber is set with pressure measuring points, and the pressure is displayed on the on-site pressure gauge. At the same time, the pressure sensor signal is introduced into the Lecher control cabinet, and the electrical interlocking protection method is: the minimum pressure value of the oil chamber measurement point is 24bar. If the pressure values of the 13 measurement points are lower than 24bar at two adjacent points or three non adjacent points, the reducer will trip. Lubrication adopts the method of forced lubrication with thin oil. The thin oil pipeline is divided into four groups, with one group consisting of four channels and the other three groups consisting of three channels. In September 2012, the main reducer tripped three times. This article analyzes the causes of this accident and introduces the process and experience of accident elimination.
1 Fault occurrence and cause analysis
1.1 Occurrence of faults
Since the production of the raw material grinding system, the main reducer has been running stably with a low failure rate. On September 29, 2012, the main reducer tripped and stopped three times. Through the alarm display of the Lecher control cabinet, it was found that the pressure points P3.11, P3.14, and P3.15 of the bearing thrust pad had three alarm lights on, the two low pressure alarm lights on, and the three low pressure alarm lights on. Check the corresponding on-site pressure gauges, which are 53B, 53E, and 53F, and the pressures are all below 24bar. Afterwards, we replaced two new high-pressure oil pumps and connected the pressure connector at point 53G in parallel with an oil pipe to point 53F to increase the pressure at that point. However, after the main reducer was turned on, there was no significant change in the pressure at the three points, and the alarm could not be cleared. The main reducer tripped again. By lowering the pressure alarm value of the thrust pad pressure point to 20bar, the main reducer can only operate. On October 12, 2012, the WPU-78E vertical reducer supplier engineer arrived at the site and opened the observation hole door of the main reducer to check the meshing and lubrication of the high-speed shaft gear. It was found that there was a broken and missing gear at the front end of the high-speed shaft, and one of the three lubricating oil holes at the front end of the high-speed shaft was not lubricated smoothly. After inspecting the pressure of 13 bearing thrust pads, it was found that the pressure was generally low and could not meet the pressure requirements for normal operation.
1.2 Fault Causes
The inspection shows that the reason for the generally low pressure of the thrust pad of the sliding bearing is due to excessive wear on the Babbitt alloy layer of the thrust pad, and there is lateral leakage between the high-pressure oil chamber and the grinding disc, resulting in the pressure not meeting the normal working pressure requirements. Due to the connection between the grinding disc of the mill and the planetary frame through cylindrical pins, the planetary frame bears the weight and grinding pressure of the grinding disc. Therefore, the thrust pad in the planetary frame absorbs the axial load generated during the grinding process. Due to the low rotation speed of the vertical grinding mill, which is 32.04r/min, the lubrication of the thrust pad can be set to static pressure lubrication, that is, before the mill starts, high-pressure oil is injected into the thrust pad, and the pressure oil is used to lift the grinding disc and bear the rolling load of the grinding disc. There are a total of 13 evenly distributed thrust pads, each of which is provided with an equal amount of oil. This forms a certain thickness of oil film between the thrust pad and the grinding disc, which can evenly and stably support the grinding disc, bear the weight and grinding pressure of the grinding disc. Due to the instability of the material layer of the mill, excessive vibration of the mill feed iron, and eccentric loading of the grinding disc, the thrust pad can bear excessive or uneven axial force of the grinding disc, resulting in excessive wear of the babbitt alloy layer. When the Babbitt alloy layer of the thrust pad wears too much, the oil film between the high-pressure oil chamber and the grinding disc will be damaged, and some high-pressure oil will leak out from the side, making the pressure unstable. Due to the inconsistent wear amount of each Babbitt alloy layer, the thickness of the oil film formed, and the decrease in feedback pressure, it is impossible to obtain uniform support below the grinding disc, resulting in eccentric loading of the grinding disc, accelerating the wear of the Babbitt alloy layer of the thrust pad. In severe cases, it can also damage the main layer of the thrust pad, causing the pressure to gradually decrease and unable to meet normal operation (see Figure 1).
Figure 1 Schematic diagram of Babbitt alloy layer and grinding disc of thrust pad
2 Repair process
The solution provided by the WPU-78E vertical reducer supplier is to replace the high-speed shaft and bevel gear, and replace 13 bearing thrust pads. The tricky thing is that their supply cycle takes more than six months, which cannot meet our company's requirements. We are seeking domestic manufacturers to repair the thrust pads. On February 16, 2013, our company dismantled the gearbox and sent it to a professional gearbox manufacturer for disassembly. After removing 13 thrust pads, it was found that the Babbitt alloy layer on each pad was worn, with some pads experiencing localized wear of up to 3mm and requiring repair (see Figure 2).
Figure 2 Severely worn thrust pads
On February 18th, 13 thrust pads were sent to manufacturers specializing in centrifugal casting alloy pads for repair. Firstly, put the thrust pad on the lathe, remove the original babbitt alloy layer, and turn it to the contact surface between the babbitt alloy and the body; The second step is to put the turned thrust pad into the tin melting furnace. When the temperature reaches 220 ℃, the original tin alloy can be melted away; The third step is to hang the thrust pad twice in the tin melting furnace; Step 4: Lift the thrust pad out of the tin melting furnace and place it flat. Fix it with a wooden template around it, and place a 2mm asbestos gasket inside the template to ensure full coverage of the babbitt alloy layer. The pouring temperature reaches 280 ℃, the pouring thickness reaches 15mm, and the thickened Babbitt alloy melt drives out the bottom pores to ensure the continuity and integrity of the Babbitt alloy layer; Step 5: Roughly turn the thrust pad until the thickness of the Babbitt alloy layer is 6mm, and conduct magnetic testing to check for slag inclusion and cracks; Step 6: After passing the magnetic inspection of the babbitt alloy layer, perform precision turning and turning until the thickness of the babbitt alloy layer reaches 5mm; Step 7: Mill the thrust pad into the oil groove and oil gap on the milling machine. The oil groove is opened on the side of the thrust pad, with a width of 10mm and a depth of 1mm. The oil gap is opened at the upper oil injection hole of the babbitt alloy layer, with a depth of 0.25mm. Finally, finely grind the thrust pad on the grinder to ensure a size of 140.2mm (see Figure 3).
Figure 3 Repaired Thrust Pad
On March 3rd, after the main reducer was assembled step by step from bottom to top, 13 thrust pads were lifted in and evenly coated with red lead powder on the Babbitt alloy layer. The pads were lifted into the grinding disc, and the high-speed shaft was turned clockwise to rotate the grinding disc for more than one cycle. The grinding disc was then lifted out to check the contact between the Babbitt alloy layer and the grinding disc. Through three adjustments of the pads, all 13 thrust pads were in continuous contact with the grinding disc.
On March 8th, the on-site assembly of our company's main reducer was completed. After starting up and running, it was found that the pressure of 13 thrust pads generally increased, with the highest value being 60bar and the lowest value being 30bar, all above the pressure alarm value. We reset the pressure alarm value of the thrust pad pressure point back to 24bar, and the main reducer ran smoothly without any low pressure alarm and the Lecher mill tripped.
3 Usage suggestions
Due to the fact that the thrust pad of the main reducer of the vertical mill is a cast body, the pad body is made of 35 # steel, the surface alloy layer is a babbitt alloy layer, and the material is zchsnsb11-6. In order to minimize the wear of the babbitt alloy layer and maintain a long service life as much as possible, we recommend:
Firstly, check the temperature of the oil tank in the thin oil station of the main reducer. When it is below 20 ℃, it is necessary to turn on the heater and intermittently start the low-pressure oil pump for thermal circulation. The outlet pressure of the low-pressure oil pump needs to be higher than 2.5 bar, and the differential pressure signal on the oil filter should be monitored. Only under normal conditions can the entire system of the main reducer, including the thrust pad, be fully lubricated and guaranteed.
Secondly, before the main reducer operates, it is necessary to turn on four high-pressure oil pumps for more than 10 minutes to form a stable and equally thick oil film between the thrust pad and the grinding disc, which can effectively extend the service life of the Babbitt alloy layer of the thrust pad.
Thirdly, during the operation of the main reducer, check the lubrication of the thrust pads. An oil injection pipe is arranged between the 13 thrust pads to immerse the pad surface in lubricating oil, and the position of the overflow valve is set above the pad surface to ensure sufficient cooling of the thrust pads. It is stipulated that the oil temperature of the thrust pad during operation should be between 25 and 75 ℃, and should not exceed 85 ℃. If it exceeds this temperature, it should be stopped and inspected in a timely manner.
Fourthly, during the operation of the vertical grinding mill, the operation of the mill should be done well to minimize the vibration of the mill. The main reducer of the mill should operate smoothly, the oil film between the thrust pad and the grinding disc should be stable, and the grinding disc should be evenly stressed without any bias load, which can better protect the babbitt alloy layer.
4 Conclusion
The thrust pad of the main reducer is an imported component with a purchase price of 260000 yuan and a supply cycle of more than 6 months. By contacting the manufacturer for repair, the repair cost was less than 50000 yuan and the repair cycle was 20 days. Not only did it save costs and time, but it also achieved the expected repair effect, making it easier to obtain after-sales service and technical support. During the operation of the vertical mill, strengthen equipment inspection, observe the oil level and lubrication of the main reducer, as well as the temperature and pressure of the thrust pad. During operation, minimize the vibration of the vertical mill as much as possible. These factors can effectively protect the Babbitt alloy layer of the sliding bearing thrust pad and should not be ignored.
More about Marginal Material Specification Sheet–MG-6:
Material specification sheet – MG-6 is Ball and roller bearing steel according to EN ISO 683-17. Ball and roller bearing steel for balls and rollers of any dimension,rings and discs up to 30mm effective thickness.
