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Abstract: The looseness and fracture of the high-strength connecting bolts between the thrust pad, its adjusting pad and the pad seat of an imported vertical mill reducer are not caused by factors such as material, manufacturing process and installation quality, but by cavitation caused by insufficient structural design. The cavitation phenomenon is completely due to the structural design problem at the root, that is, the outer edge of the thrust pad seat and the thrust pad mounting hole are too closely matched, and there is basically no gap. During maintenance, change the structure of thrust pad seat, increase the fit clearance between the outer edge of thrust pad seat and thrust pad mounting hole, and eliminate the preconditions for cavitation. After maintenance, the operation effect is good, and no abnormality is found after half a year of operation.
Key words: reducer; Thrust pad; cavitation
0 Introduction
As a grinding equipment, vertical mill is widely used in mines, thermal power and cement production. Vertical mill reducers at home and abroad are composed of box, planetary carrier, gear, shaft and bearin. At present, China has realized the localization of vertical mill reducer, and the design and manufacturing level has been recognized by the domestic and foreign engineering circles.
The internal transmission system of an imported large vertical mill reducer is divided into three stages: the first stage is bevel gear pair transmission, and the second and third stages are planetary epicyclic gear train transmission structure. After several generations of improved design and manufacturing, the reducer has become more and more reliable, but the loosening and fracture of high-strength connecting bolts between thrust pad, thrust pad adjusting pad and thrust pad seat have become a common phenomenon. This is the case with seven similar models used by a large domestic group. After many times of optimization design, the number of bolts was increased to 6, the diameter of bolts was increased to M24, and the performance grade was increased to 12.9. This situation has not been solved, which has brought great hidden dangers to the use of the vertical mill reducer. The original manufacturer sent a number of service engineers and experts to conduct field research, but has not given a reasonable explanation.
Our company received the maintenance task of the main reducer of the vertical mill in April 2020. After careful study, we found an effective way to solve the problem. This paper summarizes this.
1 Thrust pad surface height detection
Our company disassembled and maintained the main reducer of a company's imported raw material vertical mill in the middle of April 2020. The 12 thrust pads in the reducer of the vertical mill are distributed in a circle, and the distribution diagram is shown in Figure 1.
Fig. 1 Distribution diagram of thrust pad
Before reassembling the thrust pad, observe the contact marks on the alloy surface of 12 thrust pads, and determine that the height of the pad surface of the reducer is basically the same according to the contact marks on the pad surface. The structure diagram of thrust pad, thrust pad adjusting pad and thrust pad seat after installation is shown in Figure 2.
Fig. 2 Schematic diagram of thrust pad installation structure
Among the 12 thrust pads, the 9 # thrust pad seat has cracks. Because the manufacturing process of the original manufacturer is not clear, in order to verify whether the pad seat is deformed due to the release of internal stress due to cracks after the pad seat is cracked, the height of all thrust pad surfaces (i.e. the coplanarity of pad surfaces) has been preliminarily tested during assembly. Test method: use a 1500 mm vernier caliper to stick the two tile surfaces, two people press the two ends of the vernier caliper by hand, and the other person uses a 0.03 mm feeler gauge to test, but the gap is not plugged; For No. 9 bearing, we have tested it in both directions of No. 8 and No. 10 bearing, and no clearance has been measured. This detection method is easy to detect the gap with large height difference. If 0.03 mm feeler gauge cannot plug the gap, theoretically, the height difference should be within 0.02 mm.
During the oil pressure detection test, it was found that the high pressure oil pressure between the thrust pads was quite different. The clearance between the thrust pads and the output flange was detected again after the oil was stopped and drained. It was found that the feeler gauges of 0.20 mm, 0.20 mm, 0.25 mm and 0.40 mm could be inserted into No. 2, 3, 8 and 9 pads respectively, indicating that the surface of No. 2, 3, 8 and 9 pads was 0.10 mm, 0.10 mm, 0.125 mm and 0.20 mm lower under this condition. Then we dropped the grinding roller and tested it with a feeler gauge in turn. The result showed that there was no gap between all bearing surfaces and the output flange, indicating that in this case, the height of all bearing surfaces was basically the same.
2. Cause analysis of thrust pad seat damage
2.1 Cavitation phenomenon
Cavitation phenomenon refers to the instantaneous collapse of bubbles in the fluid when the pressure and velocity reach the critical value, which produces great impact force and high temperature at the moment of collapse. At the moment of bubble collapse, all the energy is concentrated on the rupture point, producing an impact force of several thousand newtons or more. The pressure of the impact force is up to more than 2000 MPa, which greatly exceeds the fatigue damage limit of most metal materials, Under the repeated action of this high impact force, the solid surface will undergo fatigue spalling, which will form small pits on the surface, and then develop into sponge blocks; At the same time, due to the high temperature generated at the moment of bubble collapse, a small amount of oxygen and other active gases entrained in the bubble will cause electrochemical corrosion on the metal surface, which will accelerate the destruction of cavitation.
Bubble formation in the fluid: When the pressure of the liquid at the contact with the solid surface is lower than its steam pressure, bubbles will form near the solid surface [4]. Cavitation usually occurs in the impeller of centrifugal pump (the speed and pressure at the inlet and outlet ends of the liquid change sharply) and the valve (the change of the flow passage area before and after the liquid flows through the orifice will lead to the sharp change of the speed and pressure), and rarely occurs in other cases.
2.2 Viscosity and pressure characteristics of lubricating oil
The viscosity-pressure characteristic of lubricating oil refers to the characteristic that the viscosity of lubricating oil changes with the change of pressure. When the pressure change is small, the viscosity change of lubricating oil is negligible; When the pressure change is greater than 20 MPa, the viscosity of lubricating oil will increase exponentially with the increase of pressure. Namely:
vp=v0ebp （1）
Where:
v0 - viscosity value at 105 Pa;
vp - viscosity value when the relative pressure is p;
b - coefficient related to oil;
p - medium pressure.
Because of the viscosity and pressure characteristics of lubricating oil, when the tooth surfaces of the driving and driven gears are smooth enough, the lubricating oil is enough to separate a pair of teeth from each other and will not crack itself, that is, a thin layer of high-pressure lubricating oil film is formed. Therefore, as long as the lubrication does not fail during the gear pair transmission, the wear of the gear tooth surface will naturally stop after it is polished into a mirror surface.
2.3 Cavitation of thrust pad, thrust pad seat and thrust pad adjustment pad
As shown in Figure 3, Figure 4 and Figure 5, the back of thrust pad, thrust pad seat and thrust pad adjustment pad are respectively shown, and Figure 6 is a typical cavitation phenomenon. According to the comparison of corrosion phenomena in Fig. 3 to Fig. 6, the corrosion phenomena in Fig. 3 to Fig. 5 are obviously cavitation corrosion phenomena just like those in Fig. 6. By comparing the shape of corrosion zone in Figure 4 and Figure 5, it is found that the shape of corrosion zone is completely consistent, which means that the damage of thrust pad seat and thrust pad adjustment pad in Figure 4 and Figure 5 is not caused by unqualified raw materials and manufacturing process, and the damage zone is consistent, which means that there is repeated impact force between their damage areas, and the impact force exceeds the fatigue limit of their materials.
Fig. 3 Rear view of thrust pad
Fig. 4 shows that the thrust pad seat has cracks, and the crack depth reaches about 20 mm. See Photo 7 for crack nondestructive inspection. The crack is basically located on the symmetrical centerline of the most severe damage areas, because the bending moment generated on the symmetrical centerline when the pressure is applied on both sides is large, and the material is easy to generate fatigue cracks from the stress concentration under continuous repeated action. There are many stress concentration factors, such as material defect, machining roughness, undercut, chamfer, arc, step transition, etc. Of course, even if there is no stress concentration factor when the pressure exceeds the fatigue limit of the material, fatigue fracture will occur over time. Therefore, the damage of thrust pad seat is not caused by material and manufacturing process.
Fig. 4 Thrust pad seat
Fig. 5 Thrust pad adjustment pad
The thrust pad adjustment pad is installed between the thrust pad and the thrust pad seat, and the outer edge of the thrust pad seat is closely matched with the thrust pad mounting hole. There is basically no gap. When we remove it, we use screws to force it out through the jackscrew hole.
During the use of the reducer, the entire thrust pad is immersed in oil, and a very narrow air gap is formed between the thrust pad and the thrust pad seat. When the pressure of the oil at the contact with the solid surface is lower than its steam pressure, a large number of bubbles are formed near the solid surface, which creates natural conditions for the occurrence of cavitation.
3. Cause analysis of the difference between the measured data before and after the roll drop
How is the difference between the measured data before and after the grinding roll? Through the previous analysis, we have made it clear that cavitation has occurred between the thrust pad, thrust pad adjustment pad and thrust pad seat.
Figure 6 Typical cavitation phenomenon
Figure 7 NDT for thrust pad seat
The pressure applied to the thrust pad before the grinding roll is caused by the gravity of the planetary carrier and output flange assembly (G1) plus the gravity of the grinding disc (G2). Due to the cavitation phenomenon between the thrust pad, the thrust pad adjusting pad and the thrust pad seat, the bubble has not reached the critical condition for the bubble to collapse under the action of G1+G2, and the bubble compresses the lubricating oil wrapping it under the action of G1+G2, The lubricating oil cannot be discharged in time due to the narrow tight gap formed between the thrust pad and the thrust pad seat, and because of the viscosity and pressure characteristics of the lubricating oil, the greater the pressure exerted by the bubble on the lubricating oil, the higher the viscosity of the lubricating oil will become, and the lubricating oil will never break. Therefore, under the effect of the bubble, the long thrust pad mounting bolt will cause elastic deformation and lift the output flange. Due to the different conditions of the 12 pads and the different degree of cavitation (during maintenance, observe the disassembled thrust pad, thrust pad adjusting pad and thrust pad seat, all of which have cavitation, but the degree of cavitation is different), the height of the output flange lifted at each point is also different, resulting in the gap between the thrust pad and the output flange.
After falling off the grinding roller, because the grinding roller gravity (G3) is far greater than G1+G2, the bubble finally reaches the critical condition for bubble collapse under the combined force of G1+G2+G3, so the bubble collapses, and the output flange falls on the thrust pad support surface. At this time, check whether there is a gap between the output flange and the thrust pad surface, and the measurement result is the true value.
The cavitation phenomenon of bubble generation and collapse is the root cause of the looseness and even fracture of the high-strength thrust pad mounting bolts.
4 Conclusion
To sum up, the looseness and fracture of high-strength connecting bolts between the thrust pad, thrust pad adjusting pad and thrust pad seat of an imported vertical mill reducer are not caused by factors such as material, manufacturing process and installation quality, but by cavitation caused by structural design deficiencies. The cavitation phenomenon is completely due to the structural design problem at the root, that is, the outer edge of the thrust pad seat and the thrust pad mounting hole are too closely matched, and there is basically no gap. During maintenance, change the structure of thrust pad seat, increase the fit clearance between the outer edge of thrust pad seat and thrust pad installation hole, and eliminate the preconditions for cavitation. After maintenance, the operation effect is good, and no abnormality is found after half a year of operation.
More about KYOCM Slewing Bearing ：
Rotary bearings consist of an inner ring and an outer ring, one of which usually contains a gear. Together with the connecting holes in the two rings, they enable optimized power transmission through simple and fast connections between adjacent machine parts. Bearing raceways are designed with rolling elements, cages or gaskets to accommodate loads acting individually or in combination in any direction.
Features and advantages:
High carrying capacity
High stiffness for rigid bearing applications
Low friction
Long service life
Surface protection and corrosion resistance
Integrate other features including:
Driving mechanism
Control device
Lubrication system
Monitoring system
Sealed cassette tape
https://www.kyocm.com/products/Slewing-Bearing/745.html
2023-01-29