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Analysis of Causes of Fracture of Bolts between Ball Mill Feeder and Hollow Shaft
Sun Libo, Duan Yun
(Jinan Steel Group Corporation Zhangmatun iron ore, Jinan 250101, China)
Abstract : The reason for the breakage of the ball mill feeder and the hollow shaft coupling bolt is that the gap between the feeder and the hollow shaft is too large, and the sealing cone surface is destroyed by the slurry erosion. According to the actual production, the method of removing the broken bolt was found, the inspection time was shortened, and the measures for preventing the hollow shaft from feeding into the slurry were proposed.
Key words : ball mill; feeder; hollow shaft; coupling bolt;
CLC number : TD453 Document code : C article number : 1004-4620 (2000) 03-0017-02
1 question raised
Jinan Iron and Steel Group Corporation Zhangmayu Iron Mine (referred to as Zhangmayu Iron Mine) MQG2700×2100mm wet lattice ball mill was put into operation in January 1994. After replacing the feeder in April 1997, it only operated for 15 months, and there was a serious phenomenon of slurrying and slurry leakage at the joint surface of the ball mill feeder and the hollow shaft. The feeder and hollow shaft are connected by 12 M30×100 studs. When the bolts were tightened, two bolts were broken, and the next day, five bolts were found to be cut off during operation. When disassembling, none of the other five bolts can be smoothly unscrewed, and all of them are broken in the bolt holes.
2 Cause analysis
The ball mill is fed by a drum feeder and a hopper feeder, and the feeding portion is connected to the feeder by 12 M36 coupling bolts. During the process of dismantling the feeder, it was found that there was slurry in the cavity formed by the feeder and the hollow shaft, and the inner surface of the feeder and the inner surface of the hollow shaft were severely rubbed, and the clearance between the two was ØA and ØB. Large (as shown in Figure 1, 2). The 60° sealing cone between ØA and ØB is also seriously damaged due to the erosion of the slurry, which cannot function as a seal and support.
According to the above phenomenon, the analysis believes that the cause of the bolt breakage is that the sealing cone between the feeder and the hollow shaft is destroyed due to the slurry flushing, the feeder loses support at this end, and the load of the entire feeder and the feeding part is basically It is carried by 12 M30 connecting bolts to form a two-head cantilever structure with this as a fulcrum. Since the feeding part and the feeder itself have a self-weight of 3t, they have to bear a large load and an impact load of the shovel in the running process, and the assembly surface of the mounting surface ØC is a clearance fit, and the ØC axial direction The distance is small and the contact surface is small, so most of the above loads are carried by 12 coupling bolts. After a long-term operation of the bolt, fatigue occurs under the alternating load, and finally the fracture phenomenon occurs.
From the above analysis, it can be seen that the root of the problem is that the assembly gap between the feeder and the hollow shaft is too large, and the sealing cone surface is destroyed due to the erosion of the slurry, thereby causing the feeder to lose the support point near the outlet.
3 Treatment of broken bolts
 The method of removing the broken bolt is usually a method of welding the force shank, drilling a small hole in the broken bolt, tapping the reverse wire, drilling a large hole at the bolt hole, and re-tapping. According to the analysis of the site situation, it is considered that it is impossible to take out the broken bolts by the two methods of welding the force-applying handle or the counter-reverse wire; and if the method of re-tapping the large hole is adopted, the manual tapping can only be performed due to the limitation of the field conditions. The M36 bolt hole is difficult to be manually tapped, the speed is slow, and the construction period is long, which will seriously affect the production. To this end, the line was drilled at the broken bolt, and the center of the bolt was found. The Ø26 mm hole was drilled on the broken bolt by a 30-arm drilling machine, and then the M30 tap tapping method was used to successfully take out 12 Strip broken bolts.
划线 During the operation, the marking, alignment and determination of the center of the bolt are essential. Because the M30 bolt hole has a wire top size of Ø26.211mm, when drilling with a Ø26mm drill bit, there is only a gap of 0.1055mm around the hole. If it is slightly skewed, the bolt hole thread will be destroyed. However, if a Ø25mm drill bit is used, the operation will be difficult due to excessive tapping allowance.
With this method, each bolt takes about 30 to 40 minutes, and the speed is relatively fast, and the quality of inspection can be ensured.
4 preventive measures
From the above analysis of the causes, it can be seen that the root cause of the bolt breakage is that there is a problem in the size of the fit between the feeder and the hollow shaft. Therefore, in order to ensure the normal operation of the equipment, prolong the service life of the hollow shaft and prevent the occurrence of bolt breakage accidents, the following points should be noted when replacing the feeder:
(1) Ensure that the gap between ØA and ØB assembly is not too large.
(2) After the feeder and the sealing surface of the hollow shaft are tightly attached, the gap C between the hollow shaft and the end face of the feeder is left with a gap of 0.5 to 1.5 mm, so that after the hollow shaft sealing cone surface is worn and deformed, The feeder is axially moved by the fastening of the bolts to strengthen the seal between the two.
(3) For ball mills whose hollow shaft has been worn but still not used at the end of the scraping period, after replacing the feeder, the lead oil or the expanded rubber strip may be packed at the gap of the B portion to block the slurry passage. Conditional can also be used to hang glue on the surface to restore the assembly dimensional tolerance on the inner surface of the hollow shaft.