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Cause Analysis and treatment measures of crack in end cover of Ball Mill

Date:2019-07-08 13:43 Source:未知Views:

The key components of the ball mill are the rotary body containing the material, which is composed of the feed part, the cylinder part and the discharge part, and the end cover is the key part in the inlet and outlet parts, which is divided into the feed end cover and the discharge end cover.

After one year of operation of an overflow ball mill, cracks appear at the root of the end cover, and the on-site shutdown treatment has caused economic losses to the users. The site crack position is shown in figure 1, and the crack on the drawing is shown in figure 2. The crack position is located near the intersection of the end cover support and the conical end cover.

1. Causes of cracks

 

1)Manufacturing process.


When the discharge end cover of the ball mill generates a crack, the defect of the influence strength of the casting itself is analyzed in consideration of the reason of the production cycle at that time.

Therefore, through the vertical lathe, the end cover of the crack is removed from the surface, and the circular layer by layer cutting is carried out along the crack to detect whether there are defects such as slag inclusion, shrinkage loosening or crack in the casting until it is completely permeable, and no defects affecting the crack source and strength are found.

In the process of casting, the wall thickness and thickness of the end cover are about 10mm thinner than the design size of the drawing, which is one of the main reasons for the crack.

2) aspects of the installation process


During the operation, the grinding machine can bear the circulation impact and the pressing action of the material and the grinding medium, and the rotating part of the grinding machine shall not bear the extra additional force during the operation, and the degree of coaxiality between the end cover and the cylinder body and between the end cover and the bearing seat must meet the technical parameter requirements.

The size deviation of casting is the passive cause of cracks, and for the working condition of mill, the installation accuracy is the key reason for the change of stress state. Therefore, the installation accuracy and the coaxiality of the rotating part are not enough, which leads to the excessive shear force of the end cover in the process of operation, which may be another reason for the crack.

3) use and maintenance aspects


In addition to manufacturing and installation problems, use and maintenance is often one of the main causes of site quality problems, such as overload operation, foundation loosening, bolt loosening, maintenance and maintenance is not timely and so on.
 

2. Manufacturing analysis

1) improvement on end cover casting


In the process of sand molding, exhaust material is placed at R to reduce the overheating of molten steel at R and the collapsibility of molding sand. Concentrate steelmaking is used to ensure the internal quality of molten steel and reduce the internal defects of castings. The heat treatment should adjust the heat preservation process to ensure the grain refinement and the strength of the castings to meet the technical requirements.

The flange size of the end cover is large, the casting shrinkage is easy to deform, the processing should be meticulous, the non-eccentricity should be guaranteed, and the R dimension should be within the drawing range.

By purchasing the flaw detection equipment, the process suitable for crack detection, that is, the local ultrasonic flaw detection process at R, is added to the original detection process (magnetic particle flaw detection) to ensure the internal quality around the R area of the end cover.

2) strengthening process inspection


In order to ensure the smooth production process, strictly in accordance with the requirements of the technical agreement, the relevant machine addition, special process, flaw detection, painting and packaging related process documents.

3. Installation analysis


Rectification on installation:when replacing the end cover, the coaxiality of the rotating part of the mill, the coaxiality of the rotation axis and the backlash on the tooth side will be comprehensively detected and adjusted.

As shown in FIG.3, the end cover, the cylinder and the ring gear are combined to form a rotary part of the mill, the axis of which is defined as the rotation center line of the cylinder body, the pinion device, the speed reducer and the motor form the transmission part of the grinding machine, and the axis of which is defined as the center line of the transmission shaft.

How to make the rotating center line of the cylinder parallel to the transmission center line, and how to make the horizontal projection distance meet the drawing requirements is the key to the normal operation of the mill. In order to make the above two axes meet the requirements, whether their individual axis coaxiality meets the requirements is the first factor. That is, the axial coaxiality of the feed end cover, the cylinder body and the discharge end cover. Coaxiality of pinion device connected to reducer.

1) to meet the parameters and requirements of the coaxial axis of rotation.


The machining dimensional tolerances and shape and position tolerances of the end cover meet the design drawing requirements; the cylinder machining dimensional tolerances and shape and position tolerances meet the design drawing requirements; the large gear machining dimensional tolerances and shape and position tolerances meet the design drawing requirements; the machining dimensional tolerances and shape and position tolerances of the workpiece fit the positioning finishing holes to meet the design drawings and processing process requirements. In the installation process, the correction accuracy of the spindle bearing bottom plate meets the requirements of the installation instruction manual and related standards and specifications; the installation level of the spindle bearing bottom plate should not be 0.20 ≤ 1000; the relative elevation deviation of the two chassis should not be more than 0.5 mm, and the feed end should be higher than the discharge end.

For the detection and requirement of the coaxial axis of the rotating axis, the axis of the two hollow shafts should theoretically be measured indirectly on the end surface of the spindle bearing with a percentile meter on the same straight line.

All the dimensions involved above can be measured to meet the requirements of design drawings and processing technology, and the rotation axis is theoretically determined to be qualified.

2) to meet the parameters and requirements of coaxial transmission axis.


The coaxiality of the drive shaft and the motor shaft (or the speed reducer shaft) shall not be greater than 0.3 mm; the installation level shall not be greater than 1.0/1000 and shall be consistent with the direction of inclination of the mill. The horizontal error of the pinion plate, the speed reducer and the motor installation meeting the plane is less than 0.06 mm. And the coaxial degree of the transmission shaft line can be determined to meet the design requirement.

To meet the parameters and requirements of the rotating axis of the cylinder parallel to the transmission axis:

As shown in figure 4, the meshing layout diagram of the ball mill size gear set is as follows: serial number 1 is the center distance of the gear pair, sequence number 2 is the horizontal elevation difference between the two axes, and the serial number 3 is the horizontal projection size of the two axis. These three lines theoretically form a closed triangle, which determines the parallelism of the two axis lines and affects the normal operation of the mill.

The parallelism deviation between the transmission axis and the rotating axis of the cylinder should not be greater than 0.15mm per meter. The detection method is based on the tooth side clearance of gear meshing, and Table 2 is the requirement of tooth side clearance.

Along the direction of tooth width, the clearance between the two ends of the tooth side is adjusted and detected according to the requirement that it is not larger than 0.15mm per meter.

4. Maintenance analysis


In addition to manufacturing and installation problems, it is necessary to strictly control the overload operation in use and maintenance, often check whether the foundation and bolt control are loose, overhaul and maintenance in time, and so on.

To sum up, the machining quality, installation accuracy and field adjustment of the workpiece are the key to the normal operation of the ball mill. In order to ensure the manufacturing quality of the end cover, strengthen the testing methods and means, ensure the installation and maintenance in place, in order to ensure the smooth operation of the field equipment, the process control is carried out from the aspects of processing, installation and maintenance.

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