01 problems

Line 2 of a company’s raw material mill is FLSmidthATOX50 roller mill, which was put into use in July 2007. There are three rollers, each roller is equipped with a pull rod head, as shown in Figure 1. The A end is connected with the piston rod of the hydraulic cylinder through the stretching rod, and the lifting of the grinding roller is completed through the hydraulic system. The B end is connected with the mill shell through a torsion bar and a rubber buffer block to cushion the grinding roller. The mill has been running normally since it was put into use, and no cracking phenomenon has occurred at the head of the roller tie rod. On August 19, 2016, the mill stopped at the wrong peak. During the inspection of the raw material mill, it was found that the #1 tie rod head was located at the side of the stretch rod (i.e., the A end) and the root cracked, with A width of about 2mm and A spreading depth of about 170mm. It can be seen from Figure 1 that the crack runs through the whole section of the tie rod head, and it can no longer be used.

Fig. 1 Fracture of tie rod head

2 Causes analysis and solutions

2.1 Cause analysis

(1) the tensile rod is one of the key components of the raw material grinding, it through at the end of the high strength bolt connection to a rod head installed on the roller, the other end by high strength bolts connected with the hydraulic cylinder piston rod, main effect is made by high pressure gas station through the piston rod, enough pressure to the roller transmission, makes the material crushing and qualified target value of raw meal fineness in order to achieve. In July 2014, there was a tensile rod fracture accident, other parts were not harmed, and the production resumed after replacement. As Smith mill design, under normal use, the rod head is not a vulnerable part, so the cause of fracture is more accidental.

From the structure of the pull rod head in Figure 1, it can be found that the crack fracture originates from the connection between the pull rod head and the flange plate. This position belongs to the stress concentration area, and it is in the air channel where the stretch rod passes through the grinding shell, so it is not easy to seal. In the harsh working conditions in the grinding, the erosion is more serious. According to the drawing, the original width of the cross-section at this position is 160mm, and the field measurement when cracking is about 120mm. Although the wear amount is less than 1/3, the tensile strength has been reduced. Under the action of the external load alternating stress, the fatigue zone expands and cracks appear. If it is not found in time, the fracture occurring in the production process will lead to a major equipment accident.

(2) Based on the analysis of production and use, when the equipment was overhauled in January 2016, the grinding disc was surfaced. During the operation, the vibration of the mill was relatively large. According to the process requirements, the retaining ring is raised about 10mm, and the vibration value is reduced. In the later period, due to the shortage of sulfuric acid slag of iron correction raw materials, other iron correction raw materials were used to replace the ingredients. The larger particle size and poor wearability led to the aggravation of vibration and the superposition of many factors, which eventually led to the crack of the tie rod head.

2.2 Solutions

(1) Under normal circumstances, the tie rod head is not a vulnerable part, but considering the long supply cycle of spare parts for roller mill in the early stage, one spare part is reserved and replaced in time, without affecting the production of the kiln system.

(2) Check the other two roller heads and carry out ultrasonic flaw detection to prevent them in advance. Every year in the middle repair and overhaul, in a planned way to pull rod head, stretch rod, torque rod and other parts of the test, to ensure the stable operation of production.

(3) Check the air leakage point in time, especially the stretching rod from the grinding shell through the position, easy air leakage and material leakage, must be plugging treatment, in order to extend the service life of the components.

(4) In the central control operation, the method of increasing the feeding amount by pressure is not allowed to increase production, to ensure that the normal grinding pressure parameter is not more than 11MPa, and the vibration value of the mill is controlled below 2.0mm/s; Strictly control the feeding quantity to ensure the stability and progressiveness of the grinder material layer.

(5) Strictly control the particle size of the material, control the variety of the material, try to use sulfuric acid slag type iron correction raw materials, and ensure that the particle size of the grinding material is within the specified range.

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