Abstract: According to the structural characteristics of rod mill and rod and wire production technology, the reasons of rolling mill burning bearing were studied from the aspects of rolling mill assembly and production control, and the control measures of lubrication, assembly and process were formulated. Through production practice, the failure of burning bearing was reduced, the downtime was reduced, and the production operation rate was increased.

Key words: bar; Burning bearing; Assemble; technology

1 Introduction

The special steel bar wire rolling mill is a short stress wire rolling mill structure, and the rolling mill bearings are axial thrust bearings and radial bearings. Cylindrical roller bearings are usually used to bear radial loads, and tapered roller bearings of thrust design bear axial loads. Most of the bearings are lubricated and cooled by means of oil and gas lubrication or oil mist lubrication. During the daily assembly, the quality of bearing installation and maintenance is not in place, which leads to the burning of bearings after the rolling mill goes online. During the rolling process, the working environment of the rolling mill bearing is harsh, the temperature is high and the dust is large. Some varieties of rolling load, bearing seat vibration, resulting in rolling mill bearing damage. In recent years, with the wear and aging of rolling mill bearing block, the failure of burnt bearing presents an increasing trend. The bearing burning phenomenon of rolling mill can only be replaced in advance, resulting in increased roll consumption, and the output is affected by shutdown.

2 Bearing structure characteristics and bar production technology of rolling mill

The bearings of short stress rolling mill all adopt rolling bearings, whose structure consists of inner ring, outer ring, cage and rolling element. A number of rolling elements are installed between the inner and outer rings, which are kept at a certain distance by a cage. The inner rings are usually installed on the neck of the roll and rotate synchronously with the roll. The bearing structure diagram is shown in Figure 1.

Short-stress line rolling mill adopts four-row cylindrical roller bearings, which bear large radial load and high limit speed, but can not bear axial force. The operation side of the rolling mill is double row angular contact ball bearing, which mainly bears axial thrust and can properly bear small load radial force, mainly limits the axial displacement of the roll, prevents the axial movement in the production process, resulting in abnormal deformation of the rolled piece, and is used with four row cylindrical roller bearings.

Brief introduction of bar production technology. The first bar workshop adopts 18 high-stiffness short-stress line rolling mill layout, the rolling mill composition is: rough rolling Φ610×6, medium rolling Φ430×6, finishing rolling Φ430×2+Φ370×4, horizontal and vertical alternating layout, of which the 16th and 18th rolling mills are horizontal and vertical convertible. The production process adopts ellipse-round pass design, and the process parameters are shown in Table 1 (diameter 12 as an example). Among them, rough rolling 2# frame pressure is larger, 5# load is larger, 17# and 18# line speed is larger, there is short-term overload phenomenon.

3 Cause analysis and measures of burning bearing

During the rolling process, the normal temperature of the bearing is 30 ~ 50℃. When the bearing temperature rises sharply and the rolling mill current fluctuates, the rolling mill bearing will burn. Statistics of rolling mill burning bearing failures in the workshop in recent two years mainly focus on bearing lubrication, rolling mill assembly, bearing installation and rolling force control [1].

3.1 Bearing lubrication

It is mainly reflected in insufficient oil and gas lubrication or no oil and gas. Oil and gas lubrication oil supply mixer oil supply quantity is small, oil pipe oil leakage, virtual connection, oil and gas distributor individual blockage; Bearing seat oil seal and water seal seal is not good, bearing seat oil leakage or water, resulting in esterification of lubricating oil resulting in bearing oil shortage;

The wrong axial regulating roll seriously causes the oil supply hole of the bearing seat and the inner bearing oil hole of the axial regulating thread pair to be unable to align, resulting in the poor supply of bearing oil.

3.2 Mill Assembly

It is mainly reflected in the abnormal axial force of thrust bearing. During roll assembly or mill adjustment, the wrong roller direction is overadjusted, resulting in no clearance between roll and thrust bearing; When the end cover of the non-transmission side is assembled, the bolt pressure is too large, resulting in too small thrust bearing clearance; The bearing weight device of vertical rolling mill is actually connected, and the weight of the connecting shaft is all pressed against the roll; The drawbar assembly tilts causing the roller box to side.

3.3 Bearing Installation

It is mainly reflected in the installation cleanliness of bearings and the installation quality of bearing inner sleeve. Rolling mill water into the oxide and other foreign matter, resulting in bearing rolling body pitting; The bearing damaged after treatment is not thorough, bearing chamber damaged; The diesel oil used for cleaning the bearing is too dirty, the compressed air used for purging the bearing is not pure, there are water and foreign bodies; Excessive interference between inner sleeve and roll neck; Inner casing fatigue after repeated heating.

3.4 Rolling pressure control [1]

In the production, it was found that the bearing burning frequency of K1 and K2 sorties in finishing rolling and the second and fourth sorties in rough rolling was higher. From the process parameters in Table 1, it can be seen that the rolling speed of K1 and K2 sorties in finishing rolling is higher, and the material pattern matching of 2 and 4 rough rolling racks is large, resulting in large rolling pressure, rolling force is transferred to the rolling mill bearings through the rolls, and the outer ring of the bearing is broken under the huge impact load. Eventually the bearing was damaged.

4 Improvement Measures

4.1 Certifying oil and gas supply

Improve the check system of rolling mill bearing oil and gas lubrication system, strengthen the check; Check the bearing seal every time the roll is changed, change the seal periodically, and apply sealant at the end cover of the roll assembly; When the mill is assembled, ensure that the adjusting roll device is at zero position according to the process requirements, and ensure that the upper and lower rolls are aligned before pressing the rubber sheet and the roll box side bolts.

4.2 Eliminate thrust bearing force

The length of the roll body must meet the tolerance range required by the standard, especially the roller ring of combined materials such as tungsten carbide; The bolts of the base and the end cover of the rolling mill should be tightened diagonally and with even force to prevent deviation and over-tightening; Check the gap between the vertical roller connecting shaft and the roll neck. If there is no gap, inform the maintenance personnel to deal with it in time.

4.3 Improve bearing cleanliness

After the failure of burning bearing, the roller box should be thoroughly treated to keep the cleanliness; Timely replacement of diesel or other decontamination measures; Check the quality of pressure air regularly to ensure that the pressure air is free of impurities such as water and oil.

4.4 Ensure roll inner sleeve quality

Check roll neck and inner sleeve strictly according to process size; Check whether there is discoloration phenomenon, if there should be timely replacement.

4.5 Standard down procedure

Reasonable matching of the reduction amount, appropriate adjustment of the reduction amount of individual sorties, reduce the load, control the current of each sortie within the rated current; Strictly control the billet temperature according to the heating process, ensure the billet in the furnace time, and ensure that the temperature difference between the head and the end is within 30℃; Eliminate rolling of low-temperature steel and blackhead steel, ensure the normal deformation resistance during rolling, and minimize the impact load of bearing outer ring.

5 Conclusion

Based on the analysis of rolling mill structure and bar production process, aiming at the problem of rolling mill bearing burning, the oil and gas supply can be guaranteed, bearing cleanliness can be improved and roll inner sleeve quality can be improved, the process rules of material type pressing can be optimized, bearing assembly and maintenance of rolling mill can be strictly and standardized, bearing burning failure can be eliminated and roll consumption can be reduced. At the same time, the downtime is reduced and the operation rate is increased.

【Reference 】

[1] Song Jiang, Jiang Wudong. Analysis and Solution of Bearing Fault in Rolling Mill [J]. Shandong Metallurgy, 2014,36 (1) : 74-75.