Abstract:  The finishing mill guide device of feed roller bearing failure problem analysis shows that the main failure form is the wear and fracture, damage and contact fatigue, etc., the main reason is that the bearing cooling is not enough, in use process by increasing the diameter of the cooling system cooling water jet and the increase of cooling water in upper and lower sides of the guide roller plate jet to solve the above problems, The service life of guide roller bearing is increased by 6 times.

In wire rolling, more than 50% of production failures are caused by guide devices. Guide roller and bearing are two key parts of inlet guide device, and they are also parts with high consumption and frequent replacement. Bearing failure is directly related to the life of guide roller. Guide roller bearing failure not only leads to equipment accidents, but also seriously restricts production, and increases manufacturing cost and labor intensity. Therefore, it is of great economic significance to analyze the failure causes of guide roller bearings and take corresponding measures to improve the service life of guide roller bearings and reduce the consumption of guide roller bearings to some extent, which is conducive to the normal production and the reduction of production costs.

1. Working conditions of guide roller bearings

Guide roll bearings of finishing mills work under harsh conditions such as high speed, high temperature and poor cooling conditions. The bearing speed is about 4000rmin and the working temperature is 350℃ or even higher. In order to reduce the cost, domestic deep groove ball bearings are generally installed in the guide rolls of finishing mills, and the material is GCr15. Bearing size is selected according to the size of the guide roller, its working temperature is generally required to be less than 120℃, and to bear the periodic impact load of the billet passing through the guide roller at high speed, grease lubrication. According to statistics, the average service life of guide roller bearing of finishing mill is about 4h.

2.Failure form and cause analysis

The main failure forms of guide roller bearing are wear, fracture, burning loss and other early failure.

2.1 Wear Failure

The wear of guide roll bearing is mainly abrasive wear, which occurs on the main friction surfaces inside the bearing, including the guide surface between the cage and the inner ring, and the contact surface between the rolling body and the inner and outer ring. It is mainly caused by the invasion of foreign bodies such as rolling chip (iron oxide) and water, and lubricants with low cleanliness. Due to the work environment is extremely bad guide roller bearing, high pressure water cooling of guide roll at the same time, the mill scale on the billet into guide roller bearings, the rolling dust and dirt in the lubricant, it forms a hard grits with high-speed rotation of the guide roller bearing, exacerbating the bearing rolling body and inner and outer ring raceway of wear, premature bearing failure.

2.2 Fracture failure

The fracture failure of guide roll bearing is mainly the fracture of cage and the cracking of inner and outer ring. Bearing cage fracture is caused by the continuous heat energy from the rolled parts to the bearing through the guide roller in the production process, while the guide roller bearing can not be cooled, after a period of time, the bearing is in a high temperature state. Long running in high temperature condition, the grease in the bearing will lose its lubrication, even affect the functioning of the roller, bearing also can produce larger thermal stress, coupled with the high speed, the inner and outer ring and cage no fracture is likely to occur in the process of assembling, installation tilt or cooperate with tight, increased the possibility of a fracture.

2.3 Burn loss failure

The burning loss of guide roller bearing is mainly manifested as grease failure, ring and rolling body bite. Guide roller bearings operate at high speed without cooling. The bearings not only need to accept the heat from the billet through the guide roller, but also produce a lot of friction heat due to their high speed rotation. Therefore, bearings are usually used at abnormal working temperature, which is as high as 350℃ or more. At such a high working temperature, the grease of guide roll bearing is easy to burn out and deteriorate, losing normal lubrication performance, resulting in a sharp rise in friction temperature, making the rolling body and raceway annealing and softening, or even melting, resulting in the ring and rolling body stuck, there may be a rolling body split open, cage scattered and burned.

2.4 Contact fatigue failure

Contact fatigue failure of guide roller bearings is mainly manifested as pitting damage of rolling body or inner and outer raceway. Under the condition of installation, lubrication and good maintenance, guide roller bearings bear a large number of repeated changes in contact stress, corresponding to the rolling body inner and outer ring raceway produced repeated changes of extrusion stress, with the extension of working time, rolling body and inner and outer ring raceway will produce contact fatigue spalling, such as pitting, pitting, etc.

3. Main measures taken

Through the analysis of various failure forms of guide roller bearings, it is found that the main factor affecting the service life of guide roller bearings is the poor cooling condition, which leads to the high working temperature of guide roller bearings, and the grease is easy to deteriorate at high temperature and lose lubrication. Therefore, the premature failure of guide roller bearings can be prevented by improving the cooling system of guide and guide devices.

(1) Increase the diameter of the cooling water jet of the cooling system and increase the cooling water pressure of the guide device, which can not only solve the blockage of the jet mouth, but also realize the cooling of the guide roller bearing.

(2) Add cooling water jet ports on the upper and lower sides of the guide roller support plate, which can not only realize the cooling of the bearing, but also realize the cooling of the guide roller circumferential surface. The guide device with the above improved measures has been used in the rolling line installation test of a rolling mill, and good results have been achieved. The test results are shown in Table 1.