Abstract: The paper introduces the full hydraulic thermal straightener of medium and thick plate and its roller system (roller box) structure, analyzes the damage causes of the support roller bearing of the thermal straightening roller box, mainly introduces the lubrication maintenance and improvement plan, and explains the clear causes of the damage of the support roller bearing in the past, providing an important reference for the maintenance and improvement of the roll box of the same type of straightening machine.

Key words: thermal straightener; Roll system; Support roll bearing; Hold to death; Oil-gas lubrication

0 Introduction

The hot straightener is one of the most important process equipment in the production line of medium and thick plate, which is responsible for leveling the steel plate after rolling (after water cooling), improving the flatness of the steel plate and making it meet the delivery requirements. The hot straightener is in the main flow of rolling line, the operation of the equipment is directly related to the output and product quality of the whole line. After 2003, the number of domestic medium and thick plate production lines increased rapidly, 90% of the hot straightening machine are using the all-liquid four-fold structure, the form of hot straightening machine is mainly composed of frame, transmission device, roll system (roll box), adjusting mechanism, of which the roll system (roll box) is the core component of the hot straightening machine. A factory 4300 thick plate production line hot straightening machine for the design of the company, a well-known domestic straightening machine equipment manufacturer and the company cooperated with the production of the straightening machine roller system for the original imported parts. At the end of 2006, the hot straightening machine was put into operation, and in June 2007, the support roller bearing was damaged for the first time. After the bearing was damaged, the support roller was stuck and the straightening roller surface was damaged. If the straightening continued, the steel plate surface would be indentation, which seriously affected the product quality. From June 2007 to 2018, the service life of the support roller bearing of the hot straightening machine roller box is extremely unstable, and the service life of the machine varies from a few days to a few months, and the problem of the support roller bearing damage occurs in less than a week after the newly repaired roller box is installed, which seriously affects the production. In order to solve the problem of the bearing damage of the roll box support roller of the hot straightening machine, the technicians of the factory have communicated with a number of wide and thick plate production lines with the same structure of the hot straightener, and there are similar problems to different degrees. This paper summarizes the experience of fault treatment for hot straightening roller box bearing in recent years, and analyzes the damage forms, main damage causes and preventive measures of the roller box support roller bearing of hot straightening machine, focusing on the bearing lubrication Angle, and provides reference for similar fault treatment and prevention.

1 Thermal straightening parameters and structure

1.1 Main Parameters

Straightener type: four-fold full hydraulic straightener; Maximum straightening force (parallel roll gap, static pressure) : 3200t; Straightening roller body length: 4300mm; Straightening roller diameter: 285mm; Number of straightening rolls: 11; Straightening roll distance: 300mm; Number of supporting rolls: 60; Support roller bearing model: 24130CCW33/C3; Maximum straightening speed: ±2.5m/s; Straightening steel plate temperature: 450~1100℃

1.2 Main Structure of thermal straightener (Figure 1)

1.3 Upper and lower roller system (roller box) assembly structure

The assembly structure of the upper and lower roll system (roll box) is shown in Figure 2 and Figure 3. In order to adapt to the high temperature steel plate straightening, the bearings of the hot straightening roller are lubricated by REBS oil and gas, and the straightening roller is cooled by water. A set of rollers consists of 120 supporting roller bearings and 22 straightening roller bearings. In the process of steel plate straightening, the straightening roller is in direct contact with the steel plate, and the supporting roller supports the straightening roller, and the supporting roller bears the straightening force, and acts on the frame through the bearing and bearing seat. Due to the direct contact between the straightening roll and the steel plate, the surface state of the straightening roll directly affects the surface quality of the steel plate. Each work roll is supported by 6 supporting rollers (3 closest to the entrance outlet).

2 Fault Symptoms

The phenomenon after the bearing of the support roller of the straightening machine roll box is shown in Figure 4. Abnormal wear of the surface of the straightening roller (the width is close to the width of the support roller body), serious wear and blue burn marks appear on the support roller body. Most of the bearing lock failures occur in the upper roll box, and the four support rolls in the middle of the upper roll box are the main ones.

3 Insufficient lubrication causes damage to the bearing of the support roll

So far, the main reasons for the damage of the support roller bearing of the hot straightening roller box in the factory are: insufficient lubrication, excessive load, wrong thickness of the calibration plate, water content in the compressed air, and too large difference in roll diameter. According to the analysis of the failure data of the roller box since production, The Times of bearing damage caused by insufficient lubrication are the most. The thermal straightening roll box of this factory was designed by Simak Company in 2004-2005. The bearing of the support roll is lubricated by oil and gas, and the internal distributor of the bearing seat is used to evenly distribute the oil of the 5 bearings of the connected bearing seat, as shown in Figure 5.

Under normal circumstances, the hot straightening roller box needs to be disassembled and maintained regularly, including the grinding roller surface, bearing inspection, seal replacement and other items, for the lubricating oil road. In the repair process, the pipeline and distributor are often only purged with compressed air to check whether each lubrication point is outgassed, and it is impossible to detect whether the oil and gas distribution is uniform. After research on the internal distributor structure of the faulty roll box, it is found that the internal structure of the distributor is complex, as shown in Figure 6 and Figure 7, there are 18 small holes (aperture 0.6mm), 4 holes lead to the current bearing, and the remaining 14 holes continue to pass to the next level. According to the principle of the distributor, if a small number of individual holes are blocked, such as two holes in the four holes are blocked, it will cause the current bearing lubricating oil amount to be reduced to 1/2, similar to the blockage problem is difficult to detect abnormalities through pressure air purging, which will cause the abnormal distributor to continue to be on the machine, and then cause the bearing to be rapidly damaged due to insufficient lubrication. When the disassembled distributor was placed upright for 2d, it was found that black liquid, similar to ink, flowed out (FIG. 8 and FIG. 9), which was analyzed and believed to be due to the carbonization of lubricating oil caused by long-term baking of the upper roll box by high temperature steel plate. In particular, the thermal radiation of the upper roll box was significantly greater than that of the lower roll box, which could also be related to the fact that most of the damage problems of support roll bearings occurred in the middle of the upper roll box.

To sum up, uneven distribution of lubricating oil is easily caused by incomplete cleaning and dredging of carbides accumulated in the built-in distributor during roll box maintenance, resulting in damage to some bearings due to insufficient lubrication (FIG. 10).

4 Maintenance and cleaning method of 4 roller box oil and gas distributor

For the maintenance of dispensers (including built-in and external dispensers), communicate with REBS technicians. The correct maintenance and cleaning method is to soak each dispenser in the coal oil tank for 2d. After soaking, a large amount of black matter appears at the bottom of the coal oil tank (Figure 11). Then use clean pressure air forward and reverse purge, and then rinse in the new kerosene. In addition, the O-ring at the joint of the built-in distributor sub needs to be replaced with a new part each time it is repaired.

5 roller box oil and gas lubrication pipeline layout improvement scheme

5.1 Shortcomings of the original design built-in oil and gas lubrication distributor

The thermal straightening roller box designed by Simak uses the form of a built-in distributor installed in the connected bearing seat (FIG. 12a). The built-in distributor has the following main shortcomings:

(1) The price is expensive, a set of roller box, only the built-in distributor needs more than 300,000 yuan.

(2) High disassembly and installation accuracy requirements.

(3) The distributor itself is a non-removable structure, and it is difficult to thoroughly clean and maintain.

(4) The built-in distributor of the upper roller box bearing seat is in a high temperature environment for a long time, which is easy to accumulate carbon, and it is easy to block because the through hole is only 0.6mm.

5.2 Design scheme and advantages of oil-gas lubricating oil road for new hot straightening roll box

At present, the newly designed oil-gas lubrication pipeline layout scheme is as follows: cancel the built-in distributor, add two external distributors, and pass through each lubrication point through stainless steel pipeline (diameter of 6mm×1.5mm) (FIG. 12b). The advantages of the new scheme are as follows:

(1) The price of the external distributor is only 1/3 of that of the built-in distributor.

(2) The external distributor location temperature is low, there will be no carbonization blockage problem.

(3) The external distributor has simple structure, can be disassembled and thoroughly cleaned, and is easy to maintain.

5.3 Application of SKF new sealed high temperature grease bearing in the roller box of hot straightening machine

In order to solve the problem of bearing damage life of the hot straightening roller box, the newly developed and upgraded version of the sealed bearing of SKF has been well used in the hot straightening machine of a steel mill 5000 production line and a steel mill 4200 production line. Has the following advantages:

(1) The elimination of the oil-gas lubrication system can completely eliminate the cost of thin oil consumption and eliminate the maintenance cost of the oil-gas lubrication system.

(2) Remove the influence of bearing underlubrication caused by oil and gas pipeline blockage.

(3) The use of high temperature grease filling, to ensure the bearing self-lubrication effect, reduce the impact of temperature on bearing lubrication.

(4) Using the upgraded SKF sealed bearing, the actual service life of the bearing is greater than that of the open bearing, which can be reused many times.

6 Other bearing damage causes

In addition to lubrication causes, the clear causes of damage that have occurred are:

(1) The model coefficient setting is wrong, resulting in overload.

(2) High strength steel head tail plate shape problem, resulting in single bearing overload.

(3) The thickness of the calibration plate is wrong. Resulting in roll gap calibration error, resulting in overload.

(4) Frequent overstraightening capacity curve for ultra-limit steel plate straightening.

(5) Water in the compressed air of oil-gas lubrication.

(6) Support roll diameter difference is too large.

7 Conclusions

This paper introduces the structure of the most widely used full hydraulic hot straightener for medium and thick plate rolling mill, analyzes the problem of the bearing lock of the hot straightener roll box support roller caused by lubrication, and provides the key points for maintenance. At the same time, the latest lubrication pipeline layout is compared with the original layout. It also points out the other 6 main reasons that cause the bearing lock of the support roller of the straightener, and provides an important reference for the maintenance and improvement of the roll box of the same type of straightener.

【 Reference 】

[1] Li Chan, Sun Qiguo. Structure Design and Performance Simulation of Oil and Gas Distributor [D]. Beijing: North China University of Technology, 2013.

[2] CUI Fu. Straightening Principle and Straightening Machinery [M]. Beijing: Metallurgical Industry Press, 2005.

[3] Xie Pengfei, Dong Zhiwei. On the repair and maintenance of oil-gas lubrication Distributor [J]. Hebei Metallurgy, 2016 (4) 77-79.