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Abstract: This article introduces the fully hydraulic hot straightening machine for medium and thick plates and its roll system (roll box) structure. It analyzes the causes of damage to the support roll bearings of the hot straightening roll box, with a focus on lubrication maintenance and improvement plans. It also explains the causes of previous damage to the support roll bearings, providing important reference for the maintenance and improvement of roll boxes of similar straightening machines.
Keywords: Hot straightening machine; roll system; Support roll bearings; Hold on tightly; Oil and gas lubrication
0. Introduction
Hot straightening machine is one of the most important process equipment in the medium and thick plate production line, responsible for leveling the rolled (water-cooled) steel plate, improving the flatness of the steel plate, and making it meet delivery requirements. The hot straightening machine is located in the main process of the rolling line, and the operation of this equipment is directly related to the output and product quality of the entire line. After 2003, the number of domestic medium and thick plate production lines rapidly increased, and 90% of the hot straightening machines adopted a full liquid quadruple structure. This form of hot straightening machine mainly consists of a frame, transmission device, roll system (roll box), and adjustment mechanism, among which the roll system (roll box) is the most core component of the hot straightening machine. The hot straightening machine for the 4300 medium thick plate production line of a certain factory was designed by Ximark Company, and a well-known domestic straightening machine equipment manufacturing factory cooperated with Ximark Company to manufacture it. The straightening machine roll series is an original imported part. At the end of 2006, the hot straightening machine was put into operation. In June 2007, the first occurrence of support roll bearing damage occurred. After the bearing was damaged, the support roll got stuck, damaging the straightening roll surface. Continuing to straighten would cause surface indentation on the steel plate, seriously affecting product quality. From June 2007 to 2018, the service life of the support roll bearings in the roll box of the hot straightening machine was extremely unstable, ranging from a few days to several months on the machine. There have been multiple instances of damage to the support roll bearings within less than a week of using newly repaired roll boxes, seriously affecting production. Regarding the issue of damage to the support roll bearings of the hot straightening machine roll box, the technical personnel of the factory have communicated with multiple wide and thick plate production lines with the same structure of hot straightening machines, all of which have similar problems to varying degrees. This article summarizes the experience of handling faults in hot straightening roll box bearings in recent years, analyzes the forms of damage, main causes of damage, and preventive measures for the support roll bearings of hot straightening machine roll boxes, and focuses on in-depth analysis and introduction from the perspective of bearing lubrication, providing reference for similar fault handling and prevention.
1. Introduction to hot straightening parameters and structure
1.1 Main parameters
Straightening machine form: quadruple fully hydraulic straightening machine; Maximum straightening force (parallel roll gap, static pressure): 3200t; Straightening roll body length: 4300mm; Straightening roll diameter: 285mm; Number of straightening rolls: 11; Straightening roll spacing: 300mm; Number of supporting rolls: 60; Support roll bearing model: 24130CCW33/C3; Maximum straightening speed: ± 2.5m/s; Straightening steel plate temperature: 450-1100 ℃.
1.2 Main structure of hot straightening machine (Figure 1)
Figure 1 Main structure of hot straightening machine
1.3 Assembly structure of upper and lower roll systems (roll boxes)
The assembly structure of the upper and lower roll system (roll box) is shown in Figures 2 and 3. To adapt to the straightening of high-temperature steel plates, the hot straightening roll system bearings are lubricated with REBS oil and air, and the straightening roll is cooled by water inside. A set of rolls has 120 supporting roll bearings and 22 straightening roll bearings. During the steel plate straightening process, the straightening roll comes into direct contact with the steel plate. The supporting roll supports the straightening roll, which bears the straightening force and acts on the frame through bearings and bearing seats. Due to the direct contact between the straightening roll and the steel plate, the surface condition of the straightening roll directly affects the surface quality of the steel plate. Each working roll is supported by 6 supporting rolls (with 3 rolls closest to the inlet and outlet).
Figure 2 Assembly structure of upper and lower roller systems (roller boxes)
Figure 3 Side view of upper and lower roller systems (roller boxes)
2. Fault symptoms
The phenomenon after the support roll bearing of the straightening machine roll box is damaged is shown in Figure 4, with abnormal wear on the straightening roll surface (width close to the width of the support roll body), severe wear on the support roll body, and blue burn marks. In the vast majority of cases, bearing locking faults occur in the upper roll box, and the four groups of supporting rolls in the middle of the upper roll box are the main ones.
a) Wear of straightening roller surface b) Support roller stuck
Figure 4 Damaged support roller bearing
3. Insufficient lubrication causes damage to the support roll bearings
The main causes of damage to the support roll bearings of the hot straightening roll box in the factory since its production are: insufficient lubrication, load exceeding limit, incorrect calibration plate thickness, water content in compressed air, and excessive roll diameter difference. According to the analysis of data on roll box failures since production, it is believed that insufficient lubrication causes the most frequent damage to bearings. The hot straightening roll box of the factory was designed by Westmark Company from 2004 to 2005. The supporting roll bearings are lubricated with oil and air, and the oil amount of the five bearings of the connected bearing seat is evenly distributed using a built-in distributor in the bearing seat, as shown in Figure 5.
Figure 5 Layout of lubrication distributor for support roller bearings
Under normal circumstances, the hot straightening roll box needs to be regularly disassembled and maintained, including grinding the roll surface, bearing inspection, sealing replacement, and other items, specifically for lubricating oil circuits. During the repair process, compressed air is often only used to blow and clean the pipelines and distributors, checking whether each lubrication point has air output, and it is impossible to detect whether the oil and gas distribution is even. After dissecting and studying the internal distributor structure of the faulty roll box, it was found that the distributor has a complex internal structure, as shown in Figures 6 and 7. There are a total of 18 small holes (with a diameter of 0.6mm), 4 of which are connected to the current bearing, and the remaining 14 holes continue to be connected to it? First level. According to the principle of the distributor, if a small number of holes are blocked, such as 2 out of 4 holes, it will cause the current lubricating oil amount of the bearing to be reduced to 1/2. Similar blockage problems are difficult to detect through compressed air blowing, which will cause abnormal distributor to continue to operate, leading to rapid bearing damage due to insufficient lubrication. After standing the dismantled distributor upright for 2 days, it was found that black liquid flowed out, similar to ink (Figure 8, Figure 9). Analysis suggests that this is due to the carbonization of lubricating oil in the upper roll box caused by long-term high-temperature steel plate baking, especially the significant thermal radiation of the upper roll box is greater than that of the lower roll box. This can also be related to the fact that most of the branch support roll bearings are damaged in the middle part of the upper roll box.
Figure 6 Splitting of Distributors
Figure 7 Distributor Structure
Figure 8 Carbides inside the distributor
In summary, during the maintenance process of the roll box, it is easy to encounter uneven distribution of lubricating oil due to incomplete cleaning and unclogging of accumulated carbides in the built-in distributor, resulting in some bearings being damaged due to insufficient lubrication (Figure 10).
Figure 9 Distributor Assembly Structure
Figure 10 Bearing damage caused by insufficient lubrication
4. Maintenance and cleaning methods for roll box oil and gas distributors
Regarding the maintenance of distributors (including internal and external distributors), the correct maintenance and cleaning method communicated with REBS technicians is to soak each distributor in a kerosene tank for 2 days. After soaking, a large amount of black material appears at the bottom of the kerosene tank (Figure 11). Then use clean compressed air to blow forward and backward, and then rinse in new kerosene. In addition, the O-ring at the connection of the built-in distributor short section needs to be replaced with a new one every time it is repaired.
Before and after soaking
Figure 11: A large amount of black material appears at the bottom of the kerosene tank
5. Introduction to the improvement plan for the arrangement of oil and gas lubrication pipelines in the roll box
5.1 Disadvantages of the original design with built-in oil and gas lubrication distributor
The support roll bearing lubrication of the hot straightening roll box designed by Ximark Company adopts the form of installing an internal distributor in the connected bearing seat (Figure 12a). The internal distributor has the following main disadvantages:
a) Original design b) New plan
Figure 12 Layout of Oil and Gas Lubrication Pipeline
(1) The price is expensive, and a set of roll boxes with only built-in distributors costs over 300000 yuan.
(2) High precision is required for disassembly and installation.
(3) The distributor itself is an indivisible structure, making it difficult to thoroughly clean and maintain.
(4) The built-in distributor of the upper roll box bearing seat is prone to carbon accumulation when exposed to high temperatures for a long time. Due to the through-hole being only 0.6mm, it is extremely easy to block.
5.2 Design scheme and advantages of oil and gas lubrication oil circuit for new hot straightening roll box
The current layout plan for the newly designed oil and gas lubrication pipeline is to cancel the built-in distributor and add a second level external distributor through stainless steel pipelines（ Ф The advantages of the new solution are as follows:
(1) The price of an external distributor is only one-third of that of an internal distributor.
(2) The temperature of the external distributor is low, and there will be no carbonization blockage problem.
(3) The external distributor has a simple structure, can be disassembled and thoroughly cleaned, and is easy to maintain.
5.3 Application of SKF New Sealed High Temperature Lubricating grease Bearing in Hot Leveling Machine roll Box
To solve the problem of bearing damage and lifespan in the hot straightening roll box, SKF's latest upgraded version of sealed bearings has been well used in the hot straightening machines of a steel plant's 5000 production line and a steel plant's 4200 production line. It has the following advantages:
(1) Canceling the oil and gas lubrication system can completely eliminate the cost of thin oil consumption and maintenance costs of the oil and gas lubrication system.
(2) Remove the impact of oil and gas pipeline blockage on bearing lubrication.
(3) Using high-temperature grease filling to ensure the self-lubricating effect of bearings and reduce the impact of temperature on bearing lubrication.
(4) Using upgraded SKF sealed bearings, the actual service life of the bearings is longer than that of open bearings and can be reused multiple times.
6. Other causes of bearing damage
In addition to lubrication reasons, the clear causes of damage that have occurred before are:
(1) The model coefficient setting is incorrect, causing overload.
(2) The problem of the shape of the high-strength steel head and tail plate causes overload of a single bearing.
(3) Calibration board thickness error. Causing errors in roll gap calibration, resulting in overload.
(4) Frequent over straightening capability curve for over limit steel plate straightening.
(5) The compressed air lubricated with oil and gas contains water.
(6) The diameter difference of the support roll is too large.
7. Conclusion
This article introduces the structure of the fully hydraulic hot straightening machine, which is widely used in medium and thick plate rolling mills. An in-depth analysis is conducted on the locking problem of the support roll bearing of the hot straightening machine roll box caused by lubrication reasons, and targeted maintenance points are provided. At the same time, a comparison was made between the latest lubrication pipeline layout and the original layout. And point out six main reasons for the locking of the support roll bearings of the straightening machine, providing important reference for the maintenance and improvement of roll boxes of the same type of straightening machine.
2024 May 3rd Week KYOCM Product Recommendation:
Rotary bearings consist of an inner ring and an outer ring, one of which usually contains a gear. Together with the connecting holes in the two rings, they enable optimized power transmission through simple and fast connections between adjacent machine parts. Bearing raceways are designed with rolling elements, cages or gaskets to accommodate loads acting individually or in combination in any direction.
Features and advantages:
High carrying capacity
High stiffness for rigid bearing applications
Low friction
Long service life
Surface protection and corrosion resistance
Integrate other features including:
Driving mechanism
Control device
Lubrication system
Monitoring system
Sealed cassette tape
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2024-05-17