Application analysis of oil and gas lubrication technology for rolling mill work roll bearings

Xia Cuili1 Zhao Guangyu2 Huang as 3

(1. Liaoning University of Science and Technology, Benxi, Liaoning, 117002; 2. Benxi Iron and Steel (Group) Company Cold Rolling Plant, Benxi, Liaoning, 117000; 3. Benxi Iron and Steel (Group) Equipment Department, Benxi, Liaoning. 117000)

Abstract: This article introduces the characteristics and key points of installation, operation and maintenance of the oil and gas lubrication system of the work roll bearings of the rolling mill in Bengang Cold Rolling Plant, and proposes solutions to common faults during operation.

Keywords: cold rolling mill; work roll bearing; oil and gas lubrication

The CDCM line at Benxi Steel's cold-rolling plant was the first imported combined pickling and tandem-rolling line in China and was internationally advanced. Its stand 1#–4# work-roll bearings, which are 4-row tapered roller bearings, use REBS oil-air lubrication. The work-roll bearings are arranged in tandem and contain multiple friction interfaces; roll-neck seals also require lubrication. The oil must therefore penetrate all friction interfaces rapidly while supplying different quantities to the bearings and roll-neck seals. Furthermore, emulsion washes over the bearing housings and inevitably enters them, endangering the bearings.

REBS's TURBOLUB oil and gas lubrication system can successfully solve the above lubrication problems of tandem bearings. The author of this article discusses the characteristics of oil and gas lubrication systems and issues that should be paid attention to in operation and maintenance.

1 Characteristics of oil and gas lubrication system design

1.1 How oil and gas lubrication technology works

The continuous action of compressed air is used to drive the lubricating oil to continuously flow along the inner wall of the pipeline to form a turbulent oil-air mixture, which is supplied to the lubrication point in the form of fine oil droplets. This process is oil-air lubrication. Compressed air is supplied continuously, and oil is supplied intermittently. The intermittent time and oil supply amount can be set according to the consumption of each lubrication point.

The oil and gas lubrication system is a comprehensive equipment integrating machinery, electricity and gas. The main equipment is:

(1) Oil supply part: fuel tank volume 500L; there are two oil supply air pumps (one for use and one for backup). The displacement of the air pump is 6ml per stroke, and the normal working pressure is about 6ＭPa.

(2) Compressed air part: The compressed air passes through the filter, solenoid valve, and pressure reducing valve to set the pressure to 0.3-0.4MPa. It reaches turbulent flow when mixed with lubricating oil in the oil and gas mixing block. The compressed air pressure must not be lower than 0.2MPa.

(3) Lubricating oil distribution and mixing part: There is a progressive distributor at the outlet of the air pump. The lubricating oil is accurately distributed by the progressive distributor and then enters the downstream 4 group oil and gas mixing block, where it is mixed with compressed air to form an oil and gas flow, and the oil and gas flow is transmitted to each lubrication point through the pipeline.

(4) TURBOLUB sleeve modular oil and gas distributor: installed in the work roll bearing seat, the oil and gas flow is secondary distributed to the bearings and sealing parts by the TUR-BOLUB oil and gas distributor according to the required amount.

(5) Pipeline: A stainless steel pipe with an outer diameter of Φ10ｍｍ is connected to the lubrication points at both ends of the work roll bearing seat. The pipeline layout has no impact on the transportation of oil and gas.

(6) Electrical control and monitoring device: The system is equipped with many electrical monitoring devices to detect flow, pressure, working time, stop time of the air pump, display components and system faults, etc.

1.2 Advantages of oil and gas lubrication technology

(1) The gas-liquid two-phase oil film of oil-gas lubrication greatly improves the load-bearing capacity of the oil film, reduces friction loss, improves lubrication efficiency and the service life of the bearing.

(2) Oil and gas are continuously supplied to the bearing seat. The gas-liquid two-phase oil film produced by the oil and gas is fresh, and the load-bearing performance is not damaged.

(3) Compressed air flows into the bearing seat and overflows from the bearing seat, which can take away a large amount of heat and effectively reduce the temperature of the bearing; at the same time, it also enhances the sealing performance of the bearing seat. The compressed air can maintain a positive pressure of 30-80kPa in the bearing seat, so that external emulsions and other dirt cannot invade the bearing seat and harm the bearing.

(4) Accurate oil supply prevents excess lubricant from filling the inner cavity of the bearing, which reduces viscous resistance and lubricant consumption. It does not pollute the emulsion, rolling mill and surrounding environment like dry oil lubrication.

(5) There is no limit on the size of the equipment to be lubricated, the piping is simple and the layout of the piping is not restricted by the direction.

(6) The system has few moving parts, reliable operation, and low maintenance, which greatly reduces the operation and maintenance costs of lubricated equipment.

(7) The monitoring means are complete and the degree of mechanical and electrical integration is high, which prevents the bearing from running without lubrication and makes maintenance easy.

1.3 System parameter setting

(1) Lubrication quantity calculation formula for each bearing

After calculating the oil supply cycle, it can be found that Tp = 110s. After a period of operation observation, setting Tp = 80s can meet the requirements of equipment lubrication.

2 Installation, operation and maintenance points

When installing and using the oil and gas lubrication system, the Benxi Iron and Steel Cold Rolling Plant drew on and absorbed the practical application experience of other plants and adopted many improvement measures to ensure the normal operation of the oil and gas lubrication system. This is briefly summarized as follows:

(1) Compressed-air quality is especially important to the stable operation of an oil-air lubrication system. If the compressed air is too dirty, contaminants can easily block the oil-air distributors and seriously harm the system. A two-stage filtration arrangement with automatic drains was therefore added during installation: an AFF22B-10 main-line filter (3μm) and an AAM550-10 oil-mist separator (0.3μm). Both have filter-element clogging indicators, allowing the degree of clogging to be checked promptly and the timing of element replacement to be determined. These two filtration stages remove virtually all dust, moisture and oil from the compressed air. Filtration performance is excellent, and no failures caused by dirty compressed air have occurred.

(2) Increase compressed-air pressure and flow. Compressed air is a natural coolant. It maintains a certain positive pressure inside the bearing housing, and there is a large pressure difference between this internal pressure and the pressure of the compressed air supplied to the housing. This large pressure difference can continuously cool the bearings, and a higher compressed-air flow provides a greater cooling effect. The system pressure-reducing valve was therefore adjusted to raise the compressed-air pressure to 40KPa. The diameter of the compressed-air piping was also increased to raise the airflow and pressure difference, improving bearing cooling.

(3) The oil and gas pipelines use stainless steel pipes, which can reduce the workload of inspection and maintenance and avoid tank burnout failures. Since the oil and gas pipelines in the rolling mill frame area have been in contact with emulsion vapor for a long time, if carbon steel pipes are used, the pipelines will be corroded and damaged over time, causing leakage, resulting in insufficient lubricating oil reaching the lubrication point, resulting in box burnout.

(4) As production increased, with annual output rising by 300,000 t, rolling speed also increased. To meet actual requirements, the air pump's oil-supply interval was therefore reduced from once every 80 s to once every 60 s, increasing the amount of lubricating oil supplied to the bearings and correspondingly improving cooling.

(5) All seals should be made of materials compatible with the selected oil, with fluorinated rubber or nitrile rubber being generally preferred.

(6) Keep the quick connector at the bearing seat clean and prevent dirt from entering. Otherwise, the dirt will block the quick connector and cause the quick connector to not be inserted in place. The oil path will be blocked or the opening will be too small, resulting in insufficient oil entering the bearing and causing box burn. In addition, dirt will clog the filter in the bearing seat, causing insufficient oil to enter the bearing, resulting in box burning.

(7) Strengthen the assembly quality of the bearing seat to avoid box burn due to poor assembly of the bearing seat or entry of stolen goods, resulting in poor oil and gas lubrication or insufficient oil volume.

(8) Strengthen routine inspections. Check daily that system air and oil pressures are normal, the distributors move freely, and both oil and air are present in the transparent plastic tubing connected to each oil-air mixing block, forming a swirling, turbulent flow. If the oil-air flow is too fast and does not form a swirl, the piping is too long, pressure loss is excessive, and lubrication is poor. If the oil-air flow is too slow, the system may be blocked and oil circulation restricted. In that case, check whether the oil-air distributors, quick couplings, bearing-housing strainers, and air filters are contaminated or blocked, and whether the quick couplings are fully inserted.

(9) During regular maintenance, check whether there is leakage in the system pipeline (after the quick connector at the bearing seat is disconnected, if there is oil and gas flowing in the transparent plastic tube connected to the oil and gas mixing block, it means there is leakage in the pipeline), whether the joints are deformed, and whether the hose is worn. Contaminated quick connectors need to be cleaned in time, and damaged hoses, joints and pipelines need to be replaced in time.

(10) If the machine is shut down for a long time for maintenance (annual maintenance, overhaul, etc.), the compressed air source should be turned off so that the pneumatic pump cannot output lubricating oil (6MPa or above) to prevent the lubricating oil from pouring back into the compressed air pipeline due to the pipeline outlet being closed (the quick connector is unplugged).

3 Conclusion

To ensure stable operation, the oil-air lubrication system was improved in its design, manufacture, installation, servicing and maintenance. Following these improvements, it has operated essentially without failures. After commissioning, the oil-air lubrication system improved lubrication of the work-roll bearings. Statistics show that oil consumption after the retrofit is 1/10 of that with grease lubrication. Failures caused by inadequate lubrication, such as bearing burnout, housing deformation and roll-neck seizure, have been largely eliminated, delivering good economic benefits.

References:

【1】Shanghai Lebos Lubrication Technology Co., Ltd. REBS oil and gas lubrication system user manual［Ｍ］.

[2] Run Tonghai, He Lidong · Gas-liquid two-phase fluid cooling and lubrication technology and its application [M] · Harbin: Harbin University Press, 1995.

Source: "Lubrication and Sealing"

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