Bearings for Steel Industry

The main characteristics of modern rolling mills are large-scale, high-speed, heavy-duty, continuous, and automatic. Modern large-scale rolling mills, especially large-scale plate and strip continuous rolling mills with automatic control of plate shape and plate thickness, mostly use oil film bearings, which are used as work rolls in rolling mills. Bearings or back-up roller bearings are called rolling mill oil film bearings. These bearings are low-speed heavy-load, medium-speed medium-load, or heavy-load bearings. The rolling mill oil film bearing is one of the bearings with the largest load capacity among the oil film bearings. Because the American Morgan Company first developed oil film bearings and commercially used in rolling mill equipment. So it is also called MORGAN BEARING.

There are many types of oil film bearings, and they are widely used, such as bearings used in steam turbines, generator sets, ball mills, fan bearings, astronomical, aviation, and aerospace equipment.

Oil film bearings are mainly composed of tapered sleeves, bushings, thrust bearing parts, sealing systems, locking systems, and other parts. Oil film bearings have many characteristics: large load-bearing capacity, strong impact resistance, long service life, wide speed range, small structural size, and low friction coefficient.

General rolling mill machinery can be divided into steel rolling mills and non-ferrous metal rolling mills (mainly aluminum rolling mills and copper rolling mills). The rolling products can be divided into wire rod rolling mills, profile rolling mills, strip rolling mills, and plate rolling mills (divided into wide and thick plates, medium-thick plates, hot-rolled plates, and cold-rolled plates), etc. The bearings used in rolling mills today mainly include rolling bearings and oil film bearings. Different types of bearings are selected according to the different properties and uses of the rolling mill. Generally, steel rolling mills and copper rolling mills mainly use oil film bearings, and aluminum rolling mills are accustomed to using rolling bearings.

The rough rolling and intermediate rolling units of the wire rod mill mainly use rolling bearings. For example, the section steel rolling mill and the bar rolling mill mostly use rolling bearings. Pre-finish mills and finish mills mainly use oil film bearings.

Currently, strip rolling mills in China are still dominated by rolling bearings. Still, due to their low load capacity and short service life, they are gradually developing into oil film bearings.

Plate rolling mills, general medium and heavy plate rolling mills, and hot continuous rolling mills all use oil film bearings. Most cold rolling mills use oil film bearings, and some use rolling bearings. Among them, Japanese cold rolling mills promote the use of most, Western Europe, the United States, Russia, China, etc. Rolling bearings are rarely recommended in other countries.

Oil film bearing classification Oil film bearings are divided into static pressure oil film bearings, dynamic pressure oil film bearings, and dynamic and static pressure oil film bearings according to the formation mechanism.

Hydrostatic oil film bearings rely on external pressure to form pure liquid friction. They require a more complex oil supply system and are rarely used. Its working principle is that the journal and the bearing are completely separated by a bearing medium with a certain pressure supplied from the outside, thereby reducing the relative friction between the journal and the bearing. The relative sliding speed does not limit the formation of the medium film, and at various speeds ( Including zero speed), it has a large carrying capacity. Depending on the medium, static pressure bearings can be divided into liquid static pressure bearings and gas static pressure bearings.

Dynamic pressure oil film bearings are based on the dynamic pressure principle of fluid. The rotating shaft continuously brings lubricating oil with a certain viscosity into the converging wedge-shaped gap. Since the lubricating oil is compressed during the process of being sucked into the converging wedge-shaped gap by the shaft, resistance force is generated. When the lubricating oil moves in the direction of the load, When the sum of the pressures is enough to balance the external load, a complete pressure oil film is formed between the shaft and the bearing, causing the surfaces of the shaft and the bearing to break out of contact, forming pure liquid friction.

The dynamic and static pressure hybrid bearing is a new type of multi-oil wedge oil film bearing that combines the advantages of hydrodynamic and static pressure bearings and overcomes their shortcomings. It uses the throttling principle of the hydrodynamic bearing to generate a large enough static pressure bearing capacity in the pressure oil chamber, thereby overcoming the dry friction that occurs when the hydrodynamic bearing starts and stops, causing the wear of the spindle and the bearing, and improving the spindle And the service life and accuracy maintenance of the bearing; most bearing oil chambers adopt a shallow cavity structure. After the spindle is started, the dynamic pressure bearing capacity and static pressure bearing capacity formed by the shallow cavity step effect are superimposed, which greatly improves the spindle bearing capacity. The multi-cavity opposed structure greatly increases the stiffness of the spindle; the homogenization effect of the high-pressure oil film and good anti-vibration performance ensure that the spindle has high rotation accuracy and smooth operation.

Lubrication of rolling mill oil film bearings

Oil film bearing is a radial sliding bearing that uses lubricating oil as the lubricating medium. Its working principle is: during the rolling process, due to the action of the rolling force, the roller journal is forced to move, and the center of the oil film bearing and the journal are The center is eccentric, causing the gap between the oil film bearing and the journal to form two areas, one is called the divergence area (the gap gradually increases along the journal rotation direction), and the other is called the convergence area (the gap gradually decreases along the journal rotation direction). ). When the rotating journal brings the viscous lubricating oil from the divergence area into the convergence area, the bearing gap decreases from large to small along the journal rotation direction, forming an oil wedge, which generates pressure in the lubricating oil.

The load-bearing oil film is also called pressure oil film. It balances the load, isolates the journal and sleeve, converts the solid friction between metals into molecular friction inside the liquid, and reduces friction and wear to a minimum. Therefore, it can operate within the maximum range. Meet the requirements of bearing pressure, impact resistance, conversion speed, rolling accuracy, structural size, and service life.

During the operation of the rolling mill, mixed lubrication may occur due to vibration, excessive water inflow, insufficient oil supply, or problems with oil quality.

When the rolling mill is running normally and smoothly, it is fluidly lubricated. Therefore, the lubrication characteristic of oil film bearings is mixed lubrication in which the above three conditions exist alternately.

Common faults and countermeasures of rolling mill oil film bearings

Although the structural principle of oil film bearings is not very complicated, its manufacturing precision is high, and the requirements for installation, use, and maintenance are also very strict. For this reason, the assembly, inspection, use, and maintenance of oil film bearings directly affect their use effect and service life, as well as the normal operation of continuous large-scale production at the company's work site. The main failure modes of oil film bearings include:

Wear, rust, scratches, flaking, plastic flow, cracks, melting, regular cracks, edge wear. Judging from the common failures of oil film bearings on-site, they are mainly reflected in the following aspects: oil leakage, water intrusion into the bearing, heating and burning of the bearing, and abnormal wear. Bearing Failure is an important factor causing bearing failure:

Oil leakage

Oil leakage from rolling mill oil film bearings is mainly divided into oil leakage from the bearing body and oil leakage from connecting pipe fittings.

Water enters the bearing

The on-site observation of water intrusion into the oil film bearings of the rolling mill is mainly due to an increase in the liquid level in the oil tank of the oil supply system. Without replenishing oil, it means that a large amount of cooling water has penetrated. Water can be drained from the fuel tank drain port.

The main reasons for water intrusion are:

• The bearing rotary seal is aged and damaged, which leads to water intrusion.

• The unreasonable position of the exhaust hole can cause a slight negative pressure to appear at the rotary seal after the system has been running for some time and water ingress.

• Steam from the oil tank Water has entered the heating pipe or cooling water pipe.

The consequences of water intrusion are mainly to cause a large amount of corrosion and scrapping of the maintenance bushing, reduce the strength of the oil film, and weaken the dehydration performance of the lubricating oil. Shorten the service life of oil products.

The countermeasures that can be adopted in response to the above situation are:

• Comprehensively inspect and replace the water seals of all bearing boxes that have been removed from the machine.

• Track the bearing boxes that are running online, and pay close attention to the liquid level changes in the oil tank and the time and amount of Water regularly released from the oil tank every day.

• Redesign and modify the position and shape of the air inlet.

That is to say. The inlet and exhaust holes are changed from the oil return joint to the upper side of the bearing box end cover, thus avoiding the pressure change inside the bearing caused by the pressure change at the inlet and outlet after the system is started.

Bearing heat and burnout

During on-site operation, the oil film bearing will suddenly heat up under certain abnormal conditions, the temperature will rise sharply, and smoke and a large amount of water vapor will be produced. Finally, the motor will be unable to drive, or the transmission link will be damaged and be forced to shut down; this is "Burning," which is a phenomenon that occurs when the bushing babbitt is melted during the operation of the rolling mill. There is also a situation where the Babbitt alloy is found to be melted when the bearing is disassembled.

The main reason for its Failure may be illegal operation. The rolling mill is started before the lubricating oil is circulated; the lubricating oil system suddenly stops supplying oil during the operation of the rolling mill, such as pipeline blockage, oil pipe joints falling off, etc.; when the rolling mill speed increases to the normal rolling speed, the lubricating oil supply Serious deficiencies; the rolling pressure is too high, the rolling speed is too high; the relative gap of the oil film bearing is too small or too large; the viscosity of the lubricating oil is too small or the lubricating oil is seriously emulsified, and the water content is too high, resulting in boundary friction and difficulty in discharging heat. Causing the babbitt alloy to melt.

Preventive measures that can be taken: Strengthen maintenance to avoid lubrication system accidents. Mainly conduct regular inspections of automatic control and self-locking devices; regularly check the clearance of bearings and make timely adjustments and replacements; strengthen on-site inspections, key parts, and regular inspections by dedicated personnel; transform the previous pipelines and pipe joints to prevent Failure caused by pipeline leakage.

Installation steps of rolling mill oil film bearings Assembly of rolling mill oil film bearings. When assembling the oil film bearing of the rolling mill, first clean the bearing seat, oil film bearing taper sleeve, bushing, and auxiliary accessories. Do not use scrapers and abrasives during cleaning. Use a turning machine to turn over the cleaned bearing seat so that the bearing seat hole is vertical, the outside of the roller, and the opening upward.

The general installation method is to lower the taper sleeve intermittently during the process of inserting the taper sleeve into the bushing hole, measure the four circumferential points from the bearing seat to the edge of the taper sleeve, and adjust the lifting equipment so that the four-point measurement values The same, and then slowly lower the taper sleeve to the installation position. When the taper sleeve is about halfway into the bushing, rotate the taper sleeve so that the keyway is above the horizontal centerline of the bearing seat.

Pre-assemble the static pressure hose, quick-change joints, connecting joints, and seals of the rolling mill oil film bearing, then pass the static pressure hose through the opening on the bearing seat and connect one end to the bushing static pressure elbow. It should be remembered that after installation, the quick-change joint must be lower than 1/8 of the bearing seat surface.