The Oil Film Bearings Lubrication on Rolling Mill

Jul 07, 2023

1. Working principle of rolling mill oil film bearing

Oil film bearings are a kind of radial sliding bearing with lubricating oil as the lubricating medium. Its working principle is that during the rolling process, due to the action of rolling force, the journal of the roll shaft is forced to move. The center of the oil film bearing and the journal are eccentric so that the gap between the oil film bearing and the journal forms two areas: one is called the divergence area (the gap gradually increases along the direction of journal rotation), and the other is called the convergence area (the gap gradually decreases along the path of journal rotation). When the rotating journal brings the viscous lubricating oil from the diverging area to the converging area, the bearing gap changes from large to small along the direction of journal rotation, forming a kind of oil wedge, which generates pressure in the lubricating oil. The resultant force of the pressure at each point in the oil film along the rolling direction is the bearing capacity. When the rolling force is greater than the bearing capacity, the eccentricity between the center of the journal and the center of the oil film bearing increases. In the convergence area, the bearing clearance becomes steeper along the journal rotation direction, the minimum oil film thickness becomes smaller, the pressure in the oil film becomes more extensive, and the bearing capacity becomes larger until it reaches equilibrium with the rolling force, the center of the journal no longer shifts, and the oil film bearing is completely separated from the journal by lubricating oil, which theoretically forms full fluid lubrication.

From the working principle of the oil film bearing, one of the most important parameters of the oil film bearing is the minimum oil film thickness. If the minimum value of oil film thickness is too small and the metal impurity particles in the lubricating oil are too large and the dimension of the metal particles is numerically greater than the minimum oil film thickness, when the metal particles pass through the minimum oil film thickness with the lubricating oil, it is like causing metal contact. In severe cases, the bearing bush will be burned. In addition, if the minimum value of oil film thickness is too small, it is easy to cause metal contact between the journal and the oil film bearing in the event of an accident such as stacking steel, resulting in the burning of the bearing bush.

The value of the minimum oil film thickness is related to the structural size and material of the oil film bearing, the machining accuracy of related parts, the installation accuracy of the oil film bearing system, the kind of lubricating oil, the rolling force, etc. The bearing oil film is also called the pressure oil film. To balance the load, isolate the journal and the shaft sleeve, convert the solid friction between the metals into the molecular friction inside the liquid, and reduce the friction and wear to a minimum. Therefore, it can meet bearing pressure, impact resistance, conversion speed, rolling accuracy, structural size, service life, and other requirements. The four elements of rolling pressure, rolling speed, bearing clearance, and lubricating oil viscosity are matched to form an uninterrupted and stable bearing oil film to realize hydrodynamic lubrication to meet the friction and lubrication of the rolling mill under different operating conditions.

Therefore, the lubrication of oil film bearings often manifests in the following three forms: 

(1) When starting or stopping, although there is lubricating oil between the shaft and the bearing because the moving speed is equal to or approaches zero and hydrodynamic lubrication has not yet formed or gradually disappeared, the shaft and the bearing must be in direct contact, and at this time they are in a state of marginal lubrication or even semi-dry friction. 

(2) During the operation of the rolling mill, mixed lubrication may occur due to vibration, excessive water intake, insufficient oil supply, or oil quality problems. 

(3) When the rolling mill runs normally and smoothly, it is fluid-lubricated. 

Therefore, the lubrication characteristics of oil film bearings are mixed lubrication, in which the above three situations exist alternately.

2. Lubricating oil characteristics of rolling mill oil film bearings

In order to meet the needs of high speed, heavy load, automation, large-scale, and high production of iron and steel enterprises, solve the lubrication requirements of oil film bearings of rolling mills, and meet the increasingly harsh working conditions, there are higher requirements for lubricating oils used in oil film bearings of rolling mills. The lubrication characteristics of rolling mill oil film bearings determine that lubricating oil must meet its performance requirements in order to ensure the normal operation and continuous production of the rolling mill. Therefore, the lubricating oil needs to have the following properties:

(1) Excellent viscosity-temperature performance (high viscosity index), which can still achieve normal lubrication of various lubrication parts when the bearing temperature fluctuates greatly. 

(2) Excellent anti-emulsification performance (that is, water separation), which can quickly separate water from oil during long-term use.

(3) Good anti-wear and extreme pressure performance When a small amount of water is mixed into the oil during operation, an oil film can still be formed to maintain heavy-duty and anti-wear performance. 

(4) Good anti-wear, anti-rust, and anti-foam properties prevent the lubrication system from rusting, blocking the oil circuit, causing wear, and insufficient oil supply. 

(5) Good oxidation stability, cleanability, and filterability, so that the oil passage of the lubrication system is smooth and the lubrication is normal.