Oil film bearings, also known as liquid friction bearings or ideal sliding bearings, are one of the key core components of modern rolling mills. With the continuous development of science and technology, users have increasingly higher quality requirements for rolled products, especially strict precision requirements for thin plates, and rolling speeds have also increased. For example, the speed of high-speed wire rods has exceeded the 100m/s level.
Due to their advantages over rolling bearings, such as larger load-bearing capacity (generally more than 3 times that of rolling bearings), longer service life (theoretical lifespan of 10-15 years), wide speed range, and strong impact resistance, oil film bearings are increasingly widely used in the rolling industry.
An oil film bearing is a radial sliding bearing that uses lubricating oil as the lubrication medium. Its working principle is as follows: During the rolling process, the rolling force causes the roll neck to move, creating an eccentricity between the center of the oil film bearing and the center of the neck. This forms two regions in the gap between the oil film bearing and the neck: one is the divergent zone (where the gap gradually increases in the direction of neck rotation), and the other is the convergent zone (where the gap gradually decreases in the direction of neck rotation). As the rotating neck carries viscous lubricating oil from the divergent zone into the convergent zone, the bearing gap decreases along the direction of neck rotation, forming an oil wedge that generates pressure within the lubricating oil.
The resultant force of the pressure at each point in the oil film along the rolling direction is the load-bearing capacity of the oil film bearing. When the rolling force exceeds the load-bearing capacity, the eccentricity between the neck center and the oil film bearing center increases. In the convergent zone, the bearing gap becomes steeper along the direction of neck rotation, the minimum oil film thickness decreases, the pressure within the oil film increases, and the load-bearing capacity rises until it balances the rolling force, after which the neck center stops shifting. The oil film bearing and the neck are completely separated by lubricating oil, theoretically achieving full fluid lubrication.
From the working principle of the oil film bearing, the most critical parameter in the oil film bearing system is the minimum oil film thickness. If the minimum oil film thickness is too small, and the metal impurity particles in the lubricating oil are too large (with particle sizes numerically larger than the minimum oil film thickness), the metal particles will cause metal-to-metal contact as they pass through the minimum oil film thickness zone with the lubricating oil. In severe cases, this can lead to bushing burnout. Additionally, if the minimum oil film thickness is too small, metal contact between the neck and the oil film bearing is likely to occur during accidents such as steel stacking, also resulting in bushing burnout. The size of the minimum oil film thickness is related to factors such as the structural dimensions and materials of the oil film bearing, the machining accuracy of related components, the installation accuracy of the oil film bearing system, the type of lubricating oil, and the magnitude of the rolling force.
The sealing structure of oil film bearings should be improved as much as possible to prevent water ingress. However, for oil film bearings in water-prone areas, especially those in high-speed wire rod rolling mills (which are in dynamic sealing conditions), water ingress is inevitable.
When water enters the oil film bearing oil, it can hydrolyze and deactivate certain additives. It may also form turbulence, disrupt the normal fluid lubrication during operation, cause abnormal wear, easily corrode equipment, emulsify the oil, and deteriorate its demulsibility, oxidation resistance, and anti-foaming properties. Furthermore, water can combine with oxidation products and sludge generated during oil use to form gelatinous oil masses, which may clog filters.
Solutions should be implemented from two aspects: improving the sealing structure and enhancing the wear resistance of sealing rubber rings.
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