1. Analysis of the causes of damage to SKF support roller bearings
The radial bearing of the support roll of the continuous rolling mill adopts cylindrical roller bearings. During the rolling process, the axial force acting on the work roll and the support roll is borne by the work roll bearing and the support roll thrust bearing respectively, causing abnormal damage to the support roll bearing. The main reason is that the axial force is too large, causing damage to the axially positioned locking plate.
① Causes of axial force
Ordinary four-high plate and strip mills can be considered to have no axial force in an ideal state. Because of the crossed rolls of the PC mill, a part of the axial force will be generated during the PC input process, so it is considered in the selection of the support roll of the PC mill When a part of the axial force is reached, the rolling mill will inevitably generate axial force due to a series of factors such as the rolling mill manufacturing and assembly errors, the transverse thickness error of the raw material, and the wear and deformation of related parts during the use of the rolling mill.
② Main factors of axial force
Due to the effect of the offset distance between the rollers, during the rolling process, the support roller is always close to the side of the entrance arch, and the work roll is close to the side of the exit arch. Analyze the site situation and search for the cause of the huge axial displacement of the support roller. It was found that the position of the PC cross head was off, which caused the work roller axis and the support roller axis to be non-parallel, and the two roller axes produced a certain angle
The surface linear velocity of the work roll is decomposed along the axis of the support roll and the axis perpendicular to the axis, the resulting component Vb is equal to the linear velocity of the support roll surface Vb, that is, Vb=Vwcosa =Vb1, and the component Vwc of Vw along the axis of the support roll Vwc=Vwsina. During the rolling process, the rollers are affected by the rolling force P, and the contact surface of the two rollers is a narrow belt. On this narrow belt, the two rollers have a relative sliding speed Vwe along the axis of the supporting roller, which can be known according to the friction theory. : The support roller must give the work roller a sliding friction force opposite to the Vwc direction, and it itself is also subjected to a sliding friction force opposite to the Vwc direction, that is, the axial force generated by the crossing angle.
③ Other factors that produce axial force
In addition to the axial force caused by the non-parallel axis, the following factors also produce axial force:
A) The taper of the roll causes the angular velocity of the work roll and the supporting roll to be different at the two ends, which makes the axes cross;
B) The assembly of the archway and the roller seat;
C) The friction coefficient of the oil film bearing of the backup roll is increased or the lubrication conditions of the oil film bearing at both ends are different;
D) The horizontal component of the work roll head caused by the inclination of the cardan shaft
However, these forces have little effect compared to the frictional forces generated by the cross-axis of the rollers.
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2. Examples of damage to SKF support roller bearings
① The back-up roller bearing retainer ring scratches and causes
The abrasion between the flange of the ring and the end surface of the rolling element is called a jam. Due to the hardness of the rolling element, the surface of the retaining ring is generally rough, accompanied by a small coating, and the surface of the inner ring is horizontally scratched.
Causes of scratches on the retaining ring and inner ring:
(1) The size design of the retaining ring is unreasonable, and the contact damage during the assembly process;
(2) Intrusion of hard foreign body;
(3) Deflection of rolling elements caused by bearing tilt;
(4) The rolling elements slip.
② Analysis of bearing indentation and wear
The surface pits caused by the jamming of solid foreign objects or impacts and the shedding tissue formed by scratches during installation, which produce indentation and wear under the action of relative sliding friction between bearing parts. Continuous wear will change the size and shape of the parts, increase the matching clearance of the bearing, and deteriorate the topography of the working surface, thereby losing the rotation accuracy and making the bearing unable to work normally.
The main reasons are excessive load, poor installation (non-linearity), large moment load, and poor sealing.
③ Analysis of the wear of the support roller bearing
The operating time of the support roller bearing is 12-15 days per cycle, and the cumulative time of online operation is about 5 months per year. The oil is disassembled and inspected every 12 to 16 months, and the load area is rotated. After disassembly and inspection, we found that the wear condition of the bearing is quite different from the wear pattern of bearings used in other occasions.
There are two main reasons for the abnormal wear of this rolling mill support roller bearing:
(1) The large internal clearance of the bearing reduces the contact angle, the number of rolling elements involved in the load during operation is small, the local contact stress is too large, the bearing capacity is reduced, and the oil film is damaged, and metal-to-metal contact produces adhesion. Wear.
(2) The large clearance causes the rolling elements to not make close contact with the outer ring raceway when they exit the load zone, and the rolling elements are not purely rolling. The tentacles of the cage increase the load carrying capacity and reduce the wear of the bearing.