Abstract: The rolling mill gear seat bearing was originally made of sliding bearing with split casting bearing alloy. After the rolling mill speed was increased to meet the market requirements, serious heating of gear seat and burning accident of sliding bearing occurred from time to time, so the production could only be maintained at medium and low speed. In addition, when the sliding bearing is replaced, the gear shaft needs to be re-polished. The replacement cycle is generally about 15 days, which is very unfavorable to production. After the gear seat bearing of Φ600 mm rolling mill was changed from sliding bearing to four-row cylindrical roller bearing, the production efficiency was greatly improved, the replacement cycle was shortened, and no rolling bearing damage was found in the overhaul for one year.

Key words: four-row cylindrical roller bearing; Gear seat bearing; Sliding bearing; design

The plain bearing has the characteristics of smooth operation, no noise, small radial size, impact resistance and large bearing capacity, so the original gear bearing is widely used in the split cast bearing alloy plain bearing. But in the process of using the sliding bearing, the accuracy requirements (especially the horizontal accuracy) are high, that is, the two ends of the gear shaft must be in a horizontal line, otherwise it will change the contact clearance between the journal and the bearing due to the tilt of the drive shaft, thus destroying the formation of the oil film. Both ends of the bearing bush installation requirements are demanding, time-consuming and laborious. The working temperature of gear seat bearing is required to be less than 60℃. When the rolling speed is increased, the friction power consumption increases, leading to the bearing heating and then damage phenomenon is very prominent. In addition, the installation and maintenance of a gear seat needs about 15 days, the labor intensity is large, seriously affect the production time.

Many domestic research institutions and user manufacturers have done extensive research on the heating problem of rolling mill gear seat bearing [1-5], and the analysis shows that the heating of gear seat bearing is mainly caused by the characteristics of use occasions, the change of service conditions and the inherent weakness of sliding bearings. Although improving lubrication conditions and adding wear reducer can reduce heating to a certain extent, the effect is very limited, and the problems of installation and maintenance can not be completely solved.

Rolling bearings have a series of advantages such as small starting friction, less wear, easy lubrication, wide operating temperature range and easy maintenance, and are used more and more widely. In Baotou Iron & Steel Group Steel mill, four rows of short cylindrical roller bearings bearing large radial loads have been used to replace plain bearings and achieved good results. The following is an example of the transformation of gear seat bearings of Φ600 mm rolling mill.

1. Selection of rolling bearing

The force analysis of herringbone gear at work is shown in Figure 1. As the friction between tooth surfaces is relatively small, the effect of friction is ignored in the analysis in Figure 1. The normal force Fn in the normal occlusal plane perpendicular to the tooth surface can be decomposed into circumferential force Ft, radial force Fr and axial force Fa.

Where: T is the transfer torque; d is the diameter of gear indexing circle; α is normal surface pressure Angle; Beta is the pressure Angle of the dividing circle. According to the force analysis on the herringbone gear, it can be seen that the two axial forces on the herringbone gear cancel each other, the values of the circumferential force Ft and radial force Fr increase with the increase of the torque T, and the radial force Fr on the driving wheel and the driven wheel points to the center of the gear respectively. With the increase of the radial force Fr, the radial force on the gear shaft also increases.

According to the working condition and force analysis of rolling mill gear seat bearing, it can be known that the rolling mill gear seat bearing only bears large radial force while working, but not axial force. The four-row cylindrical roller bearing has large radial bearing capacity, stable performance, convenient and simple installation and debugging, and because the inner raceway of the bearing is cylindrical, higher machining accuracy can be obtained, so it is decided to choose the four-row cylindrical roller bearing as the rolling mill gear seat bearing.

2. Structural design of bearing

The industrial standard JB/T5389.1-2005 stipulates the structure and technical conditions of four row cylindrical roller bearings for rolling mill. In view of the situation of Φ600mm unit in Baotou Iron & Steel Group Company, the FCDP structure of four row cylindrical roller bearings is selected. FCDP structure consists of two inner rings and two outer rings with flat retainer rings. The schematic diagram and its model are shown in Figure 2.

Rolling mill gear seat bearing reform is mainly to meet the market requirements to improve the rolling mill speed. In addition, the rolling mill gear seat bearing in the work to bear a certain impact load, considering the working condition of the rolling mill gear seat bearing, should choose the surface carburizing, quenched surface hardness is high, and the core hardness is low, can withstand the impact load material, here choose G20Cr2Ni4A steel. The dimensions of four row cylindrical roller bearings are mainly designed according to the original size conditions of gear seat. Because the gear shaft wear degree of the unit is different, under the premise of checking the strength of the gear shaft, the size of the rolling bearing can be determined according to the size of the gear shaft and gear shaft seat hole.

In order to maximize the bearing capacity, hollow rollers welded by struts are used for the rolling body. Because there is impact load in the working process of gear seat bearing, it means that rollers should bear different degrees of edge load in the working process. In order to avoid or reduce the influence of the edge load, the roller should be convex. In the trial production process, the modified roller with the plain line as the arc correction line is used. The cross-section shape of rollers is shown in Figure 3, and the convex metric is determined by equations (4), (5) and (6).

Where: W refers to roller load, which is generally calculated by taking the maximum roller load in bearings; δ is the convexity value of the roller, Lw is the length of the roller; t is the length of the trimming arc; Rc is the radius of the dressing arc; t1 is the chamfer length.

3 groups are selected in the original clearance range of bearings to the upstream clearance; The fit between the journal and the bearing inner ring is interference fit, and the tolerance of the journal is P6. The fit between the bearing seat hole and the bearing outer ring is a clearance fit, and the tolerance of the bearing seat hole is G7. In the process of processing, it is also necessary to take into account the difference in the expansion coefficient of shaft and seat hole material, and control the fit tolerance at different temperatures to meet the installation and fit requirements of rolling bearings.

3.Trial Situation

The trial production of four columns of cylindrical roller bearings installed on the Φ600mm unit, improve the rolling speed, continuous production is not found bearing seat heating phenomenon, stable operation, a year later when the main engine overhaul still not found bearing damage, and bearing installation, replacement is convenient, the original maintenance time is shortened 3 ~ 5 days, save the transformation time; At the same time can also be the original frame and gear shaft reasonable use, save costs.

References:

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