Abstract: The reasons of premature failure of roll bearing in rough rolling mill of high speed wire rod production line were analyzed, and the corresponding technical improvement measures for the installation, sealing, lubrication, cleaning and inspection of roll bearing were put forward.

Key words: roll bearing; Failure analysis; Assemble; seal

Horizontal and vertical alternating arrangement of rolling mill in roughing and middle rolling zone of a high speed wire rod production line of a company. Due to the harsh working environment, high load, high working temperature and frequent impact, the roll bearing in the rough rolling zone often fails prematurely, and even the roll and bearing seat are scrapped. This not only affects the production efficiency, but also greatly increases the consumption of spare parts, resulting in an increase in production costs. In order to reduce the premature failure of roll bearing, the assembly, use and maintenance of roll bearing are analyzed.

1 Cause Analysis

FC6490240 four-row cylindrical roller bearings and 764H deep groove ball bearings are selected for the bearing steel high-speed wire roughing rolling unit. The installation diagram is shown in Figure 1.

The clearance of the bearing determines the size of the bearing working bearing zone. Too large clearance leads to a small bearing working bearing zone, that is, a small number of rollers to be carried, resulting in reduced bearing life [1]. In the assembly process, due to the defects in the design of the fixed wire sleeve (FIG. 2a), the locking nut and the fixed wire sleeve often do not fit well, and the bearing is not tightly compressed, resulting in the axial movement of the bearing inner ring, resulting in the change of the bearing clearance, and thus the premature failure of the bearing.

There are many kinds of friction in roll bearing operation, and the most serious part of the friction is the bearing area of the outer raceway. Due to the existence of radial clearance, the roller is in a rolling and sliding state in the non-bearing region [2]. When the roller enters the bearing zone, its rotation suddenly accelerates, and strong friction occurs with the raceway. The bearing area also bears the impact load of rolling steel. The sliding friction caused by the contact deformation between roller and raceway is the key factor for the damage of bearing parts. Therefore, the lubrication of bearings is particularly important. However, due to the harsh working environment of the roll bearing, it is easy to cause lubrication failure, so that the friction and wear of the contact surface inside the bearing are intensified, and eventually lead to premature bearing failure.

Poor sealing is also a major cause of premature bearing failure. Due to the extremely harsh working environment of the roll bearing, the "maze" of the labyrinth ring and the poor sealing effect of the sealing end cap (FIG. 3a), the external dirt invaded the bearing seat along with the cooling water, contaminated the grease, resulting in lubrication failure, resulting in premature bearing failure, and in serious cases, damaged the roll, bearing seat and other parts.

2 Improvement Measures

2.1 Installation Improvement

The material of the locking hook is changed from Q235 steel to 40Cr steel, and the keyway matching the fixed wire sleeve with the locking nut is changed from 1 to 6 evenly distributed, as shown in Figure 2b. After the roller bearing is installed, the locking nut, fixing wire sleeve and bearing gland can press the inner and outer rings tightly. After the roller bearing is assembled, the axial clearance should be measured as a whole and controlled at 0.45 ~ 0.60mm. Due to the change of bearing clearance caused by wear, the clearance can be adjusted by increasing or decreasing the bearing outer ring gland gasket, so that the bearing clearance is always in a reasonable range.

2.2 Sealing Improvement

The working environment of roll bearing in rough and middle rolling area is harsh, so the ideal bearing seal can effectively prolong the service life of roll bearing and improve the operation rate of equipment. Therefore, the sealing ring (FIG. 3) and the sealing end cover (FIG. 4) are improved, and a water seal (FIG. 5) is added to the sealing end cover. The improved sealing ring fits tightly around the water seal of the sealing end cap. At the same time, a waterproof cap is added to the outer edge of the sealing ring (FIG. 3b) to reduce as much as possible the impurities such as iron oxide and dust into the bearing seat with cooling water.

2.3 Lubrication improvement

Make a reasonable lubrication cycle, add new grease to the bearing regularly to ensure good lubrication of the bearing; At the same time, the contaminated grease can also be squeezed out of the bearing seat to clean the bearing; And when the old fat is discharged, it can also take away the dirt at the sealing place of the end cover.

2.4 Cleaning and inspection before assembly

(1) Before each assembly, the bearing and bearing seat should be cleaned and the oil circuit should be dredged (the oil accumulation in the oil circuit should be cleaned with high pressure air to keep the oil circuit unblocked). Check the bearing ring, rolling body and cage and other parts, such as wear, scratches, etc., should be replaced in time. Check the J-shaped oil seal to ensure it is undamaged and elastic. Check the surface finish and wear condition of the contact part between the sealing end cover and the J-shaped oil seal. The surface should be smooth without defects and the wear degree should be less than 1.2mm to ensure the normal operation of the J-shaped oil seal [3].

(2) Check the wear condition of the inner hole of the bearing seat before assembly, including absolute size and cylindricity. If the size exceeds the standard, it needs to be repaired to the standard size before use.

(3) Check the wear of the roller and bearing parts before assembly. If the size exceeds the standard, it cannot be put into use and must be repaired to the standard size before use.

(4) When installing all bearing caps, when the gasket group is adjusted, it must be sealed with sealant to prevent bearing water.

(5) Check whether the elastic damping body fails to ensure that it is normal.

3 Closing remarks

Through the improvement, the average service life of the roll bearing of the roughing mill has been greatly increased, the consumption of tons of steel and the hot downtime of the bearing have been greatly reduced, and the smooth operation of the production has been ensured.

【 Reference 】

[1] Wan Changsen. Rolling bearing analysis [M]. Beijing: China Machine Press, 1987:23-26. (in Chinese)

[2] Fang Shixing. High speed wire mill equipment technology [M]. Beijing: Metallurgical Industry Press, 1997.

[3] Zhou Jiannan. Rolling bearing for steel rolling machinery [M]. Beijing: Metallurgical Industry Press, 2001.