How to choose the suitable tapered roller bearings for mining trucks
Aug 26, 2022
As one of the important equipment necessary for open-pit mining, mine trucks have the characteristics of short haul and heavy load. In order to improve the reliability of mining trucks, tapered roller bearings are mostly used for their hubs. So how to choose the suitable tapered roller bearing for mine trucks?
In addition to the factors of system design matching and installation adjustment, the following three factors determine the performance of tapered roller bearings:
1. Bearing material
2.Surface finish of bearing raceways and rollers
3. Bearing raceway surface modification.
When considering the material of the bearing, two points are mainly considered: the purity of the steel and the heat treatment process of the steel.
When the bearing is in normal operation, when the surface of the raceway has peeling of 6mm², we call it a failed bearing. The time the bearing runs is its fatigue life. The occurrence of spalling is mainly due to the existence of non-metallic inclusions inside the bearing steel. When the bearing is working, non-metallic inclusions will cause stress concentration inside the steel, resulting in weak points. As the bearing runs day after day, these weak points slowly develop into fine cracks, and finally cause the surface of the raceway to peel off. Therefore, the purity of the steel is very important for the bearing. The smaller the particle volume of the inclusions in the steel and the less the number of particles, the smaller the probability of cracks and the higher the fatigue life of the bearing.
The bearings produced by our company meet the ISO9001 standard, and the selected steel is also high-precision steel, which greatly improves the reliability of the bearing under high load. In addition to the purity of the bearing steel, the heat treatment process of the bearing also has a considerable impact on the performance of the bearing. CHG has brought in heat treatment line of the word-class AICHELIN technology, pusher furnace carburizing quenching and roller hearth furnace secondary quenching, and can visually display its entire production, monitor and record automatically its processing parameters and quality data. The treated steel is called ordinary hardened steel. The surface of the treated steel is harder and wear-resistant than the ordinary hardened steel, and the interior is softer. The surface of the steel produces compressive stress, which can buffer the impact load and reduce the possibility of impact cracks.
Tapered roller bearings used in mining trucks are mainly used in axle packs and wheel ends, where the working environment is very harsh, often accompanied by shock loads and heavy loads. Coupled with the unavoidable abrasive particles generated by the casting itself, as well as the external contamination caused by the possible failure of the oil seal, these have brought a huge test to the bearing itself. In the case of abrasive particles and heavy loads, the raceway surface of the bearing will inevitably produce pits. If a fully hardened bearing is selected, under the dual influence of the brittleness of the material itself and the surface tensile stress, the development of this pit will be It is relatively rapid, causing the bearing to fail prematurely before reaching the fatigue life. If carburized bearings are selected, due to the toughness and surface compressive stress of the material itself, the pit is not easy to expand, so that the bearing can still achieve a good fatigue life. Based on the above reasons, we should try to use carburized bearings when selecting bearings for mining trucks.
2. Surface finish of bearing raceways and rollers
In addition to the material factors just mentioned, another major factor that affects the bearing performance is lubrication. According to statistics, 70-80% of premature bearing failures are caused by improper bearing lubrication. Usually we use oil or grease to lubricate the bearing. When the lubrication system works on the bearing, an oil film will be produced on the metal surface. If we observe the bearing raceway with a high magnification magnifying glass, we can see that the raceway surface is not smooth, but has many small metal high points, like "mountains". If the "mountain" between the raceway and the raceway is in direct contact when the bearing is working, the "mountain" will break, and the surface of the raceway will be micro-flaked, which will eventually cause damage to the bearing surface. The purpose of the oil film is to keep the small "mountains" between the raceways and the rollers out of direct contact. To play this role, the thickness of the oil film must exceed the sum of the roughness of the two contact surfaces. When we find that the oil film is not thick enough to separate the two metal surfaces, there are two options: use a more viscous oil or grease; reduce the roughness of the raceway surface. Because mining trucks generally use SAE80W-90 oil bath lubrication on the axle bag, and use No. 2 grease with EP additives on the wheel end. Under the influence of heavy load and low speed, insufficient oil film thickness may occur. If it is not possible to change the viscosity of the gear oil, we can improve bearing performance by improving the surface finish of the raceways and rollers.
3. Bearing raceway surface modification
When the bearing is under force, the center lines of its outer ring and inner ring may not be in a straight line, and there may be an included angle, which will cause the bearing to be misaligned during operation. This misalignment may be caused by improper installation, or it may be caused by slight deformation of the shaft or bearing seat under heavy load, it may be caused by the error of the shaft and bearing seat during machining, or the above factors comprehensive. The rated load marked on the sample of the tapered roller bearing means that the included angle should be less than 0.0005 radian when the bearing is working. When the misalignment angle is less than 5/10,000 radian, it will not affect the life and performance of the bearing, but when the angle exceeds 5/10,000 radian, it will have a certain impact on the performance of the bearing. This misalignment will cause stress concentration in the bearing raceway, so that the bearing cannot reach the designed fatigue life and fail prematurely. This failure mode usually occurs at the edge of the raceway and exhibits a diagonal pattern up and down 180 degrees.
Bearing damage caused by this misalignment is very common in the bearing application of mining trucks, mainly due to the large load of mining trucks, which causes shaft deformation. When this kind of bearing damage occurs, if it is not carefully analyzed and attributed to the insufficient bearing capacity of the bearing, the result is often unsatisfactory by simply increasing the size of the bearing. For the case where the bearing life is eventually low due to the excess of the allowable stress on the raceway boundary, the bearing with raceway modification can be selected. This kind of bearing can solve the problem of stress concentration at the edge of the raceway without increasing the size of the bearing, so that the stress on the surface of the raceway is always within the required range of the material.
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