Writer: Eric Bearing Limited
1.Analysis of bearing damage and causes
The reasons for bearing damage are complex. Bearings often work at high temperatures, which will reduce the strength of the bearings; frequent overloading of bulldozers causes frequent axial impacts on the bearings, causing plastic deformation of the balls and raceways and damage; lubricating oil Dirty cleaning can cause early wear on the bearings. These reasons can be attributed to the operator's overuse and improper maintenance of the bulldozer, which makes the bearings work for a long time under very harsh conditions. Various adverse effects act on a bearing at the same time, causing early damage to individual bearings. In order to reduce the failure rate of the torque converter to zero, special requirements have been formulated for the design, manufacture, selection and installation of bearings.
2.Special requirements for bearings in design, manufacture and selection
In order to improve the reliability of this pair of bearings under harsh working conditions, special requirements are put forward in the design, manufacture and selection of bearings:
(1)Selection of contact angle
IKO bearings are angular contact ball bearings. Angular contact ball bearings can bear both radial and axial loads. The axial load capacity is determined by the contact angle. The greater the contact angle, the greater the axial load capacity. Angular contact ball bearings are available in three contact angle specifications, namely 15°, 25° and 40°. The original TY320 hydraulic torque converter uses a bearing with a contact angle of 25°. A bearing with a contact angle of 40° is suitable for bearing larger Axial load, this pair of bearings must bear a large axial impact load when the bulldozer is under heavy load, so we use 7215B/DB bearings with a contact angle of 40°. (The post code B means the contact angle is 40°)
(2)Optimized design of inner ferrule
There are two specifications for the inner ring of 7215B/DB bearings. The diameter of the shoulder on both sides of an inner ring raceway is the same. Since the angular contact ball bearings can only bear axial loads in one direction, the inner ring raceway The shaft shoulder on one side will not be subjected to axial load, and the shaft shoulder on the other side cannot bear a large axial impact load. Through optimized design, under the same weight, the diameter of the shoulder on both sides of the inner ring raceway of another specification is different. The diameter of the shoulder on the side that does not bear axial load is reduced, and the shaft on the side that bears axial load The shoulder diameter is increased, so that this type of bearing can withstand greater axial impact loads. Therefore, we choose the inner ring with the optimized design of the latter specification.
(3)Requirements for cage materials and styles
The mechanical stress suffered by the cage comes from friction, strain and inertia, and it is also eroded by certain lubricating oil and its aging products, organic impurities and other chemical substances, so the choice of cage material affects the reliability of bearing operation Great. Because the bearings work at higher temperatures, phenolic laminated cloth tube solid cages or nylon cages cannot be used, and they are frequently subjected to large axial impact loads, so we choose 7215BM/DB bearings with brass solid cages. (The post code M means brass solid cage). Due to the different diameters of the shoulders in the bearing and the outer ring raceway, the space left for the cage is not large. If an inner ring-guided cage is used, the thickness of the cage is thinner. In order to strengthen the strength of the cage, the cage is designed as a ball-guided cone-shaped cage.
(4)Determination of the amount of preload deformation δ
Angular contact ball bearings installed in pairs are pre-tightening deformations that have been taken into account in bearing production. A certain amount of pre-tightening deformations are worn off on the end faces of the inner or outer rings of the two matching bearings When the pair of bearings is installed on the bearing parts, the corresponding end faces are pressed with the fastening device, and the two bearings are in a pre-tensioned state.
The purpose of preloading the bearing is: to increase the rigidity of the bearing; to correctly position the rotating shaft in the axial and radial directions, to improve the rotation accuracy of the shaft; to reduce the vibration and noise of the bearing parts; to prevent the rolling and revolution slippage of the rolling elements; Large internal load area of the bearing improves the rationality of the internal load distribution of the bearing; compensates the change of the internal clearance of the bearing due to wear; a reasonable preload can prevent vibration ripple damage and reduce friction and wear; extend the bearing life. IKO LM132332 UU bearings online , pls click here :
However, if the amount of preload deformation is too large, the friction torque will increase when the bearing is in operation, resulting in overheating and reduced service life of the bearing. Therefore, it is necessary to select an appropriate amount of pre-tensioning deformation according to the load conditions and use requirements, and determine the appropriate amount of pre-tensioning deformation after testing.
The actual bearing's preload deformation is different in the four states of working state, installation state, natural state and manufacturing state. The increase in the temperature in the working state increases the preload deformation, and the interference between the hole and the shaft in the installation state also increases the preload deformation, so the preload deformation of the bearing in the manufacturing state is smaller than the preload in the working state. The amount of deformation is determined by the temperature in the working state and the tolerance of the mounting hole and the shaft. After actual installation and test operation, we have determined the most suitable preload deformation.
3.Special requirements for bearings in installation process
Improper operation during installation is an important hidden danger that causes early damage to the bearing. We also put forward special requirements in bearing installation process:
(1) The bearing is not allowed to be installed by knocking, but must be installed by pressing.
(2) The special tooling designed by the press-in method must be used to install the bearings.
(3) When pressing into the bearing outer ring, the hole shoulder under the bearing hole of the pump wheel connecting disc must be padded to prevent internal injury at the root of the hole shoulder when pressed to the bottom.
(4) Before pressing into the inner ring, the gap between the single-position positioning cover and the bearing end face meets the assembly process requirements, as shown in Figure 3.
(5) When pressing into the inner ferrule, a guide device must be used to prevent damage to the shaft surface caused by pressure deviation.
(6) The two bolts of the inner ring pressure plate must be tightened after the 36 bolts of the pump wheel and the pump wheel connecting plate are tightened.
(7) After installation, there is no abnormal friction feeling when rotating the pump wheel connection disk. When rotating relative to the output shaft, it can be a little pre-tensioned interlocking feeling, and can not bite the output shaft.