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1. Various mechanical scars, such as abrasion, scratch, crush and collision, will cause poor bearing installation, eccentric load and stress concentration, and reduce the rotation accuracy and service life.
2. Rust, black skin and pitting. The latter two are defects that are easy to store moisture and dirt, which are easy to develop into rust. Corrosion is the pollution source leading to poor installation, early wear and fatigue. Serious corrosion will scrap the bearing.
3. All kinds of cracks, such as raw material cracks, forging cracks, heat treatment cracks and grinding cracks, will become the source of stress concentration and expand rapidly in the later operation of the bearing, resulting in bearing fracture, which has a great impact on the service life and workability of the bearing.
4. Peeling and folding, the local combination of these two defects with the base metal is not firm, and decarburization or carbon deficiency often exist around them to varying degrees. The material is easy to collapse, dent or wear, which is very unfavorable to the service life and accuracy of the bearing.
5. For the riveting or welding quality of the cage, mainly observe whether the rivet head is offset, skewed, loose, missing meat or “double eyelid”, whether the welding position is correct, whether the welding spot is too large or too small, and whether the rolling element is stuck due to poor welding or excessive welding.
In fact, when considering the limit speed of NTN bearing, not only the type of NTN bearing, but also the components of the bearing, such as the material, form, design and process factors of cage, but also the sliding friction inside the bearing, the storage and maintenance of lubricant, the stability of rolling element operation, etc, Sometimes it will become the key to whether the bearing can reach the limit speed. The life requirements and reliability requirements of the bearing are considered uniformly. Generally, the reliability coefficient is 90%, which means that 90% of NTN bearings operating under the same working conditions still maintain normal state when reaching the predetermined working life L10. In order to obtain higher reliability, such as 96%, the required life of NTN bearing must be reduced, for example, the life of L10 must be reduced to L4.
Generally speaking, for the main shaft bearing, the high requirement is that it has the same or slightly higher service life as the whole machine, and the low requirement is that the service life is longer than the overhaul period of the main shaft components. In fact, the comprehensive consideration scheme that can meet the performance requirements of the main engine without causing the main shaft system to be too large and cost-effective is mostly adopted. For most machines, There are quite successful precedents for reference. It should be emphasized that the failure forms of NTN bearings, as well as wear, lubrication failure and fretting abrasion, shall also be considered in the type selection calculation. Among them, the wear life must be checked, and other failure forms must also be considered if involved.
Note that the expected fatigue life and wear life of NTN bearing are not only related to lubrication, but also directly related to the cleanliness of the installation position of the bearing, the bearing itself and the lubricant used for NTN bearing, but also related to the form and effect of sealing. The rated life of the selected NTN bearing must adapt to the conditions of lubrication, cleanliness and sealing provided, Improving these conditions can also improve the actual service life of NTN bearings.
When considering the load factor, it should be noted that the service life of the bearing is more sensitive to the load than the speed. Therefore, for multi-stage transmission shaft, if possible, the larger deceleration should be placed in the later stage; For the main shaft bearing, if possible, properly adjust the distance between the external load action point and the bearing fulcrum, strive to reduce the load of the main shaft NTN bearing, and strive to ensure that the load transmitted to the main shaft NTN bearing is stable and avoid impact and vibration, which requires that the shape and position accuracy of the bearing installation part is high enough, and the clearance and fit of the NTN bearing are selected correctly, The installation accuracy of the bearing is good.