the limit speed, the required service life and the scheduled working load, which are the three performance requirements that the selected bearing must meet. The limit speed is the high allowable working speed of the bearing. Beyond this limit, the operating temperature of the bearing will continue to rise, the lubricant will dry up, and the bearing will be stuck. Therefore, the limit speed of the selected bearing should be 10% higher than the predetermined high working speed of the bearing. When the bearing type has not been selected, it is advisable to calculate the average diameter DM of the bearing by the sum of the shaft and seat hole diameter in the part where the bearing is to be installed. Multiply DM by the predetermined high working speed n of the bearing, and the obtained DMN value shall not be 10 / 9 times higher than the limit speed of the selected bearing.
However, the limit speed of SKF Bearing is not invariable, but directly related to the lubrication mode. For example, the limit speed of the bearing under the oil lubrication condition listed in the bearing sample is N0, the limit speed of the same bearing under the grease lubrication condition is 0.8n0, and it is 11.5n0 under the oil mist lubrication condition; Under the condition of constant volume non repetitive circulating oil lubrication, it is 0.85n0 for continuous operation of small oil tank and 0.95n0 for intermittent operation of large oil tank. The lubrication method equivalent to N0 value is the repeated circulating oil lubrication with filter and heat sink. Note that instead of adopting this lubrication method or other lubrication methods with the same effect, the above N0 value in the bearing sample cannot be taken as the actual limit speed of the bearing, but N0 should be multiplied by a proportional constant according to the specific lubrication method and the above example, Take the obtained value as the actual limit speed of the bearing;
Although the actual limit speed of the bearing can be improved by improving the lubrication, the equipment cost and maintenance cost due to improving the lubrication and the space occupied by the lubrication equipment should be taken into account to comprehensively weigh the gains and losses and make a choice. In fact, when considering the limit speed of SKF Bearing, not only the type of 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. Usually, the reliability coefficient is 90%, which means that 90% of the bearings operating under the same working conditions still maintain normal state when reaching the predetermined working life L10. If you want to obtain higher reliability, such as 96%, you must reduce the required life of the bearing, for example, you must reduce the life of L10 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 type selection calculation should also consider the failure forms of bearings, as well as wear, lubrication failure and fretting abrasion. 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 the 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 by the bearing, but also related to the form and effect of sealing. The rated life of the selected SKF Bearing must adapt to the conditions of lubrication, cleanliness and sealing provided, Improving these conditions can also improve the actual service life of the bearing. 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 bearing, and strive to ensure that the load transmitted to the main shaft bearing is stable and avoid impact and vibration.
This requires that the shape and positioning accuracy of the bearing installation part is high enough, the clearance and fit of the bearing are selected correctly, and the installation accuracy of the bearing is good.The more the SKF Bearing calculation load selected in the type selection design can represent the actual load of the bearing, the more correct the calculation result is. Therefore, it is necessary to study the actual load of the bearing extensively and deeply. If the bearing not only bears a large load, but also stands still without rotation, this state is more unfavorable. A small indentation will be formed between the rolling element and the raceway and will soon develop into pitting corrosion, resulting in early failure of the bearing. In this case, it is necessary to verify whether the rated static load of the selected bearing can exceed the equivalent static load of the bearing.
The other extreme case is just the opposite. The bearing operates at high speed but does not bear load. In this way, the rolling element often slides on the raceway instead of rolling, resulting in rapid rise of bearing temperature, decomposition and deterioration of lubricant, and rapid failure of bearing. Therefore, the bearing needs a small load to prevent sliding. In some cases, the axial force and radial force generated by the equipment itself, the weight of parts such as gears and even the weight of the shaft, and the stress of relevant parts must also be considered. They are all calculated in the afterload and load borne by the bearing, which are often easy to be ignored and omitted. In many cases, the actual load of the bearing increases significantly due to the influence of vibration. When the coaxiality error of the bearing is large (misalignment), the actual load of the bearing will also increase. Therefore, the calculated load value must be corrected