1. The fit and clearance ofhigh-speed bearings Since high-speed bearings must meet the requirements of both high-precision bearings and high-temperature bearings, the following two points should be taken into account when considering their fit and clearance:
( 1 ) Dimensional changes and hardness changes when rising from room temperature to high temperature;
( 2 ) Changes in force system and shape caused by centrifugal force at high speed.
In short, it is difficult to maintain the accuracy and working performance of the bearing through the selection of fit and clearance under high speed and high temperature conditions.
In order to ensure that the raceway deformation is small after the bearing is installed, the interference fit should not be too large. The centrifugal force at high speed and the thermal expansion at high temperature will either offset the normal pressure of the mating surface or make the mating surface loose. Therefore, the interference fit must be carefully calculated under the premise of considering the above two factors. The effective interference fit at normal temperature and speed may be invalid for high-speed bearings.
If the contradiction between the calculated results is too great (usually only occurs at ultra-high speeds), only dual lubrication measures, combining under-the-ring lubrication and hydrostatic lubrication, can be adopted. This approach may allow the bearing's DMN value to exceed the 3 million mark.
When considering the clearance of high-speed bearings, not only the above factors should be considered, but also the influence of thermal expansion of the shaft on the clearance. The bearing is required to have the best clearance under working conditions, that is, under working temperature. This clearance is formed when the center of the inner and outer ring ball grooves are precisely aligned. Since high-speed bearings strive to reduce relative sliding and internal friction, it is best not to use the method of relative axial misalignment of the inner and outer rings to adjust the clearance of the ball bearing.
When considering the interference fit and clearance of the bearing, it is necessary to note that the material becomes soft and easily deformed at high temperatures, as well as the possibility of certain permanent deformation caused by multiple temperature changes from room temperature to high temperature.
2. Requirements for host-related parts
High-speed bearings require that the rotating system in which the bearings are located undergo precise dynamic balancing, and that the parts of the shaft and seat hole where the bearings are installed have dimensional accuracy and form and position accuracy that are higher than general requirements, especially the coaxiality and the perpendicularity of the shoulder to the seat hole or journal. When considering these issues, the high-speed and high-temperature factors during the operation of the bearings must also be taken into account.
The shaft support system requires both high rigidity and as light a mass as possible. To overcome this contradiction, measures such as reducing surface roughness and increasing surface strengthening can be taken to increase support stiffness, and hollow shafts can be used to reduce system mass.
3. Development example of ultra-high-speed bearings Let's take the development of ultra-high-speed HA type tapered roller bearings as an example.
( 1 ) Problem statement
In gas turbines and some machine tools and engineering machinery, high speed and large axial load will shorten the service life of ball bearings, while the axial load capacity will be insufficient and the axial clearance will be difficult to adjust if short cylindrical roller bearings are used. It is hoped that tapered roller bearings can be used to overcome this problem.
( 2 ) Key technical issues that must be resolved
The technical key to improving the high-speed limit of tapered roller bearings lies in improving the lubrication condition between the large rib of the inner ring and the large end face of the roller. This part is most prone to severe wear and burns at high speeds, which is the main reason for limiting its high speed.
( 3 ) Solution
In tapered roller bearings of ordinary structure, it is difficult for lubricating oil to flow through the contact area between the large rib of the inner ring and the large end face of the roller, while this area has relatively large sliding movement and is exactly where lubricating oil is most needed.
Therefore, Japan and other countries have developed HA- type tapered roller bearings. The ribs of this type of bearing are on the outer ring, so that the circulating lubricating oil can lubricate the contact area between the outer ring rib and the large end face of the roller. At the same time, some oil can be stored here even when it is stationary, avoiding the accident of oil-poor burns during starting. However, several oil drain holes are opened on the outer ring rib as needed to avoid the oil from being retained in a certain part without a discharge channel, causing power loss and excessive temperature rise due to oil stirring. Since there is no rib on the inner ring, the temperature is reduced, thereby reducing the possibility of creep between the mating surfaces between the inner ring and the shaft. This type of bearing structure adopts an outer ring guide method for the cage, so that the cage can guide the roller to operate normally without skewing more smoothly, avoiding vibration and excessive wear, which is also conducive to high speed.
( 4 ) Effects that can be achieved
The dmn value of this HA type bearing can reach 2 million, which is twice as high as that of ordinary structures. For example, this type of bearing (model I-IA30205 ) used for the main shaft of the gas turbine reducer can ensure a 1000N axial load.2LThe bearings are supplied with No. 4 turbine oil in a circulation cycle at an oil supply rate of / min , and their operating speed can reach 60,000 rpm without causing burns.
