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Preload and stiffness of super-precision bearing layout design and application examples

by:JNSN     2021-12-20
In practical applications, a large number of different types of spindle bearings have been used. The choice of spindle bearing type and arrangement depends on the characteristics of the application: such as turning, milling, grinding, etc. However, operating conditions and economy also play a decisive role in the final selection and technical requirements. In order to achieve high precision requirements (P4 or higher), bearings generally must be preloaded or at least run under conditions of zero clearance. The temperature rise at the highest speed should be as low as possible (speed index, up to n~dm u003d 2~106 mm/min. under grease lubrication, and 3,1~106 mm/min. under oil lubrication.) . The above requirements can be achieved through the use of ultra-precision bearings and corresponding precision matching components. The following information can help select the most suitable bearing and bearing arrangement in the application, including the following aspects: preload, stiffness, contact angle, rolling element size and material, bearing spacing, sealing, bearing design steps, bearing arrangement comparison, bearing samples. Preload: Rigid preload bearings are sensitive to the temperature difference between the shaft and the bearing seat, especially when the bearing spacing is small. If the floating bearing fails to float in the axial direction, the pre-tightening force in the bearing set will be greatly increased, and even the internal clearance of the bearing will be completely eliminated; radial stress will also be generated, especially for the spindle bearing with a contact angle of 15°. These phenomena can occur in cylindrical roller bearings and sliding-fit spindle bearing sets. In comparison, if elastic preloading is used, a spindle bearing with a contact angle of 25° and a bearing set with a larger bearing spacing are not so sensitive to temperature changes. Generally, the operating temperature of ceramic ball bearings is lower, and the temperature rise ΔT in a rigid preload system is smaller than that of steel ball bearings. The speed reduction factor (see Table 2) is used for rigid preloaded bearings. In the elastic preload (through spring or hydraulic pressure), due to the reduced thermal sensitivity, the bearing speed can reach the value in the bearing parameter table. The pre-tightening force must at least reach the pre-tightening force value of the medium preloaded bearing (with the suffix M) (see the bearing parameter table). Stiffness: The stiffness of the bearing system is affected by the diameter of the shaft, the number of bearings, the size of the bearings, the preload and the contact angle. Although the radial stiffness of a 15° contact angle bearing is 10% higher than that of a 25° contact angle bearing, its axial stiffness is 45% lower. If you consider the entire spindle bearing and cantilever system, since the support span of the 25° contact angle bearing arrangement is larger than that of the 15° contact angle bearing, the radial stiffness of the former is better than the latter. Due to the influence of the installation, the rigidity of the rigid preloaded bearing after installation is higher than the data in the sample. During operation, the bearing ring expands due to high-speed centrifugal force and thermal expansion of the shaft and inner ring, thereby increasing the rigidity.
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