Q: Selection principle of C spindle bearings.
A: The model of spindle bearings should be selected mainly based on stiffness and speed. If there are other requirements, other factors should also be considered, such as load capacity, vibration resistance, noise, etc.
For bearings of the same size, line contact roller bearings have higher stiffness than point contact ball bearings, but the limit speed is lower. Angular contact ball bearings can be used in multiple combinations to improve stiffness, but the limit speed will be reduced accordingly. Some bearings can only bear radial loads, such as cylindrical roller bearings, or only used to bear radial loads, such as deep groove ball bearings. Some bearings can only bear axial loads, such as thrust ball bearings, or only used to bear axial loads, such as bidirectional thrust angular contact ball bearings. Some bearings can bear both loads, such as angular contact ball bearings and tapered roller bearings. Among them, double-row tapered roller bearings can bear bidirectional axial loads in addition to radial loads.
The diameter of spindle bearings is generally selected to be relatively large, and the load is relatively small. Therefore, spindle
bearings are mostly selected from light, extra light and ultra-light series bearings. The selection can be made according to the following principles.
(1) When the speed is medium, the load is large, and the rigidity is required to be high, a line contact roller bearing can be used, such as a double-row cylindrical roller bearing, a tapered roller bearing, etc. The limit speed of a double-row cylindrical roller bearing is higher than that of a tapered roller bearing. Double-row cylindrical roller bearings can only bear radial loads and must be used in conjunction with thrust bearings. The limit speed of thrust bearings is relatively low. If used in conjunction with double-row cylindrical roller bearings, the latter's high-speed performance often cannot be brought into play. Therefore, if the required speed is high, a bidirectional thrust angular contact ball bearing should be used. Tapered roller bearings can bear radial and axial loads and can simplify the structure of the support part, but tapered roller bearings generate more heat and have a lower limit speed. Using hollow tapered roller bearings can increase the limit speed and reduce heat generation.
For medium and small lathes, milling machines, and boring machines, if the speed is high, multiple angular contact ball bearings can be used. Since the axial cutting force of these machine tools is unidirectional, three bearings are often used in a group. The first two are connected in series, with the large opening facing outward, and the third one facing inward, back to back with the first two, as shown in the figure below.
Although angular contact ball bearings are point contact bearings with low rigidity, they can bear radial loads together after being assembled and preloaded. The first two bearings in series bear the axial cutting load, and are preloaded back to back with the third bearing. This type of machine tool has a large axial load, so bearings with a contact angle of 25° are often used. The rigidity of this structure is lower than that of double-row cylindrical roller bearings or tapered roller bearings, but the allowable speed is higher. This is the case for most machine tools, such as various lathes, milling machines, boring machines, CNC lathes and machining centers.
The above three types of spindle bearings and their combinations
Tapered bore double-row cylindrical roller bearings, various types of tapered roller bearings (including single-row, double-row, hollow rollers) and multiple assembled angular contact ball bearings have formed a situation of parallel development and mutual complementation, each playing its own advantages and overcoming its shortcomings. For example, for double-row cylindrical roller bearings, bidirectional thrust angular contact ball bearings have been developed to match them in order to give full play to their high-speed performance; for cylindrical roller bearings, hollow tapered roller bearings have been developed to solve the cooling problem, thereby reducing temperature rise and increasing the maximum speed; angular contact ball bearings use multiple assemblies to increase rigidity, etc.
(2) High-speed spindles can use angular contact ball bearings, such as the grinding wheel spindles of internal grinders, thread grinders, gear grinders, etc. If the load is large or the rigidity is high, a variety of bearing configurations can be used. Considering that the axial load is usually small and the grinding is symmetrical, bearings with a contact angle of 15° are often used, and two are assembled in a group. See the figure below.
(3)For spindles that are mainly subjected to axial loads and have low precision requirements, deep groove ball bearings can be used in combination with thrust bearings.
(4) For places where radial dimensions are limited and precision requirements are not high, such as the spindles of multi-axis drilling combination machines, needle roller bearings can be used.
