Preloading and Stiffness of FAG Super Precision Bearing Arrangement Design and Application Example

In practical applications, a large number of different types of spindle bearings are used. The choice of FAG spindle bearing type and arrangement depends on the characteristics of the application: eg turning, milling, grinding, etc. However, operating conditions and economics also play a decisive role in the final selection and technical requirements. In order to achieve high precision requirements (P4 or higher), bearings must generally operate with preload or at least zero clearance. The temperature rise at the highest speed should be as low as possible (speed index, up to n~dm = 2~106 mm/min. under grease lubrication, up to 3,1~106 mm/min. under oil lubrication). . The above requirements can be achieved by using super-precision bearings and correspondingly precise mating components. The following information can help select the most suitable bearing and bearing arrangement in the application, including the following: • Pretensioning • Stiffness • Contact angle • Rolling element size and material • Bearing spacing • Sealing • Steps in bearing design • Comparison of bearing arrangements • Bearing samples.

Pre-tightening: Rigid preloaded bearings are sensitive to changes in the temperature difference between the shaft and the housing, especially when the distance between the bearings is small. If the floating bearing fails to float in the axial direction, the pre-tightening force in the bearing group will greatly increase, and even the internal clearance of the bearing will be completely eliminated; radial stress will also be generated, especially for a spindle bearing with a contact angle of 15°. These phenomena can occur in cylindrical roller bearings and sliding-fit spindle bearings. In contrast, if the elastic preload is used, the bearing set with the contact angle of 25° and the larger bearing spacing is chosen to be less sensitive to temperature changes. In general, the operating temperature of ceramic ball bearings is lower, and the temperature rise ΔT is lower in the rigid preload system than in the steel ball bearings. The speed reduction factor (see Table 2) is used for rigid preloaded bearings. In elastic preloading (by spring or hydraulic pressure), the bearing speed can reach the value in the bearing parameter table due to the reduced heat sensitivity. The pre-tightening force must at least reach the value of the preloading force of the medium preloaded bearing (with suffix M) (see the bearing parameter table).

  Stiffness: The stiffness of a bearing system is affected by the diameter of the shaft, the number of bearings, the size of the bearings, the preload, and the size of 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 the entire spindle bearing and cantilever system is considered, the radial stiffness of the former is better than the latter because the bearing span of the 25° contact angle bearing arrangement is larger than that of the 15° contact angle bearing. Rigid preloaded bearings are stiffer than the data in the sample due to installation. In operation, due to the high-speed centrifugal force and the thermal expansion of the shaft and the inner ring, the bearing rings expand and the rigidity increases.