Bearings for high speed compressors
High speed compressors, which are typically dry air twin screw compressors, operate at ndm values greater than 750,000. Normal design and precision bearings are not suitable for operation at such high speeds since the centrifugal forces are too great and can lead to very high induced loads and thus low bearing lives.
We have developed special high speed bearings suitable for operation to ndm values of 1,200,000. The bearings are EC cylindrical roller and special sets of angular contact ball bearings which have high precision and running accuracy . The roller bearing supports the radial load. The angular contact ball bearing set consists of a pair of bearings, mounted either face-to-face or backto-back, with the thrust bearing having a contact angle between 30° and 40° and the back-up bearing having a contact angle between 15° and 20°. The difference between the contact angle of the thrust bearing and backup bearing is between 10° and 20°. With this design, the internal force due to centrifugal force is small, so the induced axial force in the bearing system is minimized.
This arrangement of cylindrical roller and angular contact ball bearings for compressors is patented by us. The arrangement is available royalty free to customers. The bearings are made to order and inquiries should be directed through Applications Engineering.
In high speed applications, it is also possible to use a four-point contact ball bearing as a thrust bearing. In
such cases, the four-point contact ball bearing should have P5 running accuracy. The cage should be either a machined brass or machined light alloy outer ring guided cage.
In comparison with the above mentioned bearing sets, a single four-point contact ball bearing has larger axial clearance and thus requires a larger rotor end clearance since the bearing axial position is affected by the centrifugal loads of the balls.
Bearings having ceramic rolling elements (hybrid bearings) can also be used. The reduced mass of the
ceramic rolling element compared to steel elements allows the hybrid bearing to be used at higher speeds. This will allow lower preloads since the risk of skidding due to gyroscopic ball movement is reduced.