Measuring Axial Thrust with BeMoS® CS
Cross-section of a centrifugal pump; purple: impeller wheel; green: cylindrical roller bearing; orange: double-row angular contact bearing
Centrifugal pumps use the centrifugal force to transport fluids. These pumps have one or more impellers (purple), a bearing for radial support (green) and an angular contact ball bearing (orange) to accommodate the axial forces generated by the impeller.
The front bearing (green) absorbs only radial forces and is usually either a cylindrical roller bearing or a plain bearing.
The hydraulic system is designed for smallest possible generation of axial forces. The remaining axial force that cannot be compensated even by pressure balance valves is termed axial thrust. This axial thrust is absorbed by the rear bearing (orange). The rear bearing system is usually a double-row angular contact ball bearing or comprises two angular contact bearings in O- or X- configuration; in rare cases a four-point bearing is employed. The reason for using angular contact bearings is to accommodate potentially large forces generated by the pumping action. A double-row design is necessary to counteract axial thrust reversal which may occur under certain conditions. In such a case, the impeller wheel is pulling rather than pushing.
These pumps are designed for smallest possible axial thrust at their operating point. That is accomplished through certain geometries and transverse channels. Therefore, if the pump is operated in a different manner or passes through the operating point due to certain events (valve closed, pump is being started up etc.), the axial thrust changes.
Axial forces are of central importance in fluid pumping processes; but monitoring them requires a great effort, involving, for example, the integration of load cells into the shaft. Besides requiring design changes, this approach has a second disadvantage: it is not suitable for series production.
Axial thrust also reduces the life of the angular contact ball bearing considerably. Large axial forces shorten the bearing life and hence the service life of the pump dramatically.
With BeMoS® CS it is possible to determine both the cage speed and the shaft speed very accurately. From the ratio of these two quantities the contact angle of the angular contact ball bearing, which contains information about the axial thrust, can be calculated.
BeMoS® CS can be employed in series production without requiring measurement-related changes in the pump structure.