The fan impellers may only be used within the respective limit values for speed and temperature. The limit values do not take into account if an unbalance subsequently occurs during operation (e.g. due to shaft runout, contamination, temperature distortion, shock loads, etc.). Due to the residual unbalance, which may change, regular checking and, if necessary, rebalancing of the impeller, is required.
The limit values only apply if no bending or torsional vibrations (>30% of the permissible drive torque) occur during operation. Due to complex operating conditions, an exact definition of these is often not possible, so that the design is dependent on assumptions. The influences that cannot be defined or specified are taken into account empirically by us by means of a safety factor. In order to minimize the residual risk, the customer must carry out a test under real operating conditions. We expressly point out the necessity of such tests in order to verify our recommendation. The forces and stresses acting on the wheel body can be subdivided according to typical causes as follows:
Transport and assembly/disassembly:
- Shocks and jolts during transport
- Force exerted during installation, e.g. by pushing on, pressing on, reaming onto the shaft, etc.
Operation:
- Centrifugal force and compressive force
- Switching on and off or acceleration and deceleration during starting and stopping (e.g. with direct drive with a motor >5.5kW, impact torques of up to 12 times the nominal torque occur during startup), must be taken into account when selecting wheels and hubs or specified when ordering.
- Plant-side vibrations and shocks
- Dynamic oscillations
- Temperature effects
- Pumping medium
- Corrosion and wear during operation and standstill
- The residual imbalance resulting from the initial unbalance of the wheel and its increase during installation and operation (e.g. distortion due to settling, temperature effects, foreign body deposits, wear). Due to the changing residual unbalance during installation and operation, regular checking and, if necessary, rebalancing is required.
- Torsional vibrations during operation due to drive or resonance
- Natural frequencies
All of the above forces are of a transient and dynamic nature and are difficult to accurately record and predict. It must be ensured that the additional loads are kept as low as possible.