Rotor balancing in situ
The aim of balancing is to reduce the vibration level at the rotational frequency of a rotating machine.
This procedure involves improving the distribution of rotor masses so as to reduce the eccentricity of the centre of gravity and thereby the resulting centrifugal forces.
Generally, unbalance is corrected by adding or removing compensating masses to the rotor at prescribed locations in order to counter the effect of the initial unbalance.
The balancing procedure involves testing the effect of masses on the balancing planes defined by us (flange of a wheel, fan blades, predicted balancing holes or slopes, etc.) in order to determine the number and location of the correction masses using our calculation software.
This procedure reduces production losses on site as the machine does not need to be disassembled.
In condition-based maintenance, the vibration level at the rotational frequency of a rotating machine is an indicator of the condition of its mechanical parts.
When the vibration level is high, it accelerates the wear of machine parts, such as bearings, slide bearings and pipes, but it also induces various other problems, such as noise pollution and vibrations which are detrimental to nearby premises.
The main cause of this high vibration level is most often “unbalance”, which is a rotating force generated by an unbalance of the rotating mass, or the eccentricity of the centre of gravity in relation to the rotation axis. All rotating machines have a level of residual unbalance due to the design and assembly, which is why a manufacturer or repairer must comply with rotor balancing quality criteria laid down standards and/or customer specifications when a machine leaves the workshop. We do however find unacceptable vibration levels related to unbalance in situ caused by:
- the dynamic behaviour of the structure and shaft (resonance frequencies),
- balancing criteria taken into account that is inappropriate for the installation,
- loss of material,
- the absence of complete mobile balancing in the workshop
- the replacement of rotor parts with incorrectly balanced components (turbine blades, etc.)
Balancing in situ has the advantage of reducing production losses as the machine does not need to be disassembled and the rotor does not need to taken to a workshop.
Insofar as we have the necessary access to perform balancing in situ, we distinguish two different types of balancing service:
- Rigid rotors involve machines whose service speed is less than their first critical speed which includes small and medium size motorised fans and pumps and “low-speed” machines. The rotor is not subject to deflections which makes this type of balancing in 1 or 2 planes much easier.
- Flexible rotors involve machines whose service speed is more than their first critical speed which includes combustion steam turbine-generators, large motorised fans and pumps and other “high-speed” machines. In this case, the balancing procedure is more complicated because of the elastic deflections of the rotor and requires specific expertise.
After carrying out the balancing operation, our teams determine the number of balancing planes to install according to the type of shaft, which then makes it possible to determine the number of measurement points and parameters required to correct it.