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4. Machine balancing in situ.

Question 4.2.

Sometimes during balancing it is impossible to reduce the vibration of assembled machine on the rotation frequency. In technical literature such result is considered as action of not inertial oscillation forces with the rotor rotation frequency. What are these forces? The question was asked by the representatives of the maintenance teams of the petrochemical industry.

The question is answered by Alexej V. Barkov:

On the rotor rotation frequency in different machines can act forces of different nature, viz. practically all types of forces of mechanical, hydro- and aerodynamic, electromagnetic and other origins. The forces and pulsating moments of mechanical nature include:

  • rotating pressure forces applied to the rotor support when there is a misalignment or bending of the shaft line which is comprised of two or more rotors connected by couplings;

  • the kinematic forces acting in the direction of the rotor weight force when the rotating friction surfaces have a nonuniform wear;

  • the parametric forces acting in the direction of the rotor weight force when the rigidity of the rotor changes with rotation angle;

  • rotating radial forces in the couplings when the axis of the torque moments do not coinside;

  • friction forces pulsating with the rotation frequency, foremost in the bearings of rotors connected by couplings, due to shaft line misalignment or bending.

 

The forces and moments of hydrodynamic and aerodynamic nature include:

  • the rotating total vector of forces applied to the blades of impellers and turbine wheels that have different hydro- (aero) dynamic characteristics;

  • rotor auto oscillations in the bearings, synchronized with the rotor rotation frequency.

 

The forces and pulsating moments of electromagnetic nature include:

  • the radial forces acting in the electric machine because the axis of rotor rotation does not coincide with its geometric axis (dynamic eccentric air gap);

  • pulsating moments because of double air gap eccentricity (dynamic and static) in the electric motor.


Some of the mentioned forces on the rotating frequency can be compensated partly or entirely by the inertial forces in the balancing process. Because of this it is better to balance the assembled machine. Besides this some of these forces depend on the load, so in multy mode machines it is necessary to optimize the balancing weights for all the necessary machine operation conditions.

 

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