Notes on detecting Unbalance

Unbalance is a condition where a “shaft’s centreline” and mass centreline don’t coincide or when the centre of mass does not lie on the axis of rotation. This can be visualised as the a heavy spot somewhere on the shaft.

There are three types of unbalance.

  1. Static
  2. Couple
  3. Dynamic — which is a mix of static and couple unbalance — when the rotor is not narrow compared to its diameter or in other words — when the rotor is long compared to its diameter.

With unbalance, we expect to see high amplitude of 1x (fundamental) in the signature. Usually, for horizontal machines, the vibration levels are approximately equal in the radial directions (vertical and horizontal). However, we might expect to see higher vibration in the horizontal direction, which is due to change of stiffness. It can be quite different for a vertically mounted machine where we may find the vibration in the flow direction appreciably higher than the tangential direction, which is perpendicular to the flow direction.

The time waveform should be a smooth sine or wave. If it is not, then other problems like misalignment, looseness , bearing wear might be present. It is a convention to view time waveform and signatures relating to unbalance in velocity.

Phase is a confirmatory indicator. Generally the phase difference between the vertical and horizontal directions are 90 degrees apart (+/- 30/40 degrees).

Static Unbalance

For static unbalance we expect to see large amplitude peaks at 1x in the horizontal and vertical directions with very low amplitude 1x peak in the axial direction. However, if the amplitude of 1x in the horizontal direction is more than 2 times the amplitude of 1x in the vertical direction then foundation looseness or resonance may be suspected.

For static unbalance, phase difference between vertical and horizontal would be 90 degrees and the phase difference at bearings at either end of rotor will be zero (that is “in-phase”) since forces on both bearings are always in the same direction.

Couple Unbalance

Like static unbalance, we would also expect to see large amplitude 1x peaks in the vertical and horizontal direction with a low 1x axial component. Again the phase difference between the horizontal and vertical would differ by 90 degrees +/- 30 degrees

For pure couple unbalance, phase at bearings at either end of the rotor will be 180 degrees out of phase.

However, if a rotor suffering from couple unbalance, is statically balanced it may seem to be perfectly balanced but when rotated it would produce centripetal forces on the bearings and they would be of opposite phase. In such cases a two plane balance is required to correct couple unbalance.

Dynamic Unbalance

Common form of unbalance. A combination of static and couple unbalance. Normally happens in rotors that are long compared to their diameters. Generally a two plane balancing correction is required to correct dynamic unbalance. However, in practice a single plane balance would also prove sufficient.

While phase difference between vertical and horizontal direction on any bearing would be 90 degrees out of phase, the phase difference at bearings at either end of the rotor will be between 30 to 150 degrees out of phase.

Unbalance of overhung machines

Examples — close coupled pumps, axial flow fans and small turbines.

In this case we would expect the 1x component of a signature to be high in all the three orthogonal directions — vertical, horizontal and axial. We see a high 1x axial since unbalance creates a high bending moment on the shaft that causes the bearing housing to move axially. Hence the 1x axial is generally the highest amongst the three directions.

Phase difference

As usual, the phase difference between the vertical and horizontal directions would be around 90 degrees. But axial readings on both bearings will be in phase (zero difference) — since they tend to move in the same axial direction. Similarly, phase readings on both bearings will be in phase.

Unbalance in Vertical Machines

Example: Vertical Pumps like Cooling Water Pumps in Power Plants.

Spectrum will show a high amplitude peak at 1x running speed in the radial direction — horizontal or tangential direction. This is due to variation in stiffness along the radial direction. Generally, the vibration amplitude at the top of the motor (Motor NDE) would be higher than the rest of the machine.

To distinguish motor unbalance from pump unbalance, it may be necessary to de-couple the motor and pump and run the motor solo while measuring 1x. If the 1x component is still high then unbalance exists in the motor; else it is in the pump.

Phase: Look for 90 degrees phase shift between readings taken 90 degree apart. All readings taken in the same direction should be in phase.




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