Understanding and diagnosing the three types of imbalance

Experts agree that almost half of all machinery problems are caused by imbalance.

Imbalance occurs when the shaft’s mass centerline does not coincide with its geometric centerline. There are three types of imbalance: static imbalance, couple imbalance and dynamic imbalance (a combination of the first two).

Three types of imbalance 0901d19680559d5e

Static imbalance

With static imbalance, only one force is involved. To observe this force, place the rotor on a frictionless surface. The rotor turns until the heavy spot is located at six o’clock. The term “static” implies that this type of imbalance can be observed at rest.

Static imbalance 0901d19680559d5d

Couple imbalance

Unlike static imbalance, couple imbalance cannot be measured at rest. With couple imbalance, two equal forces (weights) are 180° from each other, causing the rotor to appear balanced at rest. However, when the rotor rotates, these forces move the rotor in opposite directions at their respective ends of the shaft. This causes the rotor to wobble, which produces a 180° out-of-phase reading from opposite ends of the shaft.

Couple imbalance 0901d19680559c80

Dynamic imbalance

In reality, almost all imbalance is dynamic imbalance. Dynamic imbalance is the combination of static and couple imbalance. On simple machines, there is usually more static imbalance than couple imbalance. On more complex machinery, with more than one coupling or several spots on the rotor where imbalance can occur, couple imbalance is usually the bigger factor.

When balancing a machine, always balance out the static imbalance first, and then take care of the coupling imbalance. When balancing for coupling imbalance, the user is forced to balance in multiple planes.

Factors that cause imbalance

Imbalance can be caused by a number of factors, including improper manufacture, an uneven build up of debris on the rotors/vanes/blades, or the addition of shaft fittings without an appropriate counterbalancing procedure. With pumps, uneven wear on impellers is indicated as imbalance. Key characteristics of vibration caused by imbalance are:

  • It is a single frequency vibration whose amplitude is the same in all radial directions
  • It is sinusoidal, occurring at a frequency of once per revolution (1x)
  • The spectrum generally does not contain harmonics of 1x running speed, unless severe
  • Amplitude increases with speed up to the first critical speed of the machine

Effects of imbalance on bearings

Imbalance usually causes the bearing to carry a higher dynamic load than its design specification, which in turn causes the bearing to fail due to fatigue. Fatigue is the result of stresses applied immediately below the load carrying surfaces and is observed as spalling away of surface metal.

Diagnosing the problem

Use overall vibration, FFT spectra and phase measurements to diagnose imbalance problems.

FFT spectrum analysis

Vibration caused by pure imbalance is a once per revolution sinusoidal waveform. On an FFT spectrum, this appears as a higher than normal 1x amplitude. While other faults can produce a high 1x amplitude, they usually produce harmonics as well. In general, if the signal has harmonics above once per revolution, the fault is not imbalance. However, harmonics can occur as imbalance increases or when horizontal and vertical support stiffness differ by a large amount.

Phase analysis

Use phase measurements to help diagnose imbalance problems. It’s important to note that all phase readings are ±30° because of mechanical variance.

  • Sensor shows 90° phase shift between the horizontal and vertical positions
  • For predominantly static imbalance, there is usually no phase shift across the machine or coupling in the same measurement position

Summing it up

  • If the radial measurement’s 1x amplitude is high and harmonics (except vane passing) are less than 15% of the 1x, then there may be imbalance
  • If the majority of vibration is in the radial plane, and the 1x amplitude is medium to high in amplitude, and the phase from the vertical and horizontal measurements differ by 90°, ±30°, then there may be imbalance
  • If there is a non-synchronous peak corresponding to the 1x running speed of a coupled machine, then there may be imbalance on the other machine
  • If the primary vibration plane is both axial and radial, and the machine has an overhung mass, and the axial phase measurements across the machine are in phase, then there may be imbalance.

Note: It is important to note increasing imbalance forces place increasing loads on nearby bearings. If the bearing’s specified load is exceeded, damage can occur and the bearing’s life will be drastically reduced.