Symptoms: 2X Axial + (Vertical & Horizontal)

Symptoms: 2X radial, small 1X radial (V & H) If the misaligned shaft centerlines are parallel but not coincident, then the misalignment is said to be parallel (or offset) misalignment. Misalignment is common due to poor alignment practices or because of thermal growth, shifting foundations, pipe strain, etc. The vibration does not always change in predictable ways when shafts are misaligned.

If the misaligned shaft centerlines are parallel but not coincident, then the misalignment is said to be parallel (or offset) misalignment.  Misalignment is common due to poor alignment practices or because of thermal growth, shifting foundations, pipe strain, etc. The vibration does not always change in predictable ways when shafts are misaligned.  Parallel misalignment produces both a shear force and bending moment on the coupled end of each shaft. Spectrum measurements are very useful; however it can be confused with other faults. Phase readings provide additional evidence. 

Types of Misalignment

There are two types of Misalignment: Angular and Offset

Offset Misalignment is characterised by: 

Spectrum:

• Radial vibration @ 2x rpm.
• high radial amplitudes low Axial amplitudes
• High peaks at 1X, 2X, 3X, and even 4X and 5X in the radial direction due to the parallel (offset) misalignment.

• 2X can be quite high compared to 1X vibration.

• The presence of 3X, 4X, 5X etc. will depend on coupling type and degree of misalignment.

  The peaks may be higher in vertical at one end of the component (e.g. motor) but higher in horizontal at the other end of the same component.

• We expect to see a high peak in the spectrum at 2X turning speed of the shaft. It is often higher in the horizontal direction compared with the vertical direction due to stiffness. 

Waveform:

• The waveform should be very sinusoidal forming a W pattern.

Phase:

• Radial phase across coupling shifts significantly (> 60°).
• Compare vertical to horizontal phase – there should be a 90° phase shift. 

Angular Misalignment is characterised by: 

  Spectrum:

•  similar vibration symptoms to Offset
• high axial vibration with both 1x and 2x rpm
• Coupling construction will often greatly influence the shape of the spectrum

 Waveform:

• sinusoidal 1X vibration.

 Phase:

• 180° Out-of-phase across the coupling
• Compare vertical to horizontal phase – there should be a 90° phase shift.