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Identifying Misalignment

Definition of misalignment:
Misalignment is the deviation of relative shaft position from collinear axis of rotation when equipment is running at normal operating conditions.

How is misalignment recognized?

shaft angular error

Misaligned shafts - parallel offset

Misaligned shafts - angular error

 

There are several symptoms indicating misalignment. By keeping your eyes open you can find them without any special equipment.

  • Premature bearing, seal, shaft and coupling failure
  • Excessive radial and axial vibrations
  • High casing temperature at or near the bearings or high discharge oil temperature
  • Excessive amount of oil leakage at the bearing seals
  • The coupling is hot while it is running and immediately after unit shutdown. Look for rubber powder inside the coupling shroud
  • Loose foundation bolts
  • Loose or broken coupling bolts
  • Excessive amount of grease on the inside of coupling guard
  • Similar pieces of equipment are vibrating less or seem to have longer operating life
  • The shafts are breaking (or cracking) at or close to the inboard bearings or coupling hubs

Misalignment effects on bearings and seals

The bearings operating life span is directly affected by the forces it is exposed to. The slightest misalignment can generate excessive forces to the bearings and seals. A misaligned machine causes stress to both bearings and shafts.
As an effect of this the seals open up, allowing lubrication leakage and contamination to enter. All together the bearings lifetime is dramatically shortened.

Misalignment and power consumption

Misalignment has a direct impact on the power consumption. Documented cases have shown savings ranging from 2 to 17%. The thermogram below shows two different couplings and the amount of heat generated due to misalignment.

Thermogram of misaligned machines

Effects of base conditions and thermal variations

Even if you align your machines properly there is a risk that they break due to misalignment. If the foundation where the machine is mounted is warped you may have a soft foot condition, i.e. one of the machine feet has a poor contact with the floor. It is of major importance that this is corrected before performing the alignment.
If the alignment is performed when the machines are cold, and no compensation for thermal growth is considered, the machines will probably be running severely misaligned at running temperature.

 thermal expansion of machine

Differences in temperature can cause severe misalignment due to the fact that machine casings and pipes expand with raised or lowered temperature.