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August 3, 2016

MACHINERY VIBRATION PROBLEMS AND SOLUTIONS, PART 5, MECHANICAL LOOSENESS

By Stan Riddle

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Hawk 1st photoMechanical looseness is a very common machine vibration diagnosis.  It is often overused-becoming a “catch all” category for undiagnosed vibration.  It can be caused by a multitude of things that may seem to have no relationship to each other, such as:

 

Loose bolting                                               Coupling looseness or backlash

Belts Too Loose                                           Cracked welds

Improper bearing or component fits      Mechanical Impacting

Piping or duct work problems                  Rubs

Loose fasteners                                            Wear

Severe bearing damage

The list is almost limitless.  But the proper response is the same-find it, and fix it.  When a diagnosis of looseness is given, the best method for determining looseness is observation.  Look for cracks, broken welds, improper skid to floor fits, loose set screws, worn or undersized shafts and keys, excessive wear-any of these can be signs of looseness.

LOOSENESS FIXES

  • Strobe LightInspection.  Many looseness problems are caused by improper sizing, fits, assembly, clearances, or manufacturing errors.  Advanced vibration analysis can often narrow down the problem, but ultimately, visual inspection and troubleshooting skills are the main requirements for diagnosing looseness.  A strobe light is an excellent tool for helping to diagnose a looseness problem.  Setting the flash frequency to very near (but not on) the forcing frequency being measured can make a loose component quite visible.

 

  • Fan photoFastening and Welds. It is not uncommon for fasteners, or bolting, to be improperly assembled and tensioned.  Fastened connections should be checked and confirmed.  Looseness can also be caused by broken or cracked welds.
  • Connections. Any piece of metal that connects or mounts onto another piece of metal is suspect to looseness.  But the experienced mechanic should look for common-sense problems.  What does the most work?  What is suspect to becoming loose?  As in many troubleshooting applications, start with the easy and obvious.

OTHER PROBLEMS SOMETIMES INCORRECTLY DIAGNOSED AS LOOSENESS

  • Imbalance
  • Misalignment
  • Resonance

 

 

2 Comments

  1. ahmed metwaly on October 23, 2016 at 10:57 pm

    how can I confirm loose problem by phase analysis?/



  2. Stan Riddle on October 27, 2016 at 10:20 am

    Resonance is most easily found by performing impact testing with a modal hammer, doing a coast down, and looking for amplitude and phase testing (phase should shift 180 degrees when crossing a resonant frequency), or by temporarily changing mass or stiffness (by adding or removing weight, or adding or removing bracing).