Archive for the ‘Signature/Vibration Analysis’ Category

SLEEVE BEARING

December 19, 2008

  • Later stages of sleeve bearing wear will give a large family of harmonics of running speed
  • A minor unbalance or misalignment will cause high amplitudes when excessive bearing clearances are present
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MECHANICAL LOOSENESS

December 19, 2008

MECHANICAL LOOSENESS (A)

  • Caused by structural looseness of machine feet
  • Distortion of the base will cause “soft foot” problems
  • Phase analysis will reveal aprox 1800 phase shift in the vertical direction between the baseplate components of the machine

MECHANICAL LOOSENESS (B)

  • Caused by loose pillowblock bolts
  • Can cause 0.5, 1, 2 and 3X RPM
  • Sometimes caused by cracked frame structure or bearing block

MECHANICAL LOOSENESS (C)

  • Phase is often unstable
  • Will have many harmonics
  • Can be caused by a loose bearing liner, excessive bearing clearance or a loose impeller on a shaft

MISALIGNED BEARING

December 19, 2008

  • Vibration symptoms similar to angular misalignment
  • Attempts to realign coupling or balance the rotor will not alleviate the problem.
  • Will cause a twisting motion with approximately 180 deg phase shift side to side or top to bottom

OTHER SOURCES OF HIGH AXIAL VIBRATION

a. Bent Shafts
b. Shafts in Resonant Whirl
c. Bearings Cocked on the Shaft
d. Resonance of Some Component in the Axial Direction
e. Worn Thrust Bearings
f. Worn Helical or Bevel Gears
g. A Sleeve Bearing Motor Hunting for its Magnetic Center
h. Couple Component of a Dynamic Unbalance

BENT SHAFT

December 19, 2008

  • Bent shaft problems cause high axial vibration
  • 1X RPM dominant if bend is near shaft center
  • 2X RPM dominant if bend is near shaft ends
  • Phase difference in the axial direction will tend towards 1800 difference

PARALLEL MISALIGNMENT

December 19, 2008

  • High radial vibration 1800 out of phase
  • Severe conditions give higher harmonics
  • 2X RPM often larger than 1X RPM
  • Similar symptoms to angular misalignment
  • Coupling design can influence spectrum shape and amplitude

ANGULAR MISALIGNMENT

December 19, 2008

  • Characterized by high axial vibration
  • 180 deg phase change across the coupling
  • Typically high 1 and 2 times axial vibration
  • Not unusual for 1, 2 or 3X RPM to dominate
  • Symptoms could indicate coupling problems

ECCENTRIC ROTOR

December 19, 2008

  • Largest vibration at 1X RPM in the direction of the centerline of the rotors
  • Comparative phase readings differ by 0 deg or 180 deg
  • Attempts to balance will cause a decrease in amplitude in one direction but an increase may occur in the other direction

Diagnosing Unbalance

December 19, 2008

􀂋 Vibration frequency equals rotor speed.
􀂋 Vibration predominantly RADIAL in direction.
􀂋 Stable vibration phase measurement.
􀂋 Vibration increases as square of speed.
􀂋 Vibration phase shifts in direct proportion to measurement direction.

OVERHUNG ROTOR UNBALANCE

December 19, 2008

􀂋 1X RPM present in radial and axial directions
􀂋 Axial readings tend to be in-phase but radial readings
might be unsteady
􀂋 Overhung rotors often have both force and couple
unbalance each of which may require correction

COUPLE UNBALANCE

December 19, 2008

􀂋 1800 out of phase on the same shaft
􀂋 1X RPM always present and normally dominates
􀂋 Amplitude varies with square of increasing speed
􀂋 Can cause high axial as well as radial amplitudes
􀂋 Balancing requires Correction in two planes at 180o