- 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
Showing posts with label Signature/Vibration Analysis. Show all posts
Showing posts with label Signature/Vibration Analysis. Show all posts
Friday, December 19, 2008
SLEEVE BEARING
MECHANICAL LOOSENESS
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
- 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
- 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
- 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
- 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
- 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
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.
Read more...
OVERHUNG ROTOR UNBALANCE
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
Read more...
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
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
Read more...
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
SIGNATURE ANALYSIS
* Which frequencies exist and what are the relationships to the fundamental exciting frequencies.
* What are the amplitudes of each peak
* How do the peaks relate to each other
* If there are significant peaks, what are their source
Read more...
* What are the amplitudes of each peak
* How do the peaks relate to each other
* If there are significant peaks, what are their source
Subscribe to:
Posts (Atom)
