Bent Shaft

General Symptoms:

  1. High overall vibration in the axial direction in displacement and velocity parameters
  2. Generally we would get 1N in the axial direction if the bend in at the centre of the shaft
  3. We may also get 2N in the axial direction if the bend in near to the coupling.
  4. Vertical and Horizontal axis measurements will also often reveal peaks at 1N and 2N but the key to catch a bent shaft is to pay attention to what we get in the axial direction.

Reasons of bending:

  1. Excessive heat. E.g. in motors that are overheated for various reasons, like for example, loose connections of the terminals. Also refer to the problem of Rotor Bow .. here.
  2. Physically bent or run out
  3. Sag of a long shaft — also called catenary. For example — turbine shaft.
  4. Half critical speed — a phenomenon seen in horizontal machines operating close to the earth’s resonant frequency

Phase:

Phase measurement is an effective test to confirm presence of bent shaft. Phase at 1N measured in the axial direction at opposite ends of the components will be 180 degrees out of phase.

However, if the phase measurements are taken around the shaft we would find that they are all in phase since the shaft will appear to be moving back and forth in the axial direction.

Spectrum:

In addition to the prominent presence of 1N and 2N in the axial direction we would also find higher than normal 1N and 2N peaks in the radial directions.

Time waveform:

In this case time waveform would not prove to be a good indicator for bent shaft. However, a sinusoidal waveform is expected in the axial direction if the vibration is predominately 1N. In the case of a predominate presence of 2N there would be a “wobble” depicting the classic “M” or “W” pattern depending on the phase angle, if the bend is closer to the coupling.

by

Dibyendu De

dde@rgbwaves.com

9836466678

 

 

 

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Note on Raised “Noise Floor”

In a spectrum, if the entire noise floor is raised, it is possible that we have a situation of extreme bearing wear.

If the noise is biased towards the higher frequencies in the spectrum then we may have process or flow problem like possible cavitation, which may be further confirmed by high acceleration measurement (or filtered acceleration measurement) on the pump body on the delivery side (since high frequency waves are always localized).

Smaller “humps” may be due to resonance (possibly excited by anti-friction bearing damage, cavitation, looseness, rubs or impacts) or closely spaced sidebands arising from other defects. A high resolution measurement (or graphical zoom and a log scale) may reveal whether the source is problems that exhibit sidebands or a problem of resonance. If  machine speed can be changed, (for e.g.motor connected to VFD drives) the resonant frequency would not move – but the other peaks would. Sidebands will typically be symmetrical around a dominant peak – e.g. 1X, 2X, 2x LF (100 or 120 Hz) etc indicating different faults.

Interestingly, the time waveform would reveal the reason as to why the noise floor has been raised.

We would see signs of looseness, severe bearing wear, rubs, and other sources of impacts in the time waveform. We must make sure that there are 5 – 10 seconds of time waveform if we suspect an intermittent rub (e.g. white metal bearings of vertical pumps or loose electrical connection of motor terminals) or if we suspect flow turbulence or cavitation.

If the time waveform looks normal (making sure there is a high Fmax (following Niquist criteria) and we view the waveform in units of acceleration then increase the resolution in the spectrum to 3200 lines or higher in case we are seeing a family of sidebands (like the sidebands we find around gear mesh frequency or rotor bars).

But if a natural frequency is being excited (necessary condition for resonance) then we have to perform a bump/impact test or a run-up/coast down test to confirm the situation.

Notes on Belt problem as seen in Vibration Analysis

General Problem: Belt is worn out or is loose on the pulley. 

How to detect it in a vibration spectrum: We would find peaks at “belt frequency” (or better known as “fundamental belt pass frequency”) and its harmonics. The highest amplitude peak in the series will often be the twice the belt rate frequency.

The fundamental forcing frequency for such a problem is known as the “belt rate” or “fundamental belt pass frequency”. It is the rate at which a point on the belt passes a fixed reference point. It is always less that the speed of either pulleys (driving and driven).

Calculation of Belt Pass frequency as follows:

Driven RPM =Driving RPM x Driving sheave diameter/Driven sheave diameter

Belt freq = Pi x Sheave RPM x Sheave diameter/Belt length = PixDxN/BL

Where Pi = 3.1416

Spectrum: Look for the belt rate peak (sub-synchronous) and harmonics.  Sometimes the belt rate peak may be cut off by the high pass filter, but the harmonics will be present. Remember we are looking for the 2 times belt rate frequency to confirm the problem.

Time waveform: If the belts are simply worn then the time waveform will not be the best analysis tool.  If a belt has a distinct point of damage then there will be an event in the waveform once per belt revolution. This provides a useful distinction to discern the exact nature of the problem.

Strobe: A strobe is a very useful tool.  If you use the strobe to freeze the movement of the belts then you can inspect them without stopping the machine.  You can also detect slip on multi-belt systems.