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.

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Dibyendu De

 

 

 

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Misalignment

Typical Symptoms: High 1x in the axial direction and 2x in the radial directions; at time 3 x is also present in severe cases (e.g. when coupled to coupling imbalance).

Reasons for misalignment:

  1. Skill
  2. Thermal growth
  3. Movement of foundation

Types of misalignment:

  1. Parallel misalignment — we would find strong presence of 2x component in radial direction along with 1x in the axial direction.  This is because two opposing forces act together at the coupling — both trying to align the shafts to each other.
  2. Angular misalignment — we would find strong presence of 1x component in the radial direction along with strong 2x in the axial direction. This is because angular misalignment produces a bending moment on both shafts.
  3. However, vibration patterns don’t change in very predictable patterns as described in points 1 and 2 above. This is because there is usually a mix of the two different types of misalignment. In addition foundation problem and stiffness (directional or variable) create further complexity in the situation.
  4. The 1x and 2x components would be strong in the radial directions (V and H) but these components would be in phase.

Usually we would find high 1x peak in the axial direction with small 2x and 3x peaks depending on the “linearity” of the vibration. There may be both 1x and 2x (at times accompanied by 3x) in the radial directions.

Time waveform in the axial direction would be dominated by sinusoidal 1x vibration

Phase: Motor and say Pump would be out of phase axially due to angular misalignment (across the coupling in the same direction).

 

Dibyendu De

Eccentric Stator

General Symptom: 2Lf (Lf = Line frequency)

Stator problems would create high vibration at 2Lf. Stator eccentricity produces uneven stationary air gap between the rotor and stator that produces a very directional source of vibration.

Soft foot is often the cause of eccentric stator.

Other key indicators:

  1. 2Lf peak would be comparably high
  2. For a 2 pole motor this peak would be close to 2N (N= running speed). Would need sufficient resolution to separate them
  3. A spectrum may reveal beating — 2Lf and 2N peaks may appear to rise and fall if we don’t have sufficient resolution to separate them.
  4. Time waveform  — a combination of 2N and 2Lf would reveal a beat type pattern if the time period covers more than a few seconds. If the time period isn’t long enough, then we would see a wobble or take on the classic M or W shapes due to combination of 1N, 2N and 2Lf.
  5. Thermal images would reveal heat bands in the direction perpendicular to the direction of high vibration
  6. Vibration would be highest at the point where the stator is closest to the rotor. Move the accelerometer around the motor housing to see if the peak is high in one or two locations.

Dibyendu De

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.