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).

 

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Eccentric Gears

Typical Symptoms: 1x radial (in Vertical and Horizontal directions)

Like eccentric pulleys, Eccentric gears generate strong 1x radial components, especially in the direction parallel to the gear.

They would also generate sidebands of the running speed of the eccentric gear around the GMF (gear mesh frequency). However, harmonics of GMF may also be generated (depends on the severity of the problem). Natural frequency might also be excited.

Time waveform: The waveform will have combination of 1x running speed of input and output shafts plus strong gear mesh vibration modulated by the running speed of the shaft having the eccentric gear.

Phase: Not applicable.

Eccentric Pulleys

Typical Symptom: High 1x in the direction parallel to belts. Though 1x component can be found on both Vertical and Horizontal directions.

Instead of the typical Vertical and Horizontal directions it is best to choose the directions parallel and perpendicular to the belts.

The high 1x can be found on both sub-assemblies (e.g. the motor and fan). Since the motor and the fan would run at different speeds we would also find two distinct peaks on the signature corresponding to the motor and fan running speeds. Confirmation about which pulley is eccentric can be obtained by removing the belts and checking for the presence of high 1x on motor in the direction parallel to the belts.

Time waveform would be sinusoidal when viewed in velocity.

Phase: Phase reading taken parallel and perpendicular to belts will either be in phase or 180 degrees out of phase.

 

Improving Inherent Reliability of a System

The inherent reliability of a system is determined by the system’s design. It means that the design of the system would determine the upper limit of reliability the system exhibits during operation. Suppose, for example, a system, with the best possible maintenance is able to achieve availability of say 90% we can say that this is the upper limit of the system’s capability that is determined by its design. A good “preventive maintenance” plan can never improve a systems inherent reliability. In other words, preventive maintenance, contrary to what many believe, cannot make a system “better”. It may, at best, only help realise the inherent reliability as determined by the physical design.

Hence the suggested process to “improve” the inherent reliability of a system, may be framed as follows: –

Understand the dynamics through tools like vibration analysis
Monitor changes and rate of change
Eliminate unnecessary maintenance tasks
Change the design of the system interactions to eliminate inherent “imperfections” and revise the maintenance plan.

In most cases, this would be the general approach.

Until we can effectively undertake some design changes (Design Out Maintenance – DOM) or take measures to eliminate inappropriate maintenance actions (Review of Equipment Maintenance – REM) it would not be possible to go beyond inherent reliability of an equipment, specially if it is undesirable in the business context. For example, a vertical pump of a power plant kept failing very frequently or had had to be stopped quite often when vibration shot beyond the trip limits. This behaviour of the system is determined by the design of the system. Unless the design (specifically the interactions between components) is corrected for improvement; the system (vertical pump) would continue to behave in that manner for all times. Likewise if the MTBF of a machine is say 90 days, it would not be possible to considerably improve the MTBF way beyond 90 days unless some undesirable interactions (which I call system “imperfections”) are corrected for improvement and a proper review of existing maintenance system is carried out. 

Such “imperfections” can be both physical and non-physical. Design features, most importantly, the interactions between physical/non-physical components are arguably the most important characteristic of a system that determine a system’s inherent reliability.

In addition, there are many physical design features that influence reliability like redundancy, component selection and the overall integration of various pieces of the system.

In the context of RCM, design extends far beyond the physical makeup of the system. There are a number of non-physical design features that can affect, sometimes profoundly, the inherent reliability of a system. Among these are operating procedures, errors in manufacturing, training and technical documentation. When a proper RCM analysis is conducted on a system or sub-system, there’s a good chance that the resulting maintenance actions will enable the system to achieve its inherent reliability as determined by its physical design features. However, if the inherent reliability is below user’s expectation or need then the design features are to be improved to achieve the desired level of inherent reliability.

Moreover, if unwarranted maintenance tasks are eliminated as it will greatly reduce the risk of suffering the Waddington Effect. There is also a good chance that if operating procedures, training, technical documentation and so forth are found to negatively impact inherent reliability, these issues will be identified and corrected. As evidenced by the Waddington Effect. In virtually every case, less than optimal, non-physical design features almost always have a negative impact on inherent reliability. Therefore, in RCM analysis a through review of existing maintenance plan (REM) along with DOM is necessary to improve inherent reliability of a system.

In brief, right amount of Condition Based Maintenance (CBM) tasks, Scheduled Inspections (which is a part of CBM activity) REM and DOM would not only help us realise the inherent reliability as determined by the physical design but also improve it, if the original inherent reliability is below business expectation.

 

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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.

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Eccentric rotor

Symptom: Pole pass sidebands around 1x N (N=running speed) and 2xLf (Lf = line frequency)

Eccentric rotors will produce a rotating variable air gap between the rotor and the stator which induces a pulsating source of vibration. We would see 2xLf. However, there will also be pole pass sidebands around the 2xLv and 1xN peaks. 1xN is expected to be high.

Note: Pole pass frequency is the slip frequency times the number of poles. The slip frequency is the difference (in terms of frequency) between the actual RPM and the synchronous speed.

Presence of pole pass sidebands around 1N and 2Lf is the key indicator of this fault. One needs sufficient resolution to see those sidebands. Else we would either miss them altogether or mistake them for resonance (a broadening of the base of the peak).

Waveform: Time waveform that covers many seconds of time will reveal the pole pass frequency modulation. Due to lack of impacting the waveform will smooth and will be a combination of the 1N and 2Lf frequencies of vibration.

Phase: Not applicable for this fault unless eccentric forces are high in magnitude.

 

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Eccentricity in general

Symptoms are generally 1x radial (Vertical and Horizontal for a horizontally mounted machine).

Eccentricity occurs when the centre of rotation is offset (like offset misalignment) from the geometric centreline of a gear, motor rotor or a pulley.

It would generate strong 1x radial peak — in the direction parallel to the rotor/gear/pulley. This condition is common and mimics unbalance.

For gear eccentricity we would see 1x sidebands

For motor rotor eccentricity we would see pole pass sidebands.

Time waveform would be sinusoidal when viewed in velocity. Vibration from gear will also have gear mesh vibration and modulation of the turning shaft of the offending gear.

Phase: If belt driven, phase readings taken parallel and perpendicular to belts will either be in phase or 180 degrees out of phase. For a direct driven component, vertical and horizontal readings will be 90 degrees out of phase.

Rotor Bow

General symptom: 1x radial (Vertical and Horizontal direction of horizontal machines)

Usually a rotor bow in a motor looks like a static imbalance. Broken bars and loose connections (at motor terminals and at MCC) cause motors to heat up (localized) owing to uneven current flow through the phases causing rotor bow — uneven weight distribution around the rotor’s centreline. Hence we see high amplitude peak at 1x running speed in the radial and horizontal directions.

Localized overheating can be seen on the motor body through infrared thermal imaging.

The effect of can also be seen on the rotating air gap — a high peak at 2xLf with pole pass sidebands around 1x and 2x peaks. The 2x peak often comes up when the effect is more severe.

The time waveform would be sinusoidal when viewed in velocity.

Phase: expect 90 degree shift between vertical and horizontal axes. The inner race will move in and out once per revolution with a bent shaft

Notes on detecting Unbalance

Unbalance is a condition where a “shaft’s centreline” and mass centreline don’t coincide or when the centre of mass does not lie on the axis of rotation. This can be visualised as the a heavy spot somewhere on the shaft.

There are three types of unbalance.

  1. Static
  2. Couple
  3. Dynamic — which is a mix of static and couple unbalance — when the rotor is not narrow compared to its diameter or in other words — when the rotor is long compared to its diameter.

With unbalance, we expect to see high amplitude of 1x (fundamental) in the signature. Usually, for horizontal machines, the vibration levels are approximately equal in the radial directions (vertical and horizontal). However, we might expect to see higher vibration in the horizontal direction, which is due to change of stiffness. It can be quite different for a vertically mounted machine where we may find the vibration in the flow direction appreciably higher than the tangential direction, which is perpendicular to the flow direction.

The time waveform should be a smooth sine or wave. If it is not, then other problems like misalignment, looseness , bearing wear might be present. It is a convention to view time waveform and signatures relating to unbalance in velocity.

Phase is a confirmatory indicator. Generally the phase difference between the vertical and horizontal directions are 90 degrees apart (+/- 30/40 degrees).

Static Unbalance

For static unbalance we expect to see large amplitude peaks at 1x in the horizontal and vertical directions with very low amplitude 1x peak in the axial direction. However, if the amplitude of 1x in the horizontal direction is more than 2 times the amplitude of 1x in the vertical direction then foundation looseness or resonance may be suspected.

For static unbalance, phase difference between vertical and horizontal would be 90 degrees and the phase difference at bearings at either end of rotor will be zero (that is “in-phase”) since forces on both bearings are always in the same direction.

Couple Unbalance

Like static unbalance, we would also expect to see large amplitude 1x peaks in the vertical and horizontal direction with a low 1x axial component. Again the phase difference between the horizontal and vertical would differ by 90 degrees +/- 30 degrees

For pure couple unbalance, phase at bearings at either end of the rotor will be 180 degrees out of phase.

However, if a rotor suffering from couple unbalance, is statically balanced it may seem to be perfectly balanced but when rotated it would produce centripetal forces on the bearings and they would be of opposite phase. In such cases a two plane balance is required to correct couple unbalance.

Dynamic Unbalance

Common form of unbalance. A combination of static and couple unbalance. Normally happens in rotors that are long compared to their diameters. Generally a two plane balancing correction is required to correct dynamic unbalance. However, in practice a single plane balance would also prove sufficient.

While phase difference between vertical and horizontal direction on any bearing would be 90 degrees out of phase, the phase difference at bearings at either end of the rotor will be between 30 to 150 degrees out of phase.

Unbalance of overhung machines

Examples — close coupled pumps, axial flow fans and small turbines.

In this case we would expect the 1x component of a signature to be high in all the three orthogonal directions — vertical, horizontal and axial. We see a high 1x axial since unbalance creates a high bending moment on the shaft that causes the bearing housing to move axially. Hence the 1x axial is generally the highest amongst the three directions.

Phase difference

As usual, the phase difference between the vertical and horizontal directions would be around 90 degrees. But axial readings on both bearings will be in phase (zero difference) — since they tend to move in the same axial direction. Similarly, phase readings on both bearings will be in phase.

Unbalance in Vertical Machines

Example: Vertical Pumps like Cooling Water Pumps in Power Plants.

Spectrum will show a high amplitude peak at 1x running speed in the radial direction — horizontal or tangential direction. This is due to variation in stiffness along the radial direction. Generally, the vibration amplitude at the top of the motor (Motor NDE) would be higher than the rest of the machine.

To distinguish motor unbalance from pump unbalance, it may be necessary to de-couple the motor and pump and run the motor solo while measuring 1x. If the 1x component is still high then unbalance exists in the motor; else it is in the pump.

Phase: Look for 90 degrees phase shift between readings taken 90 degree apart. All readings taken in the same direction should be in phase.

 

 

External Noise in Vibration Analysis

Quite often, vibrations external to a machine (emanating from other machines or structures) can be transmitted through the foundation (from other machines) and structural supports (e.g. grinding frequencies generated from grinding of materials). It can also be transmitted through liquids (e.g. water hammer, turbulence) and air (acoustic pressures, electromagnetic radiation).

In most cases, low frequency vibrations are transmitted in this manner. This is because low frequency vibrations travel great distances.

In case such transmitted vibrations match the resonant frequency of the machine or any of its components, vibrations are greatly amplified (resonance).

Such vibrations can damage components like anti-friction bearings through a phenomenon called false brinelling if the affected machine is in the stand-by mode.

It is wise to suspect presence of such external noise if a frequency peak is found in a vibration spectrum (FFT) which can’t be identified or appears strange.

In that case we can check whether any machine near to the machine of interest exhibits that particular frequency. Or we can stop the machine to check whether the unusual or odd frequency still appears on a stationary machine. Alternatively, we can stop other local machines (usually not possible) to see whether the odd frequency disappears from the signature.

In case, the frequency happens to coincide with 2x, 3x, 8x harmonics of 1x (fundamental frequency) then we may use time synchronous averaging to see whether the amplitude contributed by the external noise averages away.