Phase Characteristics

PhasePhase provides the direction of movement at a given moment in the vibration cycle, which helps determine how different locations on the machine, usually bearings, are moving relative to one another.


Imagine a snapshot of a machine operating and imagine being able to see arrows drawn at each bearing indicating what direction that bearing is moving in at that precise moment. From this you can determine if the bearings are moving in the same direction at the same time (in unison or 'in phase') or not.

Phase helps determine how different locations on the machine (different bearings, usually) are moving relative to one another.

Rules for Measuring Phase

Rule #1: Phase is a relative measurement.

Rule #2: Phase is collected at ONE frequency at a time.

Rule #3: The analyser must be able to detect a vibration signal at that frequency.

Since the angle is calculated based on the arrival of a sinusoidal peak from the vibrating component, there must be an amplitude peak to get a phase angle.

Clock face readings - 3:00; 7:30; 12:00; etc. are sufficient for general phase analysis. More precise phase uses (i.e. balancing) requires specific angular references (0 - 360°).

What Tools Are Used for Measuring Phase

A 'Phase-Triggering' Strobe Light. This type of strobe light differs in two ways from a conventional, hand held strobe light. It either feeds frequency information (its flash rate) into the analyser or has its flash rate set by the analyser. It can also be triggered by a vibration signal from the analyser. Alternatively a Photoelectric Eye or related mechanism might provide a 1x rpm pulse from the shaft.

How is Phase Measured with a Strobe Light?

Step #1 - Mount TransducerThis involves several steps - each one designed to satisfy one of the rules for measuring phase. We must start with a couple of assumptions:

Step #1 - Mount Transducer

Step #2 - Locate a Reference Marks

Step #3 - Tune the strobe to 1x rpm (the pertinent frequency)

The Phase Reading is Set Up - What Is It Measuring?

So far, nothing. The strobe light is being triggered by an internal trigger on either the analyser or the light itself - it is acting exactly like a typical, hand-held strobe light. There is one final step which must be performed:

Once this switch activated, the strobe stops to using its internal trigger to flash and starts to use the vibration sinusoid being detected. Therefore:

Let's look at a graphic example of how this process works and why it is important.

Radial Phase Analysis Around A Single Bearing:

Phase Angle 1Radial Phase Analysis Around A Single Bearing:What is happening in the animation here?

We now have 1 piece of phase data on this bearing at a frequency of 1x rpm:

With the transducer vertical, our phase angle is 10:30

Phase Angle 1 -Radial Phase Analysis Around A Single Bearing:What has been changed in this animation ?

What is Now Happening ?

With the transducer vertical, our phase angle is 10:30.

With the transducer horizontal, our phase angle is 1:30.

What Do These Phase Angles Mean?

We have checked the radial vibration at 2 angular locations around a single bearing. We found two important pieces of information. The first is:

What does this Mean?

What is the second piece of Information?

What does this Mean?

Let's examine these facts more closely and see how they were arrived at from the phase measurements we recorded.

Facts Established by Phase:

- Fact #1: The vibration IS (or IS NOT) coming from the shaft.

Frequency Confirmation

Frequency confirmation simply means you are confirming the source of the vibration is the rotor that appears frozen under the strobe flash and it is the ONE use for a single phase reading.

Frequency confirmation is a simple test that requires only a few seconds to perform but can be crucially important during field testing.

- Fact #2: The shaft is moving in an elliptical orbit approaching circular.

Before explaining this, you must understand what is meant by the 'shaft orbit'. Consider the following:

Figure 1

Fact #2: The shaft is moving in an elliptical orbit approaching circular

Figure 2

Fact #2: The shaft is moving in an elliptical orbit approaching circular

Figure 3

Fact #2: The shaft is moving in an elliptical orbit approaching circular

When the amplitudes are relatively equal, you can see how the 'orbit' approaches a circle. But let's look at the other extreme. What if the horizontal amplitude were 'X' and the vertical amplitude (0.00 ips or mm/sec). The 'orbit' would be a straight line - completely linear.

So we have examined the extremes that are possible:

These extremes are equally unlikely. Everything in between is an ellipse. We can therefore realistically use the rule of thumb that all orbits are elliptical.

However, it can be helpful in determining the nature of the problem to know whether the orbit is approaching circular or linear (which, it should be noted, can also be determined to a large degree by examining amplitude values). Let's return to Fact #2.

To illustrate, let's examine what a 'flatter' (more linear) orbit might reveal during a phase analysis:

Phase Liniar

What Is The Significance Of The Orbit Shape ?

A more circular ellipse (i.e. when you move the transducer a certain angular amount, the mark shifts an equal amount) usually indicates:

A flatter ellipse (i.e. when you move the transducer a certain angular amount, the mark either does not shift or shifts 180°) usually indicates:

These are Rules of Thumb and a number of variables such as structural strength (which can influence amplitude values in one direction versus another) much be considered. Phase analysis, however, can provide some important information related to how the components are moving.

Radial Phase Analysis Across Two Adjacent Bearings

Bearings in Phase

Bearings in Phase

Up to now, we have only analysed radial phase readings taken on a single bearing at different angular locations. What about comparing adjacent bearings?

Bearings out of Phase

Bearings out of Phase

What is the significance of this?

Axial Phase Analysis Around A Single Bearing

Axial Phase

Phase analysis can also reveal some important information when performed in the axial direction. Let's examine what is happening in the animation here:

Just as our radial phase analysis provides important information on the 'shape' of the movement radially, an axial phase analysis provides important information on the 'shape' of the movement axially. In the above animation, we have found there is no phase shift as we move the transducer around the face of the bearing. But how else could it be moving?

The bearing is moving axially in a 'planar' motion (not twisting on the shaft).

What is the significance of this?

Axial Phase

Misalignment typically causes this type of movement

The bearing could be moving as shown here - a 'twisting' action on the shaft or in the housing. Let's examine this animation:

How can this information help the analysis?

The most likely source of 'planar' axial motion (as in the previous animation) is:

The most likely sources of 'twisting' axial motion (as shown above) are:

Completing an Axial Phase Analysis

An 'axial phase analysis' is a procedure that is conducted one bearing at a time. On a smooth running machine, all axial phase readings (on any bearing at any angular location) will be similar. On a machine with high axial vibration, the following procedure should be used if possible:

However, transducer orientation must be accounted for:

Understanding Transducer Orientation

The "orientation", or direction, of the transducer is extremely important. The reason for this is simple.

Understanding Transducer Orientation

The animation here shows both transducers oriented (pointed) in the same direction:

The transducers have the same orientation and generate the same phase angles so they are in-phase.

It is common to collect phase readings across a coupling. In that case, you will almost always have the transducer orientation shown below - the transducers are oriented in opposite directions.

Understanding Transducer Orientation Understanding Transducer Orientation

The Most Common Use of Phase: Diagnosing Misalignment


How Amplitude Units Affect Phase Angles

How Amplitude Units Affect Phase Angles

Once you start a phase analysis, you should never change the amplitude units you are using. Although we have been creating examples using displacement units, velocity units are the most versatile and commonly used. Let's review how phase angles are determined:

The key to why amplitude units affect phase angles is that:

Using displacement units, the peak will arrive when the transducer is displaced the maximum amount in the '+' direction. The mark is at 10:30.

How Amplitude Units Affect Phase Angles

Now let's look at velocity units - the most commonly used. When will the peak arrive? Remember what we are measuring - the speed of the bearing in one direction.

Using velocity amplitudes, the peak occurs when the bearing is moving towards the transducer at the fastest speed. At that moment, the mark is at 7:30. That is 90° different than what we measured with displacement units.

How Amplitude Units Affect Phase Angles

Finally, let's look at acceleration units. When will the peak arrive in that case?

Using acceleration units, the peak will arrive when the transducer is reacting (pushing back) in response to being displaced. Since the reaction (pushing back) must be in the '+' direction (up), the displacement would be at a maximum in the '-' direction. Comparing these images (from the 'Amplitude' Section) will help you understand why the peaks arrive at different moments for different amplitude units.

How Amplitude Units Affect Phase Angles How Amplitude Units Affect Phase Angles How Amplitude Units Affect Phase Angles

When is this information important?

How is Phase Collected with a Photo-eye?

How is phase collected with a photoeye?

The procedure for collecting phase with a photo-eye is somewhat different than with a strobe light. Let's measure phase at 1x rpm as shown here.

Step #1 - Mount Photo-eye & Trigger

The first step in measuring phase is to properly set up the photo-eye. It must be mounted rigidly next to the shaft so it can detect a trigger mark rotating on the shaft. The mark is often a piece of reflective tape. With some modern detectors (like 'Lasertachs'), pattern recognition is used and reflective tape is often not needed. The trigger gives the analyser a 1x rpm pulse (gives it the frequency).

Step #2 - Mount Transducer

Mount the transducer at the location and direction desired.

Step #3 - Instruct Analyser to collect a phase measurement

A keystroke will tell the box to collect a phase reading.

How does the photo-eye calculate the phase angle?

What is "Time Synchronous Averaging"?

It is a procedure that differentiates between synchronous and non-synchronous frequencies. When applied properly, it is a powerful tool.

Further Information

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