DynaVibe Blog

So, What is a Vibration Survey Anyway?

 Saturday, January 16, 2016

Much has been written in this blog about vibration analysis: what it is, how it works and the benefits.  The DynaVibe GX2 is the solution from RPX Technologies that simplifies performing a vibration analysis, or vibration survey as they are often called.  If you’ve never seen the output of a vibration survey, you might find it helpful to see an example. 

The following graph is the actual output of a vibration survey taken using a DynaVibe GX2 of a Rotax 912 engine:

Sample airplane engine vibration survey

This Velocity Mode chart shows the vibrations that were detected, measured in Inches Per Second, during the vibration survey.  We’ve annotated the chart so you can easily see what the presence of these vibrations indicate:

  • A 1-per vibration of about .7 IPS indicates propeller imbalance
  • A 1.2-per vibration of about .8 IPS indicates carburetor imbalance
  • A 2-per vibration of about 1.7 IPS indicates a gearbox issue
  • A 2.43-per vibration of over 2 IPS indicates a front-back cylinder imbalance

The threshold of tolerance for a vibration is about .2 IPS. Anything greater should trigger some action to identify and resolve the vibration(s).  With the information a vibration survey provides, resolving these vibrations becomes much easier because there is certainty about their cause.  Let’s look at each of the vibrations this survey identifies in more detail.

The 1-per Vibration

A 1-per vibration is caused by propeller imbalance.  Dynamically balancing the prop should eliminate a 1-per vibration.  Dynamic prop balancing is always a good place to start when resolving vibrations, because it eliminates the most likely source of vibration.  Because this particular vibration survey reveals multiple vibration sources, dynamically balancing the prop will help, but other vibrations remain. 

The 1.2-per Vibration

The presence of a 1.2-per vibration can mean different things.  On a standard engine, this vibration would appear on the survey as a half-per vibration, but since the Rotax engine in this example is geared, this vibration manifests as a 1.2-per.  Typically, this vibration indicates a combustion problem.  It can also indicate that the carbs are out of balance.  The way to know for sure is to complete at least one more vibration survey at a higher RPM.  If the 1.2-per vibration stays consistent as you vary the RPM, then a cylinder combustion problem is the likely cause.  If, however, the intensity of the 1.2-per vibration varies as RPM changes, a carburetor imbalance is the most probable cause.   Out of balance carbs tend to show the most pronounced vibration at mid-RPM levels.  In this example, a second vibration survey (not shown) confirmed the suspected carb imbalance, so balancing the carbs would resolve this vibration.

The 2-per vibration

The 2-per vibration in this example measured 1.7 IPS, a very significant vibration. A 2-per vibration is related to something that rotates with the prop.  Since this engine had a 1-per vibration, it is known that the prop was out of balance.  The recommended approach is to first dynamically balance the prop and then complete another vibration survey to see if the intensity of the 2-per vibration (or any others) has changed.  In this case, the 2-per vibration persisted even when the prop was balanced.  Since the Rotax 912 is a geared engine, attention fell on the gearbox as the vibration source.  In this example, testing the friction torque of the gearbox revealed that it was well over the manufacturer’s maximum, which led the mechanic to contact the manufacturer for a resolution.

The 2.43-per vibration

The most significant vibration shown on this survey, the 2.43-per, is probably caused by the difference in length of the intake manifold.  This engine is designed with the front cylinder having a longer flow path than the back cylinder.  This difference in front-back airflow causes a torsional vibration in the engine, and the DynaVibe GX2 measure this vibration by utilizes two accelerometers mounted in different places on the engine.  This vibration typically diminishes almost to zero as RPM is increased to standard cruise levels.  This assumption was easily verified by doing another vibration survey at cruise RPM, proving the assumption true.  For this reason, the presence of this engine’s 2.43-per vibration was not a concern.

Summary

Here’s what you should take away from this vibration survey example and discussion:

  • Persistent vibration is never a good thing.  Even small ones, over time, can damage engines, airframes or instruments.
  • Vibrations are often complex.  As this example shows, four different vibrations were revealed in the survey.  It’s nice when the vibration source is an out-of-balance prop and dynamic balancing resolves it, but it doesn’t always work that way.
  • Troubleshooting vibrations can be easy.  With the right equipment, such as the DynaVibe GX2, you can identify the frequency and magnitude of vibrations that exist.  With this information, it becomes easy to pinpoint the source without resorting to trial-and-error, speculative maintenance procedures. 

Vibration surveys are also an excellent preventative maintenance strategy.  Doing vibration surveys at regular intervals can provide an early warning of maintenance issues while they’re relatively small, before they’re noticeable or even better, before they cause a failure.  You can purchase a DynaVibe GX2 by visiting our online store, or if you wish to learn more, just call us at 469.708.8779 or enter your email address in the form below and we’ll respond. 


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