Product Tip of the Month

Product Tip of the Month

April 2000

Model 2020 ProBalancer Analyzer

Vibration Surveys for Piston Engines

Without doubt, the most frequently asked question since the introduction of the Model 2020 ProBalancer Analyzer is, "What do all these spikes mean?" Because the Model 2020 is a full spectrum analyzer, new owners want to use it to track down vibration and noise problems. The Model 2020 can be used to find problems with mechanical looseness, misalignment, as well as unbalanced props and shafts. Piston engines are complicated and produce many vibrations including harmonic and impulse vibration from the firing of the pistons. Not all vibrations are abnormal, especially for a typical six cylinder opposed engine like those found on most private general aviation aircraft today. If you don’t have a good understanding of vibration and balancing, a recommended book, written in common "wrench-bender" language, "The Practical Aviation Vibration and Noise Handbook," by Jack E. Foster could help. This 276-page book covers everything from propeller balancing to jet engine fan trim balance and helicopter rotor track and balance. You can obtain one from Arrow Press of Tucson, Arizona.

An excerpt from the book's section on piston engine vibration follows.

"For example, in an IO-470, which is a six cylinder, four cycle engine there are three firing pulses per revolution so there is a three times RPM vibration. If the engine has a three-bladed propeller, the blade pass frequency is also three times RPM, making it impossible to separate the two.

There are a lot of other harmonics of the crankshaft speed that can be present in piston engines. It is not uncommon to see one times, two times, three times, and higher multiples of the RPM in a spectra. Half order vibrations are also common such as ½ X, 1 ½ X, and 2 ½ X the RPM. No piston engine manufacturer that I know of, provides any vibration analysis data for their engines. Consequently, no one knows what is normal vibration for piston engines.

Generally speaking, whole number harmonics, such as 1X, 2X etc., are usually classed as mechanical vibrations and half order vibrations, ½ X, 1 ½ X etc., are classed as operational vibrations.

Things that have vibration characteristics at whole number multiples of crankshaft speed are:

2X RPM – worn connecting rod bearings

3X RPM – worn main bearings

2X RPM – excessive piston to cylinder clearance

2X RPM – unbalanced reciprocating forces caused by different weight pistons or connecting rods.

Things that can cause vibrations at half orders are:

Camshaft vibration

Cylinder misfiring

Looseness.

No matter what type of engine you work on, turbines or piston, if the task is to analyze engine vibration, it is imperative to learn as much as possible about the engine. Find the calculated vibration frequencies and obtain vibration spectra. Try to obtain vibration spectra for any engine problems that may be encountered and keep good records.

If frequently working with one type of engine, build a database of spectra and information and you will soon become the resident expert in diagnosing engine vibration problems."

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