Measurements Using Accelerometers

APPLICATION NOTES

Making vibration measurements on reciprocating machinery is sometimes taken for granted. The accelerometer is attached without regard to end results in order to facilitate data collection speed. Care must be taken to insure quality vibration traces that will give the analyst the greatest insight into machinery malfunctions.

The form of the vibration signal (pattern) is perhaps the most important means of presenting vibration data for analysis. Many factors play an important role in acquiring these patterns with repeatable and accurate results.

The techniques explored here and the understanding of accelerometers will help in obtaining the best vibration information from the 6300 Portable Reciprocating Machinery Analyzer.

We must first look at how the accelerometer works in order to understand the factors that hinder accurate repeatable results. There is a small piezoelectric crystal structure located in the accelerometer and an inertial reference mass. By applying a force to this crystal a small signal is produced. The signal is amplified and transformed into a usable analog signal representing forces in the machinery.

Accelerometers are inertially referenced transducers, and as such, measure absolute machine structural motion relative to free space. The typical frequency response is from 0.5 Hz to 14 kHz for the accelerometer supplied with the Model 6300 analyzer.

The frequency response is constrained by the mechanical resonance of the sensing element assembly and the low frequency filters used in the amplifier. The resonance is determined by the stiffness of the sensing element and the reference mass.

Acceleration is a function of the frequency squared, and at low frequencies of vibration the amplitudes of acceleration are very small. The amplitudes may be so small that the signal will be lost in the noise level of the transducer. For example, if a 900 rpm machine has a synchronous vibration of 5 mils peak-to-peak displacement, this yields about 0.05 g's zero to peak acceleration. At a typical accelerometer sensitivity of 100 mV/g zero to peak, the amplitude of the signal output would be only 5 millivolts zero to peak. However, at high frequencies, even low displacement amplitudes produce high acceleration amplitudes.

As one can see, an accelerometer is very sensitive to forces applied to it; particularly ones at higher frequencies. With this in mind, we will look at the many methods of mounting an accelerometer and the affects on the signal that is captured for analysis.

Magnetic mounting is used most often by analysts when taking periodic measurements as it facilitates hands free operation. Accurate results can be obtained relatively quickly if the analyst takes careful precautions when mounting the magnet to the machine. To work properly, magnetically mounted accelerometers should be used on flat, finished surfaces free of paint, oil, and debris. If the surface is not flat, a rocking motion can occur causing erroneous measurements. Remember that a rocking motion of only 0.01 inch at 900 rpm will produce an equivalent false vibration@ of almost 0.12g and only 0.06 inch rocking at 900 rpm will produce nearly 0.69 g!

Other factors affecting good vibration measurements are:

Surface finish is the most important underlying issue. Rough surfaces can produce unwanted resonance in the resulting pattern giving false information for the analyst to act on. A surface texture of no greater than 32 micro inches (RMS) provides for meaningful and repeatable measurements.

Rounded head bolts produce rocking motion and/or and unsecured (loose) mounting that does not allow the entire accelerometer to come in contact with the machine. Additionally, some of the actual machine forces may not act upon the accelerometer as it is not in full contact with the machine surface.

Painted (chipped) surfaces can also produce a rocking motion or unwanted resonance in the pattern. As mentioned above, some of the actual machine forces may not act upon the accelerometer. Also, the paint thickness forces the magnet away from the metal which weakens the magnetic attraction to the metal.

To obtain the desired surface specifications, the machine surface must be prepared or have a finished mounting disc bonded to it. The latter is usually preferred because it requires less labor.

Magnet strength can play an important role in obtaining good vibration information. Magnets come with keepers (generally a large flat washer) attached to the bottom. These keepers should remain on the magnets when not in use. Magnets tend to loose their strength over time without keepers. A weak magnet will not hold the accelerometer in place tightly against the machine surface. A loss of contact with the machine surface means not all the forces will be transferred to the accelerometer resulting in lower amplitude and frequency content in the pattern obtained. A weak magnet can also allow the accelerometer to vibrate against the surface of the machine producing unwanted signals.

Thus, magnet mounted accelerometers can produce good, repeatable measurements if they are mounted securely.

Hand held accelerometer readings are the least acceptable. This usually entails the analyst placing the accelerometer on a stinger and applying pressure to the accelerometer to keep it in place while taking the measurement. However, each analyst, will place a different amount of pressure on the accelerometer, producing varying results each time. This will lead to an inconclusive analysis and poor trending capability.

The mounting technique used will determine the results obtained. In general, there are four configurations for mounting accelerometers: threaded studs, adhesives, magnets, and stingers (see figure 1). As you can see, the stinger cuts off most of the high frequency content while the stud mounting provides the best response.

Figure 1

Consistent placement of the accelerometer each time will also help in obtaining accurate repeatable results. A marking should be placed at each collection point location such that each analyst using the analyzer will obtain the same expected result each time.

An example of good mounting technique is shown below. The accelerometer is mounted using a magnetic base on a smooth surface. Notice the well defined patterns.

An example of poor mounting technique is shown below. The accelerometer is once again mounted using a magnetic base, but this time on a rough surface. Notice the loss of information and poorly defined engine events.