Typically, Ingersoll Rand air compressor gear transmission systems generate a complex broadband vibration spectrum, with frequencies ranging from below the shaft rotational speed up to several times the gear meshing frequency (number of teeth × shaft revolutions per minute).

At the low-frequency end of the spectrum are the shaft rotational speed and its harmonics. Usually, the amplitude at the fourth or fifth harmonic of the Ingersoll Rand air compressor shaft speed decreases to below 1% of that at the fundamental frequency, making it negligible for many practical applications.

The next set of frequencies are the mid-frequencies, which typically appear between the rotational speed frequency and the meshing frequency. These mid-frequencies are a series of frequencies synchronized with multiples of the rotational speed frequency of one or two shafts, making them easily explainable based on the compressor's operation.

Additionally, there is at least one scenario where these frequencies coincide with the natural frequencies of the gear transmission components themselves, which could very well be due to resonance amplification caused by a very low-level excitation.

In many cases, changes in the amplitude of mid-frequencies can serve as highly sensitive early indicators of gear transmission failure in Ingersoll Rand air compressors.

The gear meshing frequency (gear speed × number of teeth) and its harmonics are usually the most prominent components in the housing vibration and acoustic spectra recorded from the gear transmission system of Ingersoll Rand air compressors.

The meshing frequency can be a single prominent frequency or may be surrounded by sidebands on both sides, which are frequencies spaced at intervals of the shaft rotational frequency around the meshing frequency.

The amplitude at the meshing frequency can vary significantly from one tooth to another, depending on the number of teeth, transmission ratio, surface roughness, and load.

As a general rule, the greater the number of teeth, the smaller the transmission ratio, the finer the tooth surface roughness, and the lighter the load on the teeth, the smaller the amplitude at the meshing frequency.

Typical amplitude values at the meshing frequency for Ingersoll Rand air compressors usually range from 4 to 8 G in acceleration; however, it is not uncommon to encounter amplitudes exceeding this range even under non-abnormal conditions.

The amplitude at the meshing frequency is often used as a measure of condition, but normal variations due to load changes can mask any trends caused by changes in condition.

Three acceleration characteristic diagrams recorded from a gas turbine generator reducer were measured under three operating conditions: rated speed with the circuit breaker open, approximately half load at 9.5 MW, and full load at 15.5 MW at room temperature.

Note that there is a significant change in the amplitude at the meshing frequency from no load to full load. Since the characteristic diagrams were recorded over about 15 minutes when the machine was presumably operating satisfactorily, such variations are likely only due to load changes and the resulting temperature changes in the reducer components.

Also note the relative changes in the amplitude of the harmonics of the meshing frequency from no load to full load.

At 9.5 MW, the characteristic diagram shows smaller amplitudes at the second and fourth harmonics, combined with a larger amplitude at the third harmonic, suggesting a square wave, possibly due to a sudden force during engagement and disengagement.

At full load, this characteristic disappears and is replaced by a more normal harmonic sequence with decreasing amplitudes, indicating a slightly distorted sine wave. Unfortunately, the recording process was not continuous, so the changes in amplitude at the meshing frequency throughout the loading process could not be observed.