Sound can be a very subjective thing. What may sound “unusual” to one individual may very well sound “normal” to another.

As a method of diagnosing a potential problem with a gearbox or gearmotor, the sound coming from it should not solely be relied upon to determine whether or not a problem exists. The measurement of sound, however, may provide valuable clues to the gearbox operator, the equipment manufacturer and supplier as to its condition.

In terms of the measurement of sound, many companies offer specialized equipment intended to measure sound pressure levels. The size and functionality of such equipment varies greatly and, as a result, the cost varies greatly as well. In an industrial environment, it is common to utilize hand-held meters, often called decibel (or dB) meters, for such evaluations, and they provide a portable and economical way of measuring sound pressure levels. As with any sensitive piece of measuring equipment, calibration to assure measurement accuracy is vital. Given this, specialized sound calibrators are also available for use with the aforementioned dB meters to assure result accuracy.

Operational sounds are generated a variety of ways: the mesh of the gears, the rotation of the bearings, the splash of the lubricant, the operation of oil pumps and the interaction of the gearbox itself within the machine structure — just to name a few — all contribute to the normal overall sound generated during its operation.

In considering only the gear components, the tooth finish of the gearing itself contributes to the sound that is generated during operation. As an example, helical gearing that has been hobbed only would tend to be “louder” than a comparably sized helical gearset that has had a post-hobbing, tooth finishing operation conducted on it. In this regard, shaving or grinding are finishing operations performed on the gear teeth to minimize, or eliminate altogether, the rough tooth surface finish that the hobbing operation often generates. It is important to note that, in terms of tooth finish, the rotation speed of the gearset also plays a significant roll in the sound that is developed. Very little difference, for example, would be noted between a hobbed only gearset that is rotating at 30 RPM verses a ground finished gearset of the same size also rotating at 30 RPM. Conversely, if the rotational speed of the gearset were to increase, one would note a marked increase in sound from the unfinished set in comparison to the finished one.

The quality to which the gearset is manufactured also influences the sound that it develops during operation. For clarification, the quality of a gear is often based on certain tooth characteristics such as tooth profile, tooth lead and tooth index. The following picture details these individual parameters as they relate to a gear:

Various organizations (i.e.: AGMA, DIN, JIS) have established quality rating standards based on tolerances associated with each of these parameters. Given these and their influence on how gears in mesh interact with each other, it is reasonable to say that, all else being equal, a gear set manufactured to a lower quality rating would likely generate more sound than a comparably sized gear manufactured to a higher quality rating.

Design choices to minimize operational sound are not limited to those aspects discussed thus far. There are other factors that a designer can consider that would serve to actively lessen sound. Incorporation of these sound-reducing options, however, may yield unfavorable consequences in other areas of the design. Ultimately it is up to the gear designer to clearly understand and prioritize his/her design intent. A firm understanding of the intent will allow the designer to balance the design variables accordingly such that all aspects are being achieved in ways that are both physically and financially viable.

Considering the gearbox as a whole: certain noises could also emanate from it that may provide an indicator of an undesirable situation. As an example, nicks or dings on the teeth of the gear(s) could easily yield a constant “clicking” sound as the face of the offending tooth (or teeth) come into contact with the face of the mating gear. Typically, gearbox manufacturers “run-test” a product after it has been assembled but prior to its shipment. The purpose of this run test is to ensure that the unit is operating normally. Sumitomo Drive Technologies, for example, conducts a comprehensive battery of tests on each assembled gear unit prior to its leaving the factory. Performance data (which includes sound) are measured, evaluated, and compared against nominal values to ensure that the product is operating within established limits.

How to Measure
While the theory of sound discussed so far has been informative and perhaps even interesting, the question remains as to how all of this information can be applied to real-world situations. As noted previously, noise can be a very subjective thing. To call a gearbox manufacturer to say that a gearbox is “noisy” may be a frustrating experience for the user if he/she cannot quantify their claim of noise more clearly.

Fortunately, we have defined the governing mathematical formula for sound pressure. This formula, combined with some applied mathematical operations, provides us with a means to more clearly define the sound that an installed gearbox is generating in application. What is described as follows is a method of subtracting an ambient or background noise from a total measured value thereby allowing one to determine the sound pressure level from a particular source.

To begin, let LP,BACK and LP,SOURCE represent the sound pressure levels of the background and source respectively. Then,




and:

LPTOTAL = the total measured sound pressure value.

With these sound pressure levels defined, it can be mathematically proven that:



In essence, this equation provides the ability to calculate the sound from a source when the total sound (LPTOTAL) and background sound (LPBACK) are known through individual measurements.

To estimate the sound that is being generated by a gearbox, one first needs to measure the total sound in the area where the gearbox in question is located. About this, two points need to be made – first: the gearbox should be running while this measurement is being taken and second: typically, the meter should be held at a distance of 3 feet (1 meter) from the unit of concern. Once this measured value (LPTOTAL) is noted, turn the gearbox in question off and again measure the sound level in the same area where the first measurement (LPTOTAL) was obtained. This second measured value is LPBACK. To calculate the sound that the gearbox is generating in operation (LPSOURCE), simply insert the measured values into the defined equation to obtain the result.

As an example, say that there exists an interest in determining the sound that a gearbox is making while running in an application. Let’s further say that, using a handheld sound power level meter, it has been determined that the total measured sound is 77 dBA. This value is LPTOTAL and it is measured around the area of the gearbox in question while this gearbox is operational.

Finally, let’s measure the sound pressure level (in the same area as LPTOTAL was measured) when the gearbox in question is off. This value is LPBACK and, for the purposes of this example, let us say that the measured value is 73 dBA.

Inserting these values into the equation:



Thus, using a scientific calculator, it is determined that LPSOURCE = 74.8 dBA.

Depending on the application and the set-up, it may be possible to distinguish between motor sound and gearbox sound using the same mathematical procedure previously detailed. To begin, one would first need to disconnect the gearbox from the driven machine. Once disconnected, and with the motor and gearbox operational, proceed measuring the sound being generated by the combination of the gearbox and motor - this value is LPTOTAL. Then disconnect the motor from the gearbox and again measure the sound being generated while the motor is running. Using the previously defined nomenclature, this value would be LPBACK (which essentially is the sound generated by the motor itself). Inserting these measured values into their respective locations in the equation would result in a value (LPSOURCE) that would be the calculated sound being generated by the gearbox by itself. This calculated value can now be compared against the measured value obtained from the motor itself to isolate more clearly which source is influencing the overall sound that is being heard.

For new equipment installations, it is recommended that baseline sound measurements are obtained and noted for future reference. Subsequent measurements of sound could be then compared to this baseline value for evaluation purposes.

Conclusion
Clearly, situations do exist where one can readily identify that a problem exists with a gearbox based on the sound (or more appropriately “noise”) that it is making during its operation. Conversely, other situations also exist where “normal sounding” and “abnormal sounding” are simply too ambiguous to be of any diagnostic value. By understanding what “sound” is and applying some of the fundamental techniques addressed in this paper, the means does exist to quantify more clearly the sound that a gearbox or gearmotor is generating. This calculated or measured value, by itself, may not solely be used as a diagnostic tool. It’s knowledge, however, may provide significant clues to the OEM or gearbox manufacturer regarding the condition of the unit in question. In turn this may provide a starting point in root cause analysis and, ultimately, corrective action measures.

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This is an expanded version of an article that first appeared in print. Todd Bobak is research and development project engineer with Sumitomo Drive Technologies.