Using Sound to Diagnose Gearbox Problems
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 very well provide valuable clues to the gearbox operator, the equipment manufacturer, and/or the gearbox supplier as to the condition of the unit in question. This article will briefly explore the fundamentals of sound and how it is measured. Additionally, a method of isolating sound from ambient conditions will be addressed.
Sounds from a Gearbox
From the perspective of gearboxes (or gearmotors), operational sounds are generated through 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 by a gearbox 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.
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.
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.
American Gear Manufacturers Association – AGMA 914-B04 Gear Sound Manual. 2004
American Gear Manufacturers Association – AGMA 6025-D98 Sound for Enclosed Helical, Herringbone and Spiral Bevel Gears. 1998
Basaraba, Bruce., Archer, James, IPT’s Rotating Equipment Handbook, IPT Publishing and Training Ltd. 1995
Halliday, David., Resnick, Robert, Fundamentals of Physics. John Wiley & Sons. 1988