Designing Quiet Gears

The gear design phase is the easiest, least costly, and most effective way to influence gear noise.

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In today’s industrial and commercial environment, gear noise is increasingly undesirable. Conversely, quiet gears are necessary and important in nearly every industry and for every application.

For example, in the electric-car industry, quietness in all aspects of the vehicle is critical. Quiet gears and gearboxes are particularly important in electric cars because these vehicles have no internal combustion engine to generate additional noise to cover up the gear and gearbox noise. There is only an electric motor, which is scary quiet, that by design does not cover up or compete with the noise produced by the gears or gearbox. In this market, noisy gears would not be accepted or tolerated.

Much research, thought, and development have gone into the effort to reduce gear noise. This effort is best and most effectively addressed in the design phase; however, other phases that also affect gear noise positively or negatively are the manufacturing and assembly phases.

Regarding the assembly phase, it is well-known that simply assembling and then disassembling and reassembling the same gearbox has an effect on gear noise, sometimes in the same order of magnitude as the differences between two different gear pairs. While all these stages do, in fact, influence gear noise, the gear design phase is the easiest, least costly, and most effective way to address this.

Gear noise involves multiple factors. It originates from the impacts of gear teeth coming into contact in the mesh cycle. Sometimes a gear contacts its mating gear tooth and enters the mesh cycle before or after it is expected to, and this causes the gear tooth to either instantaneously speed up or slow down the mating gear tooth because of this out-of-position and location situation. The gear excitation and resulting noise can then be transmitted to the other adjacent components such as shafts, bearings, housings, etc. And then it can be radiated away from the gearbox and to the listeners’ ears.

Here, I will concentrate on the impacts of the gear teeth entering and leaving the mesh cycle, because it has the highest rate of return in gear noise reduction among all the elements of design.

In general, the maximum gear tooth contact and minimum deflection is best. Gear teeth with no errors and with little to no deflection would be extremely quiet.

Contact ratio also plays an important role in this process. Higher contact-ratio gears, whether spur or helical, have more on average gear teeth in contact at any one time and less deflection.

Helical gears have contact in both the transverse and the axial planes. This lessens the gear tooth impact and provides smoother engagement because of the higher total contact ratio.

High contact-ratio spur gears, which I discussed in the April 2017 Tooth Tips column in Gear Solutions, have some of the same advantages as helical gears. Helical gears also can be designed with a high contact ratio in the transverse plane.

In either spur or helical gears, high contact ratio (HCR) is defined as greater than 2.0 in the transverse plane. Achieving a high-quality level and greater accuracy is important in HCR gears because of the multiple teeth in contact.

Isotropic superfinishing also reduces noise due to reduced friction and a smoother gear tooth surface.

Designing gears for high strength versus low noise sometimes requires opposite design characteristics due mainly to the reduced contact ratio, coarser diametral pitch, and fewer gear teeth typical of most high-strength gears.

In general, the goal is to achieve low tooth-to-tooth transmission error, which relates to gear teeth entering the mesh to each other in the exact position and location that they should, as well as controlling gear tooth deflections. The Ohio State University Gear Lab has developed a program that defines and measures tooth-to-tooth transmission error called the Load Distribution Program (LDP). This program takes into account macro and microgeometry effects as well as speed, torque, and other related factors.

Microgeometry is typically defined as tip and root relief on the flanks of the gear teeth. Optimizing this microgeometry is beneficial and important in reducing gear noise because it eases the entry and exit of the gear teeth in the gear mesh.

In summary, contact ratio is important and, to some degree, the higher the better. Although a high contact-ratio spur gear set benefits from a contact ratio of greater than 2.0, it is also thought that a gear set with any contact ratio above 1.8 is beneficial in reducing gear noise. A quiet gear set is the result of a high contact-ratio spur or helical design, is isotropic superfinished, and has a high quality level.

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Rick Miller
is president/sole member of Innovative Drive Solutions LLC, which provides gear design engineering services for optimizing gears, gearboxes, and other geared devices. Miller has created over 300 original gearbox system designs. He is a member of AGMA, SAE, and ASME, and he holds three patents. He is vice chair of the AGMA Vehicle Gearing Committee. For more information, visit www.innovativedrivesolutions.com.