Removing a small amount of material at the gear tooth tip can help ease the tooth into rolling contact with other teeth so the transmission runs more quietly.
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Think of the racket a lawn mower under your control creates. You can experience the same noise level by standing 10 feet from a standard double-reduction speed reducer. The sound measures about 90 dBA. This is on the verge of being quite loud, but combined with equipment like pumps, fans, motors, and conveyors, noise in occupied sectors of industrial plants can easily reach 140 dBA – a dangerous level equivalent to a jet taking off. Noise abatement is therefore an essential practice nowadays, to promote worker productivity and to stem the very real danger of psychological and physiological strain, including, of course, permanent hearing damage.

OSHA guidelines stipulate that employees are spared noise levels above 90 dBA over the course of an eight-hour day (1910.95 occupational noise exposure). At levels above 85 dBA, therefore, employees are required to have hearing protection such as earplugs and are given regular hearing tests.

But it’s best if the noise itself can be toned down.

Go to the source

As a gear drive does its work, all the components – gears, bearings, shafts, cooling devices, lubrication, and housing – vibrate. The vibrating structure causes pressure pulsations (otherwise known as sound) in the surrounding air. If any one or, more likely, a combination of these components get out of sync, the noise can get excessive.

The machine’s quality may be high; most gearbox manufacturers put significant effort into designing for noise reduction in concurrence with specific applications. But there are numerous factors that can add to a gear unit’s noise level, ranging from gear mesh friction to improper lubrication.

Most speed reducers in satisfactory running condition will generate 85 to 95 dBA, while there are some low-noise applications with gears that emit between 80 and 85 dBA. But the acoustics of the plant itself can also affect the loudness.

To diminish vibration, engineers can change the type of gear teeth, refine the geometry of a particular type of gear tooth (the profile as well as the surface finish), and improve lubrication.

Gear tooth selection is a critical factor in the design of a geared transmission. The point and length of contact between meshing teeth is key; the more constant and uniform the contact, the lower the friction that helps create noise. This is true for all gears – spur, helical, bevel, and so on.

Helical gearing is popular for high speeds because it has the best maximumto- minimum contact-length ratio, and noise problems are often solved simply by switching to this type of gear, as long as it’s appropriate for the design and application.

While the geometry of the tooth profile can be revised to perform more quietly, concerns of compromised strength can complicate designation of the optimum tooth geometry. For example, increasing the tooth height to achieve greater overlap or mesh can actually reduce the gear’s load capacity, because the teeth act as beams when transmitting shaft torque.

If the gear is weakened because of an altered tooth design, it might seem logical to use a bigger gear to make up for lost strength. However, larger gears operate at higher pitch line velocities (the point of contact between gear teeth, measured in feet per minute) and actually produce higher noise levels. Thus, the situation would only be worsened.

Not to worry. Modification of existing teeth is still a viable option for manufacturers and in-house maintenance specialists.

Tip relief, or the removal of a small amount of material near the tip of the gear tooth, mitigates interference to help ease an incoming tooth into contact with other teeth.

In addition, crowning (sometimes called barreling) can be done across the width of the gear face. This involves reducing the material on either end of a gear tooth to produce a more oval tooth profile.

Both methods decrease friction and, therefore, noise. But be aware, such tooth modifications may end up reducing the gear’s durability and capacity. Furthermore, excessive profile modifications can actually increase the sound levels.

Better gear manufacturing and finishing techniques also help decrease loudness in the gearbox. Find out whether gears are hardened before cutting and finished after heat treatment. This process reduces errors and inconsistencies that can cause noise. As for cutting and surface-finishing techniques such as hobbing, shaving, grinding, and lapping, each lends to an individual noise characteristic. Generally, as would be expected, the finer the finish, the quieter the gearing.

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