Making a Loud Case for Reduced Gear Box Noise
Imagine the sound of a lawn mower, close-up. That's the noise level
one is subjected to standing only 10 feet from a standard double-reduction
speed reducer. In acoustical terms, the sound measures about 90
decibels (dB(A)).
Ninety dB(A) may or may not seem loud, but combined with sounds from other equipment (for
example: pumps, motors, conveyors, compressors and fans), noise levels within certain plant
quarters can easily reach 140 dB(A) -- a dangerous level equal to that of a jet during take-off.
Is it any wonder why so many companies are implementing noise abatement programs as part of
their environmental and work-place safety strategies? The plain and simple fact: over extended
periods of time, high noise levels can reduce worker productivity, or worse yet, possibly damage
employees' hearing.
For this reason, current U.S. Occupational Safety and Health Administration (OSHA) guidelines
(1910.95 Occupational noise exposure) stipulate that employees not be subject to noise levels
greater than 90 dB(A) over the course of an eight-hour shift (see Table 1). In fact, at levels
greater than 85 dB(A), employees must be provided with hearing protection, for example,
earplugs, and be given a regular hearing loss test...all at company expense.
Yet, there are ways to eliminate costly noise-related problems within the work environment.
Company medical expenses and administration hassles can be reduced by finding the sources of
loud plant noise and developing noise control and abatement tactics.
Understanding the Source of Noise
One source of objectionable noise may be the various gear drive units found within a facility. After
sound tests are conducted, if a gear drive is judged "noisy", the logical step is to determine a.) the
source of sound generation, and b.) how the noise is being transmitted. Take a speed reducer, for
example. As a reducer turns, all of its components -- gears, bearings, shafts, cooling devices and
lubrication, even its housing -- begin to vibrate. This structural vibration causes pressure pulsations
in the surrounding air which a human ear detects as sound. However, anyone, or more likely, a
combination of those components may get out of sync and cause excessively noisy vibrations.
This is not to say the quality of the machine is suspect. On the contrary, during design and
production, gear box manufacturers consider the speed reducer's application and specifications in
order to make the quietest, most efficient machine possible. However, due to any number of
factors, ranging from gear mesh friction to improper lubrication, component vibration and noise
can occur. Most speed reducers operate with sound levels ranging from 85 to 95 dB(A) with gear
designs for specific low-noise applications achieving sound levels of 80 to 85 dB(A). Bear in
mind, of course, the one noise factor outside the gear manufacturer's control is the acoustical
environment of the plant itself.
Controlling the Noise Source
The vibration generated by gear mesh action creates the impetus for the most noise within a gear
box. Noise level and frequency are affected by:
- Type of gear teeth
- Gear tooth geometry
- Finished gear tooth surface
- Lubrication
Factors such as transmission load and motor speed affect noise levels as well. Often, noise
controlling factors can't be altered due to the machine's performance requirements.
Gear Tooth Selection
Understandably, gear box manufacturers pay close attention to gear tooth selection, design and
geometry, taking into consideration at what point and for how long the gear teeth should come into
contact with one another. The more constant and uniform the contact, the lower the friction forces
which cause noise. This is true for all types of gear designs: spur, helical, spiral bevel or straight
bevel.
The reason helical gearing is used on many high speed gear units is because this design offers the
best maximum-to- minimum contact length ratio. It comes as no surprise, therefore, that problems
with noise and vibration have often been solved by switching from spur to helical gearing.
However, you should consult with your gear drive manufacturer to determine whether or not this
solution is appropriate.
Gear Tooth Geometry
While gear teeth themselves can be designed to reduce noise, compromises in strength complicate
the selection of optimum tooth geometry. For example, increasing the height of a tooth to achieve
greater overlap or mesh can actually reduce the gear's ability to transmit load. If "weakening" of
the gear occurs because of an altered tooth design, it seems logical to simply use a larger gear to
make up for lost capacity. However, larger gears operate at higher pitchline velocities (point of
contact between gear teeth measured in feet per minute) and actually produce increased noise
levels; thus, the situation would only be worsened. But not to worry. There are still some options
available to your gear drive manufacturer or in-house maintenance department, mainly the
modification of existing gear teeth. Tip relief, or the removal of a small amount of material near the
tip of the gear tooth, can ease an incoming tooth into contact with other teeth (See Figure 1). In
addition, crowning (sometimes called barreling) can be done across the face width of the gear.
This involves reducing the material on either end of a gear tooth to produce a more oval tooth
profile.
In both cases, reduced friction equals reduced noise. But be warned, such tooth modifications
may end up reducing the gear's durability capacity. And of course, if excessive profile
modifications are done, one can actually increase rather than decrease the unit's noise levels.
Smooth Finished Surfaces
Better gear manufacturing and finishing techniques also help reduce gear box noise levels. You
should be aware whether the gears in your machinery are hardened before cutting and finished
after heat treatment. This process will reduce errors and inconsistencies which can cause noise. As
for surface finishing techniques, such as hobbing, shaving, grinding or lapping, each creates its own
noise characteristic. Normally, the finer the finish, the lower the noise level. Ask your gear drive
manufacturer which finishing method they'd recommend for your applications.
Thicker Lubricants
Noise developed by friction forces not only varies with the roughness of the gear surface, but also
with the thickness of the lubricant film. While higher viscosity oils and greases can cut down on
noise, they may not be well suited for the conventional gear unit. Always consult with your gear
manufacturer regarding proper gear box lubrication. Better to reduce noise levels using other
methods than to tamper with thicker, less effective lubricants.
Other Sources
Finally, one may be able to control some gear box noise by re-considering the unit's housing
material. The housing itself is not the source of the noise since it must be excited to vibration by
rotating elements. However, using stiffened or ribbed housings is an economical alternative which
may help combat resonant frequencies that contribute to objectional noise levels.
(For a list of system conditions that may cause excessive noise, see "Trouble Shooting Noisy
Drives")
Controlling Noise Transmission
Controlling the source of gear box noise is not always feasible due to economic considerations or
the unit's performance requirements. If this is the case, often times controlling the transmission of
air-or structure-borne noise from the unit will accomplish your abatement goals. In effect, you will
either be redirecting the vibration and noise away from employees or absorbing and converting the
energy altogether. It should be noted, the path of sound waves does not always travel from the
source of the noise directly to the ear. Sound waves are usually reflected from the floor, walls or
ceiling. Methods for "capturing" these scattered sound waves involve any number of devices, for
example:
- Vibration Isolators
- Sound Absorbing Panel
- Total Enclosures
- Damping Devices
An important distinction must be made between damping and isolation. While damping converts
vibrational energy into thermal energy, isolators reduce the vibratory force transmitted between
structures. Often times, even experienced engineers misunderstand this relationship.
Vibration Isolators
Vibration isolation requires the placement of a resilient material between the unit and its mounting
structure. This configuration reduces the amount of structure-borne noise which is
normally transmitted to the mounting and then radiated in the form of air-borne sound. Vibration
isolation with soft mountings is only effective at the lower frequencies and may create shaft
alignment problems.
Panels
Sound barriers with absorbing panels, on the other hand, interrupt the
air-borne path between the unit and receiver. Under certain conditions, one or two panels
properly placed can solve noise problems. These panels prove most effective when dealing with
high frequencies.
Total Enclosures
In extreme cases, total enclosure of the gear drive may be necessary. This air-tight
structure typically consists of a rigid outer wall and a sound-absorbing inner wall, responsible for
preventing noise build-up within the enclosure. Total enclosures are gaining a wide acceptance
because the gear drive unit as well as the driving and driven equipment can be encased within the
structure.
Damping
Noise radiated by vibrating surfaces of a gear drive unit can also be treated with damping devices.
These devices are made of a resilient material, usually a sheeting of honeycomb construction with,
perhaps, a metallic (lead) outer layer. In addition, double housing walls with damping particles
such as sand or metallic shot placed between the walls have also been used effectively. One
caveat, however: total enclosures, damping sheets and double walls restrict air flow creating a
thermal problem. As a result, ventilation (without allowing noise to escape) or water cooled heat
exchangers are required, which can be complicated.
Finally, there are some very simple means of obtaining noise abatement goals. Besides requiring
employees to use ear plugs or other protective devices, placing them further from excessively
noisy equipment or areas (when possible) is an obvious solution. In addition, if noise radiation is
sharply directional, changing the gear drive's angular position away from employees can also prove
quite useful.
Summary
In accordance with OSHA regulations, the number one priority in any noise abatement program is
to protect your employees and to provide a safe, comfortable work environment. While gear
drives and driven equipment are not the only "culprits" that can contribute to excessive noise within
a plant, they do generate vibration and noise which can be reduced. From proper gear tooth
selection to modification, from using sound barriers and damping devices to simply following
recommended maintenance procedures, there are measures your company can take to protect
personnel, production and profitability. Consult with your gear drive manufacturer to determine
which of those steps is most practical and economically feasible.
Permissible Noise Exposures Taken from OSHA Regulation 1910.95 Occupational Noise Exposure.
| Duration per day |
Sound level dB(A)* hours |
8 |
90 |
6 |
92 |
4 |
95 |
3 |
97 |
2 |
100 |
1-1/2 |
102 |
1 |
105 |
1/2 |
110 |
1/4 or less |
115 |
| * In noise abatement, the units most frequently used are adjusted decibels (dB(A)), which relate
measurements to the human hearing range. |
| Table 1 |
Trouble Shooting Noisy Drives
Selecting a manufacturer who can help design, install and maintain original or retro-fit power
transmission equipment is the first step towards quieter plant operations. However, one also needs
to be able to identify possible system conditions which are causing noises within your gear drives.
Listed below are possible causes of excessive noise and suggestions for remedies:
- Shaft Misalignment -- Both input and output shaft misalignment can cause noise within a speed
reducer. Misalignment may be present on start-ups or on drives which have operated for a long
time.
- Coupling Wear -- Usually the result of shaft misalignment or improper lubrication.
- Cascading Load by the Application -- If driven equipment operates with less than constant
velocity, gear mesh oscillation can occur resulting in an erratic noise. To correct, disconnect the
output coupling and operate the drive. Listen if the noise goes away or changes its pattern.
Typically, this problem occurs on new start-ups or with system upgrades.
- Bumps on Pinion or Gear -- May occur at installation or after repair. The noise will seem random.
Inspect gear teeth for tiny, shiny spots.
- Flexing Foundation -- A solid, flat foundation is needed to support the reducer and its transmitted
torque. Check for flexing at start-up or after a system upgrade.
- Loose Foundation Bolts -- Inspect bolts for proper tightness. Loose bolts will allow the reducer to
move or deflect about the foundation and result in noise.
- Excessive Tooth Wear -- Listen for a rumbling noise. Compare both sides of the tooth profile. If
there is a significant difference in the shapes, contact your manufacturer.
- Failed Bearing -- First measure axial float. If the float is within specifications, visually inspect for
surface distress. Catching roller bearing damage early can minimize related damage.
- Torsional Problem -- Every system has a critical range of operation either below or above normal
operation speed. A torsional problem will produce an erratic noise and may show up at a new
start-up or after system upgrade. If suspected, contact your manufacturer.
Today, there is a variety of good monitoring equipment which can measure noise and vibration
levels. It's designed to provide hard copies or vibration signatures which can establish a history or
base line for your drives. Once again, when all other checks prove negative, do not hesitate to
contact your manufacturer for assistance.
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