Some tips for controlling and damping noisy equipment and minimizing employee exposure help mills meet OSHA guidelines

Gearbox Noise Reduction Efforts Pay Off in Safer Mill Workplace

BY RICHARD SCHUNCK and JOHN LISIECKI

Maintenance

Imagine the sound of a lawn mower up close. That's the noise level an employee is subjected to standing only 10 ft 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 (e.g., pumps, motors, conveyors, compressors, and fans), noise levels within certain mill areas 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 workplace safety strategies? The plain and simple fact is that 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 (Table 1). In fact, at levels greater than 85 dB(A), employees must be provided-at company expense-with hearing protection (such as earplugs) and be given a regular hearing loss test.

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 NOISE SOURCE. 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 the sound generation source and how the noise is transmitted.

A speed reducer can be used as an example. As a reducer turns, all of its components-gears, bearings, shafts, cooling devices, lubrication, and even its housing-begin to vibrate. This structural vibration causes pressure pulsations in the surrounding air that a human ear detects as sound. However, any one component, or more likely a combination of 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, gearbox manufacturers consider the speed reducer's application and specifications 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 gearbox. Noise level and frequency are affected by the following:

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 cannot be altered due to the machine's performance requirements.

Gear tooth selection. Understandably, gearbox 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 that 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 that 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, a mill should consult with its 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.

However, there are still some options available to a gear drive manufacturer or an in-house maintenance department that primarily concern 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 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 gearbox noise levels. Mills should be aware whether the gears in its machinery are hardened before cutting and finished after heat treatment. This process will reduce errors and inconsistencies that 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. A gear drive manufacturer should be consulted on which finishing method should be used for an individual application.

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. A gear manufacturer should always be consulted regarding proper gearbox lubrication. It is better to reduce noise levels using other methods than to tamper with thicker, less effective lubricants.

Other sources. Finally, a mill may be able to control some gearbox noise by reconsidering 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 that may help combat resonant frequencies that contribute to objectionable noise levels. (For a list of system conditions that may cause excessive noise, see sidebar on this page.)

CONTROLLING NOISE TRANSMISSION. Controlling the source of gearbox 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 abatement goals.

In effect, a mill will either be redirecting the vibration and noise away from employees or absorbing and converting the energy altogether. It should be noted that 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, including the following:

Vibration isolators

Sound-absorbing panels

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 (Figure 2). This configuration reduces the amount of structure-borne noise that 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 (Figure 2), 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 (Figure 3). This air-tight structure typically consists of a rigid outer wall and a sound-absorbing inner wall, responsible for preventing noise buildup within the enclosure. Total enclosures are gaining 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.

There is 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 employees farther 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.

RICHARD SCHUNCK is senior engineer and JOHN LISIECKI is manager, product development and applied technology, The Falk Corp., Milwaukee, Wis.

Troubleshooting noisy drives

Selecting a manufacturer that can help design, install, and maintain original or retrofit power transmission equipment is the first step towards quieter mill operations. However, one also needs to be able to identify possible system conditions that are causing noises within gear drives. 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 startups or on drives that 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 startups 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 startup 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 the manufacturer.

Failed bearing. First, measure the 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 startup or after system upgrade. If suspected, contact the manufacturer.

Today, there is a variety of good monitoring equipment that can measure noise and vibration levels. It is designed to provide hard copies or vibration signatures that can establish a history or baseline for a mill's drives. Once again, when all other checks prove negative, do not hesitate to contact the manufacturer for assistance.