Vibration equipment noise: steps toward abatement.

Some mechanical changes to foundry vibrators can help reduce noise to comply with OSHA sound exposure requirements.

Once upon a time, factory noise was seen as a sign of productivity, that there was profitable work going on. But as the metalcasting industry continues to improve foundry working conditions and comply with OSHA requirements, this traditional notion is being rethought.

Medical science has discovered that high workplace noise can induce costly physical side-effects, including hearing impairment, headaches, fatigue, heart rate changes, high blood pressure and increased thyroid and adrenaline production.

Excessive noise also contributes to poor job performance in the form of inattention to speech communication, missed instructions and warning signals and higher accident rates. Noise especially interferes with performance of complex tasks that require high concentration.

Mechanical vibration inducers are valuable tools for foundries, aiding in shakeout and sand handling. They reduce particle-to-particle and material-to-structure friction, allowing material to flow more freely and feed more easily. But vibrators cause some of the most conspicuous noises in foundries.

While there are a number of alternatives to vibrators, they continue to be one of the most practical materials handling tools available. This article examines the causes of vibrator noise and suggests some steps to lower that noise.

Vibration Noise Causes

There are some general rules of thumb about vibration equipment noise: air vibrators are noisier than electric vibrators; linear vibrators are noisier than rotary vibrators; and impacting piston vibrators are noisier than nonimpacting piston vibrators (Table 1). In air vibrators, the more air pressure put through the vibratory device, the faster the exhaust speed and the more noise produced. For every 20 psi increase in air pressure, the noise level typically rises 3-6 decibels (dB).

Table 1. Typical Vibrator Noise Output Ranges

                                        dBA

Linear     Impacting Piston            95-115
           Spring Cushioned Piston     80-100
           Nonimpacting Linear         60-85

Rotary     Ball                        75-95
           Roller                      75-90
           Turbine                     60-75
           Electric                    65-75

There are a number of other possible sources of vibration noise. These include:

* oscillation ("oil canning") of walls, and loose or flexing vibrator mount;

* metal-to-metal impact (hammering) of linear piston vibrators;

* revolution of bearings or eccentric weights inside rotary vibrators;

* "hum" of the electric motor;

* echo of empty space or material within the vessel;

* the noise of material movement.

In addition, the amount of material within the bin has a considerable impact on the volume of noise; the less material, the more noise. Also, the shape of the vessel, the proximity of other equipment, and the construction of the entire plant building itself influence the amount of noise generated.

Each structure - whether shakeout, vessel, bin, hopper, screen or feeder - has its own resonance and noise pattern due to its design, construction and material load. A common problem is that a vibrator may be operating within acceptable noise requirements, but when attached to a particular structure, the noise level is raised above expectations.

To control this oil canning noise, the integrity of the structure can be improved by tightening bolts, strengthening welds or adding reinforcing plates. The problem is that sometimes, the same oil canning that causes the noise also transmits the vibration into the material.

Noise Control Methods

A variety of actions can be taken to control vibration noise in foundries. The overall success of any of these options will depend on the variables of each situation.

Muffling the exhaust - A modest amount of noise reduction can be obtained by the attachment of a muffler to the exhaust port of an air vibrator. These devices are composed of a set of baffles that reduce the escape velocity, and hence the sound, of the exhaust. A similar procedure uses an air line to direct exhaust air away from the operation and operating personnel. The noise is then vented outside, or in an open area like the plant roof. Care must be exercised with these options so as not to increase to the "back pressure," reducing the force output of the vibrator.

Improving the mount - Care must be taken to curtail noise originating from loose mounts or cracked welds. Bolts should be tightened to the specified torque. The structure itself can be reinforced around the mount, stiffening the point of application, so more energy is transmitted into the material instead of dissipating through flexure.

On rotary vibrators, a hard rubber pad 0.5-1 in. thick can be installed between the vibrator and the bin wall. This will quiet mount noise while allowing much of the vibratory motion to pass into the material.

Reducing the speed - The higher the operating speed, the more noise produced. Adjusting vibrator frequency downward or switching to a larger vibrator (swinging a larger eccentric) that operates at a lower speed but produces the same force will lower the noise level.

"Oversizing" the vibrator in this way will allow you to reduce operating speed and noise. By choosing a unit one or two sizes larger than the one normally recommended, the air pressure can then be lowered so the vibrator runs at less than maximum speed.

Changing the schedule - By changing the vibration schedule, a good deal of noise control can be achieved. For example, vibrators can run intermittently rather than continuously. Installing automatic controls may allow you to reduce "on" time to a minimum. In some cases, 10 minutes on and 10 minutes off may provide flow as consistently as continuous operation.

Relocating the vibrator - It may be possible to move the vibrator to a spot on the bin where it creates less noise, or is heard less. It might simply mean moving it around to the back of the bin, away from the work station.

Enclosing the Vibrator - Another option is to enclose the vibrator in a foam-lined box that reduces the amount of noise released. However, some sort of port must be left to vent exhaust air, which allows some noise to escape. The enclosure may cause a buildup of heat, create maintenance difficulties and dampen the vibratory force. And of course, it will do nothing to reduce noise coming from the structure.

Typical Sound Levels

Perception      dBA     Sound Source

Painful         140     Jet Engine
Deafening       110     High Speed Train, Boiler Factory
                100     Auto Horn at 20 ft, Pneumatic Drill at 10 ft
Very Loud        90     Noisy Factory, Loud Street, Heavy Truck
                 80     Very Noisy Restaurant, Auto at 40 mph.
Loud             70     Average Restaurant, Noisy Office
Comfortable      60     Average Conversation at 3 ft
Faint            40     Library, Quiet Street
Very Faint       20     Whisper
                  0     Threshold of Hearing
Table 2. OSHA Allowable Noise Dose

Sound Level             Duration Per Day
(dBA)                       (hours)

90                             8
92                             6
95                             4

Sound Level             Duration Per Day
(dBA)                        (hours)

97                             3
100                            2
102                           1.5

Sound Level             Duration Per Day
(dBA)                        (hours)

105                            1
110                           0.5
115(*)                        0.25

* Maximum steady level permitted

"Canceling" the Noise - Active noise cancellation entails using a digital signal processor to generate an equal and opposite copy of the noise wave to neutralize the original sound. This "antinoise" only works on repeating low frequency noises (below 1500 Hz).

To date, little work has been done with active noise cancellation to determine its suitability to controlling vibrator noise. The future may see increased application of these systems, particularly where one antinoise device can "cover" the noise from vibrators and other equipment at the same time.

Updating the Vibrators - An obvious way to reduce vibrator noise in your foundry is to switch to quieter vibration equipment. One option would be to use electric units.

There are also now a number of "state-of-the-art" air vibrators that offer significant noise control benefits. These units are more effecient, with less air volume and pressure to cause exhaust noise. They also rely on nonimpacting pistons and lightweight aluminum housings, which also aid quiet, efficient operation. Depending on the application, these new vibrators may cut as much as 20 dB from a foundry's sound level.

OSHA Compliance

As seen in Table 2, OSHA limits a worker's daily exposure to 90 dBA for eight hours (The "A" in dBA refers to the A scale that simulates the response of the human ear). Higher levels can be sustained for shorter intervals, such as 95 dBA for four hours. Workers moving about the facility are exposed to varying doses, so employees are allowed a series of partial doses of noise, but the sum should not exceed 90 dBA in an eight hour shift.

OSHA insists on the use of engineering steps in noise abatement, and so should foundries. Because hearing conservation programs for each employee are complex and costly, as much as possible should be done to reduce overall shop noise. While engineering changes are also expensive, full compliance dictates that both routes be taken to reduce workers' exposure to dangerous noise levels.