Introduction

During commissioning of a welded beam plant, it was noted that extremely high noise levels were occurring in the finished products area. Whenever the finished welded beams were stamped with their identification numbers, the entire area was cleared of personnel. The only person remaining in the immediate area was the operator of the cartridge operated Piston tool used for stamping the beams. Even the earmuffed operator found the stamping operation intolerably loud.

The stamping tool is operated by the detonation of a cartridge which propels the stamp mounted on a piston into the surface of the steel beam. This sharp blow causes a short, but extremely loud impulsive noise.

Investigation

It was decided to observe the stamping procedure at close range. Each welded I-beam is over ten metres long and a metre deep. The beams are placed on their sides with the web horizontal. The gun was placed on the flat of the web and fired. The client was under the impression that the noise came from the tool and had inquired from the supplier where he could obtain a quieter stamper.

The sound pressure levels were measured at one metre from the stamping tool, and the sound pressure level was in excess of 135dB(A). The noise level would have been higher still at the operator's ear, as they had to lean into the tool. Since most earmuffs will only provide about 25dB protection, this meant that for hearing conservation purposes the operators would have had to limit their use of the stamping tool to only a few minutes per day.

To assure the client that the stamping tool was not the major source of the problem, the tool was test fired away from the beam. The sound level of the tool itself was quite low, and thus not the primary source of the disturbing noise.

It was suggested to the client that the noise level could be reduced by stamping the welded beam at a different location. The client stated that this could not be done and requested that an alternate solution be found. An attempt was made to reduce the noise of stamping by clamping a mass damper to the centre of the beam. This resulted in only 3dB noise reduction.

The client was again approached to alter the stamping location. The reasoning behind this suggestion was that by stamping at the centre of the web, this caused the beam to ring "like a bell".

When a beam is caused to vibrate, maximum movement occurs at the centre of the web, and along the edges of the flanges. By striking the beam at its vibrational nodes, either in the centre of the web, or at the outside edge of the flanges, the beam will freely vibrate. If a beam is struck at its antinodes, namely the junction of the web and the flanges, the beam is not induced to vibrate, and will not ring.

Conclusion

The effect of striking the beam at different locations was demonstrated to the client. Stamping the beam in at the end face, close to the junction of the web and the flange, rather than the middle of the web, produced a noise reduction of in excess of 20 dB.

The client immediately issued instructions that this new location was aceptable and the beams were stamped at the ends. The procedure manuals were modified accordingly. With a reduction of greater than 20 dB this permits an operator (wearing earmuffs) to operate the stamping tool frequently with no fear of hearing damage.

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