10.7 BLANKING PRESS NOISE

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A typical blanking press is a massive machine, with a mass on the order of

125,000 kg (125t or 275,000 lbm or 137.5 tons). The unit in this case study

was mounted on four footings set on heavy concrete piers (Salmon et al.,

1975). The press produced automobile chassis steel sections of 1

4-inch

(6.4 mm) thick steel with a width of approximately 254mm (10 in) and

lengths between 2.4m (8 ft) and 3.0m (10 ft). The normal operation involved

30 strokes per minute.

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TABLE 10-1 Transmission Loss for the Muffler Discussed in

Sec. 10.6

Octave band center frequency, Hz

63 125 250 500 1,000 2,000 4,000 8,000

Lp for LA of 85dBA 102 92 85 79 76 75 75 77

TL for LA of 85 dBA — — 8 18 27 31 35 36

TL provided, dB 10 8 12 20 27 32 43 50

Copyright © 2003 Marcel Dekker, Inc.

10.7.1 Analysis

The vertical acceleration levels for the press pier as a function of frequency

in the acoustic frequency range are shown in Fig. 10-10. These values are

peak acceleration levels averaged over an 0.6-second sampling time. The

peak values are less than the maximum acceleration values because of the

averaging over the sampling time.

The sound pressure level spectrum at the operator’s location is shown

in Fig. 10-11. The operator was located approximately 1.2m (4 ft) from the

press. These octave band sound pressure level readings were obtained from

measurements when the press made a single stroke, so the measurements

corresponded to the peak values averaged over an 0.6-second sampling time,

as was the case for the acceleration data. The sound pressure level indication

Case Studies in Noise Control 495

FIGURE 10-10 Vertical acceleration levels referenced to 10 mm=s2 on the support

pier: (1) before and (2) after installation of vibration isolators.

Copyright © 2003 Marcel Dekker, Inc.

on the ‘‘slow’’ setting on the sound level meter was approximately 10 dB

lower than the peak values shown in Fig. 10-11. The A-weighted sound level

calculated from the data in Fig. 10-11 was 105 dBA, whereas the data measured

on the ‘‘slow’’ setting of the sound level meter was 95 dBA before any

noise control procedure was applied. This value corresponds to an allowable

exposure time of 4 hours according to the OSHA criteria.

The estimation of the acoustic energy radiated from a vibrating surface

of complicated shape due to impact loading must generally be carried

out through a numerical analysis (Beranek and Veґ r, 1992). Reduction of the

vibration transmitted from the machine to the foundation will usually result

in some reduction of the noise radiated by the foundation, however.

496 Chapter 10

FIGURE 10-11 Sound pressure level spectrum (peak values averaged over 0.6 second)

for the blanking press noise at the operator’s location: (1) before installation of

the vibration isolators and (2) after installation of the vibration isolators.

Copyright © 2003 Marcel Dekker, Inc.

10.7.2 Control Approach Chosen

Based on the speed of the press and its mass, special vibration isolators were

designed for placement under the four feet of the press. The vertical peak

acceleration levels (averaged over 0.6 seconds) on the pier after the vibration

isolators were installed are shown in Fig. 10-10. There was no large reduction

in the vibration levels in the octave bands from 31.5Hz to 1000 Hz;

however, there was significant attenuation in the octave bands above

1000 Hz.

The sound pressure level spectrum at the operator’s location after the

vibration isolators were installed is shown in Fig. 10-11. The octave band

sound pressure levels actually increased somewhat for the range from

31.5 Hz to 250 Hz; however, the octave band sound pressure level was

reduced in the higher octave bands. Sound in the octave bands from

about 250 Hz to 8000 Hz is more significant, as far as damage to the

human ear, than noise in the lower frequency range, as discussed in Sec.

6.2. The A-weighted peak sound level calculated from the data in Fig. 10-11

was 99 dBA, and the A-weighted rms sound level measured with the ‘‘slow’’

setting on the sound level meter was approximately 89 dBA.

Although the noise generated by the press after vibration isolators

were installed was less than the OSHA limit for 8-hour exposure

(90 dBA), noise radiated by adjacent presses contribute to the operator’s

work noise exposure. These other operational noise sources for the press

would be controlled separately from the vibration isolation.

The impact of the blanking press produces vibration in the floor of the

building and in the press structure. Often the background noise around a

press that has no vibration isolation is a result of induced vibration of the

building structure, which is probably caused by the anchor bolt aftershock.

Vibration isolators can act to reduce the building structure vibration.

10.7.3 Cost

The vibration isolators selected for this problem were not stock or off-theshelf

items, but were specifically designed for this case. The cost of the

isolators was approximately $5000. The cost for in-plant labor to install

the vibration isolators was approximately $3000.

10.7.4 Pitfalls

One of the major problems in using vibration isolators to reduce noise

radiated from a foundation of complicated shape is that the prediction of

the amount of noise reduction is usually quite difficult and/or expensive.

Although some reduction in support-generated noise will be achieved by

Case Studies in Noise Control 497

Copyright © 2003 Marcel Dekker, Inc.

vibration isolation, the actual noise levels are best determined after the

vibration isolators have been installed.

Some degree of vibration isolation is usually recommended for such

items of machinery as presses. The life of the dies is usually increased and

maintenance problems are usually decreased after the machine has been

treated to reduce vibration. Foundation failures, anchor bolt breakage,

and fracture of the press feet are usually significantly reduced by vibration

isolation of the press.

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