Vibrating conveyors, hoppers, separators and screens, not mention bowl feeders, generate high levels of noise that is difficult to attenuate using conventional techniques such as acoustic enclosures and barriers. These are not only high cost, but are often impractical (e.g. maintenance access) and not very effective, particularly where noise is dominated by low-frequency sound.
The alternative approach to vibratory system noise reduction is engineering source control that is low-cost, effective and often improves productivity. It is based on a detailed understanding of the noise-generating mechanisms and the following diagnostic process.
This approach is equally effective whether the noise problem is occupational hearing damage risk or environmental noise nuisance.
There are 4 potential constituents that can contribute to the noise from these vibrating systems.
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Once the contributions from each noise element have been diagnosed and ranked, it is then possible to determine the optimum engineering noise control technique. This approach and the accompanying technology can be extraordinarily effective, including a world record project that cut the noise from a large vibrating screen by a factor of 100,000 (50dB) without affecting normal productivity or access...
There are a number of vibratory mechanisms that are used to sieve, separate or convey product.
There is a fundamental difference in approach depending on whether the noise problem is occupational or environmental.
This will almost always be caused by the low fundamental frequency and the first couple of harmonics, not by the higher frequency noise components. The best (and low-cost) solution is to reduce the noise radiation by the dominant components via isolation damping or detuning. In the latter case, significant attenuations can sometimes be achieved simply by changing the speed slightly.
The overall dB(A) will usually be dominated by the higher harmonics. The first step is to check the condition of the bearings (1 and 2 above) as any wear will dramatically increase the number and amplitude of the harmonics that can increase the overall noise level by as much as 10dB - 20dB. In the case of electromagnetic vibrators, check the clearances to ensure there is no "hammering". In all cases, tune the speeds and forces to provide maximum productivity for minimum input. It is not uncommon for vibratory feeders and conveyors to be poorly adjusted and hence much noisier than necessary. Once correctly adjusted, lock-off the control.
If there is considerable broadband noise when run empty, then review items 1 and 2 below. If the dB(A) noise level increases dramatically when the vibrating hopper, conveyor or bowl feeder is run with product, then review items 3 and 4 below.
The following examples illustrate the process.
Vibratory separator noise reduction
At 105dB(A), new food industry vibratory separators were too noisy to install under a Buy Quiet policy. The above diagnostic process showed the noise was mid-high frequency dominated by a combination of product bowl and product/product impacts plus some high-frequency radiation from the vibrating mechanism.
We worked with the supplier to develop a set of engineering modifications that cut the noise by 16dB(A) without affecting normal operation, hygiene, maintenance, or access. The modifications were installed at at virtually no cost when built into the machines and have since become the new standard low-noise product.
As a result, PPE use could be restricted to areas local to the separators rather than throughout the whole department.
Pneumatic vibrators or impact hammers generate very high levels of high-frequency sound e.g. when used to prevent product bridging on hoppers or to facilitate product feed.
Many of the same diagnostic and noise control techniques described above are also applicable to vibrating mould-filling plant, whether it be concrete or chocolate moulds. In this case, however, the process performance is very dependent on both the frequency and amplitude of the vibration that is fed into the product to fill the mould effectively. The mitigation process is therefore predicated on tuning the vibration fed into the product to optimise the mould-filling quality whilst reducing the noise. In many cases, this process will improve both throughput and product quality.
The 2 most common areas where vibration is used to fill moulds are concrete (tiles, blocks etc) and chocolate (bars and sweets). In both cases, the principles and the diagnostic processes are the same.
The success of this approach is demonstrated by the following examples...
Concrete mould vibrator
Sound file: concrete tile vibratory mould filling noise reduction
A new unit due to be installed to manufacture specialist concrete tiles was highlighted as being unacceptably noisy (99dB(A)) as part of the Buy Quiet process. As a result of the diagnostic process, the system was re-tuned using simple engineering modifications (a fitter for a half day and <£100 materials). The result was not only a 15dB(A) reduction, but also a 25% reduction in cycle time.
Chocolate moulding vibrators
A noise and vibration control project on a small chocolate mould filling production line not only cut the noise by >5dB, but it also reduced the standard deviation in chocolate weight variation from 1.2gms down to 0.2gms/sweet. This provided a very considerable cost saving...