Vibratory feeder, hopper, sieve and conveyor noise reduction techniques
Vibrating conveyors, hoppers, separators and screens, not mention vibratory 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.
What are the features of vibratory feeder, conveyor or screen noise?
There are 4 potential constituents that can contribute to the noise from these vibrating systems.
- low-frequency tonal noise (hum): at the operating frequency and harmonics. Typically at a few Hz up to 100Hz. Hoppers are like massive bass speakers and this noise can travel for miles and cause structural rattles
- broadband mechanical noise: generated by the vibrating hopper, bowl, separator screen or conveyor structure
- broadband product noise: generated by product or component impacts on the bowl, conveyor or sieve or by product-product impacts
- high-frequency tonal and/or broadband noise: when pneumatic vibrators are installed
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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...
Low-frequency tonal noise reduction - vibrating mechanisms
There are a number of vibratory mechanisms that are used to sieve, separate or convey product.
- crankshaft - con-rod: the vibrating hopper or conveyor is physically connected to a rotating crank. Rotational speed tone plus harmonics.
- out-of-balance drivers: typically motor-driven balance weights bolted to the structure. Rotational speed hum plus harmonics.
- electromagnetic drivers: simple electromagnets imparting vibration - usually mains frequency at 50Hz plus harmonics on vibrating conveyors or bowl feeders
There is a fundamental difference in approach depending on whether the noise problem is occupational or environmental.
Environmental noise
This will almost always be caused by the fundamental low-frequency component plus 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.
Occupational noise
The overall dB(A) will usually be dominated by the higher harmonics. The first step for mechanically driven systems 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" ( a common problem for vibratory bowl feeders). 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.
Broadband mechanical or vibrating product noise
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.
- Looseness: check for looseness in the mechanical components and drivers. If present, it generates a toothcomb of harmonics up the noise spectrum (the noise signatures show if this is the problem without inspection). If present, then maintenance will provide a large noise reduction.
- Mechanism impacts: check for impacts due to insufficient clearances as components wear. Maintenance.
- Product impacts on conveyor/hopper surfaces: use high-efficiency damping and/or bespoke high-frequency isolation to achieve 10dB or more attenuation with no effect on operation. The materials used are hygienic so this is a very effective option in the food and pharmaceutical industries.
- Product/product impacts: if you have proved that this is the dominant source, then the only noise control option is to screen or enclose - or fit a simple lid on vibratory bowl feeders.
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 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.
High-frequency noise
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.
- pneumatic drivers - rotating: air-driven out-of-balance ball-race. The dominant source is almost always the unsilenced (or insufficiently silenced) air exhaust. Fit effective silencing, zero backpressure if required to provide 20dB - 30dB or more attenuation with no effect on operation.
- pneumatic drivers - impact: air-driven impactors. In many cases, only the low to mid-frequency components are responsible for effective operation. Experiment with high-frequency vibration isolation (creating a low-pass vibration filter) and high-frequency damping. This can provide 10dB - 20dB attenuation without affecting the operation.
Vibratory mould noise reduction
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.
- measure the dwell time and the vibration signatures
- rank the noise-radiating surfaces
- where practical, vary the amplitude and frequency of the mould vibration and evaluate the effects on product quality
- design modifications to ensure that the maximum vibration at the optimum frequency is fed into the mould whilst the minimum is fed into the surrounding structures
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...
