Exposure of employees to the noise emitted by a machine tool is highly undesirable:
- for employees themselves:
- deteriorating working conditions, causing difficulties in communicating and in perceiving audible warning signals
- causing discomfort, fatigue, stress or even hearing loss
- for employers:
- resulting, depending on the context, in loss of quality of workshop production, absenteeism, difficulties in filling some positions or even costs for compensation for deafness when recognized as being of professional origin
It is therefore advisable that the regulations in the matter of reduction of the exposure of employees to noise - opportunely taking into account the various disadvantages linked to excessive sound levels in work premises - are respected.
In Europe, it is based in particular on Directive 2003/10/EC of the European Parliament and of the Council of February 6, 2003 concerning the minimum health and safety requirements relating to the exposure of workers to the risks due to physical agents (noise):
- for the daily noise exposure level L EX, 8h: 80 dB(A) and 85 dB(A) are the thresholds, respectively lower and upper, triggering action[1],[2]
- 87 dB(A) is the authorized limit value (i.e. to be exceeded in no case)[1],[2]
In France, it is supplemented by the decree of August 30, 1990 taken for the application of article R. 235-11 of the labor code and relating to the reverberation control of work premises, in the case of a level daily sound exposure greater than 85 dB(A) with regard to the decrease in sound level by doubling the distance to the source DL that must respect minimum values:
- in a room empty of any machine or production facility: 2 dB(A) to 4 dB(A) depending on the floor area S (the pivotal values being 210 and 4600 m2 respectively, with a logarithmic interpolation for intermediate values [3] )
- in a room after installation of machines and production devices: 3 dB(A) to 4 dB(A) depending on the floor area S (the pivotal values being respectively 210 and 1000 m2, with a logarithmic interpolation for intermediate values [4])
In a workshop with machine tools, reducing the exposure of employees to noise often requires action at the source, which alone can limit the intrinsic characteristic of the acoustic power of such noise sources [5], the use of sound-absorbing materials to cover the walls of the room reflecting the sounds - instead of absorbing them as is desirable - not being in general sufficient, on its own, to obtain sufficiently low sound levels.
ITS will participate in the reduction of the exposure of employees to the noise emitted by a machine tool near Vienne (France, Auvergne Rhône-Alpes region).
The aim will be to reduce noise levels at the locations provided for driving and monitoring the production line and also - in the same workshop - at other more or less close workstations and in circulation aisles.
For a large press-lathe assembly, allowing the transformation of the metal that aluminum is, which constitutes a key stage in production, it is indeed the action of reducing noise at the source mentioned above which, during an on-site diagnosis, was considered to be a priority.
It will be a question of realizing a complete enclosure of the machine tool by means of an envelope combining, in terms of acoustics, two fundamental properties:
- sound reduction, i.e. the ability to limit the transmission of noise from the interior space (where the noise source will be) to the exterior of the enclosure; a single (multi-frequency) sound reduction index Rw=35 dB will be considered for opaque parts (fixed, removable or mobile) and for translucent parts
- sound absorption, i.e. the ability to limit the amplification of sound levels inside the enclosure (inherent in the creation of limits to sound propagation, at the origin of the reverberation phenomenon) which would be counterproductive - all other things being equal - to obtain sufficiently low noise levels outside the enclosure; a sound absorption coefficient reaching 1 i.e. 100% at medium and high frequencies (i.e. in the 1/1 octave bands with a central frequency between 500 Hz and 4 kHz) will be considered for opaque parts (fixed, removable or mobile), and the glazed parts will be in sufficiently low proportion not to dramatically lower the average
Thus, the reduction of employees exposure to noise emitted by the machine tool in question will be achieved by means of a metal frame (steel) and:
- for opaque parts (fixed or easily removable sub-assemblies):
- industrial soundproofing panels with an absorbing face (modular, removable)
- acoustic doorsets (some pivoting, others sliding)
- for translucent parts (with glazing guaranteeing residual safety in the event of accidental breakage):
In addition, the air renewal of the acoustic enclosure will be carried out by means of openings equipped (to limit the transmission of noise) with ventilation silencers (with a fan, connected to a duct-network, to extract hot air from the cabin).
The design study of these different sub-assemblies (selection of materials, assembly techniques, layout) will be carried out meticulously, for optimal consideration of the different technical requirements of the soundproofing project:
- for acoustics: with sizing of the various sub-assemblies allowing the achievement of the most homogeneous performance possible, satisfying the objective of noise reduction all around the enclosure (without weakness of any element or of any connection between elements)
- for ventilation: with sizing of the various aeraulic components for evacuation of the heat output dissipated by the manufacturing process respecting the permissible temperature rise (a ventilator is a noisy piece of equipment, the noise of which must also be limited)
- for ergonomics of workstations outside the enclosure and for the maintenance of enclosed hardware: with the most appropriate choices (e.g. location, dimensions):
- for glazed surfaces - for visual process monitoring -
- for access (taking into account frequency and methods of opening, clearance, securing)
- for the interfaces (conveyors for raw material supply, product outlet and waste evacuation)
- for removable sub-assemblies (taking into account the modalities and the duration of handling operations)
- for lighting
- for optimizing the conditions of implementation: a high level of prefabrication will make it possible to minimize the immobilization time of the machine-tool during the sound enclosure installation works on site
- for durability: very thick exterior facings (1.25 mm minimum) for the modular soundproofing panels and the acoustic door sets, connections, accessories and mechanisms with the best quality for the opening parts (access, hatches, removable elements) will contribute to the solidity of the construction, and its resistance to time (and to stresses of all kinds, sometimes accidental in an industrial environment)
As with the press-lathe set considered in the context of the project reported in this article, other machine tools (e.g. CNC) often require soundproofing equipment similar to that described above (e.g. non-combined lathes and presses, saws, milling machines, grinders, drills, boring machines, other machining centres, special machines, transfer or assembly machines), and soundproofing (for the reduction of exposure to worker noise) is not the only functionality that can be expected from noise reduction at source.
In fact, protection against the risk of employees accidents (including in the case of projections, splinters or even tools break), dust collection are all features offered by such noise reduction equipment, whatever whatever the name given to them apart from enclosure e.g. casing, canopy, hood, cabin.
Reducing the exposure to the noise emitted by a machine-tool is therefore likely to contribute to the overall improvement of the working conditions of employees; it requires diversified skills and experience counts for the development of effective solutions, satisfying all the technical requirements of each project: the human resources of ITS and of its business partners are largely provided, and their fields of intervention cover all industrial sectors.
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[1] with a reference value of 20 μPa
[2] A-weighting takes into account human hearing sensitivity
[3] DL = 1.5 log S - 1.5 in dB(A)
[4] DL = 1.5 log S - 0.5 in dB(A)
[5] variable according to frequency; often expressed in decibels, with a reference value of 1 pW[2]