CONTROL OF ULTRASONIC NOISE AT WELDING WORKSTATIONS

Transcription

1 CONTROL OF ULTRASONIC NOISE AT WELDING WORKSTATIONS Bozena Smagowska and Dariusz Pleban Central Institute for Labour Protection-National Research Institute, Department of Vibroacoustic Hazards, Czerniakowska 16, Warsaw, Poland Noise with components of high audible frequency (10-20 khz) and low ultrasonic frequency (20-40 khz) is determined in Poland as ultrasonic noise. This noise is a hazardous factor which cause harmful and annoyance effects on the human body particularly in the working environment. Ultrasonic welding machines are one of main sources which emit of this type of noise. This paper presents exposure to ultrasonic noise at chosen workstations with ultrasonic welding machines. Test results of the study confirmed that this type of noise is a hazardous factor in the working environment. Prevention activities to reduce the emission of ultrasonic noise at workstations are recommended. This paper presents examples of technical solutions reducing the sound pressure level at workstations with ultrasonic welding machines. 1. Introduction Resilience oscillations in the frequency range from 20 khz which propagation takes the form of acoustic waves in the gas, liquid and solid are called ultrasounds [1]. The broadband noise containing high audible (10 20 khz) and low ultrasounds frequency (20 40 khz) is defined as ultrasonic noise [2]. The use of ultrasounds in various branches of industry (including automotive, textile, electro- technical, processing) have resulted in the increase of use of ultrasonic transducers in various processes, such as ultrasonic welding, ultrasonic cutting, ultrasonic cleaning, ultrasonic soldering, diagnostics, medicine and many others [3]. One of the main uses of these transducers is ultrasonic welding, i.e. the joining of elements of devices or structures [4, 5]. These elements are mainly manufactured from a plastic that has a high capacity for transmitting mechanical vibrations; in other words a material with a high modulus of elasticity. Ultrasonic welding machines are composed of the following systems: mechanical system (table, column), pneumatic system and ultrasonic vibration generating system (generator, transducer, waveguide, sonotrode tool). During ultrasonic welding the energy penetrate into the human body by contact with worked object (e.g. through the hands during welding of electric wire) or by air. Typical frequencies for ultrasonic welding are: for large elements 20 khz and for small elements 30 khz, 35 khz and 40 khz. The evenness of the ultrasonic welding depends on the even distribution of the vibration amplitude at the front of the sonotrode. Ultrasonic welding time does not exceed 1 2 s and depends on the type of plastic, and on the thickness and shape of the elements to be joined [4]. Ultrasonic welding machines belong to a group encompassing sources of technological ultrasonic noise. The few specialist literature sources concerning the effect of ultrasonic noise on humans in the work environment demonstrate that this type of noise may affect hearing (may cause hearing loss) and may also have a negative impact on the vestibular system, which may manifest itself in headaches and dizziness, impaired balance and nausea [6]. In order to prevent any adverse effects of ultrasonic noise exposure, Maximum Admissible Intensities (MAI) values have been determined 1

2 [7].The highest sound pressure level (SPL) in the ultrasonic welding machines working space occurs most frequently within the band frequency for ultrasonic welding generation and depends, among other things, on the type of welded materials (the highest SPL for ultrasonic noise occurs for homogenous combinations and the lowest SPL for heterogeneous connections)[4]. Moreover influence on the SPL values are time of single welding or diameter welded elements (in the case welding of electrical wires) [8]. The paper presents the results of ultrasonic noise exposure at selected ultrasonic welding workstations, examples of a reduction in its harmful effects through a decrease of noise emissions in the welding working space. 2. Measurement methods and assessment of ultrasonic noise at workstations Ultrasonic noise measurements for the assessment of worker exposure at a given workstation are carried out at places that are typical for a particular job, taking into account all actions performed and the regular operating conditions using whatever tool, machine or piece of equipment that would be the source of such noise [9]. The method of assessing the exposure of the noise (especially ultrasonic noise) at workstations consists of comparing the measured values used for noise assessment to the admissible limits. The MAI values of ultrasonic noise concerning to health protection are specified in applicable dispositions [7]. The method used for measurements of noise in the ultrasonic frequency range is defined in the procedure which was developed by the Central Institute for Labor Protection National Research Institute (CIOP-PIB) [10]. The quantities characterizing ultrasonic noise for assessment are: equivalent SPL in the one-third octave frequency bands with the center frequencies from 10 khz to 40 khz, normalized to 8-hours using, Lf,eq,8h (or equivalent SPL in the one-third octave frequency bands with the center frequencies from 10 khz to 40 khz normalized to week Lf,eq,w, maximum sound pressure level determined in the one-third octave frequency bands with the center frequencies from 10 khz to 40 khz, Lf,max. 3. Test results The aim of test was determining ultrasonic noise exposure at 15 workstations of ultrasonic welding machines. Ultrasonic welding machines were either single header (welding machines No 1, 2 and 5) or double header (the rest machines). Some of them were equipped with shields within the device working space (machines No 1, 2, 3, 4, 5, 8). The nominal frequency of the tested ultrasonic welding machines was 20 khz. The Svan 912AE sound analyzer, ¼ diameter microphone and B&K type 4230 sound calibrator were used for measurement of noise parameters in ultrasonic frequency range. The measurement were carried out during the welding of electrical wires for the automotive industry and plastic elements for household appliances. The distance between the operator and the ultrasonic welding machines depended on the type of the ultrasonic welding machines. It was from 0.3 m to 0.5 m. Measuring results of ultrasonic noise values at ultrasonic welder workstations are presented in Figures ICSV23, Athens (Greece), July 2016

3 Figure 1: Equivalent SPL at workstations of ultrasonic welding machines without causing No 1 and No 2 and ultrasonic welding machines with causing No 3 and No 4. (where L feq,8h,adm - admissible equivalent SPL in the one-third octave frequency bands with the center frequencies from 10 khz to 40 khz, normalized to 8-hours) Figure 2: Equivalent SPL at workstations of ultrasonic welding machines No 5 10 Figure 3: Equivalent SPL at workstations of ultrasonic welding machines No ICSV23, Athens (Greece), July

4 Figure 4: Maximum SPL at workstations of ultrasonic welding machines No Discussion Figure 5: Maximum SPL at workstations of ultrasonic welding machines No The results show that, for most workstations of ultrasonic welding machines (with the exception of machines No 1, 4, 5) the highest exceeding MAI values of equivalent and maximum SPL are in the 1/3 octave band with a centre frequency of 20 khz. The results presented in the Fig , show that equivalent SPL exceeds the MAI values for all workstations of ultrasonic welding machines (with the exception of machines No 1, 4, 5, 8). The maximum SPL exceeds the MAI values of for all workstations of ultrasonic welding machines (with the exception of machines No 1, 2, 3 and 4) (Fig. 4.and 5). The highest exceeding the admissible SPL of equivalent SPL (31.2 db) and the maximum SPL (18 db) are in the 1/3 octave band with a centre frequency of 20 khz. They were measured at workstation No 6. The results of measurements for ultrasonic noise values at workstations of ultrasonic welding machines in cases of open doors shield (machines No 2. and 3.) and closed doors shield (machines No 1. and 4.) are presented in Fig.1. The results show that incorrect using of shield causes (door remains open) increase SPL at working environment. An exceedance of the equivalent and maximum SPL within all bands of normalized frequency range did not occur at workstation No 5. when assembling welded elements. The lack of exceeding the ultrasonic noise MAI values at this workstation results from the use of a shield in the sonotrode s working space. 4 ICSV23, Athens (Greece), July 2016

5 5. Example of preventing In order to limit the risk of ultrasonic noise at workstations, the spreading of noise emissions should be limited. Technical methods, such as enclosures, shields and sound absorbing insulation materials are one of the ways of limiting the spread of noise[11, 12].At the workstations of ultrasonic welding machines (where the welding of electrical wires takes place), plastic enclosures were used (machines No 8 and No 11). The results of ultrasonic noise value measurements at the workstations of ultrasonic welding machines with an enclosure (machine No 8 - Stapla Octopus Servo) are provided in Table 1. Based on the obtained ultrasonic noise parameter values, it should be noted that after the use of an enclosure no exposure limit values were exceeded, either for the equivalent SPL or for the maximum SPL. Table 1: Measuring results and assessment ultrasonic noise at workstations of welding machine Stampla Octopus Servo. Parametr The center frequency of one-third octave frequency bands, in khz Protection in db without enclosure Lfeq,8h with enclosure Lfeq,8h Lfeq.8h.adm without enclosure Lf max with enclosure Lf max Lf max,adm where: L feq,8h - equivalent SPL in the one-third octave frequency bands with the center frequencies from 10 khz to 40 khz, normalized to 8-hours in db L feq.8h.adm- admissible equivalent SPL in the one-third octave frequency bands with the center frequencies from 10 khz to 40 khz, normalized to 8-hours in db L f max - maximum SPL determined for the one-third octave frequency bands f, in db. L f max,adm - admissible maximum SPL, determined in the one-third octave frequency bands, in db. In the case of a Branson ultrasonic welding machine (No 11), the main cause for exceedance of the maximum SPL at the workstation was the lack of sealing of the enclosure (including an opening in the front wall). Therefore, the enclosure walls were covered (inside) with a polyurethane foam and the opening in the front wall was sealed. The results of ultrasonic noise value measurements at this workstations before and after the enclosure walls were sealed are provided in Table 2. Table 2: Measuring results of ultrasonic noise at workstations of welding machine Branson. protection Parametr The center frequency of one-third octave frequency bands, in khz in db without enclosure Lfeq,8h with enclosure Lfeq,8h Lfeq.8h.adm without enclosure Lf max with enclosure Lf max Lf max,adm ICSV23, Athens (Greece), July

6 After the enclosure was sealed, the exposure limit values of the equivalent and maximum sound pressure levels at this workstation were not exceeded. Sealing the enclosure resulted in a 7 db decrease in the maximum SPL in the nominal frequency band, i.e. 20 khz. The use of an enclosure for machine and sealing of the existing enclosure ultrasonic welding machine have reduced the ultrasonic noise emissions at operating stations, and thus decreased the exposure of employees to the hazard. In case when the operator may come into contact with the welded element protective gloves must be used. Additionally wearing of hearing protectors should be used. 6. Summary In instances where there is an ultrasonic welder workstation without an enclosure, there is a risk of exceeding the MAI values for ultrasonic noise. For any employee operating such a device it is mandatory to use individual hearing protectors. Workstations located around welding machine at which exceed MAI values for ultrasonic noise are exceeded, should be shielded by sound absorbing barriers. The ultrasonic welding machines have to be placed as far from the walls and other sound reflective surfaces as possible. The rooms were these devices operate must be marked in order to protect other employees against the noise. Employees operating ultrasonic machines should be trained in the correct and safe operation of theirs. Moreover, they should be notified of the dangers of noise exposure and occupational health and safety. 7. Acknowledgements This paper has been based on the results of a research task carried out within the scope of the third stage of the National Programme Improvement of safety and working conditions partly supported in within the scope of state services by the Ministry of Labour and Social Policy. The Central Institute for Labour Protection National Research Institute is the Programme s main co-ordinator REFERENCES 1 Indulski, J., Higiena Pracy, [Work Hygiene; in Polish] IMP, Łódź (1999). 2 Pawlaczyk-Łuszczyńska, M., Koton, J., Augustyńska, D., Hałas ultradźwiękowy Procedura pomiarowa, Podstawy i Metody Oceny Środowiska Pracy, [Ultrasonic noise -Measuring Procedure, Principles and Methods of Assessing the Working Environment; in Polish], 28(2), 89-95, (2001). 3 Śliwiński, A., Ultradźwięki i Ich Zastosowania, [Ultrasounds and Their Use; in Polish], WNT, Warszawa (2001). 4 Matusiak, J., Szłapa P., Wyciślik, J. Hałas słyszalny i ultradźwiękowy przy zgrzewaniu ultradźwiękowym metali, Przegląd Spawalnictwa, [Ultrasonic and audible noise during ultrasonic welding of metals, Welding Technology Review; in Polish], 10, 52-60, (2013). 5 Smagowska, B. Ultrasonic noise sources in the work environment, Archives of Acoustics, 38(2), , (2013). 6 Smagowska, B., Pawlaczyk-Łuszczyńska, M. Effects of action of ultrasonic noise on the human body a bibliographic review, International Journal of Occupational Safety and Ergonomics, 19(2), , (2013). 7 Rozporządzenie Ministra Pracy i Polityki Społecznej z dnia 6 czerwca 2014 r. w sprawie najwyższych dopuszczalnych stężeń i natężeń czynników szkodliwych dla zdrowia w środowisku pracy, Dz.U poz.817, [Regulation of the Minister of Labour and Social Poli- 6 ICSV23, Athens (Greece), July 2016

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