VIBRATIONS AT THE WORKPLACE PRACTICAL CASES OF EVALUATION

Transcription

1 VIBRATIONS AT THE WORKPLACE PRACTICAL CASES OF EVALUATION Jorge Preto; Clotilde Lages; Luís Conde Santos dblab, Laboratório de Acústica e Vibrações, Lda. luis.conde@absorsor.pt; clotilde.lages@absorsor.pt; jorge.preto@absorsor.pt Abstract The European Directive 2002/44/CE, on vibration exposure at the workplace, has been transposed to the Portuguese Law by DL 46/2006 which aims at promoting the improvement of working conditions in order to guarantee a higher level of protection of health and safety of workers. This paper presents practical cases of vibration exposure evaluation at the workplace, both for WBV (whole body vibration) and for HAV (hand-arm vibration), covering different types of industries. Results are presented for several types of vibrating equipments, taken from a wide number of accredited measurements performed by dblab s laboratory, according to ISO /2:2001 and NP ISO :1997. Keywords: vibration, hand-arm, whole body, health, exposure. 1 Introduction Until the publication of the Portuguese ordinance on human vibration exposure at the workplace, DL46/2006 of 24 th February 2006, which transposed the European Directive 2002/44/CE of 25 th June 2002, no specific regulations were available in Portugal that could ensure workers protection against vibration exposure risks. This legal publication was an important milestone because, for the first time, legal requirements were available for this major source of professional diseases, establishing the need for periodic evaluation of vibration exposure of workers as well as exposure limits and action values. Vibration is a harmful physical agent which affects workers and can be found in almost every activity, particularly in construction and public works, extractive industry, forest exploitation, foundries and transportation. The effects on workers health and safety may include musculoskeletal, neurological or vascular disturbances, among others. 1

2 The hand-arm vibration (HAV) effects are well known amongst the medical community and have been studied for a long time. These effects include vascular diseases known as white fingers syndrome, Raynaud s professional syndrome and traumatic disease of the blood vessels (vasospastic). The development of this syndrome relies on many factors such as: the vibration level exposure, the duration of daily exposure, the accumulated duration of the exposure (in hours, months or years), the workplace temperature and more specifically the hand temperature, the working procedure and the ergonomics of professional tasks. Whole-body vibration (WBV) has been less studied than HAV so its effects are less well known than those of HAV. The only exception is the transmission of vibration through a seat when the worker is seated, whose effects are quite known in the medical community. As stated in ISO :1997, such effects may lead to an increase in health risk related to the spinal cord and the nervous system of affected segments. This increase may be caused by the spine s biodynamic behaviour: horizontal dislocation and torsion of the spinal segments. An uncommon mechanical tension and/or nutrition disorder and its transmission to the spinal disc tissue may lead to a degenerative process in lumbar segments (deforming spondylosis, intervertebral osteochondrosis, deforming arthrosis). It also worsens some pathological disorders of the spine. Possible harmful effects on the digestive system, the genital / urinary system and the female reproductive organs may also occur although they are less understood. In any case, changes in human health produced by whole-body vibration take many years to appear. For more information we recommend the reading of the Good Practice Guides on hand-arm vibration [3] and on whole body vibration [4]. 2 Measurement procedure 2.1 Hand-arm vibration The measurement and evaluation techniques of human exposure to hand-arm vibration are described in ISO :2001 [5] and ISO :2001 [6] international standards. These measurements are carried according to the coordinate system represented in the figure below. (a) Figure 1 Coordinate system used for hand-arm vibration, in: (a) gripping a cylindrical bar and (b) contact upon a spherical surface [1]. (b) Measurements are carried with a vibration analyzer which can measure weighted acceleration in each direction x, y, z (a hwi ) and the global value of effective values of weighted acceleration in frequency (a hv ), determined according to orthogonal coordinates and given by the following expression: 2

3 a = a + a + a hv 2 hwx 2 hwy 2 hwz (1) Commonly, the measurement is done taking into account the different situations of exposure of the worker and both of his hands, selecting in the end the result of the hand with the highest value. The daily exposure to vibration is expressed in terms of the global value of equivalent continuous vibration, frequency-weighted, for an 8 hour period, A(8). It is calculated according to the following expression, using a value of 8 hours for the reference period T 0 and an exposure period T i for the duration of each task i: (2) Where: a hvi global value of vibration for the i th task; n number of partial exposure to vibration periods; T i duration of i th task T 0 reference period of 8 hour 2.2 Whole-body vibration The measurement and evaluation techniques of human exposure to whole-body vibration are described in ISO :2007 [7] international standard. The determination of whole-body vibration exposure level is based on the calculation of personal daily exposure for an 8 hour period, defined as a continuous equivalent acceleration. These calculations are performed according to the coordinate system represented in the figure below. Figure 2 Coordinate system used for whole-body vibration: seated, standing and recumbent positions. 3

4 The effective weighted acceleration is determined for each axis x, y and z allowing to calculate the vibration level based on the highest effective value of frequency-weighted acceleration, measured according to the three orthogonal axis. In this case and taking into account different human body responses to horizontal and vertical vibration, particularly the seated position with vibration transmitted through the seat, the frequency weight for x and y axis (W d filter and multiplying factor k=1.4) is different from the one applied to z axis (W k filter and multiplying factor k=1). The daily exposure to vibration, A(8), is presented in meters per square second and calculated using the following expression: Where: a wi vibration amplitude for an exposure with a duration T i n number of partial vibration exposure periods T 0 duration of the reference period of 8 hours (28800 seconds) The vibration transmitted to the body is measured between the body and the contact surface. The vibration transducer is triaxial and must be located in a way that shows the vibration in the interface between the body and the vibration source. 3 Practical cases of evaluation Within its scope of activity as an accredited laboratory for the evaluation of human exposure to hand-arm and whole-body vibration, dblab has performed a growing number of tests, related with a natural increase in demand by companies and other organizations whose workers are exposed to vibration and need to comply to the new regulations. As a consequence, the number of accumulated tests allows us to summarize some of the results obtained for several equipments and machines. These results are presented below. 3.1 Evaluation of hand-arm vibration The equipments and machines presented for the evaluation of hand-arm vibration are quite usual and a reasonable number of samples are available for them. The following machines will be analysed: grinders (37 samples), rock drills (18 samples), vibratory rammers (9 samples), geared head drill press (21 samples) and cutting saw (30 samples). 4

5 Z X Y X Z Y Figure 3 Example of triaxial accelerometer mounting in a grinder: left hand and right hand Figure 4 Example of hand-arm vibration measurement for the left hand of a worker using a rock drill Table 1 Example of vibration measurements for a grinder and a rock drill (small samples collected from the group of measurements presented in the following tables) Workplace Machine / Equipment Hand-arm a hwi (RMS. m/s²) a hv a hv MAX Direction m/s² m/s² x y z P1 Grinder Left 7,15 3,81 4,08 9,07 Right 2,53 3,28 5,86 7,17 9,07 P2 Rock drill Left 14,3 6,14 3,39 16 Right 15,7 6,07 2,92 17,1 17,1 5

6 Table 2 Average, maximum and minimum values and standard deviation for left hand measurement tests Left hand (a hwi ) Machine x y z Average MAX MIN Average MAX MIN Average MAX MIN deviation deviation deviation Grinder 5,3 9,5 1,3 2,5 3,6 7,2 1,0 1,7 4,8 10,9 0,7 2,9 Rock drill 18,7 21,9 14,3 4,0 6,2 7,3 5,2 1,1 4,9 6,2 3,4 1,4 Geared head drill 0,4 0,6 0,2 0,2 0,4 1,3 0,1 0,5 0,9 3,1 0,1 1,1 Vibratory rammer 9,7 16,5 5,8 4,7 5,5 8,6 4,0 2,1 12,7 16,9 9,3 3,2 Cutting saw 1,4 3,2 0,2 1,0 1,1 2,8 0,2 1,0 1,6 4,1 0,2 1,5 Table 3 Average, maximum and minimum values and standard deviation for right hand measurement tests Machine Average MAX MIN deviation Right hand (a hwi ) x y z Average MAX MIN deviation Average MAX MIN deviation Grinder 6,3 14,6 1,7 4,1 4,8 16,3 1,6 3,4 5,8 14,7 1,8 3,3 Rock drill 14,2 21,9 5,0 8,5 5,2 6,1 4,3 0,9 6,8 11,4 2,9 4,3 Geared head drill 0,4 0,6 0,2 0,2 0,4 1,3 0,1 0,5 0,9 3,1 0,1 1,1 Vibratory rammer 9,7 16,5 5,8 4,7 5,5 8,6 4,0 2,1 12,7 16,9 9,3 3,2 Cutting saw 1,3 3,2 0,1 1,0 1,4 4,2 0,2 1,5 1,7 4,5 0,1 1,6 Table 4 Average, maximum and minimum values and standard deviation for global values GLOBAL (a hv ) Machine Average MAX MIN deviation Grinder 11,1 23,3 3,9 5,3 Rock drill 20,8 23,4 17,1 3,3 Geared head drill 1,1 3,4 0,3 1,2 Vibratory rammer 17,4 21,2 13,9 3,7 Cutting saw 2,7 6,03 0,33 2,3 The Portuguese law (DL 46/2006) defines an exposure limit value of 5 m/s 2 and an exposure action value of 2,5 m/s 2. Using the limit value, the results presented in the above table show that for average values of a hv of the entire set of samples, a grinder should not be used more than approximately 100 min a day, whereas a rock drill should not be used more than 30 min by a single worker in a daily basis. In the same way but using maximum values for the analysis, the exposure time drops to 20 min approximately for both the grinder and the rock drill. While in the first case, it seems reasonable in practice to use a grinder for not more than 100 min in a daily basis, a limitation of 30 min on the daily use of a rock drill will certainly require special measures such as rotation of workers, alternative working methods and replacement of old equipments with new ones, selecting low vibration machines certified by the manufacturer. Some of the machines analyzed in this study have high standard deviation 6

7 values showing that there is a wide range of situations in measurements and machines available in the market. Maintenance certainly play an important role here too. Education and information about the use of these equipments is an important measure for operators combined with the use of special gloves for rock drills, for example. In some cases, a change in the whole production process is to be considered, that is, avoiding the use of rock drills or limiting its use, for instance. 3.2 Evaluation of whole-body vibration For whole-body vibration evaluation, six equipments have been selected: fork-lift truck (306 samples), dumper/truck (33 samples), backhoe loader (51 samples), excavator (24 samples), wheeled excavator (12 samples) and roller (15 samples). Figure 5 Example of whole-body vibration measurement in a fork-lift truck Figure 6 Example of whole-body vibration measurement in an excavator 7

8 Table 5 Example of results collected from vibration measurements of a fork-lift truck, a dumper, a backhoe loader and an excavator Machine Axis a w (RMS, m/s 2 ) k.a w k.aw MAX (m/s²) Electric fork-lift truck Dumper Backhoe loader Excavator x 0,198 0,278 y 0,181 0,253 z 0,197 0,197 x 0,69 0,97 y 0,73 1,02 z 1,19 1,19 x 1,01 1,41 y 1,01 1,41 z 1,92 1,92 x 0,27 0,38 y 0,42 0,59 z 0,45 0,45 0,278 1,19 1,92 0,59 Table 6 Average, maximum and minimum values and standard-deviation of global values Machine k a w (m/s 2) Average MAX MIN deviation Fork-lift truck 0,69 2,7 0,24 0,40 Dumper/truck 0,67 1,19 0,20 0,24 Backhoe loader 0,95 1,92 0,59 0,29 Excavator 0,42 0,71 0,18 0,16 Roller 0,64 1,18 0,24 0,39 Wheeledexcavator 0,67 0,89 0,19 0,28 The Portuguese law indicates an exposure limit value of 1,15 m/s 2 and an action value of 0,5 m/s 2. The results presented in the tables above comply with the exposure limit value, based on average values of k a w of all samples considered and also considering an 8-hour daily working period. Still, we can observe for maximum values (selecting higher values of vibration among the equipments with higher values of vibration) that the maximum working period decreases to 7,5 hours for dumpers, 3 hours for backhoe loaders and 1,5 hours for fork-lift trucks. The other equipments can be used for 8 hours without any restriction. The values presented in the tables above clearly show major differences among several samples taken. This can be explained by different types of machines and mostly by different operating conditions. 8

9 The vibration values measured in fork-lift trucks are very diverse as shown by high standard deviation. The main reason for this difference is based on pavement conditions as pavement is obviously needed for vehicle circulation. Rough or discontinuous pavements account for dramatic increases in vibration exposure values being responsible for high vibration values. It is important to point out that in many tests the highest measured value of vibration belongs to the z-axis because the vibration is caused by vehicle rolling on rough and/or discontinuous surfaces. An effective way of reducing these vibration levels is to build small ramps to soften pavement discontinuities or change commonly-used paths to avoid vehicles passing perpendicularly to pavement joints. The z-axis value also tends to be the highest in dumpers, being reduced by applying a factor of 1,4 to x and y-axis values. The roughness of roads used by these machines (pathways to construction yards or quarries) is closely related to these values. The equipments here tested show no major differences among them toward global vibration values. Tested backhoe loaders show quite similar values in every 3 axis although results may depend on the type of operations conducted during measurements and the type of terrain machines are moving on. Some differences in global vibrations values have also been observed in different equipments as shown by high standard deviation. Excavators have the lowest average vibration value of all measured equipments. Even for the highest vibration value workers are allowed to use equipments for 8 hours a day without exceeding the limit exposure value. Only in a few cases, the action value is exceeded. Global vibration values measured in rollers present a great variation as shown by its high standard deviation. It is known these equipments are mainly used for compaction operations so vibration values strongly depend on the soil or surface they are compacting. Another important remark is that older equipments tend to transmit more vibration to the worker than new ones mainly because they have been in use for a longer time and they have no vibration reduction devices. Wheeled excavators have higher vibration values than normal excavators but the number of samples taken is not enough to make a statistical comparison as only 12 samples have been collected. Once again, vibration values depend on the type of operation and the way the machine is handled. 4 Conclusion The DL 46/2006 transposed the European Directive 2002/44/CE to the Portuguese law and has been widely used in companies and organisations since then. The aim is to promote better working conditions and ensure a better level of protection of workers health and safety towards vibration risks. This paper presented practical cases of vibrations at the workplace both for whole-body and hand-arm vibration. In each case, several equipments have been analysed. The results reveal that manually-handled equipments pose serious health risks to its users (e.g. rock drill). For instance, in average, rock drills should not be used more than half an hour a day. This is the type of machine that creates an important challenge for companies which use them and equipment-makers whose main goal is to minimize vibration transmitted to the worker. In a general way, whole-body vibration machines are less critical than hand-arm 9

10 vibration ones but there are cases to be noted. For example, one of the evaluated fork-lift trucks could not be used more than 1 hour and a half a day to comply with the exposure limit value. This situation can be explained by the age of the equipment but mainly by the pathways it uses. References [1] Decreto-Lei n. º 46/2006, of 24th February of Diário da República, number 40, I-A series 2006, pages [2] Directive 2002/44/CE of the European Parliament and the Council, 25 th June Official Journal of the European Community of , pp. L177/13 a L177/19. [3] Guide to good practice on Hand-Arm Vibration. European Commission Directorate General Employment, Social Affairs and Equal Opportunities. 12/06/2006. [4] Guide to good practice on Whole-Body Vibration. European Commission Directorate General Employment, Social Affairs and Equal Opportunities. 12/06/2006. [5] ISO :2001. Mechanical vibration - Measurement and evaluation of human exposure to hand-transmitted vibration - Part 1: General requirements. International Organization for ization. [6] ISO :2001. Mechanical vibration - Measurement and evaluation of human exposure to hand-transmitted vibration - Part 2: Practical guidance for measurement at the workplace. International Organization for ization. [7] ISO :1997. Mechanical vibration and shock -- Evaluation of human exposure to whole-body vibration -- Part 1: General requirements. International Organization for ization 10