Hydraulic Rock Breaker

Vol. 29 No. 2 2010 101

JOURNAL OF LOW FREQUENCY NOISE, VIBRATION AND ACTIVE CONTROL Pages 101 – 110

Experimental Evaluation of Whole BodyVibration exposure from Tracked Excavatorswith Hydraulic Breaker Attachment in RockBreaking operations

Alphin.M.S, K.Sankaranarayanasamy and S.P.Sivapirakasam Department of Mechanical Engineering, National Institute of Technology,Tiruchirapalli -620 015, Tamilnadu, India. E-mail: [email protected]

Received 4th March 2010

ABSTRACT Ever expanding technological growth has led to an increase in the use of trackedexcavators for construction, demolition, material handling, rock breaking etc.Excavator operators are exposed to a variety of risk factors that may lead tohealth problems. A major health hazard among operators is whole-bodyvibration. Human response to vibration is very complex and nonlinear. WholeBody Vibration in the range of 2 to 30 Hz corresponds to most of the resonantresponses of various organs and parts of the human body. The objective of thispaper is to assess whole body vibration for the tracked excavator with hydraulicbreaker. The job safety analysis conducted through questionnaires for differentindustrial vehicle operators revealed the presence of a health risk among theoperators in rock breaking machinery. To quantify the level of vibration, fieldtests are performed on four tracked excavators with hydraulic breakerattachments in two different work locations. Accelerometer, a real-time signalconditioning / processing and PULSE data acquisition software are used forvibration measurement. The frequency of vibration exposure is observed to bebetween 6.8 and 12 Hz. Acceleration levels measured were in the range of 0.87-2.2 m/s2 for a tracked excavator operator with breaker. The total vibrationexposure calculated was between 0.621 and 1.932 m/s2. The vibration dosevalue recorded was 17.6 -62.72 m/s1.75 . Whole body vibration exposure of thebreaker operator was much higher and lies beyond the upper limit as given inISO 2631-1. The ranges of vibration parameters measured were concomitantwith frequent lower back pain, other muscular-skeletal injuries like leg pain etcwhich are prevalent among these operators.

Keywords: whole body vibration, tracked excavator, ergonomics, Industrial health.

1. INTRODUCTION Whole body vibration (WBV) can affect comfort, performance and health. Itseffects depend on the magnitude of the waveform and the exposure time for theoccupant. Hence quantifying WBV by experimental investigation is of currentinterest. Whole body vibration exposure and ergonomical risk factors are significantfor acceleration of more than 0.5 m/s2 with industrial drivers according toepidemiological studies [1].There is strong evidence of increase in lumbar problemsdue to workers being exposed to WBV [2]. Acceleration in the range between 0.3-0.45 m/s2 is the ergonomic standard suggested for industrial vehicles withfrequencies mostly between 1-5Hz and for an 8 hours operating day. The dynamicsof vehicle axle and occupant suspension systems can be redesigned in order toachieve the limits [3]. A level of 0.5m/s2 was repeatedly quoted in many studies for

the relationship between whole body vibration and low back disorder [4]. Highlateral and low vertical vibration is also prevalent in railway wagons leading to lowback disorders [5]. The highest levels of vertical vibration are found in off-roadvehicles and forklifts [6], [7].

The International standard ISO 2631-1[8] for exposure assessment, shows ahealth caution zone between 0.45–0.8 m/s2 for 8 hours exposure. This implies thatthere are certain health risks for the limit value of 1.15 m/s2, for workers exposedaccording to the evaluation method suggested by EU Directives [9].

The objective of this paper is to conduct Field measurements for whole bodyvibration in tracked excavators with breaker by using a standard vibrationmeasurement system, and the data obtained such as magnitude of acceleration, rootmean square acceleration and peak acceleration are quantified from time domainand spectral analysis signatures. Vibration Dose Value (VDV) and daily exposureare calculated and interpreted with the international standards for predicting thehealth risk.

2. STUDY ENVIRONMENT The Field trials for WBV analysis were conducted at Valvanthannkottai, TrichyDistrict, India (Fig. 1). The study was necessitated because an earlier survey throughquestionnaires revealed that drivers of excavators with hydraulic breakerattachments (Fig.2) are suffering chronic problems of lower back pain and othermusculo-skeletal injuries. The drivers seats used are of conventional type with nono soecial attachments for vibration suppression. Excavator drivers operate on aneight-hour shift. At the rock breaking operation, even though the normal shift periodis 8 hours, the drivers often have extended exposure periods of up to 12 hours, withovertime work included. This results not only in longer periods of exposure, withincreased vibration dose, but also reduced time for physical recovery, repair andrecuperation. Most of the workers are staying near to the workplace where they lackbasic facilities.

Figure 1. One of the Sites where field study has been done (At Vazhavanthan Kottai in Tiruchirapalli District, India)

If they complete the work allocated for the day in a shorter time, productivityincreases, but it comes at the cost of increased vibration exposure levels andassociated health effects. This is due to the increase in driving speeds, vibrationshock loads and reduced rest periods . Tracked excavators used for rock breakingare primarily operated by means of hydraulic breaker attachments. Excavators withbreaker attachments are operated for the whole working day to break the big blocksof stones to the desired size. In some cases the stones are broken directly from rock.The breaking is possible by high compression force applied with impact. Theexcavator has to travel on very uneven paths, filled with stones.

Experimental Evaluation of Whole Body Vibration exposed from Tracked Excavatorswith Hydraulic Breaker Attachment in Rock Breaking operation

JOURNAL OF LOW FREQUENCY NOISE, VIBRATION AND ACTIVE CONTROL102 102

Figure 2. Tracked Excavator with Hydraulic Breaker Attachment

2.1 EVALUATION OF WHOLE BODY VIBRATION EXPOSURE In the absence of the domination by any axis of vibration, the total value vibrationfor a seated worker, the vibration total value (av) for the frequency-weightedaccelerations (aw) of whole-body vibration is calculated as [8]

av=(kx2 awx

2 + ky2awy

2 + kz2 awz

2)1/2 (1)

Where k = 1.4 for X and Y axes and k = 1 for Z axis. Since it is believed that the health effects of whole-body vibration are influenced

by shocks or vibration peaks, ISO 2631-1 [1997] suggests using the fourth powervibration dose method instead of the second power of the acceleration time history(i.e. r.m.s.) as the basis for averaging. The fourth power vibration dose value (VDV)is expressed in meters per second to the power of 1.75 (i.e. m/s1.75).The assessmentof exposure to whole-body vibration “is based on the calculation of daily exposureA(8) expressed as continuous equivalent acceleration over an eight hour period,calculated as the highest (r.m.s.) value, or the highest vibration dose value (VDV)of the frequency-weighted accelerations determined on three orthogonal axes(1.4awx, 1.4awy, awz)” in accordance with the International standard [8].

2.1.1 Measuring Equipment Instrumentation used for the measurements and recording of whole-body vibrationsinclude accelerometers (Bruel & Kjaer) and signal conditioning equipment (Brueland Kjaer Vibration Analyzer 3560), analogue-to-digital converters (Dell Laptop)and a data analyzer (PULSE software platform). The PULSE platform is capable ofmeasuring any type of vibration with basic tools such as FFT analysis and waterfallplots standard in all PULSE vibration solutions.

Accelerometers are the most common transducers used in whole body vibrationmeasurements, and produce an electrical output proportional to acceleration.Transducers are connected to some form of signal conditioning, to provide power tothe transducer in addition to amplifying and filtering the vibration signal before it ispassed to the data analyzer. The analogue to digital converter converts theconditioned, filtered, analogue voltage signal to a digital record. The data analyzerenables visual inspection of the acquired signals during measurement and providesa means of storing the signals in a useful digitized form in the computer memory.

The accelerometer was placed and fixed firmly at the position where themeasurement was intended to be recorded. Hardware setup in the PULSE softwarefor data acquition system was set for the corresponding Transducer. SpectralAnalysis gave the assessment from Fast Fourier Transforms (FFT) and PowerSpectral Density.

Vol. 29 No. 2 2010

Alphin.M.S, K.Sankaranarayanasamy and S.P.Sivapirakasam

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2.1.2 Vibration Test Vibrations were measured according to the coordinate system as shown in Fig.3.Vibration which is transmitted to the body was measured from the sensor placed atthe interface. Three principal areas as suggested by ISO 2632 are used formeasurements which are the supporting seat surface, the seat-back and the feet. Theduration of measurement was kept sufficiently long to ensure reasonable statisticalprecision and to ensure that the vibration is typical of the exposures which are beingassessed

Figure 3. WBV measurement direction and location

From the spectra obtained for the vibration measurement maximum amplitude andthe corresponding frequency were observed.

R.M.S acceleration was also measured using the Pulse B&K data acquitionsoftware. The values obtained are used for further evaluation. The frequency atwhich the magnitude attains maximum is observed to be between 6.8-12 Hz formost of the cases measured. Crest factor which is the ratio of the peak accelerationmeasured will be useful to predict the presence of instantaneous Shocks.

Figure 4. Accelerometer location during vibration field Test



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Figure 5. Field Test set up Showing Accelerometer, Vibration Analyzer, Connectors and Laptop

Table IVibration Measurement with Rock Breaking Machinery (For a Working Period of 12hrs)

3.0 RESULT ANALYSIS 3.1 VIBRATION ANALYSIS During the rock breaking operation amplitude of acceleration, crest factor, anddominant frequencies as measured under typical working conditions aresummarized in Table I. The maximum acceleration in the vertical axis was 3.88 m/s2

at 8.78 Hz for the measurement taken at backrest. But the highest magnitudes ofacceleration at floor and seat operator interface were 2.56 m/s2 at 7.15 Hz and 2.58m/s2 at 7.5 respectively at the X axis.

The vibration magnitude transmitted through the floor to the leg is comparativelyhigher than at the seat operator interface, so it indicates that the cushion in the seatdampens the vibration transmitted from floor to seat. The operator’s backrest wasexposed to the maximum vibration of more than 3 m/s2 in X and Z axes, Y-axisvibration exposed to the operator back was comparatively less.

Position Measured Parameters

X-direction Y-direction Z-direction

Trial Trial Trial

1 2 3 1 2 3 1 2 3

At floor

Max. accn(m/s2) 2.05 2.58 2.55 2.22 1.07 1.44 1.51 2.44 1.30

Frequency(Hz) 6.813 7.5 7.219 78.53 11.94 11.94 6.938 18.41 8.281

RMS accn(m/s2) 2.22 2.04 1.81 1.43 1.44 1.91 1.58 2.12 1.68

At seat operator

Max. accn(m/s2) 2.56 1.72 1.69 1.27 1.34 1.30 1.83 0.926 1.63

Frequency(Hz) 7.156 8.75 6.9 4.969 5.72 6.03 16.34 16.03 15.34

RMS accn(m/s2) 1.88 1.36 1.39 1.39 1.43 1.26 1.58 0.87 1.28

At back rest

Max. accn(m/s2) 2.84 2.37 1.88 1.07 1.66 2.20 3.88 2.50 1.90

Frequency(Hz) 2.375 21.81 38.56 19.28 6.625 6.063 8.781 8.56 22.78

RMS accn(m/s2) 1.58 2.08 2.3 1.41 1.35 1.68 2.71 2.35 2.02

Crest Factor 18.2 25.3 19.6 11.2 9.12 8.45 15.2 12.1 15.9

Crest Factor 29.2 24.3 22.8 9.3 8.23 10.2 17.1 19.6 15.2

Crest Factor 32.1 28.2 24.6 11.3 12.22 9.56 15.2 20.8 18.2

In order to predict the health risk associated with the operation of rock breakervehicle, the 8-h equivalent frequency weighted acceleration values A(8) arecomputed from the equation below and the values are depicted in Fig.5.

(2)

Figure 5. Total exposure, A (8)

Health risks predicted by the standard analysis procedure were compared across thecommon values that were normalized to an 8-h equivalent value. Rankings (high;moderate; low) for predicting health risks were predicted for a rock breakermachinery operator based on Health Guidance Caution Zone (HCGZ) limits fromtotal acceleration and VDV total values [8].

The vibration dose value is the cumulative dose, based on the fourth root of thefourth power of the acceleration signal with units of m/s1.75. The European directoryprohibits exposure above the exposure limit value of 21 m/s1.75 VDV. If the exposureexceeds an exposure action value of 9.1 m/s1.75 VDV a programme of continualimprovement was recommended for the employer to implement. VDV total calculatedas suggested by the European directive standard is shown in Table II. The dailyexposure (Fig. 6) is the maximum of these three values. VDV was calculated [9]. Thehighest value of VDVexp x, VDVexp y and VDVexp z will give the daily VDV.

Figure 6. VDV variation

0

10

20

30

40

50

60

70

X Y Z X Y Z X Y Z

LEG BUTTOCK BACKREST

VD

V(m

/s1.

75)

Trial -1

Trial -2

Trial -3

Average

0

1

2

3

4

5

X Y Z X Y Z X Y Z

LEG

Acc

eler

atio

n(m

/s2)

Trial -1

Trial -2

Trial -3

BUTTOCK BACKREST

Average

expx Wx

0

expy Wy

0

expz Wz

0

TA (8)= 1.4 a

T

TA (8)= 1.4a

T

TA (8)= a

T



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Table IIVibration Dose Value

Figure 7. Acceleration Range

The measured acceleration at various positions during rock breaking shows highvariation at the operator’s back compared to floor and seat-operator interface (Fig. 7).The maximum value of VDV obtained is 64.96 m/s1.75 at the backrest during the thirdtrial measurement taken with a breaker.

3.2 COMFORT RATING PREDICTION The acceptable value of vibration magnitude for comfort depends on many factorswhich vary with each application.ISO 2631 gives approximate indications of likelyreactions to various magnitude of overall vibration total values in mobile vehicles.An acceleration level less than 0.315 m/s2 is not uncomfortable, 0.315 m/s2 to 0.63m/s2 is a little uncomfortable, 0.5 m/s2 to 1 m/s2 is fairly uncomfortable, 0.8 m/s2 to1.6 m/s2 to 1.6 m/s2 is uncomfortable, 1.25 m/s2 to 2.5 m/s2 is very uncomfortable,greater than 2 m/s2 is extremely uncomfortable. The vibration total value of r.m.s.

Mea-surement Location

Axis

Total VDV (m/s1.75)

Re-marks

Trial -1 Trial -2 Trial -3 Aver-age

LEG

X 62.72 57.68 51.1 57.16 Health risk is likely

Y 40.46 40.74 54.04 45.08

Z 31.9 42.8 33.9 36.2

BUT-TOCK


Y 39.34 40.46 35.56 38.45

Z 31.9 17.6 25.8 25.1

BACK-REST


Y 39.9 38.08 47.46 41.81

Z 54.7 47.4 40.8 47.39



acceleration, in orthogonal coordinates was calculated using Eqn. 1. and tabulatedin Table III. The total vibration exposure average taken at the leg-floor interface wasthe highest with 3.46 m/s2 .

Table IIITotal Exposure (for 12hrs. working period)

The nature of work done by the tracked excavator would be the reason for totalvibration exposure at the buttocks being less than the other two positions. the majorshocks are predicted to transmit along the x axis to the occupant.

Table IVComfort analysis

Figure 8. Comfort rating

Trial awx awy awz av(m/s2) Comfort level

I 1.88 1.39 1.58 3.635 Extremely un-comfortable

II 1.36 1.43 0.87 2.89 Extremely un-comfortable

III 1.39 1.26 1.28 2.92 Extremely un-comfortable

Mea-surement Location

Axis

Total Exposure

Remarks (m/s2) A(8)

Trial -1 Trial -2 Trial -3 Aver-age

LEG


Y 2.45 2.47 3.27 2.73

Z 1.94 2.6 2.06 2.2

BUT-TOCK


Y 2.38 2.45 2.16 2.33

Z 1.94 1.06 1.56 1.52

BACK-REST


Y 2.42 2.31 2.88 2.53

Z 3.32 2.88 2.47 2.89

Calculated vibration total value av was compared from [8] standard guide to assessthe effects of vibration on comfort and perception under comfort reactions tovibration environments, it has been found that rock breaker operators are extremelyuncomfortable from Figure 8 and the acceleration values exceed 2.5m/s2. So fromthe results obtained (Table IV) it is clear that tracked excavator with breakermachinery crosses the standard limit by big margin and in many cases the exposurein the y direction is higher than the vertical axis.

4. CONCLUSION Field vibration tests were conducted with tracked excavators with hydraulicattachments for WBV assessment and evaluation. The peak frequency was observedbetween 6.8-12 Hz for most of the vibration exposures. Acceleration measured inranges between 0.87-2.2 m/s2, the total exposure calculated varies from 0.621 to1.932 m/s2, Vibration dose values recorded were between 17.6 and 62.72 m/s1.75.The Crest Factor Values reveal that there are instantaneous shocks present in theexposure. The results obtained from the field study clearly prove the possibilities offrequent lower back pain and other muscular-skeletal injuries like leg pain etc.According to the calculated values, very high levels of adverse health effects werepredicted for rock breaker operators. The assessment method for the effect ofrepeated shocks on the human body has led to an appreciation of the importance ofbiodynamic frequency, and peak accelerations in ergonomics and work placedesign. Occupational exposure to WBV and physical factors at work are importantcomponents of the multifactorial origin of physiological effects in professionalmobile machinery operators. Ergonomic evaluation and design of workenvironment in use of tracked excavators is required.

ACKNOWLEDGEMENT Authors thank for the Instrumentation facilities from the National Institute ofTechnology, Tiruchirapalli, India.

REFERENCES 1. Bovenzi Massimo, Rui Francesca, Negro Corrao, D’agostin Flavia, Angotzi

Giuliano, Bianchi Sandra, Bramanti Lia,Festa Gianluca, Gatti Silvana,PintoIole, Rondina Livia, Stacchini Nicola “An Epidemiological Study Of LowBack Pain In Proessional Drivers”, Journal Of Sound And Vibration Vol.298,2006, pp. 515-539.

2. Lenka Gallais, Michael J. Griffin,”Low Back Pain in Car Drivers:A review ofstudies published 1975 to 2005”, Journal of Sound and Vibration, Vol 298,No.3, 2006, 499-513.

3. Kroemer.K and E.Grandjean “Fitting The Task to the Human-A Textbook ofOccupational Ergonomics”, Taylor and Francis, London, 1997.

4. Bovenzi.M and C.T.J.Hulshof,”An Updated Review of Epidemiologic StudiesOn The Relationship Between Exposure To Whole-Body Vibration And LowBack Pain (1986–1997). International Archives Of Occupational andEnvironmental Health 72, 1999, pp. 351–365.

5. Johanning E., “Back disorders and health problems among subway trainoperators exposed to whole-body vibration”. Scand J Work EnvironHealth,1991, 17(6), pp. 414–419.

6. Sarah Cation, Robert Jack, Michele Oliver,James P.Dickey, Natasha K.Lee-Shee,”Six Degree of freedom whole body Vibration during Forestry SkidderOperations”,International Journal of Industrial Ergonomics.Vol. 38, No.9-10,2008, pp. 739-757.

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7. Scarlett.A.J, J.S.Price, R.M.Stayer,”Whole body vibration:Evaluation ofemission and exposure levels arising from Agriculture Tractors”, Journal ofTerramechanics. Vol.44, No 1, 2007, pp. 65-73.

8. International Organization for Standardization, ISO 2631/1:1997 –Mechanical vibration and shock – evaluation of human exposure to whole-body vibration, Part 1: General requirements, Geneva, Switzerland.

9. European Parliament and the Council of the European Union (2002) Directive2002/44/EC on the minimum health and safety requirements regarding theexposure of workers to the risks arising from physical agents



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Hydraulic Rock Breaker