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Departement Mechanical & Mechatronic Engineering•

Faculty of Engineering

Human Vibration in the WorkplaceMeasurement, Analysis and Assessment

Prof Wikus van NiekerkSound & Vibration Research GroupSound & Vibration Research Group

South African Society of Occupational Medicine15 April 2008

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Outline

• Introduction to human vibration• Standards and references• Hand-arm vibration• Whole body vibration• Risk management• Concluding remarks

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Human vibration

• Hand-arm vibration, or hand-transmitted vibration, refers to vibration entering the body at the hand, i.e. power tools and rock drills.

• Whole body vibration occurs when the body is supported on a surface, which is vibrating, i.e. the seat of an earth-moving machine.

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Vibration

• Vibration is defined as oscillatory motionoscillatory motion of a particle, body or surface from some reference position and is described by at least two quantities, one relating to the frequency, or frequency content and the other to the amplitude of the motion.

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Physical quantities for measurements

• Typically acceleration levels are measured: Rate of change of velocity

• Units: metres/second/second, m/s2

• Frequency is expressed as cycles per second, or Hertz, Hz

∫=T

wrmsw dttaT

a0

2, )(1

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Vibration characteristics

Am plitude

Frequency Duration

Amplitude

DurationFrequency

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Effect of vibration on the human

Effect

HUMAN

Vibration Vibration: AmplitudeFrequencyDurationDirection

Effects: Bio-dynamicPsychologicalPhysiologicalPathological

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Human stressors

• Interference with comfortAnnoyanceFatigue

• Interference with activitiesE.g. motion sickness, blurred vision

• Interference with healthPhysiological and pathological effects

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Pathological effects

• Whole body vibrationBack problemsGastrointestinal problemsVestibular disordersVisual disorders

• Hand-arm vibrationVascular disorders (eg. VWF)Neurological disordersBone and joint disordersMuscle disorders (eg. Carpal Tunnel)

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Reference and standards

Handbook of Human Vibrationby Prof M.J. Griffin (University of Southampton), Academic Press, London, 1990

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Reference and standards (ISO)

• ISO 2631-1 (1997) Mechanical vibration and shock - Evaluation of human exposure to whole body vibration – Part 1: General requirements

• ISO 2631-2 (1989) Guide for the evaluation of valuation of human exposure to whole body vibration – Part 2: Continuous and shock induced vibration in building (1 to 80 Hz)

• ISO 2631-4 (2001) Evaluation of human exposure to whole body vibration – Part 4: Guidelines for the evaluation of the effects of vibration and rotational motion on passenger and crew comfort in fixed-guideway transport systems.

• ISO 5349 (1986) Mechanical vibration – Guidelines for the measurement and evaluation of human exposure to hand-transmitted vibration

• ISO 5349-1 (2001) Mechanical vibration – Measurement and evaluation of human exposure to hand-transmitted vibration – Part 1: General requirements

• ISO 5349-1 (2001) Mechanical vibration – Measurement and evaluation of human exposure to hand-transmitted vibration – Part 2: Practical guidance for measurement at the workplace

• ISO 8041 (1990) Human response to vibration – Measuring instrumentation• ISO 8662-1 (1988) Hand-held power tools – Measurement of vibration at the handle. Part1: General (Parts 2 to 14

relates to specific tools)• ISO 8662-3 (1988) Hand-held power tools – Measurement of vibration at the handle. Part 3: Rock drills and

rotary hammers• ISO 8662-4 (1988) Hand-held power tools – Measurement of vibration at the handle. Part 4: Grinders

• ISO 10819 (1996) Mechanical vibration and shock – Hand-arm vibration – Method for the measurement and evaluation of the vibration transmissibility of gloves at the palm of the hand

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Legislation

• South Africa• Very little legislation and/or regulations• SANS 2631-1:1997; SANS 8662-1 to 14• Act No. 130 of 1993: Compensation for occupational injuries and

diseases act.• Schedule 3: Hand-arm vibration syndrome (Raynaud's phenomenon)

caused by vibrating equipment

• Europe• Machinery Directive• Newly implemented EU Legislation (EU Directive 2002/44/EU)

• Exposure Action Values: (Hand Arm: 2.5 m/s2 Whole Body Vibration: 0.5 m/s2)• Exposure Limit Values (Hand Arm: 5 m/s2 Whole Body Vibration: 1.15 m/s2)EAVs and ELVs all for A(8) (Eight hour daily exposure time)

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Tools and processes associated with Hand-transmitted vibration

1.7 to 3.6 % of workers in Europe and USA exposed to potentiallyharmful hand-transmitted vibration.

• Percussive metal working tools• Riviting tools - Caulking tools – Chipping Hammers - Fettling Tools

• Grinders and rotary tools• Portable grinders – Sanders – Polishers – Burring Tools

• Forestry and gardening• Chain saws - Brush saws - Barking machines - Shears

• Mining and percussive hammers• Rock drills – Hammers

• Others:• Motor cycle handles - Engraving pens - Concrete vibro-thickeners

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Effects of hand-transmitted vibration

• Vascular disorders• Vascular disorders include impeded blood circulation to the fingers and

attacks of blanching on one or more digits. Occasional necrosis and rarely, gangrene, has been reported.

• VWF, Primary Raynauds

• Bone and joint disorders• Bone and joint injuries mostly associated with percussive pneumatic

tools.

• Neurological disorders• Neurological disorders lead to numbness and reduced tactile sensitivity

that are not necessarily restricted to areas affected by blanching.

• Muscle disorders: loss of dexterity

• Other general disorders: e.g. central nervous system

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Blanching

Adapted from: Wasserman,D.E & Wasserman,J.F.

The nuts and bolts of human vibration exposure to vibration, Sound and Vibration, January 2002

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Scoring finger blanching

Griffin introduced a method of scoring finger blanching:

For more detail, see

Griffin, M.J. Handbook of Human Vibration. Academic Press, 1990.

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Evaluation of effects of vibration

• Objective tests:

• Thermal aesthesiometry: Time to perception of heat/cold• Vibrotactile thresholds: Amplitude of vibration for

perception• Cold provocation: Rewarming time measured• Grip strength• Evoked potentials: nervous system

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Location and orientation of transducershandgrip position

• Basicentric coordinate system originating in vibrating appliance• Hand grips on cylindrical bar• Measure in 3 orthogonal directions (preferably simultaneously)• As close as possible to centre of gripping zone

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Location and orientation of transducersflat palm position

• Hand presses down onto sphere

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Rock drill comparative measurements

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Quantity to be measured

• Primary quantity to be measured is the root-mean-square (rms) frequency weighted acceleration in m/s2.

• This requires the application of the Wh frequency weighting which reflects the assumed importance of different frequencies in causing injury to the hand.

• The rms value shall be measured using a linear integration method.

• Recommended that frequency spectra be obtained.

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Wh frequency weighting curve

Octave bands from 8 Hz to 1000 Hz (nominally 5.6 Hz to 1400 Hz)

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Multi-axis vibration

• It is assumed that vibration in all 3 directions equally detrimental – all 3 directions should be measured.

• The frequency-weighted rms acceleration values for 3 directions ahwx, ahwy and ahwz shall be reported separately.

• Evaluation is based on• If measurements can only be taken in 1 direction, the total

value will be estimated using a multiplying factor (typically 1.0 – 1.7 where axis of greatest vibration has been measured.)

222hwzhwyhwxhv aaaa ++=

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8h Energy equivalent frequency weighted vibration total value

Exposure to one magnitude ahv

T is total daily duration of exposure to vibration ahv

T0 is the reference duration of 8 h (28 800 s)

0)8,( )8(

TTaAa hvheqhv ==

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8h Energy equivalent frequency weighted vibration total value

Several operations with different magnitudes

ahvi is the vibration total value for ith operation

n is number of individual exposuresTi is duration of ith operation

Example:2 h – 3 m/s2

3 h – 4 m/s2

1 h – 8 m/s2

∑=

=n

iihviTa

TA

1

2

0

1)8(

[ ] 2222 /0.418342381)8( smh

A =×+×+×=

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Prevalence of vibration-induced white finger

Dy lifetime exposurein yearsA(8) daily vibration exposure

06.1)8(8.31 −×= ADy

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• It is assumed that the exposure time required to produce symptoms is inversely proportional to the square of the frequency weighted acceleration, i.e. if the vibration magnitudeis halved then the daily exposure time may be increased by factor 4.

• Exposure should not be extrapolated to very short durations and large accelerations.

• The graph is used for assessment of all biological effects of hand-transmitted vibration (not only vascular).

Prevalence of vibration-induced white finger

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Factors likely to influence the effects of human exposure to hand-transmitted vibration

No standardised methods available to account for:• The direction of the vibration• Method of working and operator skill• Operator age and health• Temporal exposure pattern (rest spells, etc.)• Coupling force and pressure on the skin• Posture of the arm• Type and condition of vibrating machines• Area and location of parts of the hands exposed to vibration• Climatic conditions• Agents affecting peripheral circulation (e.g. nicotine)• Noise

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Preventative measures

• Medical measures• Workers who may have to expose their hand to vibration

should, prior to employment• Be physically examined• Have previous history of vibration be recorded

• Workers should be advised of the risks involved • Persons with certain medical conditions might be at greater

risk and should be particularly carefully assessed• Primary Raynaud’s disease• Disease caused by impairment of blood circulation• Past injuries causing circulatory defects/bone deformity• Disorders of the peripheral nervous system• Disorders of musculoskeletal system

• Regular medical check-ups and reporting of symptoms

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• Technical measures• Choose process resulting in lowest vibration exposure• Use tool with lowest resulting vibration exposure• Carefully maintain equipment • Prevent tools from expelling cold gases or fluids• Heat handles where appropriate• Avoid tools with handle shapes resulting in high pressure• Choose tools with low contact forces• Keep mass to minimum (provided vibration not increased)• Anti-vibration gloves can be beneficial for high frequencies

Preventative measures

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• Administrative measures• Ensure adequate training• Arrange work-schedules to include vibration-free periods• Keep workers warm (where appropriate)

Preventative measures

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Introduction (Whole Body Vibration)

• Examples:• Impact road compactor• Articulated dumpers• Buldozers• Logging vehicles• Forklifts• Building vibration

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Introduction (WBV)

• Perception• Ground borne vibration (Gautrain)

• Comfort• Vehicles and equipment (Trucks and cars)

• Health• Severe vibration environment (Earthmoving

equipment)

• Motion sickness

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Introduction (WBV)

• Physiological effects of whole body vibrationBack problems (lower back pain)Gastrointestinal problemsVestibular disordersVisual disorders

• No consensus on dose-effect relationship for whole body vibration

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Human Response

1Uncomfortable>1,25 for 8 hours*Health risk

0.1Easily noticeable~0.01Perseption threshold

RMS acceleration [m/s2]

Vibration level

* EU Directive that has now become law in Europe

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Human Response

• Vibration inteference with:• Input: Vision, e.g. Display• Output: Hand control e.g. Writing

• Vision: Eye natural frequency >30 Hz (Griffin)• Can follow below 1 Hz if low velocity• 1 – 3 Hz eye will jump to anticipated position• Above 3 Hz the eye will be directed to the node

• Manual control:• Vert. ~3 – 8 Hz, Horiz. < 2 Hz (Max shoulder transmissibility)• Spilling liquid from a hand-held cup 4 Hz, Writing 4 – 8 Hz

• Vibration induced fatigue: no direct evidence yet

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ISO 2631(Introduction)

Scope• Measurement methods (for periodic, random, transient with high peak

values)• Mounting of transducers• Directions• Recumbent, seated (back, buttocks and feet), standing person

• Guidance (NO LIMITS)• Health• Comfort• Perception• Motion sickness

• Not covered• Performance (depend on task)• Single shocks (blast, vehicle accidents)• Direct transfer to limbs (e.g. power tools) – ISO 5349

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ISO 2631(Introduction)

• Frequency range:• Health, comfort and perception – 0,5 Hz to 80 Hz• Motion sickness – 0,1 Hz to 0,5 Hz

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ISO 2631(Definitions)

• Basicentric coordinate system

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ISO 2631 (Measurement)

• Seatpad• ISO 103261 Mechanical vibration – Laboratory method for

evaluating vehicle seat vibration – Part 1 Basic requirements

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ISO 2631(Frequency weighting)

Frequency weighting Health Comfort Wk z-axis, seat surface z-axis, seat surface

z-axis, standing vertical recumbent (except head) x, y, z-axes, feet, (sitting)

Wd x-axis, seat surface y-axis, seat surface

x-axis, seat surface y-axis, seat surface x-, y-axes, standing horizontal recumbent y-, z-axes, seat-back

Wc x-axis, seat back x-axis, seat back We - rx-, ry-, rz-axes, seat surfaceWj - Vertical recumbent (head)

Wf Motion sickness (vertical)

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ISO 2631(Frequency weighting)

• Frequency weighting curves

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ISO 2631(Evaluation method)

• Weighted rms acceleration (filter methods):

• 1/3 Octave and narrow band spectra method:

• Wi weighting from table in ISO 2631• ai rms values measured in 1/3 octaves

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0

2 )(1⎥⎦⎤

⎢⎣⎡= ∫

T

ww dttaT

a

( )21

2⎥⎦

⎤⎢⎣

⎡= ∑

iiiw aWa

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ISO 2631 (Analysis)

Basic method does not apply for crest factor (weighted) > 9 In this case two additional evaluation methods are applied:

• Vibration Dose Value (VDV)rms

max( ( ))wa ta

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0

4 )( ⎥⎦⎤

⎢⎣⎡= ∫

T

w dttaVDV

[ ]414∑= itotal VDVVDV

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ISO 2631 (Assessment - Health)

• Assume: Discomfort = Health effects• Total exposure rather than average: Dose• No precise limit (Because: No dose injury risk relationship)

• BS 6841 VDV 15 ms-1.75

• ISO 2631 • VDV < 8.5 ms-1.75 = No proof• 8.5 < VDV < 17 ms-1.75 = Caution• > 17 ms-1.75 = Risk

• EU machinery safety directive: report if arms > 0.5 m.s-2

• EU vibration exposure directive: action level A(8) = 0.5 m.s-2 (Limit 1.15)

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ISO 2631 (Assessment - Health)

0.1667 0.5 1 2 4 8 240.1

0.16

0.25

0.4

0.63

1

1.6

2.5

4

6.3

10

Exposure tim e [h]

We

igh

ted

ac

ce

lera

tio

n [

m/s

2]

W hole body vibration health guidance zone

2122

2111 TaTa ww =

4122

4111 TaTa ww =

VDV = 15m/s1.75

VDV = 17m/s1.75

VDV = 8.5m/s1.75

RiskRisk

CautionCautionzonezoneNo risk?No risk?

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• Multiplication factors:• Seated person

• Seat: x, y, z-axes: k = 1, rx: k = 0.63, ry: k = 0.4, rz: k = 0.2• Backrest: x: k = 0.8, y: k = 0.5, z: k = 0.4• Feet: x: k = 0.25, y: k = 0.25, z: k = 0.4

• Standing, recumbent k =1

• Calculate point values: Seat trans, seat rot, back, feet

• Calculate total value (RSS of point values)

( )21

222222wzzwyywxxv akakaka ++=

2vRSS a= ∑

ISO 2631 (Assessment - Comfort)

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ISO 2631 (Assessment - Comfort)

• Use for: Comparison between vibration exposures• Adding exposure periods:

• Acceptable values vary, but comfort guidelines:• <0.315 m/s2 is not uncomfortable• 0.8-1.6 m/s2 is uncomfortable• >2 m/s2 is extremely uncomfortable

• Perception• Highest value only, k=1 for all points and directions• 50% of alert, fit persons can just detect Wk weighted peak

magnitude of 0.015 m/s2

12 2

,wi i

w ei

a Ta

T⎡ ⎤

= ⎢ ⎥⎢ ⎥⎣ ⎦

∑∑

14 4

,wi i

w ei

a Ta

T⎡ ⎤

= ⎢ ⎥⎢ ⎥⎣ ⎦

∑∑

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ISO 2631 (Motion sickness)

• Motion of the body detected by:• Vestibular: Inner ear• Visual• Somatosensory: Sensing force or displacement

• Causes: • Variance between systems • or variance between systems and previous experience

• Frequencies < 1 Hz (0.125 – 0.15 Hz most sensitive)• aw is frequency weighted and To in seconds

• Percentage adults expected to vomit (⅓ x MSDV)%

5.00TaMSDV wz =

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Risk assessment

• Measure the vibration levels• Determine the exposure time• Assess the vibration exposure• Report! and follow-up• Mitigating steps

• Reduce levels• Reduce exposure time• Work rotation

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Risk assessment (Griffin, 1990)

IDENTIFICATION

OF PROBLEM

ISOLATION REDUCED

PROBLEM

REDUCTION AT

SOURCE

ELIMINATION

AT SOURCE

NO

PROBLEM

ALLEVIATION

VIBRATION

MEASUREMENT

HEALTH

MONITORING

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Case study (Driver of a logging truck)

• Known data?• Measurements

• Typical seat vibrations• Typical routes• Exposure time?

• Analysis• Weighted rms or VDV

values• Combine routes/ exposure

• Assessment• Levels? Caution zone

• Mitigation• Slower driving• Seat improvement• Road maintenance• Exposure time

• Follow-up• Medical examinations• Additional measurements

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Concluding remarks

• Increasing awareness of human vibration and its effects

• Many industries where operators are exposed to high levels of vibration, not only mining

• Measure vibration levels to assess risk• Reduce exposure levels by reducing vibration

levels and/or exposure time

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Concluding remarks

• SA tends to follow European standards and legislation (which are becoming more severe!)

• New EU legislation will influence export of SA products, eg. rock drills and articulated dump trucks to Europe