Non binding guide to good practice for implementing Directive

ANNEX B What is vibration?Non binding guide to good practice for implementing Directive 2002/44/EC (Vibrations at Work)B.1 WHAT ISVIBRATION?Vibrations arise when a bodymoves back and forth due toexternal and internal forces,Figure B.1. In the case of wholebodyvibration, it may be theseat of a vehicle or the platformon which a worker is standingthat vibrates, and this motion istransmitted into the body of thedriver.Figure B.1Whole-body vibrationB.2 WHAT IS MEASURED?Vibration is defi ned by its magnitude and frequency.The magnitude of vibration could be expressed as thevibration displacement (in metres), the vibration velocity(in metres per second) or the vibration acceleration (inmetres per second per second or m/s²). However, mostvibration transducers produce an output that is relatedto acceleration (their output is dependent on the forceacting on a fi xed mass within the transducer and, for afi xed mass, force and acceleration are directly related);so acceleration has traditionally been used to describevibration.The vibration transducer measures acceleration in onedirection only, so to get a more complete picture of thevibration on a surface, three transducers are needed:one in each axis as illustrated in Figure B.2.B.3 WHAT IS FREQUENCY AND FREQUENCYWEIGHTING?Frequency represents the number of times per secondthe vibrating body moves back and forth. It is expressedas a value in cycles per second, more usually known ashertz (abbreviated to Hz).For whole-body vibration, the frequencies thought to beimportant range from 0.5Hz to 80Hz. However, becausethe risk of damage is not equal at all frequenciesa frequency-weighting is used to represent the likelihoodof damage from the different frequencies. As a result, theweighted acceleration decreases when the frequencyincreases. For whole-body vibration, two different frequencyweightings are used. One weighting (the Wdweighting) applies to the two lateral axes: x and y, andanother (the Wk weighting) applies to the vertical, z-axisvibration.When considering the risks to health from whole-bodyvibration an additional multiplying factor must beapplied to the frequency weighted vibration values. Forthe two lateral axes (x and y) the acceleration values aremultiplied by 1.4. For the vertical, z-axis vibration thefactor is 1.0.B.4 WHAT VIBRATION PARAMETERS ARE USEDFOR EXPOSURE ASSESSMENT?The vibration directive allows for two vibration assessmentmethods:• the daily exposure, A(8) - the continuous equivalentacceleration, normalised to an 8 hour day, the A(8)value is based on root-mean-square averaging ofthe acceleration signal and has units of m/s²; and• the vibration dose value (VDV) is a cumulative dose,based on the 4 th root-mean-quad of the accelerationsignal with units of m/s 1.75 .Both parameters A(8) and VDV are defi ned in ISO2631-1:1997.Some examples of vibration magnitudes for commonhand-held power tools are shown in Figure B.3.Figure B.2 Vibration measurement axes84