Measuring Vibration and Tool Testing

Vibration measurement is a description of motion expressed in terms of both magnitude and direction. The magnitude of hand-arm vibration is quantified as the acceleration rate in an engineering unit known as "meters per second squared" (m/s2). For example, a measurement of 2 m/s2 means that the motion of the vibration in the hand is accelerating at a rate of 2 meters per second, every second.

It is important to know that what is being measured is the output of the tool and not necessarily the measurement of exposure transmitted to the worker. Current exposure standards and guidelines are based on tool output data and do not distinguish transmissibility rates.

For hand-arm vibration, three perpendicular directions are simultaneously measured. These measurements are referred to individually as the X, Y and Z axis with the x-axis on a plane passing through the back of the hand, the y-axis crossing the knuckles and the z-axis running parallel to the plane of the bones in the back of the hand. Quantifying hand arm vibration involves monitoring the vibration acceleration rates in all three axes (directions) simultaneously while the tool is in use. You cannot assume that a tool has a dominant axes, hence the need to measure in three axes and sum the individual measurement as the "vibration total value".

Vibration total value is a single-number summation of the three individual axis measurements. Vibration total value is calculated as the square root of the sum of the squares of each axis measurement: Asum = ? Ax2 + Ay2 + Az2

The HAV exposure laws (European Directive on Hand Arm Vibration Exposure) prescribe single number limits based on the vibration total value. The European Directive establishes 2.5 m/s2 as the action limit and 5.0 m/s2 as the maximum exposure limit for all EU member countries. The EU's action limit for HAV is the trigger for commencing medical surveillance.

Measuring vibration on site in actual working conditions produces far more accurate and realistic figures than those obtained by the manufacturer using static bench tests in factory environments. Often the actual site readings are typically 1/3 higher than those supplied by the manufacturer.

Many different scenarios are needed to obtain accurate results as the way in which tools are used, the application of the tools and the way in which they are held all effect the vibration readings. Similarly, maintenance routines, sharpness, balance and wear of blades/cutting edges produce different vibration readings. For example, the length and thickness of grass cut by a strimmer will affect the vibration reading as will the length and thickness of strimmer cord used to perform the task. The harness used to secure the strimmer, the body mass/health of the operator, and the weather conditions and angle of slope worked on will all alter the readings obtained.

On site vibration readings should always be carried out by trained, competent personnel, using specialist equipment to obtain readings from the point of operator contact (handles). HAVSRM use the latest measuring equipment to obtain accurate on site measurements for specific operator tasks. If you would like a free quotation for on-site equipment testing contact the HAVSRM team direct.