6. Vibrations and shocks

Background
Mechanical vibrations are periodically repeated reversals of the motion direction, say movements of certain points relative to an equilibrium state, after disruption of the balance.Vibrations are fully described by their direction (x-, y-and z-component), frequency (Hz) and intensity: amplitude or deflection (m) / speed (m/s) / effective acceleration (m/s²). The effective acceleration (root mean square or RMS) is calculated by squaring the positive and negative acceleration values, to count them, and to take the square root of their sum.Also the character of the vibration is important:

  • harmonic: regularly with fixed frequency and amplitude (occurring rarely, in practice)
  • stochastic: irregular, unpredictable with peaks and valleys (does occur often), short vibrations with high-intensity are called ‘shocks’

The vibration is only a risk for humans when there is contact between the human body and the vibrating object. The vibration load of a human is also determined by severalcharacteristics:

  • place of contact between the object and body: hand tools can cause hand-arm vibrations, a vibrating seat in a vehicle will lead to vibration of the entire body
  • direction of vibration: z-component is the longitudinal direction along the back, x-component is the forth and back direction, y-component is the transversal direction
  • frequency and intensity: the sensitivity of the human body towards vibration depends on the frequency; because it is difficult to determine the intensity for each individual frequency, a weighing filter is applied to express the intensity in the three directions, in one single number
  • Crest Factor: greatest acceleration divided by the RMS
  • exposure time: in hours per day, days per year, years, ...

Vibration load due to whole-body vibrations can lead to fatigue, chronic back problems, car- or sea sickness, ... Hand-arm vibrations may damage nerves and blood vessels in the hands (white finger syndrome) and can cause damage to bones, tendons and joints at the level of the hand, the wrist and the elbow.

Legislation

Standards

  • ISO 8041:2005 Human response to vibration - Measuring instrumentation
  • ISO 2631-1:1997 Mechanical vibration and shock – Evaluation of human exposure to whole-body vibration - Part 1: General requirements
  • ISO 2631-2:1997 Mechanical vibration and shock – Evaluation of human exposure to whole-body vibration - Part 2: Vibration in buildings (1 to 80 Hz)
  • NEN EN 14253:2003 Mechanical vibration - Measurement and calculation of occupational exposure to whole-body vibration with reference to health - Practical guidance
  • NBN EN ISO 5349-1:2001 Mechanical vibration - Measurement and evaluation of human exposure to hand-transmitted vibration - Part 1: General requirements (ISO 5349-1:2001)
  • NBN EN ISO 5349-2:2001 Mechanical vibration - Measurement and evaluation of human exposure to hand-transmitted vibration - Part 2: Practical guidance for measurement at the workplace  (ISO 5349-2:2001)
  • more information on http://www.nbn.be

Documentation

Tools

Websites