There are many sources of vibration in the occupational environment and from a workplace perspective we are generally concerned about the potential risk to human health. Exposure in the workplace is assessed in terms of hand-arm vibration and whole-body vibration. In addition, legal controls and criteria for workplace exposure are addressed in terms of these phenomena.
Vibration may be transmitted to the human body through the part in contact with the vibrating surface: the handle of a machine, the surface of a piece of equipment, or the seat of a mobile machine.
Hand-arm vibration (HAV) is vibration which is transmitted from work processes and equipment into workers’ hands and arms. It can be caused by operating hand-held power tools, such as pneumatic wrenches, angle grinders, road breakers, and hand-guided equipment.
Whole-body vibration (WBV) has been described as a shaking or jolting of the human body through a supporting surface (usually a seat or the floor). The vibration can potentially enter the body via a number of routes and potentially affect organs and/or human health.
Examples of WBV include exposure arising when driving or riding on a vehicle over rough terrain, operating earthmoving machines, or standing on a structure attached to a large, powerful, fixed machine which is vibrating.
Regular and long term exposure to WBV is linked to lower back pain and industries associated with WBV include agriculture, mining and construction. However, emphasis is given to HAV throughout the remainder of this article.
Vibration –basic concepts
A vibrating object moves back and forth from its normal stationary position. The number of cycles that a vibrating object completes in one second is an important characteristic called frequency. The unit of frequency is hertz (Hz) and one hertz equals one cycle per second.
Vibration magnitude can be measured using any one of three different quantities: Displacement, Velocity or Acceleration. Typically, acceleration is used particularly when dealing with occupational vibration. A typical vibration measurement system includes an accelerometer and an instrument to measure the level of vibration in accordance with ISO 5349 (with the instrumentation based on ISO 8041).
A transducer is firstly required to convert the movement of the vibrating body into a voltage which is proportional to the motion of the vibrating body. The transducer (accelerometer) will predominantly utilize piezoelectric crystals to produce an electrical voltage when subjected to an applied force. This is proportional to the applied force and the voltage is subsequently assessed and processed by the vibration meter. The measurement system incorporates the accelerometer and the standardised frequency-weighting to give a single number measurement (expressed as the frequency-weighted vibration exposure).
There have been major developments in Micro-Electro-Mechanical Systems (MEMS) and this technology has been used to develop ‘wearable personal vibration exposure meters’. Compliance with ISO 5349 and ISO 8041 is not generally claimed by the manufacturers of these meters but nonetheless studies have established their relevance in assessing and managing HAV risk.
Measuring and Assessing HAV Exposure
Many factors need to be considered when measuring vibration. The mounting of the accelerometer is very important and can often raise challenges. There are a number of methods available for mounting accelerometers, but the most important consideration is that the accelerometer is securely fixed to the surface of the vibrating body.
Typically, the measurements are taken with the accelerometer primarily fixed onto the tool handle and located close to the hand position. Ideally, the accelerometer should be located at the middle of the gripping zone (e.g. halfway along the width of the hand when gripping a power tool handle). It is at this location that the most representative evaluation of the vibration entering the hand is obtained. However, it is often not practicable to locate transducers exactly at this point (e.g. where the transducers could interfere with the normal grip used by the operator). For many measurements, the accelerometers are mounted either side of the hand or on the underside of the tool handle adjacent to the middle of the hand.
The mounting of accelerometers on a power tool or hand-held workpiece can be intrusive and will have some effect on how the operator works.