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December 8, 2010

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Physical agents – Up to scratch

Many manufacturing processes involve some form of abrasive tooling, meaning hand-arm vibration can be an issue. Nigel Willcock reminds employers of their responsibilities in this area and advises on steps to take to minimise vibration in the workplace.

Hand-arm vibration describes vibration transmitted from physical work processes into workers’ hands and arms. It can be caused by a variety of sources, including power tools, such as grinding wheels, sanders and angle-grinders used in abrasive finishing processes.

While occasional exposure is unlikely to cause ill health, frequent exposure to hand-arm vibration – usually when contact with a vibrating tool or work process is a regular part of an operator’s job function – can lead to permanent health problems. These include a range of effects collectively known as hand-arm vibration syndrome (HAVS), as well as conditions such as carpal tunnel syndrome, neurological problems, and musculoskeletal disease. According to the HSE, nearly 2 million people in the UK are at risk.1

For some people, symptoms may appear after only a few months, while for others, it may take a number of years. The symptoms are likely to worsen with continued exposure to vibration and may become permanent. Long-term effects can include pain, distress and sleep disturbance; an inability to perform fine or detailed work, or everyday tasks such as fastening buttons; a reduced ability to work in cold or damp conditions, which could trigger painful blanching attacks; and reduced grip strength, which might affect an employee’s ability to work safely.

While the effects of HAVS are permanent once the damage is done, it is preventable. The Control of Vibration at Work Regulations 2005 place a clear duty on employers to take action to ensure their staff are not exposed to a daily ‘exposure action value’ above 2.5m/s2 or an overall ‘exposure limit value’ of 5m/s2 (see panel overleaf). These vibration levels are now applicable to all tools used in the workplace.

Tooling up

With regard to abrasive processes, vibration is arguably the most significant safety issue when selecting tools and products. There are six main factors to consider, in terms of the impact on the level of exposure that an employee using abrasive products will face: the tool itself, the abrasive consumable product, the support medium (if applicable), the substrate being treated, the application method, and the actions of the employee. Optimising all of these will help ensure that the level of vibration and the exposure time for the employee are minimised.

With regard to equipment, there have been many innovations by manufacturers that have made processes safer and less physically demanding, with no negative impact on quality or productivity. Tools supplied should always be low in vibration and appropriate to the task (vibration levels can vary enormously between different tools).

The manufacturer or distributor can advise here but, for a true value of the vibration level, an independent vibration assessment needs to be made on the application. Tools should always be checked before use to ensure they have been properly maintained and repaired to avoid increased vibration caused by faults, or general wear.

Some abrasive tools, such as random orbital sanders, incorporate, by definition, a degree of eccentricity into their movement in order to achieve a randomised pattern on the substrate, usually for a subsequent coating, or finishing process. Their balance is therefore typically off-centre, meaning they place greater physical demands on the operator, as well as potentially increasing hand-arm vibration. Sometimes, however, their use is unavoidable, so it is imperative to ensure they are not used for too long a period of time.

In the case of random orbital sanders, it is critical to use a backing pad that is matched to the tool, so vibration is minimised. The tool should be supplied with a backing pad that is ‘balanced’ to work with the sander. Using a different pad from the same supplier, or a different supplier’s pad, can increase the vibration by up to three times.

Other products, such as angle grinders, are rotary and operate in a more regular pattern, so are less likely to cause high levels of vibration. However, when using a depressed-centre grinding disc, high vibration values can be encountered, so using a tool with an autobalancer is important.

Owing to significant advances in coated abrasive consumables technology, it is possible to increase quality and reduce process time – and thus vibration – with these types of product. Developments in this area include long-lasting fibre discs for weld grinding, de-scaling, heavy deburring and deflashing of flame-cut parts. Thin, flexible, resin-bonded products offer better balance characteristics than grinding discs and so can significantly reduce vibration.

Products that contain multiple layers of abrasive, with each structure exposing a new layer as it wears down, ensure wear is more even, and reduce the pressure the operator needs to apply. They can even cut process steps, thus lowering both the time spent at the abrasive stage and operator exposure to vibration.

As already noted, the backing pad – typically fixed to the equipment – is also critical. A pad that is inappropriate, or incompatible with the abrasive disc being used is likely to significantly increase vibration. Ribbed high-performance pads, checked for compatibility with the abrasive disc, will enhance reinforcement and also assist in delivering the required impact pressure, thereby reducing vibration. The aggressive cut and long life of this combination of fibre disc and high-performance backing pad means, in some instances, they can be used to replace depressed-centre grinding discs, which can wear unevenly and so place greater physical demands on the operator. Furthermore, their lack of flexibility means the vibration level is often much higher than the fibre disc and backing-pad system.

With any abrasive consumable, the type and aggressiveness of the mineral is key to reducing both process time and the force needed to be applied by the operator. Again, expert advice should be sought before a decision is made.
Grinding wheels are still widely found in many foundries, and usually create higher levels of vibration than either belts or tools. However, advances in belt technology mean that coated abrasive belts properly fitted to contact wheels can now perform most of the tasks previously undertaken using grinding wheels. They deliver a faster cut and more consistent finish, and so provide a more rapid, lower-vibration alternative.

Indeed, my company’s own research shows that grinding wheels with the same backstand can create more than twice the amount of vibration as a coated abrasive belt on a contact wheel, while changing either the backstand to a more rigid design, or the type of contact wheel used, can reduce vibration.

While the type of substrate cannot be changed, the quality of the casting process is key in determining how much work is required at the abrasive stage. In foundries, for example, good practice and optimised investment casting, or lost wax casting techniques, will help reduce the amount of work needed with an angle grinder to remove gates. Similarly, castings should be designed to minimise the number of joint lines, or to be suitable for direct machining. Inappropriate customer specifications should also be challenged to avoid unnecessary work.

Again, the nature of the application may require more or less pressure from the operator. Required operator effort will be minimised if correct decisions have been made in terms of tools, consumables and backing pads.

Finally, the operator should be trained in: the risks of HAVS and how to avoid them; the importance of correct operation and maintenance of equipment; arrangements for health surveillance; and their duty to cooperate.

Assess and address the risks

The 2005 Regulations require employers to focus on the elimination or control of vibration exposure, and the most effective means of achieving this is to seek new, or alternative work methods.

The risk assessment should involve establishing which processes and equipment create regular exposure to vibration; finding out if there are any warnings of vibration risks in equipment handbooks; and establishing whether employees have any symptoms of HAVS, and whether the equipment they are using produces high levels of vibration or uncomfortable strain on the hands or arms. The times for which employees’ hands are in contact with the equipment while it is vibrating – also known as ‘trigger times’ – should be noted.

Activities should then be grouped into high, medium or low risk – high being above 5.0m/s2; medium being above 2.5m/s2; and low being below 2.5m/s2. Existing vibration data can be used to confirm the group to which each process should be assigned. However, a better method is to use a vibration meter that uses an accelerometer placed where the operator holds the tool or part to measure the vibration in all three axes, which the meter then converts to the three-axis sum vibration value. This gives an accurate snapshot in time of the vibration level to which the operator is exposed for this particular application.

Once the vibration-level and trigger-time data have been acquired and collated, they can be entered into a vibration-exposure calculator to determine each employee’s daily exposure. Employees who work on different processes will need to have trigger times estimated for each process they are working on to calculate their total vibration exposure.

A free exposure calculator is available on the HSE website,1 which requires the input of vibration magnitude and exposure duration. It automatically calculates the partial exposure and total exposure, as well as the number of exposure points per hour, the time that will be taken to reach the exposure action value, and the time taken to reach the exposure limit value.

This will assist with the planning of work, as well as highlighting processes and equipment that are high in vibration and which therefore need to be addressed to reduce the risk to operators.

Any control actions put in place as a result of this will need to be checked on an ongoing basis to ensure they are effective. It also goes without saying that a proper health surveillance scheme should be established; this will help employees recognise if and when symptoms of HAVS start to develop, and will also highlight how well measures that have been put in place to control hand-arm vibration are working.

Reference
1    www.hse.gov.uk/vibration/hav/

Nigel Willcock is part of the abrasive systems division at 3M.
 

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