Managing hand-arm vibration in abrasive processes
16 February 2011
By Nigel Willcock, Technical Service Specialist, Abrasive Systems Division, 3M United Kingdom plc
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 the use of power tools, such as grinding wheels machines, sanders and angle grinders, in abrasive finishing processes.
While occasional exposure is unlikely to cause ill health, frequent exposure to hand-arm vibration – usually when in contact with a vibrating tool or work process is a regular part of an operator’s job function – can lead to permanent health effects. 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 musculo-skeletal disease. According to the HSE, nearly 2 million people in the UK are at risk1.
HAVS can manifest itself in a variety of symptoms, the most common being Vibration White Finger (VWF), a whitening of the fingers which can lead to circulatory, joint, muscle and nerve problems.
The symptoms can also include any combination of tingling and numbness in the fingers; the employee being unable to feel properly; loss of strength in the hands; and the fingers going white (blanching) and becoming red and painful on recovery. The latter is especially the case in the cold and wet, and may only affect the fingertips at first.
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.
Measuring hand-arm vibration
Hand-arm vibration is usually measured with a sensor containing an accelerometer that is placed on the tool or part where the operator normally holds it, and is calculated by summing the accelerations measured along the three principal axes, with the daily exposure values calculated by standardising the exposure to an eight-hour period.
What does the law state? Hand-arm vibration is an area which is becoming subject to ever tighter legislation. 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. For equipment purchased before 6 July 2007, these vibration levels came into force from 6 July 2010, so is the Regulations are now applicable to all tools used in the workplace.
What factors impact on hand-arm vibration in abrasive processes?
Vibration is arguably the most significant safety issue with regard to abrasive product selection, and with six main factors impacting on the level of exposure that an employee using abrasive products will face. These are the tool, the abrasive consumable product, the support medium (if applicable), the substrate being treated, the application, and the actions of the employee. Optimising all of these will help to ensure that the level of vibration, and the time for which the employee is exposed, is minimised.
With regard to equipment, there have been many recent innovations by the leading manufacturers which make processes safer and less physically demanding – indeed going a long way towards ensuring equipment complies with the Regulations – with no negative impact on quality or productivity.
Tools supplied should always be low in vibration – the manufacturer’s information will assist the selection process – and appropriate to the task in question. Vibration levels between different tools can vary enormously. Once again, the manufacturer or distributor can advise here, but for a true value of the vibration level then 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. However, in some applications, 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.
Due to significant advances in coated abrasive consumable 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. These resin-bonded products with their thin, flexible construction, offer better balance characteristics than grinding discs and so can significantly reduce vibration.
The latest designs contain multiple layers of abrasive, with each structure exposing a new layer as it wears down. This means wear is more even, reducing the pressure that the operator needs to apply, and can even cut process steps, so lowering the time spent at the abrasive stage and operator exposure to vibration. Even this has been taken one step further over the past 12 months with the introduction of a mineral formed of precisely-shaped, uniformly-sized and vertically-oriented triangles of ceramic aluminium oxide, available on both belts and discs. These self-sharpening triangles are designed to fracture as they wear, continuously forming new, super-sharp points and edges that slice cleanly through metal, rather than gouging or ploughing. As well as an abrasive disc, a backing pad will typically be fixed to the machine. A backing pad which 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, placing greater physical demands on the operator, and whose 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 in reducing 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, delivering a faster cut and more consistent finish, so providing a more rapid, lower-vibration alternative. Indeed, 3M’s own research shows that grinding wheels with the same backstand can create well over 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 contribute further to vibration reductions.
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 the area of tools, consumables and backing pads.
Finally, the operator should be trained in the risks from HAV and how to avoid them; the importance of correct operation and maintenance of equipment; and arrangements for health surveillance and their duty to co-operate.
What do employers need to do?
The 2005 Regulations require employers to focus on the elimination or control of vibration exposure. The most effective means of achieving this is to seek new or alternative work methods which eliminate or reduce exposure.
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 asking employees if they have any of the HAVS symptoms described above and whether the equipment they are using produces high levels of vibration or uncomfortable strains 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 also be noted.
Activities should then be grouped into high, medium or low risk – high being above 5.0 m/s2; medium being above 2.5 m/s2, and low being below 2.5 m/s2. Existing vibration data can be used to confirm which group each process should be assigned to. However, the preferred 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 then 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 has been acquired and collated, it 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 which requires the input of vibration magnitude and exposure duration. It then automatically calculates the partial exposure and total exposure shown in both m/s² A(8). It also calculates the number of exposure points per hour, the time which 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 which are high in vibration and which therefore need to be addressed to reduce the risk to operators.
Any control actions put into place as 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 to know if the first symptoms of HAVS have started to develop and will also highlight how well measures that have been put in place to control hand-arm vibration are working.
Further information can be found on the HSE website at www.hse.gov.uk. Further information on abrasive solutions from 3M can be found at www.3M.co.uk/abrasives.