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Steps To Safety

Published: 04th Sep 2013 in OSA Magazine

Slips and trips – a worldwide safety issue

Slips, trips and falls are one of the main causes of workplace injuries. Steve Rose, general manager of SATRA, China, looks at the importance of assessing footwear and flooring for slip resistance.

There is evidence that in industrialised countries nearly 40% of workplace injuries are caused by slipping and tripping accidents. Furthermore, in the general public, more than half of all reported major injuries are due to slips and trips. More than half of all footwear safety related complaints also concern slipping and tripping.

Increasing legislation is forcing manufacturers, retailers, employers and providers of public facilities to pay much greater attention to slip risk assessment and the provision of footwear and floorings with improved slip resistance. In order to be able to do this, suppliers must have a reliable method to quantify the slip resistance or coefficient of friction of their footwear or floorings. 

Slip resistance versus friction

What do we mean by ‘slip resistance’ and ‘coefficient of friction’ (CoF)? These two terms are often used synonymously when in fact they are different.

Slip resistance

The slip resistance of footwear is the ability to minimise the risk of a wearer slipping while undertaking normal activities, e.g. walking or running in the chosen environment such as the home, workplace or a leisure area. Similarly, the slip resistance of floorings is their ability to minimise the risk of wearers slipping in their normal footwear under reasonably foreseeable circumstances, e.g. walking or running on new or worn, dry or wet floors.

Slip resistance assessment has to take account of many product and user related factors. In practical pedestrian situations there are numerous physiological, psychological and environmental factors which can influence the risk of slipping accidents. For example, age, mobility, sight and gait of the individual, distractions, carrying baggage, changes of surface, obstacles, incorrect maintenance of the floor, unexpected spillage of greasy substances and so on. (See Box 1).

Coefficient of friction (CoF)

A CoF on the other hand is a specific laboratory measurement, using a test instrument, of the forces generated between a shoe soling and floor surface when slip is made to occur under a defined set of conditions. Different tests and different conditions will give different CoF values. (See Box 2).

Coefficients of friction are used as laboratory indicators of slip resistance but they do not provide the whole answer. If the CoF, measured by appropriate means, is above the level required for safe walking and, if the shoe soling or floor is of suitable material and design for the intended purpose, then it can be expected that the slip resistance properties will be satisfactory. In general, the higher the CoF the lower will be the risk of slipping, but at no point is it safe to say that slip won’t occur.

It is important to realise that one CoF cannot necessarily be predicted from another – a high dry CoF, for instance, does not necessarily indicate a high wet CoF; indeed, a low wet result is quite possible. It should also be noted that excessively high friction might cause problems of stumbling so friction should not be too high.

Despite all these complexities, in today’s world of standards, product liability legislation and compensation litigation, there is a practical need for means of product testing to ensure that at least at the design and production stage, footwear and flooring products meet minimum performance requirements. An understanding of factors influencing CoF and slip resistance, and an appreciation of the limitations of test methods is important. 

Soling materials

Many rubbers, PVCs (especially blends) and PUs can all give high CoF values. Hard materials – typically top-piece grades – and some types of unit with a lacquered finish give relatively low results and indeed are associated with slip problems.

Certain types of very hard plastic are slippery in the dry and these include nylon and hard PVC. Paint or lacquer finishes commonly used on leather, resin rubber and PU can have low friction. Aside from this, most slip risk comes from surfaces which are covered with some contaminating substance, such as water, and this is where tread pattern design can have a major bearing on performance. 

Those materials capable of being moulded clearly enjoy an advantage here. Hardness is an important factor in determining friction. Soft, or low hardness materials nearly always offer the best dry friction but, taken to the extreme, very soft compounds can grip excessively. If the sole almost adheres to the floor there is a risk of tripping, stumbling or wrenching of knee joints.

Good friction characteristics for dry underfoot conditions may not follow through when conditions are wet and improving friction in one condition does not necessarily result in a corresponding improvement in the other. This is often a source of major trouble. Soft materials can offer excellent wet friction but only if the tread design is good. Poor tread designs can dramatically reduce wet friction despite the perception of good performance of the same sole in the dry.

Moderate hardness usually offers the best compromise between dry and wet friction. While moderating the hardness may limit the dry friction, it will still be at a good level, not excessive, and wet friction is likely to be a higher percentage of it and less sensitive to pattern design and wear. Moderate hardness therefore offers a good balance between dry and wet performance and between new and worn sole treads.

Tread pattern design

Good tread pattern design is critical to slip resistance performance on wet surfaces. As with car tyres, the tread should be able to cut through and sweep away liquid contaminants to create an almost dry condition between the tread cleats and the floor.

Surface texture should not be seen as a substitute for tread pattern. Texture in the main provides little or no cutting edges and its greatest effect is to reduce contact area between the sole and floor. Generally, texture has the effect of reducing friction in both dry and wet conditions.

Soles for vigorous use and demanding wear conditions require the most careful attention to tread design. This includes sports shoes, military and hiking/trekking footwear, sailing shoes and industrial footwear for use in slippery conditions. Everyday shoes for ordinary walking conditions will also need care in design, especially where soft sole materials are being employed.

The scale and form of the tread pattern will depend on the purpose of the footwear, be it for use on manmade surfaces or natural/uneven or soft surfaces, outdoor or indoor, for walking or some other specific activity. 

In a significant proportion of footwear the main risk of slipping is on smooth surfaces, probably manmade, which are coated with water or another lubricant. In such situations the purpose of the tread pattern is simply to expel the liquid from the interacting surfaces of sole and floor so that the contact returns to a virtually dry state and can then benefit from the available dry friction. 

The best frictional contact comes from one smooth surface upon another, but large smooth surfaces will trap a film of liquid and promote dangerous aquaplaning. It is therefore necessary to provide channels in the sole at regular intervals into which the liquid can be diverted. These channels then require an unbroken route to the edge of the sole to allow the liquid to completely escape.

Roughness of surfaces

Roughness is the microscopic character of a surface. The surface roughness of floorings and footwear greatly influences CoF and slip resistance. Care needs be taken to differentiate between the terms roughness and texture. Roughness refers to the microscopic peaks and valleys of the surface and is measured in microns (µm). Texture refers to the much larger – macroscopic – surface contours or patterning, such as the fabric weave effect sometimes seen on solings, which would be measured in millimetres (mm).

Roughness is a principal factor determining the friction properties of floorings. Most of us will have experienced slippery, wet, smooth, highly polished floors such as marble or glazed ceramic tiles.

Roughness is also an important factor for solings. Rough, worn or abraded soles or top-pieces generally give higher CoF values than smooth ones and it is known that PVC can polish smooth in wear and become more slippery. The roughness of a soling will depend on the surfaces walked upon and the material type. The benefit of roughness is that the peaks project through the thin fluid film trapped between the shoe sole and floor on wet or oily surfaces, reducing the risk of aquaplaning.

Industrial footwear

One class of footwear with a very particular need for high slip resistance is that worn in industry. In testing industrial footwear we will tend to seek higher levels of performance on ordinary surfaces – to allow for pushing, pulling, carrying loads, operating on ramps – or tests will be made on more slippery types of underfoot surface.

In recent years, new standards have being developed and implemented, both test methods and associated specifications, and the European Community (EC) has been at the forefront of these. For manufacturers exporting to Europe, Personal Protective Equipment (PPE) footwear needs to be certified and type-approved for use according to Directive 89/686. To achieve this, a set of test methods (EN ISO 20344) and associated specifications (EN ISO 20345, 20346 and 20347) are available.

These have been amended since their introduction to include requirements for slip resistance. This requirement calls up an EN ISO method, EN ISO 13287. This is a machine-based method for measuring the coefficient of friction between whole footwear soles and test floorings and has been adapted from the SATRA slip resistance standard TM 144, which is used widely around the world. Different modes of contact between the footwear and floor are used and EN ISO 13287 also specifies different test surfaces. Three slip resistant ratings are possible: SRA, SRB or SRC (see Table 1), according to whether pass levels are attained on the different test surfaces.

How should tests be used?

The answer to this question is with great caution. Too much reliance should not be placed on one or two CoF results for the reasons discussed earlier – no single test should be taken as an absolute measure of the performance of a given soling or floor.

Furthermore, the performance of one shoe on a particular flooring is not necessarily an indicator of how that shoe will perform on other floorings nor, indeed, of how other shoes will perform on that floor. Test results must be interpreted in the light of experience and specific performance requirements. A knowledge of the environment in which the footwear is expected to perform is also key in determining whether the measured CoF will lead to adequate slip resistance.

Reducing the injury statistics

It will never be possible to completely eliminate all slipping accidents, but we now have the testing methodology and sufficient knowledge to be able to make a considerable impact upon the problem from the point of view of the footwear. SATRA can help identify the most suitable materials and tread designs and prove these with product testing before the footwear goes to market.

Even a satisfactory set of test results does not guarantee that the sole or top-piece will never slip under any circumstances – clearly the floor itself and many other factors come into play.

Achieving the recommended values, however, shows that the shoe is playing its part in reducing the risk. 

Footwear need not be labelled as being ‘slip resistant’ – this should go without saying. If everyday footwear is marked ‘slip resistant’, however, then we suggest that it achieves a margin of performance over the bare minimum value needed for safe walking.

Footwear should never be marked ‘non-slip’ or ‘anti-slip’ as it will never be possible to guarantee that footwear will not slip under all conditions encountered in wear.

Summary

Slip resistance assessment is a very complex subject in which measurement of friction coefficients is only one part. Footwear and floorings have to be considered together and many other factors taken into account. 

Sight should not be lost of the primary goal which is to improve safety. To achieve this all the parties concerned need to take reasonable steps to guard against foreseeable dangers. Testing and assessment is an important part of this.

The SATRA test method and test machine combination TM144/STM 603 has been developed to evaluate footwear slip resistance and is used throughout the world to test the suitability of products under a variety of conditions. Although our machine-based test cannot simulate every situation encountered in wear, it does provide a benchmark from which to evaluate products. Even a satisfactory set of results, however, will not guarantee that the sole or top-piece of a shoe will never slip under any circumstances.

Published: 04th Sep 2013 in OSA Magazine

Author


Steve Rose


Steve Rose is general manager of SATRA’s China operations and is based in Dongguan. He joined SATRA in 1978 and has considerable experience and expertise in footwear and materials research and testing.

SATRA in China
SATRA’s China laboratory in Dongguan is now fully operational allowing SATRA customers and members in the region to access its safety footwear testing services more easily. The laboratory can accept safety, protective and occupational footwear for testing to the European EN ISO 20345/6/7 suite of standards for both members and other organisations, as part of a CE marking application. 

SATRA members can also benefit from a range of other global safety footwear related tests including slip resistance testing against Canadian standard CSA Z195-09, and the newly developed US standard ASTM F2913.

SATRA’s Dongguan facility can also accept safety footwear for testing to other standards, working in conjunction with SATRA UK for specialised testing such as chainsaw resistance or flame resistance. 

As well as the purpose-built safety footwear testing laboratory, SATRA China also houses a new suite of offices, training and lecture rooms.


Steve Rose

Website:
http://www.satra.cn

Email:
info@satrafe.com

Phone:
+86 (0) 769 2288 8020

info@satrafe.com
http://www.satra.cn
+86 (0) 769 2288 8020

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