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Slip resistance and the UK Slip Resistance Research Group
223
Safety Science, 14 (1991) 223-229 Elsevier
Slip resistance and the UK Slip Resistance Research Group G.A. Kime April Dene, 128 Andlers Ash Road, Liss, Hants GU33 7LS, UK
Received 14 March 1991; accepted 22 May 1991
Abstract Kime, G.A., 1991. Slip resistance 14: 213-219.
and the UK Slip Resistance
Research
Group. Safety Science,
This paper highlights some of the problems concerned with slipping accidents and gives the history of the UK Slip Resistance Group and the use of dynamic mechanical thermal analysis (DMTA) to characterise the various slidder rubbers. Discussion concerning the theory of slipping and the influences other than the coefficient of friction that make up slip resistance, and the clarification of the parameters for a safe floor surface are presented.
1. Introduction The problem of slipping accidents is still with us (Kime, 1986) and this paper summarises current work being carried out in the UK. It is abundantly clear that slipping accidents have not reduced since the HSE wrote their book “Watch Your Step” (HSE, 1985) and although it is obvious that friction plays a significant part in slipping there are other significant factors such as surface roughness, the presence of any lubricant and additionally, movement of the whole body. Slip resistance, a property related to friction, needs to be defined in terms of a combination of these factors. Falling accidents still represent the largest single category of industrial accidents in the UK, and a major proportion of these are due to slipping. They not only cause injury but result in very large financial penalties. Manning (1988) has for a number of years indicated that the problem is not as straightforward as at present represented by the HSE report “Watch your step” (HSE, 1985 ). The situation regarding slipping accidents is even worse than at present envisaged, for if the first event of any accident was recorded as the cause of accident, and not as now reported with the second or sometimes the third events being the cause of the accident a different picture would emerge. For example, “slipped and caught hand in machine” is currently reported as a machine accident and not a slipping accident and “slipped and fell on hot stove” becomes
0925-7535/91/$03.50
0 1991 Elsevier Science Publishers
B.V. All rights reserved.
224
a burn accident and not a slipping one. Viewed in this way the ability to measure whether or not a floor surface is a potential slipping hazard acquires an even greater importance. In spite of the accident statistics, research into the causes of slipping accidents does not rank very highly in public concern; consequently there is considerable apathy to the problem. However, its importance may be judged from work carried out by Manning (1988) of the Ford Motor Co. whose recent work has confirmed earlier research which showed that slipping is the most numerous first event. Many machine accidents had their origin in an initial slipping accident. Looked at in this way slipping and the other underfoot accidents cause a disproportionate amount of disability. Manning suggests that in the future the amount of disability caused by accidents should be related to first events to provide information on causes of disability. If this information were used to educate workers, management and the general public on the hazards of untidy, uneven and slippery floor surfaces, then Manning believes that a considerable improvement in accident prevention could be achieved.
2. UK Slip resistance research group A meeting held at Rapra Technology Ltd, Shawbury, Shrewsbury, on 18 September 1984 brought together interested scientists, engineers, research associations, flooring manufacturers, government departments, and international researchers, all of whom were interested in slipping accidents. It was established at this inaugural meeting that the commonly accepted methods of measuring slip resistance do not always agree with one another and do not always correlate well with experience. Consequently, there is some doubt about the validity of the safety criteria which are sometimes applied. Following this meeting the UK Slip Resistance Research Group was set up to address this problem of test methods and the results from the first phase of the research have been published by James (1985). The first aim of the Group was to develop a standard rubber that could be used in all the various floor friction measuring equipments available in the UK. The main floor friction measuring equipments in the UK are the “Tortus”, designed and developed by the British Ceramics Research Association (BCRA), the portable skid-resistance tester developed by the Transport and Road Research Laboratory (TRRL) and also the ramp test from the Shoe & Allied Trades Research Association (SATRA) . Each of these equipments use rubber compounds specified by their designers. Dynamic mechanical thermal analysis (DMTA) was used to characterise the various rubbers and this is the subject of the following section.
225 Dynamic Mechanical Thermal Analysis. Of several devices for characterising rubber used at Rapra, one is made by Polymer Laboratories a technique known as dynamic mechanical thermal analysis (DMTA) (Wetton et al., 1986). A small strip of rubber is clamped at the two ends to two fixed points and the centre is clamped to an arm which can be vibrated at one of a number of preset frequencies. The whole assembly is contained in a small environmental chamber the temperature of which can be arranged over a wide range. Both the stress and the strain can be monitored continuously. When rubber is vibrated, energy is lost as heat. This energy loss results from the stress and strain in the rubber being out of phase with each other. The greater this phase difference the greater the energy loss. In the DMTA test the energy loss is measured as the tangent of the phase angle between stress and strain (tan 6). The stress may be resolved into two components, one of which is in phase with the strain and the other 90” out of phase. The ratio of the in-phase component of stress to the strain is called the in-phase modulus and it is this,
I
I
TIME (ANGULAR DISPLACEMENT)
IN PHASE STRESS
Fig. 1. Diagram showing the phase relationship between stress and strain. The ratio of the inphase stress to the strain is called the elastic modulus, E’ . The ratio of the out-phase stress to the strain is called the viscous modulus, E’ . tan 6= E" /E’ .
226
together with tan 6, which is recorded in the DMTA test. Fig. 1 diametrically shows the above theory. Using the information obtained from a DMTA test of the Tortus rubber, numerous rubber compounds were mixed and samples distributed to members of the group for evaluation in Tortus, TRRL, RAMP and SATRA TESTER, on various standard floor surfaces. The result of these tests and the DMTA tests were considered and a standard rubber was selected. This is now marketed as RAPRA Four S rubber. In parallel with this work all the Tortus readings obtained by members of the Group were supplied for computer analysis, the results of which appear in Kime (1986). The Group also prepared standard test methods for both the Tortus and the TRRL, instruments for consideration by the British Standards Institution and other standardising authorities. More recently the group has considered the development of a completely new piece of apparatus to evaluate the influence of the time that is involved when slipping occurs (James, 1989).
3. Theory of slipping In a previous paper, Kime (1986) discussed the influences of the coefficient of friction on slipping and showed that dynamic friction is the controlling parameter in the most dangerous situation. (This of course only deals with the pure friction involved in slipping and does not account for any other factors.) Dynamic friction is the coefficient of friction that is related to the force
V
Fig. 2.
V
SIN I3
V
V
SIN
a
227
required to keep a foot sliding at a constant speed without the aid of lubrication or loose objects (i.e. a banana skin). As soon as the heel touches the ground when walking, the speed of the foot must by very quickly stopped, or heel slip occurs and an accident can happen. The coefficient of friction is therefore an important factor at this moment. It is clear that for the same velocity of impact of the foot to floor the size of steps taken have a big influence, and it is shown in Fig. 2 that the component of speed parallel to the floor (Vcos a) increases with longer steps (V cos /3). By instinct, we always take smaller steps when walking on slippery surfaces. When liquids are present these cases are somewhat different and Proctor and Coleman (1988)state that there are then hydrodynamic squeeze film factors concerning slipping which are influenced by surface roughness. Proctor and Coleman further explain that when testing wet floors, there are considerable differences between the results obtained by different test methods due to differences in hydrodynamic effects that occur. This is but one of the factors affecting measurements and illustrates the fact that coefficient of friction alone is not sufficient to define slip resistance. This is a concept worth of further discussion.
4. Slip resistance Strandberg (1983) is of the view that when testing for “slip resistance” or “slipping” all of the following factors play an important part. (i) Contact time and normal force time so that surface roughness pattern and drainage capabilities are involved. (ii) Foot angle - for testing the most critical part of the shoe. (iii) Contact force application point at the shoe. (iv) Vertical force - for correct pressure in the contact area. (v) Sliding velocity - for correct dynamic friction forces. Many researchers in the field of floor and shoe materials talk about slip resistance and then proceed to use an instrument that only measures the coefficient of friction (COF). Andres and Chaffin (1985) gave a good analysis of the equipment used, but the majority of these pieces of equipment give only a single point measurement of COF. Wilson and Perkins (1985) make two recommendations that indicate that a single value of the COF is not only component of slip resistance, namely: “3. A single measurement of Friction is not adequate to describe Slip Resistance. Both state and dynamic friction are important, but specific relevance may depend on surface and mode of slip. 4. Assessment of Slip Resistance of shoes can depend on the floor surface used.” It will at once be apparent that the reverse of this is also true, namely that
228
the assessment of the slip resistance of a floor depends on the slider used to represent the shoe. One of the UK Slip Resistance Group members, Proctor using the theory for hydrodynamic squeeze films, has concluded that one other factor involved in slip resistance, along with COF is that of surface roughness. James (1983) introduces another component, that can go towards evaluating slip resistance, namely the time available for recovery should a slip occur. This is related to stride length, and James suggests allowing for a maximum stride length of 90 cm when making assessments for slip resistance. We therefore have to consider the factors which influence slip resistance, namely dynamic COF, sufficient for a step length up to 90 cm, surface roughness, and time to slide a certain critical distance. It it were possible to design a piece of equipment that encompasses all the above factors, the problem of evaluating slip resistance would be made easier, but I seriously think that for some time we will need to measure the factors separately and then estimate slip resistance from the separate parameters, e.g. dynamic COF, interaction with surface roughness and available time for recovery. Recent work of the Slip Resistance Group comprising walking tests on the ramp and laboratory figures obtained from Tortus and pendulum tests have shown quite clearly that no single test gives an unequivocal indication of “slip resistance”. The results must be interpreted with care and the “break points” for each test must be related only to that test. For example the GCL figures obtained with the pendulum apparatus and quoted in GCL (1971) cannot be applied to any other test without modification. Indeed, the GCL figures, if they are valid at all, relate only to tests carried out using the TRRL rubber, and if any other rubber is used in the pendulum tester then different “break points” must be used. This is an important finding for it establishes beyond doubt that the recommendations of BS 5395 (1977-1985) giving friction values for flooring materials to be used on or near stairs, have no validity unless a test method and standard slider material are specified. At this time there are no legally approved limits applying to floors tested with the Tortus apparatus, the instrument favoured by most researchers. However, field experience leads us to believe that a dynamic coefficient of friction of 0.5 coupled with a measured peak to valley roughness of at least 20 pm represents a minimum requirement for most environments. Floors not meeting these criteria should be regarded as potentially hazardous. Acknowledgements
I wish to express my sincere thanks to David Ivan James the UK Slip Resistance Group project leader for all his help and assistance with this paper. Without his criticisms and suggestions, completion would have not been possible.
229
References Andres, R.O., Chaffin, D.B., 1985. Ergonomic analysis of slip-resistance measurement devices. Ergonomics 28: 1065. BS 5395 Pt. 1-3, 1977-1985. Stairs, Ladders & Walkways. GLC, 1971. GLC Architects Dept, Slip Resistance of Floors. Item 5 in Development and Materials Bulletin, No. 43 (2nd Series). HSE, 1985. Watch your step, prevention of slipping, tripping & falling accidents at work. HMSO. James, D.J., 1983. Rubbers and plastics in shoes and flooring. The important of kinetic frictions. Ergonomics 26: 000-000. James, D.J., 1985. Slip resistance test for flooring two methods compared: Polymer testing. Elsevier Applied Science, Barking. James, D.J., 1989. Slip tests with variable load apparatus. RAPRA Technology Ltd. RTL/llGO. Kime, G.A., 1986. Slipping accidents and floor friction measurements. MOD Safety 3/49 BR 98803. Manning, D.P., 1988. Down to earth. Published Occupational Safety & Health. Proctor, T.D., Coleman, V., 1988. Slipping, tripping and falling accidents in Great Britain present and future. J. Occup. Accid. 9: 269-285. Strandberg, L., 1983. On accident analysis and slip-resistance measurement. Ergonomics 26: ll32. Wetton, R.E., Morton, R.M., Rowe, A.M., 1986. International Labators 70-81. Wilson, M.P., Perkins, P.J. 1985. Evaluation of a slip-resistance test for shoes. Ergonomics 28: 1081.
Safety Science, 14 (1991) 223-229 Elsevier
Slip resistance and the UK Slip Resistance Research Group G.A. Kime April Dene, 128 Andlers Ash Road, Liss, Hants GU33 7LS, UK
Received 14 March 1991; accepted 22 May 1991
Abstract Kime, G.A., 1991. Slip resistance 14: 213-219.
and the UK Slip Resistance
Research
Group. Safety Science,
This paper highlights some of the problems concerned with slipping accidents and gives the history of the UK Slip Resistance Group and the use of dynamic mechanical thermal analysis (DMTA) to characterise the various slidder rubbers. Discussion concerning the theory of slipping and the influences other than the coefficient of friction that make up slip resistance, and the clarification of the parameters for a safe floor surface are presented.
1. Introduction The problem of slipping accidents is still with us (Kime, 1986) and this paper summarises current work being carried out in the UK. It is abundantly clear that slipping accidents have not reduced since the HSE wrote their book “Watch Your Step” (HSE, 1985) and although it is obvious that friction plays a significant part in slipping there are other significant factors such as surface roughness, the presence of any lubricant and additionally, movement of the whole body. Slip resistance, a property related to friction, needs to be defined in terms of a combination of these factors. Falling accidents still represent the largest single category of industrial accidents in the UK, and a major proportion of these are due to slipping. They not only cause injury but result in very large financial penalties. Manning (1988) has for a number of years indicated that the problem is not as straightforward as at present represented by the HSE report “Watch your step” (HSE, 1985 ). The situation regarding slipping accidents is even worse than at present envisaged, for if the first event of any accident was recorded as the cause of accident, and not as now reported with the second or sometimes the third events being the cause of the accident a different picture would emerge. For example, “slipped and caught hand in machine” is currently reported as a machine accident and not a slipping accident and “slipped and fell on hot stove” becomes
0925-7535/91/$03.50
0 1991 Elsevier Science Publishers
B.V. All rights reserved.
224
a burn accident and not a slipping one. Viewed in this way the ability to measure whether or not a floor surface is a potential slipping hazard acquires an even greater importance. In spite of the accident statistics, research into the causes of slipping accidents does not rank very highly in public concern; consequently there is considerable apathy to the problem. However, its importance may be judged from work carried out by Manning (1988) of the Ford Motor Co. whose recent work has confirmed earlier research which showed that slipping is the most numerous first event. Many machine accidents had their origin in an initial slipping accident. Looked at in this way slipping and the other underfoot accidents cause a disproportionate amount of disability. Manning suggests that in the future the amount of disability caused by accidents should be related to first events to provide information on causes of disability. If this information were used to educate workers, management and the general public on the hazards of untidy, uneven and slippery floor surfaces, then Manning believes that a considerable improvement in accident prevention could be achieved.
2. UK Slip resistance research group A meeting held at Rapra Technology Ltd, Shawbury, Shrewsbury, on 18 September 1984 brought together interested scientists, engineers, research associations, flooring manufacturers, government departments, and international researchers, all of whom were interested in slipping accidents. It was established at this inaugural meeting that the commonly accepted methods of measuring slip resistance do not always agree with one another and do not always correlate well with experience. Consequently, there is some doubt about the validity of the safety criteria which are sometimes applied. Following this meeting the UK Slip Resistance Research Group was set up to address this problem of test methods and the results from the first phase of the research have been published by James (1985). The first aim of the Group was to develop a standard rubber that could be used in all the various floor friction measuring equipments available in the UK. The main floor friction measuring equipments in the UK are the “Tortus”, designed and developed by the British Ceramics Research Association (BCRA), the portable skid-resistance tester developed by the Transport and Road Research Laboratory (TRRL) and also the ramp test from the Shoe & Allied Trades Research Association (SATRA) . Each of these equipments use rubber compounds specified by their designers. Dynamic mechanical thermal analysis (DMTA) was used to characterise the various rubbers and this is the subject of the following section.
225 Dynamic Mechanical Thermal Analysis. Of several devices for characterising rubber used at Rapra, one is made by Polymer Laboratories a technique known as dynamic mechanical thermal analysis (DMTA) (Wetton et al., 1986). A small strip of rubber is clamped at the two ends to two fixed points and the centre is clamped to an arm which can be vibrated at one of a number of preset frequencies. The whole assembly is contained in a small environmental chamber the temperature of which can be arranged over a wide range. Both the stress and the strain can be monitored continuously. When rubber is vibrated, energy is lost as heat. This energy loss results from the stress and strain in the rubber being out of phase with each other. The greater this phase difference the greater the energy loss. In the DMTA test the energy loss is measured as the tangent of the phase angle between stress and strain (tan 6). The stress may be resolved into two components, one of which is in phase with the strain and the other 90” out of phase. The ratio of the in-phase component of stress to the strain is called the in-phase modulus and it is this,
I
I
TIME (ANGULAR DISPLACEMENT)
IN PHASE STRESS
Fig. 1. Diagram showing the phase relationship between stress and strain. The ratio of the inphase stress to the strain is called the elastic modulus, E’ . The ratio of the out-phase stress to the strain is called the viscous modulus, E’ . tan 6= E" /E’ .
226
together with tan 6, which is recorded in the DMTA test. Fig. 1 diametrically shows the above theory. Using the information obtained from a DMTA test of the Tortus rubber, numerous rubber compounds were mixed and samples distributed to members of the group for evaluation in Tortus, TRRL, RAMP and SATRA TESTER, on various standard floor surfaces. The result of these tests and the DMTA tests were considered and a standard rubber was selected. This is now marketed as RAPRA Four S rubber. In parallel with this work all the Tortus readings obtained by members of the Group were supplied for computer analysis, the results of which appear in Kime (1986). The Group also prepared standard test methods for both the Tortus and the TRRL, instruments for consideration by the British Standards Institution and other standardising authorities. More recently the group has considered the development of a completely new piece of apparatus to evaluate the influence of the time that is involved when slipping occurs (James, 1989).
3. Theory of slipping In a previous paper, Kime (1986) discussed the influences of the coefficient of friction on slipping and showed that dynamic friction is the controlling parameter in the most dangerous situation. (This of course only deals with the pure friction involved in slipping and does not account for any other factors.) Dynamic friction is the coefficient of friction that is related to the force
V
Fig. 2.
V
SIN I3
V
V
SIN
a
227
required to keep a foot sliding at a constant speed without the aid of lubrication or loose objects (i.e. a banana skin). As soon as the heel touches the ground when walking, the speed of the foot must by very quickly stopped, or heel slip occurs and an accident can happen. The coefficient of friction is therefore an important factor at this moment. It is clear that for the same velocity of impact of the foot to floor the size of steps taken have a big influence, and it is shown in Fig. 2 that the component of speed parallel to the floor (Vcos a) increases with longer steps (V cos /3). By instinct, we always take smaller steps when walking on slippery surfaces. When liquids are present these cases are somewhat different and Proctor and Coleman (1988)state that there are then hydrodynamic squeeze film factors concerning slipping which are influenced by surface roughness. Proctor and Coleman further explain that when testing wet floors, there are considerable differences between the results obtained by different test methods due to differences in hydrodynamic effects that occur. This is but one of the factors affecting measurements and illustrates the fact that coefficient of friction alone is not sufficient to define slip resistance. This is a concept worth of further discussion.
4. Slip resistance Strandberg (1983) is of the view that when testing for “slip resistance” or “slipping” all of the following factors play an important part. (i) Contact time and normal force time so that surface roughness pattern and drainage capabilities are involved. (ii) Foot angle - for testing the most critical part of the shoe. (iii) Contact force application point at the shoe. (iv) Vertical force - for correct pressure in the contact area. (v) Sliding velocity - for correct dynamic friction forces. Many researchers in the field of floor and shoe materials talk about slip resistance and then proceed to use an instrument that only measures the coefficient of friction (COF). Andres and Chaffin (1985) gave a good analysis of the equipment used, but the majority of these pieces of equipment give only a single point measurement of COF. Wilson and Perkins (1985) make two recommendations that indicate that a single value of the COF is not only component of slip resistance, namely: “3. A single measurement of Friction is not adequate to describe Slip Resistance. Both state and dynamic friction are important, but specific relevance may depend on surface and mode of slip. 4. Assessment of Slip Resistance of shoes can depend on the floor surface used.” It will at once be apparent that the reverse of this is also true, namely that
228
the assessment of the slip resistance of a floor depends on the slider used to represent the shoe. One of the UK Slip Resistance Group members, Proctor using the theory for hydrodynamic squeeze films, has concluded that one other factor involved in slip resistance, along with COF is that of surface roughness. James (1983) introduces another component, that can go towards evaluating slip resistance, namely the time available for recovery should a slip occur. This is related to stride length, and James suggests allowing for a maximum stride length of 90 cm when making assessments for slip resistance. We therefore have to consider the factors which influence slip resistance, namely dynamic COF, sufficient for a step length up to 90 cm, surface roughness, and time to slide a certain critical distance. It it were possible to design a piece of equipment that encompasses all the above factors, the problem of evaluating slip resistance would be made easier, but I seriously think that for some time we will need to measure the factors separately and then estimate slip resistance from the separate parameters, e.g. dynamic COF, interaction with surface roughness and available time for recovery. Recent work of the Slip Resistance Group comprising walking tests on the ramp and laboratory figures obtained from Tortus and pendulum tests have shown quite clearly that no single test gives an unequivocal indication of “slip resistance”. The results must be interpreted with care and the “break points” for each test must be related only to that test. For example the GCL figures obtained with the pendulum apparatus and quoted in GCL (1971) cannot be applied to any other test without modification. Indeed, the GCL figures, if they are valid at all, relate only to tests carried out using the TRRL rubber, and if any other rubber is used in the pendulum tester then different “break points” must be used. This is an important finding for it establishes beyond doubt that the recommendations of BS 5395 (1977-1985) giving friction values for flooring materials to be used on or near stairs, have no validity unless a test method and standard slider material are specified. At this time there are no legally approved limits applying to floors tested with the Tortus apparatus, the instrument favoured by most researchers. However, field experience leads us to believe that a dynamic coefficient of friction of 0.5 coupled with a measured peak to valley roughness of at least 20 pm represents a minimum requirement for most environments. Floors not meeting these criteria should be regarded as potentially hazardous. Acknowledgements
I wish to express my sincere thanks to David Ivan James the UK Slip Resistance Group project leader for all his help and assistance with this paper. Without his criticisms and suggestions, completion would have not been possible.
229
References Andres, R.O., Chaffin, D.B., 1985. Ergonomic analysis of slip-resistance measurement devices. Ergonomics 28: 1065. BS 5395 Pt. 1-3, 1977-1985. Stairs, Ladders & Walkways. GLC, 1971. GLC Architects Dept, Slip Resistance of Floors. Item 5 in Development and Materials Bulletin, No. 43 (2nd Series). HSE, 1985. Watch your step, prevention of slipping, tripping & falling accidents at work. HMSO. James, D.J., 1983. Rubbers and plastics in shoes and flooring. The important of kinetic frictions. Ergonomics 26: 000-000. James, D.J., 1985. Slip resistance test for flooring two methods compared: Polymer testing. Elsevier Applied Science, Barking. James, D.J., 1989. Slip tests with variable load apparatus. RAPRA Technology Ltd. RTL/llGO. Kime, G.A., 1986. Slipping accidents and floor friction measurements. MOD Safety 3/49 BR 98803. Manning, D.P., 1988. Down to earth. Published Occupational Safety & Health. Proctor, T.D., Coleman, V., 1988. Slipping, tripping and falling accidents in Great Britain present and future. J. Occup. Accid. 9: 269-285. Strandberg, L., 1983. On accident analysis and slip-resistance measurement. Ergonomics 26: ll32. Wetton, R.E., Morton, R.M., Rowe, A.M., 1986. International Labators 70-81. Wilson, M.P., Perkins, P.J. 1985. Evaluation of a slip-resistance test for shoes. Ergonomics 28: 1081.