A method of ranking the grip of industrial footwear on water wet, oily and icy surfaces

Safety Science, 14 (1991) 1-12 Elsevier

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A method of ranking the grip of industrial footwear on water wet, oily and icy surfaces D .P . Manning, C . Jones and M . Bruce Ford Motor Company Limited, Halewood, Liverpool L24 9LE, UK Received 10 March 1990 ; accepted 17 July 1990

Abstract Manning, D .P., Jones, C . and Bruce, M ., 1991 . A method of ranking the grip of industrial footwear on water wet, oily and icy surfaces . Safety Science, 14: 1-12 . A recently invented walking traction test method was chosen to measure the coefficient of friction (c .o.f.) of thirteen pairs of discarded working footwear. Floor surfaces were lubricated with a water-based wetting agent and four grades of mineral oil . The footwear were also tested on wet and dry ice . The maximum c .o.f. attained prior to each slip was recorded whilst walking forwards and again whilst walking backwards on the heels . Results for the thirteen footwear samples were ranked in descending order of c .o.f. for each floor and lubricant combination, and rank orders for the seven most slippery surfaces were compared . Kendall's coefficient of concordance W=0 .73 for walking forwards and 0 .79 for walking backwards ; P<0 .001 . Rank orders for each of the seven surfaces were also compared with mean rank orders. Correlation coefficients r, all reached or exceeded 0 .92 (P<0 .005) on rough plastic and stainless steel coated in 121 .4 or 88.2 cSt oil (at 17 .5°C) and on water lubricated glazed white tiles . On dry ice, r=0.49 ; P<0 .05. The correlation between mean rank orders of footwear on forward and backward walking was 0 .95 ; P< 0 .005 . The c.o.f. recorded whilst walking backwards was 37 .7°% lower than the forward walking c.0-f., supporting the hypothesis that dangerous slips are likely to occur on heel strike . There was also a significant correlation between roughness of soling and c .o .f . ; P< 0 .01 . This method of measuring c .o .f. is now being applied to eliminate the most slippery footwear with the long-term aim of selecting the safest solings for specific environments .

1 . Introduction A preceding paper (Manning et al ., 1990) described a new walking traction method of measuring grip of shoes on slippery floors . Surface roughness of new solings and floors and coefficients of friction (c.o.f .) were measured initially and repeated, following successive abrasion treatments with increasingly coarse grit to achieve roughness . The important contribution of soling roughness to c .o.f. was demonstrated . The first paper published from this laboratory (Manning et al ., 1983) reElsevier Science Publishers B.V .

ported the "improved slip resistance on oil from surface roughness of footwear". At that time, roughness was assessed subjectively by a team of observers and c .o.f. was measured by standing on an oily steel plate which gradually tilted until the feet slipped . Following the successful trial of the new walking traction test, a decision was taken to measure c .o.f. of a selection of worn working boots and shoes and also to attempt to objectively measure surface roughness of the soles and heels . 2 . Materials and method To measure c .o.f., the subject walked on a slippery surface, pulling against a set of springs anchored to the wall, until the feet slipped (Fig . 1) . A load cell interposed between the subject and the springs measured the maximum force in kilograms developed before the feet slipped. Measurements were displayed on a portable load indicator and paper chart recorder . The subject was protected from injury by a fall arrest harness and two handles suspended from a pulley which moved freely on an overhead rail . When his feet slipped, the sub-

Fig . 1 . The subject walking backwards on the heels, pulling against a spring and supported by a fall-arrest harness . The load cell is positioned between the belt and the springs .



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ject collapsed into the harness . Details of the method were described by Manning et al . (1990) . Thirteen footwear samples which had been worn in an oily environment were selected. Some had remnants of sole and heel patterns and others were badly worn on the point of the heel . One sample had been discarded after two days wear, because it was too slippery and another was worn for only four weeks due to termination of employment . The mean period of wear for the remaining eleven samples was 48 .2 weeks (range 15 to 63 weeks) . Each sample was tested on five water wet and two oily surfaces, the latter being repeated in succession with four oils having a range of viscosities from 7 .3 to 121 .4 centistokes (cSt) at 17 .5'C. With the exception of one slip-resistant rough plastic material coated with oil, these surface and lubricant combinations were the most slippery conditions found in tests of over sixty floor surfaces . The surfaces and roughness values are listed in Table 1 . The subject put on a sample of footwear after protecting the feet with polythene bags. He then walked forwards on the glazed white tiles lubricated by a squeegee application of 0 .01 % sodium dodecyl sulphate in water which ensured adequate wetting . The subject walked in short steps at a normal pace until tension in the springs resulted in a backwards slip . The measurement was repeated five times to obtain a mean c .o.f. On some surfaces, the feet did not slip at a pull of 30 kg which was the maximum the subject could generate (c.o.f. 0.42) . A stronger subject would be able to achieve higher maximum c .o.f. Daily variations in body weight were corrected to a constant 71 kg by carrying or shedding small iron weights . Following completion of the forward walking test, the surface was wiped clean and the lubricant reapplied by squeegee . The subject then walked backwards on the points of the heels recording the five c .o.f. readings (Fig. 1) . The angle between soling and the floor was approximately 15' . It was not possible to measure this accurately and it varied slightly with worn down heels . However, with practice, the inclination of the feet to the floor TABLE 1 The test surfaces Surfaces Rough plastic Stainless steel Smooth vinyl Smooth vinyl with floor polish Terrazzo Glazed wall tiles Ice

Roughness in microns (Rtm) Mean of ten measurements 19 .1 1 .2 11 .1 6 .0 6 .37 0 .44 Not recorded

was consistent . Variations in this angle were believed to be averaged by repeating the c.o.f. measurements five times for each of the fifteen floor and lubricant combinations . The maximum force which the subject could generate in this backwards walk was 20 kg (c.o.f. 0.28) . Again, a more muscular subject would generate a higher maximum c .o.f. On completion of tests on water wet white tiles, all samples were similarly assessed on the other water-lubricated surfaces . After all samples had been tested on water-lubricated surfaces, roughness was measured five times on the point of the heel and five times on the sole by each of two observers, using an electronic meter ; a diamond stylus moves 5 mm across the surface and gives a reading in microns (Harris and Shaw, 1988) . Hardness was also calculated by taking the mean of ten readings recorded by a Shore A durometer . Coefficient of friction measurements then continued on oily stainless steel and oily rough plastic, beginning with an application by squeegee of the thin 7.3 cSt oil. After all the footwear had been tested on this oil, they were wiped clean and again assessed on the 27 .4 cSt oil and then on the 88 .2 and 121 .4 cSt oil applied to the two surfaces . Following completion of the laboratory tests on water wet and oily surfaces, boots were cleaned with a water-based detergent and then taken to an ice skating rink . Coefficients of friction of all samples were recorded on dry ice and then on ice covered by a film of water applied by a garden watering can . In substitution for the fall arrest harness, a frame on wheels was used to protect the subject . Finally, the footwear samples were tested on the tilting oily steel plate (Manning et al ., 1983) . 3. Results A total of 1950 c .o.f. measurements were listed. On water-lubricated terrazzo and vinyl surfaces, including one coated with floor polish, nearly all samples reached maximum attainable c .o.f of 0 .42 on forward walking and 0 .28 on backward walking . These surfaces could not be used to detect slippery footwear . On water-lubricated stainless steel, more than half of the footwear samples also achieved maximum attainable c .o .f. On oily surfaces lubricated with the two least viscous oils, many samples reached maximum c .o.f. All the above surfaces were therefore excluded from the calculations . Mean c .o .f. results for walking forwards on the seven most discriminating surfaces are listed in Table 2 ; some samples consistently registered a higher c .o.f. on wet and oily surfaces, while others stayed at the lower end of the scale . 3.1 . Test surfaces The Kendall coefficients of concordance, W, for the results on the seven surfaces was 0 .73 for forward walking (X z -61 .32 for twelve degrees of free-

5 TABLE 2 Coefficient of friction results for thirteen footwear samples measured whilst walking forwards . Samples are identified by letters and ranked horizontally in descending order of c .o .f. A bar beneath a letter indicates that the sample reached maximum attainable c .o .f. Oil-lubricated Rough plastic 121 .4 cSt oil 0 .41 0 .40 K D 88 .2 cSt 0 .42 0 .42 D K

0 .33 F

0 .31 Gb

0 .29 Ga

0 .26 C

0 .25 E

0.24 M

0 .22 P

0.21 A

0 .19 B

0 .16 S

0 .15 I

0.39 Gb

0 .39 Ga

0 .36 F

0 .33 E

0 .31 M

0 .28 C

0 .25 P

0.25 A

0 .22 B

0 .17 S

0 .16 I

0 .29 F

0.25 Gb

0 .22 Ga

0.20

0 .20

M

C

0.20 E

0 .17 A

0 .16 P

0 .15 B

0.14 I

0 .13 S

0 .29 F

0.27 Gb

0 .25 Ga

0.25 E

0 .24 M

0.20 C

0 .17 A

0 .17 P

0 .16 B

0.12 I

0 .12 S

0 .41 K

0.40 D

0 .37 F

0.37 E

034 . C

0.32 A

0 .29 M

0 .28 P

0 .28 B

0.28 S

0 .25 I

0 .21 I

0 .20 K

0 .20 Gb

0 .20 C

0.19 E

0 .19 F

0 .19 A

0 .19 B

018 . P

0 .18

0 .17 D

0 .11

0 .20 F

0 .20 Gb

0.19 E

0 .15 D

0.14 Ga

0 .14 I

0.13 A

0 .13 P

0 .12 C

0 .11 B

0 .11

0 .07

M

S

Stainless steel 121 .4 cSt 0 .41 0 .37 K D 88 .2 cSt 0 .42 0 .40 K D

Water-lubricated White tiles 0 .42 0 .41 Ga Gb Ice Dry 0 .22 Ga Wet 0 .21 K

M

S

dom) and 0.79 for backward walking (X'= 66 .53 for twelve degrees of freedom) P<0.001 . Therefore, there is significant agreement between the rank orders of results on these seven surfaces. The mean c .o.f. results for each footwear sample on all seven surfaces were then compared with the rank order on each separate surface by calculating Spearman's rank correlation coefficient . Results are shown in Table 3 . 3 .2 . Forward and backward walking

The mean rank orders on the seven surfaces for forward and backwards walking were also compared by calculating Spearman's rank correlation coef-

6 TABLE 3 A comparison of the rank orders of mean c .o .f. values for thirteen footwear samples on seven surfaces . with those for individual surfaces Walking forwards lr)

Walking backwards (r)

Rough plastic 121 .4 cSt Oil 88.2 cSt

0 .94 0 .95

0 .96 0 .96

Stainless steel 121 .4 cSt Oil 88 .2 cSt

0 .95 0 .95

0 .92 0 .93

Water-lubricated White tiles

0 .93

0 .93

Ice Dry Wet

0 .49 0 .86

0 .77 0 .95

Oily floors

TABLE 4 Rank orders of footwear, roughness of soles and heels and hardness . (Rank order is based on the mean of all c .o .f. results on the seven surfaces) Walking forwards

Walking backwards

Rank order

C .o .l .

Roughness of sole

Hardness Shore A

Rank order

C .o .f .

Roughness of heel

K D F Gb Ga F C M A P B I S

0.35 0.33 0.29 0.29 0.28 0 .25 0 .23 0.22 0.21 0 .20 0 .19 017 0.15

41 .6 58 .0 35 .8 25 .9 28 .3 44 .9 25 .0 12 .9 20 .0 20 .0 17 .5 16 .2 27 .3

49 48 53 60 59 65 70 53 60 62 63 70 76

K D F Gb Ga E M P A B C 1 S

0 .26 0 .25 0 .23 0 .22 0 .21 0-17 0 .12 0 .12 0,1 0 .1 0 .1 0 .09 0 .07

56 .5 57 .6 39 .3 37 .6 30 .4 45 .7' 14 .5 23 .4 15 .7 17 .9 26 .3 17 .6 49 .3"

'See discussion .



C 4 I

_ .- .

0 .35

0,15 -

0 .1

-

0 .05

1

D

F

Gb

C;o

E

-

1

M

p

B

I

Fig . 2 . Mean c .o .f. of each footwear sample measured on seven surfaces . The upper curve represents walking forwards and the lower curve, walking backwards on the heels . Footwear samples are represented by letters on the X axis .

ficient; r=0 .95 ; P<0.005 (Table 4) . The c .o .f. measured whilst walking backwards on the heels is nearly always less than the c .o.f. for forward walking. This is illustrated in Fig . 2. The differences in c .o.f. between walking forwards and backwards were calculated for each sample on all seven surfaces . When maximum attainable c.o .f. was achieved, the result was excluded in order to obtain the real differences . Results are listed in Table 5 which reveals that the c .o.f. on walking backwards averages 37.7% less than for walking forwards. 3.3. Viscosity As viscosity of oil increased, c .o.f. fell . It was not possible to calculate the exact relationship between viscosity and c .o.f. because many samples reached maximum c .o.f. on floors coated with the 7 .3 and 27 .4 cSt oil and because the c.o.f. values were so low, but there appears to be an inverse relationship between viscosity and c .o.f. as shown in Fig . 3.

8 TABLE; 5 The c .o .f. measured whilst walking backwards on the heels was consistently lower than the co .f. measured whilst walking forwards . A comparison between a forward and backward c .o .f. was excluded if footwear did not slip Oily surfaces

Percentage' difference

Number of pairs included

Rough plastic 121 .4 cSt 88 .2 cSt 27 .4 cSt 7 .3 cSt

39 48 39 34

10 8

Stainless steel 121 .4 eSt SP .2 cSt 27 .4 cSl. 7 .3 cSt

53 40 40 42

11 11 10 7

Water lubricated surfaces E9oor polish on smooth vinyl Smooth vinyl Terrazzo Glazed white tiles Stainless steel

26 37 35

Ice Dry R'et

37 20 .5

4

0 0 1 8 1

13 13

Mean difference 37 .7%

3.4 . Roughness of solings

A comparison between the rank orders of roughness readings shown in Table 4 and rank orders of footwear shows that c .o.f. is significantly related to roughness of soling material (Spearman's rank correlation coefficient r=0 .701 for forward walking (P < 0 .01) and r=0 .562 for backward walking (P < 0 .025) ) . If sample S is excluded from the backward walking calculations, r=0 .527 (P-c0.005) . This is mentioned in the discussion . 3 .5 . Roughness of floors

Student's t test for matched samples, was applied to mean c .o.f. readings for each of the four grades of oil on rough plastic and stainless steel . When all results which reached maximum c .o.f. were excluded t=1 .7 (P<0 .5) . The test does reach significance if maximum attainable c .o.f. results are included but



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K

0

E

M

C

8

S

Fig. 3 . Mean c .o.f. results for thirteen footwear samples measured on stainless steel and rough plastic . The surfaces were coated with oil having viscosities at 17 .5°C of 7 .3 cSt (top curve), 27 .4 cSt, 88.2 cSt mid 121 .4 eSt, respectively. Any measurements which reached maximum attainable c .o .f. are excluded.

the conclusion is censored because the upper c .o .f. values cannot be measured by this method . A comparison between the results obtained by this walking traction test and the tilting oily steel plate showed that both methods ranked the samples, K, D, F, Gb and Ga, as the most slip-resistant and S as the most slippery sample ; r=0 .88 (Spearman's rank correlation ; P<0.005) .

4 . Discussion The consistent ranking of some samples at the high friction end of the scale and others at the low friction end is very encouraging. The highly significant correlations of rank orders are robust tests of the method due to the very small intervals in c .o .f. which determine rank order - usually the second decimal place of c.o .f. value . Also, the first five most slip-resistant samples were all formed from the same polymer, although produced by two separate manufacturers . This material had previously been found to have the best slip-resistance on oily surfaces and the fact that it also gave the best grip of the solings tested on water-lubricated surfaces, indicates that materials are available for good

to general purpose use . The soling material maintained a high c .o.f. when worn on either smooth or rough floor surfaces ; roughness was an inherent characteristic of the polymer . Three of these five were worn on smooth, oily floors (F, Gb and Ga) and the other two on abrasive oily floors . The rank order of samples would probably change if worn on different floors which altered roughness of solings but it is likely that the first five would remain at the top . The good correlation between rank orders for forward and backward walking confirms that the least slip-resistant footwear samples can be detected by this walking traction method and that it is unnecessary to routinely measure c .o.f. by walking backwards . Table 3 reveals that it is only necessary to measure c .o.f. whilst walking forwards on stainless steel or the rough plastic coated with 121 .4 or 88 .2 cSt viscous oil . The water-lubricated glazed white tiles would be a valid test surface for solings which are not oil resistant ; water-lubricated stainless steel, vinyl or terrazzo are not sufficiently slippery . All test surfaces, except the rough plastic, were the most slippery underfoot conditions found in a long series of trials and solings which grip well on the above specified surfaces should be satisfactory on all others but confirmatory tests on all available slippery surfaces would be prudent . It is gratifying that the original test rig with the tilting oily steel plate ranked the footwear samples in a similar order as the walking traction test, confirming the validility of earlier work . Measurement of c.o.f. on ice is difficult because of the very low values and the possibility of error, due to small surface irregularities of the ice . The ranking was not as consistent as on the best oily and water wet surfaces but the samples which provided the highest grip on other surfaces, were generally towards the upper ranking on ice . Forward walking on dry ice was the least representative rank order (Table 3) . Although the walking traction method of measuring c .o.f. cannot record high values, it does reveal those solings which slip below 0 .42 on walking forwards and 0.28 whilst walking backwards on the heels ; a stronger subject would he able to reach a higher c .o.f. Since 0 .4 is generally regarded as a safe c.o.f. for walking at a normal pace (James, 1983), the method can be used to select safe footwear; it measures c .o.f. prior to a slip during walking and can be repeated rapidly to obtain many readings . The reduction in c.o.f. of 37 .7% recorded during backward walking on the heels, supports the view that dangerous slips are most likely to occur on the heel strike (Perkins, 1978) . Only on wet ice, was there an exception to this statement; three of the most slip-resistant samples recorded a higher c .o.f. for backward walking on the heels than for walking forwards . Figure 2 shows that the percentage reduction of c .o.f. on the heels is much higher for the slippery footwear at the right hand end of the curve, than for the most slip-resistant samples . It was not possible to calculate the effect of viscosity but the progressive rise

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of c.o.f. with diminishing viscosity of the oily lubricant, supports the hypothesis that the heaviest, most viscous mineral oils on the floor are the most likely to cause a dangerous slip (Proctor and Coleman, 1988) . The rougher of the two oily surfaces always generated the highest c.o.f ., although the difference was not as large as one might have expected and was not statistically significant . The stainless steel roughness was only 1 .2 microns (very much less than the sole roughness) but the rough plastic measured 19 .1 microns . It appears that the electronic roughness meter is not entirely satisfactory for measuring shoe and floor roughness and in future, magnified photographs will be obtained to reveal the nature of surface roughness . The small contact area traversed by the diamond stylus of the roughness meter, causes inaccuracies in measuring surface roughness of profiled solings and when there are deep scratches and embedded grit. Nevertheless, the rough samples were significantly more slip-resistant than smooth solings, in keeping with earlier findings . Roughness of sample S was particularly difficult to assess because of a series of narrow ridges on the heel . Photomicrography showed that it was smooth by comparison with the most slip-resistant samples . Samples of currently available footwear are now being worn by volunteers and the solings will be tested at three monthly intervals, to determine the best available footwear for specific environments, with gradual elimination of the least slip-resistant solings and progression towards the safest possible footwear . It is probable that the safest solings will be selected from those which become rough and remain rough during wear . Acknowledgements We wish to thank Dr . S .E. Brill, Chief Medical Officer of Ford of Britain and Mr. T .D. Proctor of the Health and Safety Excutive Research and Laboratory Division for co-operation and permission to publish and Mr . T . Walsh who measured viscosity of the oils . This research was supported by a grant from the H&SE but the views expressed are those of the authors and are not necessarily those of the H&SE . References Harris, G .W ., Shaw, S .R ., 1988 . Slip-resistance of floors: Users' opinions. Tortus instrument readings and roughness measurement. J . Occup . Accid ., 9 : 287-298James, D .l ., 1983 . Rubbers and plastics in shoes and flooring : the importance of kinetic friction . Ergonomics, 26 (1) : 83-99, Manning, D.P ., Jones, C ., Bruce, M ., 1983 . Improved slip-resistance on oil from surface roughness of footwear. Rubber Chem . Technol ., 56 (4) : 703-717 . Manning, D .P ., Jones, C ., Bruce, M ., 1990 . Proof of shoe slip-resistance by a walking traction test . J . Occup . Accid., 12 : 255-270 .

12 Perkins, P .J ., 1978 . Measurement of slip between the shoe and ground during walking, In : C . Anderson and J . Senne (Eds . ), Walkway Surfaces : Measurement of slip-resistance . American Society for Testing and Materials, Philadelphia. ASTM STP 649 : 71-87 . Proctor, TD ., Coleman, V., 1988 . Slipping, tripping and falling accidents in Great Britain - present and future . J . Occup . Accid . . 9: 269-285 .