Upload
hoangdat
View
215
Download
0
Embed Size (px)
Citation preview
300A
1985
\57 h
Amt.
9
(g
;
\
\
t.53
F00twear Fricticn Assessed by
Walking Experiments
Lennart StrandbergNational Board of Occupational Safety and Health
Lars Hildeskcg and Anna-Lisa OttosonSwedish Road and Traf c Research Institute
(db Vag-00/) Sta tens va'g- och trafikinstitut (VTI) - 58 1 o 1 Linkc'ping
'IIIStItUtBt Swedish Road and Traffic Research Institute - 8-581 0 1 Linkc'ping Sweden
300A 1985
F00twear Friction Assessed by
Walking Experiments
Lennart StrandbergNational Board of Occupational Safety and Health
Lars Hildeskog and Anna-Lisa OttosonSwedish Road and Traf c Research Institute
Vag-06/7 Statens va'g- och trafikinstitut (VT/l - 58 1 0 1 Linkb'ping
'l St/tlltet Swedish Road and Traffic Research Institute - 8 587 0 1 Linkc'ping Sweden
Klintland Grafis-ka, Linképing 1986
Footwear Friction Assessed by Walking Experiments
by Lennart StrandbergNational Board of Occupational Safety and Health (AV)5 171 84 SOLNA Sweden
Lars Hildeskog and Anna Lisa OttosonSwedish Road and Traffic Research Institute (VTI)5-581 01 LINKOPING Sweden
PREFACE
This study was carried through with equipment and methods, originally
developed at the AV research department in cooperation between the
Associate Professors Anders Kjellberg, Hakan Lanshammar and Lennart
Strandberg. Since August 1983, Lennart Strandberg has been stationed at
the VTI, leading this and other accident research projects within the
occupational safety area.
Lars Hildeskog, now completing his studies for a Physician's degree, was
responsible for the organization and operation of the walking experi-
ments. Anna Lisa Ottosson, Research Engineer, contributed to solutions
of various problems within physics and engineering.
Adoption of Slipping and Falling research and development into the VTI
program became possible, thanks to the support from Professor Kare
Rumar during the planning and former stages of the project, which will
continue. Funds for another test series have been granted by the main
sponsor. The move, improvements, and reinstallation of laboratory
equipment (from the AV in Solna/Stockholm to the VTI in Linkoeping)
have beengoverned by the VTI Chief Designers, Matts Mattsson and Rolf
Svensson. The same tasks for the computer software were completed by
Electronics engineers Sten Lundstrom at the AV and Uno Ytterbom at
the VTI.
Various scientific problems have been solved with support from VTI
personnel, e.g. in mathematical statistics from Stig Danielsson, Chief
Researcher, in tribology from Kent Gustafsson and Evert Ohlsson, Chief
Researchers. Linguistic checks and word processing tasks were
performed by Christina Ruthger.
VTI REPORT 300A
TABLE OF CONTENTS Page
ABSTRACT I
REFERAT 11
SUMMARY III
1 BACKGROUND 1
2 METHOD 1 2
3 RESULTS 8
4 CONCLUSIONS 9
ACKNOWLEDGEMENTS 11
REFERENCES 12
VTI REPORT 300A
Footwear Friction Assessed by Walking Experiments
by Lennart StrandbergNational Board of Occupational Safety and Health (AV)5 171 84 SOLNA Sweden
Lars Hildeskog and Anna-Lisa OttosonSwedish Road and Traffic Research Institute (VTI)5-581 01 LINKOPING Sweden
ABSTRACT
The slip-resistance of 18 types of footwear (ToF) on three contaminated
floorings was assessed by experiments with 12 well-trained subjects,
walking in a triangular closed path as fast as possible.
From the lap _t_ime the average value of the friction utilization (TFU) was
computed. Ground reaction force measurements from individual steps
with sliding motions confirmed that the TFU was close to the actual
coefficient of friction.
On the most slippery surface the average time over five laps ranged
between 7 s and 20 s for the 18 ToF. The lap time varied least between
the three surfaces for the most slip-resistant ToF, from 7 s to 8 5, while
other ToF varied between 9 s and 20 s.
VTI REPORT 300A
II
Skofriktion bestémd genom gé mgfo'rsok
av Lennart StrandbergArbetarskyddsstyrelsen (AV)171 84 SOLNA
Lars Hildeskog och Anna-Lisa OttossonStatens véig- och trafikinstitut (VTI)581 01 LINKGPING
REFERAT
Halkmotsténdet for 18 skotyper (med olika ovandel, sulmaterial eller -
monster) bestémdes pé tre golvbeléiggningar téckta med halkmedel. Tolv
véiltréinade forso kspersoner gick snabbast mojligt runt respektive bana.
Skador forhindrades med fallskyddsselar. Fri-in varvliderna beréiknades
medelvéirdet av iriktionsgtnyttjandet (TFU). Kraftmé itningar fré m
enstaka steg med glidrorelser bekréftade att TFU och det aktuella
friktionstalet overensstéimde véil med varandra. P51 det halaste under-
laget varierade medelvarvtiden mellan 7 och 20 s for de 18 skotyperna 1
ca 10 signifikant étskilda TFU nivéer frén 0,25 till 0,04 respektive.
Skotypen med béist grepp var ocksé minst kénslig for variationer i
underlagets textur (TFU : 0,25 0,26), medan andra skotyper gav riska
belt stora skillnader (TFU : 0,05 0,23) mellan sléitt och skrovligt golv.
VTI REPORT 300A
III
Footwear Friction Assessed by Walking Experiments
by Lennart StrandbergNational Board of Occupational Safety and Health (AV)8 17184l SOLNA Sweden
Lars Hildeskog and Anna Lisa OttosonSwedish Road and Traffic Research Institute (VTI)5-581 01 LINKOPING Sweden
SUMMARY
Analyses of accidents and gait biomechanics by Strandberg (1983) point
at the substantial influence on human safety from the walking friction on
contaminated surfaces. However, friction meter data often lacks validity
according to a past interlaboratory comparison. Therefore, the slip
resistance of 18 types of footwear (ToF) on three contaminated floorings
was assessed by experiments with 12 well trained subjects, walking in a
triangular closed path as fast as possible without slipping and falling into
the safety harness.
From the lap time the average value of the _f_riction utilization (TFU) was
computed with a model by Lanshammar and Strandberg (1985). Ground
reaction force measurements from individual steps with sliding motions
confirmed that the TFU was close to the actual coefficient of friction.
On the most slippery surface the average time over five laps ranged
between about 7 s and 20 s for the 18 TOP. The corresponding TFU means
were about 0.3 and 0.04. The 18 TOP could be separated at about ten
significantly different TFU levels. On the other two surfaces, the lap
time variation was less pronounced and the rank order was different for
certain ToF.
The lap time varied least between the three surfaces for the most slip
resistant ToF: from 7 s to 8 5; while other ToF varied between 9 s and 20
s. The results elucidate the influence from flooring and footwear
parameters such as pattern, hysteresis, hardness and stiffness.
VTI REPORT 300A
1 BACKGROUND
Accident analyses and biomechanical measurements, see Strandberg
(1983a), as well as tribological considerations by Rabinowicz (1956) and
Moore (1972) point at the substantial influence on human safety from the
dynamic slip resistance of shoes on contaminated surfaces. According to
the Official Statistics of Sweden, about twice as many fatalities ocCur in
falling accidents as in motor vehicle accidents. However, it is unknown
how may of these fatal falls that are intiated by slipping.
In a study of traffic injury victims seeking emergency aid at various
hospitals in Sweden, Nilsson and Thulin (1983) found slipping pedestrians
to be the greatest group. Though many falling accidents occur without a
preceeding slip, there are also many injuries from slipping without
falling. Andersson and Lagerl'o'f (1983) found that falling occured in 63%
of the M000 occupational accidents with slipping in Sweden in 1979.
Though a great number of various friction meters were found in the
literature by Strandberg (1983b), no apparatus should be considered
perfectly valid, according to an interlaboratory comparison reported by
Strandberg (1985) and by Strandberg and Lanshammar (1985). Their
method for assessing reference values hasnow been modified and used in
shoe testing.
While Andriacchi et a1. (1977) have used walking speed and ground
reaction _f_orce (GRF) measurements for other purposes, this study applies
a model from Lanshammar and Strandberg (1985), where the lap _t_ime in
a triangular closed path yields an average value of the _f_riction utiliza-
tion (TFU). Since the subjects are instructed to walk as fast as possible
without slipping and falling into the safety harness, the TFU value can be
considered close to the coefficient of friction of the actual shoe. This
assumption was confirmed by GRF recordings from individual steps,
where sliding motions were detected visually.
VTI REPORT 300A
2 METHOD
The slip-resistance of 18 various types of footwear (19E) was assessed in
648 walking tests with 12 well-trained normal subjects (six male and six
female) on three contaminated floorings, each forming a triangular (90
135 135 degrees) closed path with 12 m circumference. One test
comprised five laps to be walked as fast as possible without slipping and
falling into the safety harness.
From the lap Times the test average value of the Friction Utilization
(TFU) was computed with a model by Lanshammar and Strandberg (1985).
Assuming constant vertical force and constant acceleration/deceleration
along every side of the triangle, Newtonian mechanics yield:
k
where k=l§ when the circumference of the triangular path is 12 m as in
our case.
This model yielded a correlation coefficient of 0.99 and a linear
regression coefficient of 1.05 when considering the TFU a function of
"the _f_orce plate recorded friction utilization (time average over one
stance phase)", FFU, from five steps in the 90-degree corner. Hence, the
TFU has been considered an appropriate measure of the practically
available friction. Complete force plate data were recorded at 500 Hz in
the present study, as well, together with visual observations on sliding
motions for each step in the 90 degree corner of path no. 1.
The path no. 1 coarse flooring consisted of quartz sand (particle
diameter 1.1 1.8 mm) sealed with an epoxy. resin layer, which was
covered with 90% glycerine (viscosity varying between 140 and 300 cP,
depending on relative humidity and temperature). Path no. 2 was covered
with smooth stainless steel and the same 90% glycerine. Path no. 3 had
its smooth unglazed ceramic tile flooring partly covered with glass
spheres (diametre median: 0.3 mm). See Figure 1.
VTI REPORT 300A
VTI REPORT 300A
Figure
1 TheVTIwalking
laboratorypa
ths.
a)No.
lcoarseepoxy
and
glyc
erin
e,piezo
electricforce
plat
ein
thecorner.
b)No
.2smooth
stai
nles
sst
eelandsame
glyc
erin
e(v
isco
sity
abou
t20
0cP
).c)
No.
3un
glaz
edceramic
tile
and
glas
ssp
here
swi
thave
ragediameter
0.3mm.
The 18 ToF evaluated in this study (Table 1, Figure 2) were distributed
systematically between and within sessions in 12 different permutations
(one per subject) to neutralize learning, fatigue and other order effects.
Each subject participated in six three hour sessions over about two
months.
One session occupied three subjects, testing three ToF each (nine ToF
per session to minimize influence between subjects). While one subject
was performing the five lap test, the other two changed path and ToF,
walked a few pre test laps, smoothed the lubricant or rested. The tests
were recorded by video (Figure lb), photo-cell and piezoelectric force
plate (only in path 1, Figure la) equipment.
Confidence intervals for the TFU-means over 12 subjects have been
indicated by lines in Figure 3 being 25 tall on both sides of the TFU
mean level, where 6 is the standard deviation of the mean. According to
the t distribution these lines will approximately cover a 92% confidence
interval for the ToF and TFU-mean in question. Considering two ToF
with similar (max 200% difference) o magnitudes, simple calculus shows
that the probability of reversed TFU mean order is less than 1%, if no
overlap exists between the 25-lines. See e.g.- Brownlee (1965). The
mentioned probability is less than 10% if the longer line does not reach
the other mean value.
VTI REPORT 300A
Tablel
Characteristics
oftested
footwear.
Sole
patterns
inFigure
2.SBR:
styrene-
butadiene
rub
ber,EVA:
ethylene
vinyl
acetate,
PU:
polyUrethane,PVC:
poly
viny
l
chlo
ride
,NR:
natur
al
rubb
er,
NBR:
nitr
ile
rub
ber.
Material
statements
not
checked.Paths
l3according
toFigure
l.
VTI REPORT 300A
Type
FRICTION
VALUE
SHOE
SOLE
of
Mean
12
subjects
Weight(g)
Flexibility
Material
Pattern
Hardness
Texture
A
depth
0Ordinal
FOOtwear
TFU-z
-TFU
Size
Ordinal
as
no.
path
1path
2path
34O
43
scale
stated
(mm)
IRH
scale
.227i.017
.059i.005
.l9li.009
350
450
10w)
SBR
or
EVA
1(
84
0(smooth)
.l63i.lOl
.039i.004
.l70t.004
360
490
2(medium)
SBR
or
EVA
2(m
2(
7O
0(smooth)
67
0(smooth)
66
0(Smooth)
.217i.009
.l46t.Oll
.286t.015
420
450
.l76i.009
.llBi.Ol8
.33li.015
410
470
edium)
SBR
or
EVA
medium)
SBR
or
EVA
OO Q N
r4 m co m
Ienseg
0")
V
.218t.01o
.144i.013
.329t.017
210
240
2(medium
)PU
710(smooth)
.2551.008
.246i.017
.302i.011
180
220
2(medium)
EVA
352
0(smooth)
KO
410ds
l\
.243i.012
.143i.014
.314i.008
440
490
0(rigid)
Pvc
768
.2oot.01o
.089i.006
.310i.011
420
480
0(rigid)
pU
262
-267
t.00
9.1
53t.
005
.245
i.00
641
049
01(
1OW)
pU
0(smooth)
0(smooth)
00
SBOIQ
m
O\
62
l(orange
peel)
10.2
07i.
006
.112
i.01
o.2
83i.
014
800
959
3(hi
gh)
NR11
.197
i.00
5.1
15t.
006
.314
i.01
774
082
03(
high
)NR
12.2
16i.
005
.124
:.oo
7.2
82i.
013
800
950
3(hi
gh)
NR.l
68i.
007
.094
i.01
o.2
18i.
006
780
890
3(hi
gh)
NR
54
53
orange
peel)
coarse)
sqoog
mH
l( 2(
51
l(orange
peel)
54
0(
KOme
smooth)
.249t.006
.157t.014
.264t.012
450
500
.229t.008
.054t.005
.237t.011
560
570
2(medium)
EVA
N
45
0(smooth)
2(medium)
NBR
l60
0(smooth)
VLF)r4 H
stepues
+_ aoq Teens -~4
.242t.006
.093t.005
.29oi.017
670
730
2(medium
)SBR
455
.275t.008
.189t.012
.278t.011
610
670
2(medium)
NBR
461
.250i.008
.135t.013
.263t.016
470
500
2(coarse)
0(smooth)
2(medium)
PU
466
0(smooth)
w rs mH r4 H
-xIOM
Figure 2 Sole patterns for footwear in Table l.
CAUTION: Though the pattern here (and other parameters in Table l)are the same as those of footwear recognized on the market, thematerial recipies may be different. Therefore, it must be discouraged touse the pattern for connections between the results and marketedfootwear.VARNING: Aven om monstret air detsamma som pa vissa skor i marknaden, 551 kan testskornas materialrecept vara sa annorlunda att resultat-ja'mforelser ar helt missvisande.
VTI REPORT 300A
VTI REPORT 300A
TFU
1I
GLYCERINE
0NCOARSE
EPOXY
GLYC
ERIN
EON
SMOO
THST
EEL
3E]
GLAS
SSP
HERE
SON
SMOO
THCE
RAMI
CTI
LE
0.3-
0.2-
0.1%
17
14
93
47
18
12
11
51O
16
81
15
2TOF
Figu
re3
TFU
mean
bars
and
conf
iden
celi
nes
(see
text
)fo
rth
e18
Types
ofFootwear
inTable
1andFigure
2.
3 RESULTS
Based on each subject's TFU values on the steel path (no. 2 in Figure 3),
a discrete rank number was assessed to each of the 18 ToF. The 18
means over the 12 subjects of these ToF-ranks have been used in Figure
3 to order the bars. The three bars to the very left represent the TOP
(no. 6) with the highest rank mean and the greatest slip-resistance on the
steel path, while the most slippery ToF (no. 2) can be found to the very
right. The TFU mean orders of the 18 ToF vary considerably between the
three paths, however. The TFU values are also listed in Table l.
VTI REPORT 300A
4 CONCLUSIONS
Since normal straight walking with constant speed required FFU-values
between 0.1 and 0.2 (Strandberg, 1983a), the choice of ToF appears more
important for safety on floors with a viscous contaminant (paths 1 and 2)
than with a solid one (path 3). See Figure 3, also showing the substantial
influence from. flooring roughness: the coarse (but polished) texture of
flooring no. 1 made the TFU on glycerine less sensitive to ToF than the
smooth Steel on path 2. The TFU variation on the coarse flooring (path 1)
is therefore of less importance to safety than on the smooth path 2,
which also exhibits smaller TFU values in general when compared to path
1.
The relations between results (Figures 3 and 4) and ToF characteristics
(Table 1, Figure 2) may be enlightened by three phenomena, decisive for
walking friction on contaminated surfaces: Drainage, Draping, and
Damping.
Without sole pattern (ToF l, 2, 13) or with closed cavities in the pattern
(ToF 5, 8, 15), poor bulk contaminant drainage prevented efficient sole
draping and molecular contact with the flooring asperity tips on paths 1
and 2.. On path 3, however, the glass sphere layer was probably thin
enough to avoid filling the cavities of ToF 5 and 8. Drainage of very
viscous or solid (glass spheres, path 3) contaminants requires an edge-like
pattern and a particularly small contact area. On the other hand, such
ToF offer too small draping areas to be competitive on coarse floorings.
Hence, paired comparisons of ToF 5, 11, and 8 versus 3, 12, and 9
respectively, exhibit great versus small TFU-means on path 3, and vice
versa on path 1.
On the smooth Steel path (no. 2) the coarse sole texture of ToF 16 may
have caused a small effective draping area, contributing to its compara
tively small TFU. The effect from normal pressure on actual contact
area and on the coefficient of friction has been elaborated on by
Gillespie (1965) and will not be discussed here.
A soft sole and flexible shoe may increase draping, but if there are
insufficient bulk drainage channels in the sole and flooring pattern,
VTI REPORT 300A
FRICTION
USAGE
10
elastohydrodynamic separation (Moore, 1972) may cause the draping time
to be longer than the stance time, see Figure 4. (The stanCe phase starts
with heel strike and ends with toe-off.) The significant TFU differences
between ToF 1 and 2 may be due to this phenomenon.
Test: 245 Laps: 2-5 ToF: 17 761 Laps: 1 5 ToF:
FFU: 0.27 Mean stance time:r
Test: 2 f
0.51 FFU: 0.15 Mean stance time: 0.67s
1.0
.6
.4
\ w /
\
v.
1..
r'
.
.
__ . . .- .3. _,
. . . . :I c _ .n. o -.. . n...""...- -"n\':".::::: .;;:.N 01" ' . n..;.'. .
. . - . .. - .. . -' . H" --. 1.001.". 2." ...'» z . - .-' '. . .I L. .- . _;_ .(. _,:\_..Ew'i 1114...... n o . . ... 3.. I-..._.$f : . 00.31.:Z.. ~ v, V ._ -. nu... . ' ,A ;'J .' t ' . ' . . ' "
o I ' 'o'oou..o
' . . . . . I. 04'."...'..0 . '
z' '- ....... ur'-- :.".. "Wham. - - . -..... ° 0 K'. -...,.:n.;:. |.."'2% ,, W M M u me -,,...,:;-u v w,
: .;: ... ..'::....l.." '~.o'ou..; :fo %::'o'-I:_.-"'?M .
a... o. .I 5"" .o-.p~o-.QQ.I ' '
_v -v 0. x'h
""A . A A A 4 A L A A A A A r
O N V 9 w0 O 0
TIME (FRAéTloN OF STANCE TIME)
Figure 4 Friction usage over time divided by stance duration from slidingsteps: four with ToF 17 (TFU: 0.28, FFU: 0.27, stance duration:0.51 3);five with ToF 2 (TFU: 0.12, FFU: 0.15, stance duration: 0.67 5).Same female subject in her 3rd and 6th session respectively.Coarse epoxy flooring with glycerine (path 1).
After drainage and draping, the damping or hysteresis (Moore, 1972)
properties of the sole material play an important role. Though EVA and
PVC materials appear more favourable in this respect, the great TFU-
values of ToF 17 indicate that high damping can be achieved also with
certain NBR-recipies. The great and path independent TFU~va1ues of
ToF 6 demonstrate a well-balanced and probably the safest compromise
between these slip resistant properties.
VTI REPORT 300A
11
ACKNOWLEDGEMENTS
This paper is an extended preprint from BIOMECHANICS X (to be
published in 1986 or 1987) with kind permission from Human Kinetics
Publishers, Champaign, USA.
The study was sponsored by the Work Environment Fund (Arbetarskydds
fonden). The National Board of Occupational Safety and Health (Arbetar
skyddsverket, AV) provided equipment and project manager.
The subjects participated during working hours with salary from their
employers: Postverket, Cloetta AB, Farmek AB, Arla AB.
The laboratory flooring materials were provided without cost from:
Partek Byggvaror AB (ceramic tile), Perstorp AB (epoxy resin), Platsla-
geriernas Riksforbund (stainless steel), 3M Svenska AB (Safety walk
adhesives beneath the triangular paths).
The authors wish to express their gratitude also to the members of the
Advisory Group for their numerous contributions. This group consists of
people from Arbetarskyddsn amnden, AV, Branschforeningen for Personlig
Skyddsutrustning, Foreningen for Teknisk F'o'retagsha'lsovard, Skoinsti-
tutet, Statshalsan and VTI.
VTI REPORT 300A
12
REFERENCES
Andersson, R., and Lagerlof, E. (1983). Accident data in the
new Swedish information system on occupational injuries.Ergonomics 26, pp 33-42.
Andriacchi, T.P., Ogle, J.A. and Galante, 3.0. (1977). Walkingspeed as a basis for normal and abnormal gait measurements.Journal of Biomechanics 10; pp 261 168.
Brownlee, K.A. (1965). Statistical theory and methodology. JohnWiley (Sc Sons Inc., New York.
Gillespie, T.D. (1965). Pavement surface characteristics andtheir correlation with skid resistance. Joint Road Friction
Program, The Pennsylvania State University, Report No. 12.
Lanshammar, H., and Strandberg, L. (1985). Assessment offriction by speed measurement during walking in a closed path.In D. Winter, R. Norman, R. Wells, K. Hayes, (St A. Patla (eds.),Biomechanics IX-B, pp 72-75. Human Kinetics Publishers,Champaign, USA.
Moore, D.F. (1972). The friction and lubrication of elastomers.
Pergamon Press, Oxford.
Nilsson, G., and Thulin, H. (1983). Trafiko1yckor och trafik-skadade i Ostergotland under tva veckor i mars och tvé veckor ioktober 1982. Undersokning av olika rapporterings- och regi-streringsrutiner av trafikolyckor och trafikskadade.VTI Meddelande nr 348.
Rabinowicz, E. (1956). Stick and slip. Scientific American, 194;pp 109 118.
Strandberg, L. (1983a). On accident analysis and slip-resistancemeasurements. Ergonomics 26, pp 11-32.
Strandberg, L. (1983b). Ergonomics applied to slippingaccidents. In: T.O. Kvalseth (ed.), Ergonomics of WorkstationDesign, pp 201-228, Butterworths, London.
Strandberg, L. (1985). The effect of conditions underfoot onfalling and overexertion accidents. Ergonomics 28; pp 131 147.
Strandberg, L., and Lanshammar, H. (1985). Walking slipperinesscompared to data from friction meters. In D. Winter, R.Norman, R. Wells, K. Hayes, (St A. Patla (Eds.), Biomechanics
IX-B; pp 76-81. Human Kinetics Publishers, Champaign, USA.
VTI REPORT 300A