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THE SECURITEST
: APPARATUS
TO MEASURE THE INTERACTION
BETWEEN FOOT AND SPORT SURFACE By
D. Boisnard LABOSPORT 
INTRODUCTION
: The aim
of this new test method is to assess the friction between foot and sport surface,
but also the warming up and abrasion of the skin during the friction. This
machine was built after a research in Labosport, with ANVAR funding. An improvement
was done last year in collaboration of Wassing firm in Germany. The apparatus
will be presented by a video tape. It records all along the test the vertical
force, the speed, the horizontal force and the temperature of the skid. The vertical
force is given by pressure air which involves a constant force during the test. A
coefficient of friction is then calculated. Why
we produced a new apparatus for measuring friction : - No
apparatus is suitable for synthetic turf because the distance of friction is difficult
to settle (high deformation) and not accurate, because the vertical force and
speed vary during the test, because the discrimination of different products is
difficult and because results do not correlate to the practice.
- In the European standardization,
no apparatus was proposed to determine the interaction between skin, clothes and
surface.
- We
need to wove the parameters : speed, vertical force and skid (dimension and quality).
- Existing apparatus
are using parameters far from reality of the friction in sport. So criterions
as surface rugosity or deformation are not taken into account. Apparatus must
use parameters (force, speed…) as close as possible to the practice.
Example
: comparative tests on PVC with the SRT pendulum.
| |
SECURITEST |
SRT pendulum
| | Surface
without preparation | 0.50
| 112
| | The
same surface cover by a no sliding preparation |
0.75 |
110 |
The
SECURITEST could give differences on different preparation of surfaces, not the
SRT (RRL). I
FRICTION / TRACTION : From
the documentation we know that the Coulomb laws are not applicable with rubbers
and polymers. In
1982, Bring noticed that that the coefficient of friction is the addition of an
adhesion coefficient of friction and a deformation coefficient of friction, and
that the adhesion coefficient could be higher than one. With
synthetic turf and also for surfaces with high deformation, due to the texture
of the product (pile height), there is a reactional force which increase the coefficient
of friction measured with an apparatus : COF
= fa + fd + fr This
reactional force could mean that the real term for friction is traction (term
used for the natural surfaces). Nevertheless we will adopt for the results the
term of COEFFICIENT OF FRICTION, COF. The
conclusion is that COF is for polymer materials not limited to 1 Examples
of SECURITEST measurements on different surfaces : - Synthetic
turf : Leisure-floor: only 5mm fibre length
- Synthetic
turf : PP
- Synthetic
turf : LSR3
- Synthetic
turf : Nylon
- Synthetic
turf : Plastic back: Floor with plastic back
- Synthetic
turf : SPF
- Wood
floor
- Track
| Sample
| 1
| 2
| 3
| 4
| 5
| 6
| 7
| 8
| | COF
| 0.95
| 1.25
| 0.80
| 1.25
| 1.35
| 1.45
| 1.15
| 0.4
| Sliding
device : Desmopan II
DETERMINATION OF THE TEST PARAMETERS The
sliding materials: - Crystal
rubber: Synthetic transparent rubber (Polybutadien, CB), extremely good in wear
(DIN: 30mg). used for transparent out-soles for Adidas shoes.
- Adidas
beige: TPU-Material used for the sole of soccer-boots.
- Adidas
white: TPU-Material used for the studs of soccer-boots.
- Desmopan:
Thermoplastic PU but elastomer! (TPE-U), product of Bayer.
- PVC:
Hard PVC (Palight S white), same material as used for securitest housing.
Comparative results
: see graph 3
Conclusion
: necessity to adapt a rubber near the quality of the shoe. 1.
Pressure Variation (graph 1: x-axis is gas pressure = vertical force)
We
tried out the different sliding materials on a leisure-floor. We
found a dependence (dependent on the material more or less) of COF on the pressure.
The higher the pressure the lower is COF. There seems to be a saturation
for higher pressures. All this corresponds to the literature (more than
80 N). The graph
4 shows this saturation with different floors. 
2.
Speed variation (Graph 2 : x-axis is horizontal speed)
We
tried out with crystal rubber all at 1bar (60N), 2 turns, 20turns/min. We
found out a strong dependence for small speeds (below 0.3 m/s). The higher the
speed the higher COF with about +0.1 in COF for +0.1 m/s. For higher
speeds we find a kind of a saturation (higher 0.6 m/s). III
VALIDATION OF THE SECURITEST 3.
Reproducibility (Graph 3, with y-error bars that give the standard deviation
of COF). We tried
out 8 times for each floor and each foot the reproducibility (all at 1bar, 2 turns,
20 turns per minute). Conclusions
: Very
good reproducibility. Good accuracy ; nevertheless more COF increases more the
error increases, so we have to choose a rubber device with a low COF. The
phenomenon of friction changes itself : Temperature
: We found a direct influence when we do not wait between two measurements.
For our standard condition (2 turns, 20 turns per minute, 1 bar) we found that
for the high friction Crystal Rubber a 2 minutes pause is already sufficient.
If pressure, speed and numbers of turns are increased, pauses have to be longer.
Humidity :
Humidity can have a big influence dependent on the materials. In the literature
we found for polished steel on Polyamid 66 between 10% (COF 0.5) and 80% Humidity
(COF 0.12) a monotonously declined behaviour. Other materials show no influence,
e.g. polished steel on high density polyethylene. Thus the humidity has also
to be controlled. CONCLUSIONS
: General
Statements : COF
is for polymer materials not limited to 1. To find higher values is not a
big surprise (see e.g. standard book for friction science "Tribologie"). SECURITEST
is able to classify floors in a reproducible way. As
there is a high dependence on the speed a standard should be done with a realistic
speed. At least a speed should be used that is in the saturated area of e.g.
higher 0.6 m/. Same is true for the dependence on the normal force. The
vertical force must be adjusted more than 80 N. The
standard rubber (from SRT) is not suitable for testing on synthetic turf.
The material is extremely rubbed out due to high friction. The rub-out pollutes
the turf and influences further measurement. The surface structure is changed
completely during an experiment (micro-waves perpendicular to moving direction).
The hardness
and quality of the skid must to be the same than sport shoes. The
apparatus SECURITEST is important for research because vertical force and speed
could vary, but for the tests we will take the vertical force of 80 N and the
speed of 60 cm/s. A
collaboration with biomecanic organism will be important to adjust the parameters.
The apparatus
SECURITEST classifies synthetic turf and other sport surfaces. In
the future we will improve the device to measure the temperature of the skid,
so a sport surfaces could be classified by : - its
coefficient of friction
- its
warming up during the test
- its
non abrasive characteristics
In
addition the apparatus could be used with different devices : ball, shoes, rollers,
abrasives. |
