8
FRICTION AND MECHANISM OF SLIP
Friction is the force which resists the movement of two surfaces which try to
slide or slip with reference to one another. This force acts in a direction
opposite to the possible or existing motion between the two surfaces.
Figure 1 shows the mechanism of the development of friction force. Figure
1(a) shows two components of body weight when the heel touches the floor.
Figure 1(b), an enlarged section at micro level, shows reaction offered by
the floor at different contact points. Figure 1(c) shows the overall view of
the development of friction force at the contact surfaces whereas Figure
1(d) shows the resulting forces and reactions. If these forces are not
balanced and equilibrium is not established, slip can occur. The most likely
case for the occurrence of a slip is where the horizontal component of body
weight, force P, is greater than the friction force F.
Figure 1
Explanation of friction force and stability of a body
Let us now look at the factors affecting friction and thereby the slip
resistance.
9
FACTORS AFFECTING SLIP RESISTANCE
A number of factors affect the slip resistance. Figure 2 shows various factors
affecting the slip resistance.
Figure 2
Factors affecting slip1
These factors affect the slip resistance jointly and severally. Some of these
factors may be either easier to deal with or easier to understand. Others
may not be. However, it is important not to get fixated to a factor.
9.1
Effect of Flooring
Flooring is probably the most important factor affecting the slip risk. It is
therefore important to understand the effect of the type of flooring material,
floor finish, profile and, wear and tear over a period of time.
Most flooring materials can be broadly divided in two categories – soft and
hard flooring. Rubber, vinyl and linoleum are the most common soft floor
materials whereas ceramic, stone, epoxy, wood, metal and concrete are
some of the most common hard floor materials.
Identification of right flooring material at the initial design stage or during
refurbishment can help reduce slip risk. Location, type of work, amount of
pedestrian usage and, type and amount of contamination are the slip risk
related factors which need to be considered at the design stage.
1
http://www.hse.gov.uk/slips/step/
9.2
Effect of Contamination
Contamination plays equally important role in slip resistance. HSE states
that almost all slip incidents involve some form of contamination between
the floor surface and foot. Most surfaces, when clean and dry, should
present a low slip risk2. Contamination could come from spillages, leaks,
ingress due to weather, work process or improper cleaning. Identification of
source of contamination and controlling/removal of the same should be the
highest priority to reduce the slip risk. In many cases, simple engineering
solutions or minor changes in the system of work can control/reduce the
contamination.
Contaminants can be divided in three categories.

Fluids – e.g. water, oil, milk etc.

Dry – e.g. brck-dust, flour, sand etc.

Semi-solids – fruits, chips, dough etc.
Slip resistance is affected by the individual properties of each type of
contaminant.
9.2.1
Fluid Contaminants
Reduction in slip resistance due to a fluid contaminant depends on the
surface finish and viscosity (thickness of fluid). Low microroughness is not
able to break through the film of the fluid contaminant thereby making the
floor slippery (Figure 3). On the other hand, high microroughness breaks
through the film of fluid contaminant which, in turn, allows for a solid-tosolid contact with pedestrian’s heel (Figure 4).
2
Defined as a risk of less than one in a million people would have problems walking safely.
Figure 3
Low microroughness of the floor can’t break through the fluid contamination
which makes the floor slippery3
Figure 4
High microroughness breaks through fluid contamination allowing the floor to
offer grip3
The thicker the contaminant, the rougher the floor needs to be to have a
low slip risk.
9.2.2
Dry Contaminants
Research carried out in last 10-15 years has shown that dry contaminants,
similar to their wet counterpart, reduce slip resistance of surfaces. Figure 5
shows general view of interaction between a shoe heel and dry contaminant.
Research
carried
out
in
last
decade
has
identified
three
possible
mechanisms leading to slip under the influence of dry contaminants. These
mechanisms are shown at micro level in Figure 6.
3
http://www.hse.gov.uk/slips/step/
Figure 5
Figure 6
9.2.3
General view of interaction between shoe heel and dry contaminant4
Interaction between shoe heel and dry contaminant at micro level; slip due to
(a) sliding, (b) shearing, (c) rolling4
Semi-solid Contaminants
Footwear and flooring alone are unlikely to be sufficient to control slip risk
due to semi-solid contaminants such as fruits, chips, dough etc. It is
therefore important to prevent them getting on the floor in the first place
and clean up as soon as possible if they do.
9.3
Importance of Cleaning
People rarely slip on clean, dry floors. It is therefore vital to maintain floors
in clean and dry condition through effective cleaning regime.
Effective cleaning, when carried out well, should remove contamination from
the floor and reduce the likelihood of a slip occurring. On the other hand,
cleaning, if not carried out effectively, could introduce slip hazards to an
environment. Research has shown that cleaning regimes frequently fail to
take account of these important points. Well planned cleaning regimes not
4
http://www.hse.gov.uk/slips/step/
only have a positive effect on flooring appearance and hygiene but also
reduce likelihood of slip taking place.
Different floor types require different cleaning techniques. The most
effective technique depends on the type of floor, likely contaminant and the
type of activities carried out in the area. Flooring manufacturers and
cleaning equipment suppliers can also provide useful information on
cleaning regime. It is important to note that a cleaning regime is as good as
the people who implement it and the tools they use.
9.4
Effect of Environment
Both, external and internal, environments affect the slip resistance.
9.4.1
External environment
External surfaces in typical British climate can be routinely wet throughout
the year. They could be icy or covered in snow during winter months
whereas they could be covered in leaves during autumn.
Risk assessed flooring material, cleaning regime, adequate drainage and
planning for weather changes can help mitigate slip risks.
9.4.2
Internal environment
Humidity and temperature affect the slip resistance of floors whereas factors
like lighting and noise affect how people react while walking on a surface.
Increased humidity may make a surface damp and slippery whereas low
temperatures, such as that in cold storages, can lead to slip hazards. Poor
lighting affects people’s ability to spot the hazards on the floor whereas
noisy environment can be distracting thereby diverting attention from local
slip hazards.
9.5
Effect of Human Factors
Ability to understand and react to the instructions, ability to perceive hazard
related information, type of activity undertaken, capability, behaviour and
fatigue are the most common factors affecting the slip risk. These factors
are not always controllable but they could be predictable.
9.6
Importance of Footwear
Footwear is an important factor affecting the slip risk. Use of sensible
footwear helps reduce the slip risk. Some footwear, such as high heels,
smooth soles etc. increase the slip risk and don’t form part of “sensible
footwear”5 group. It is important to note that sensible footwear is different
from slip resistant footwear.
Different types of contaminants require different types of footwear. A slip
resistant footwear suitable for wet environment is not necessarily suitable
for oils and solid contaminants.
Generally a softer sole and close packed tread pattern works well in the
indoor environments (Figure 7). A more open tread pattern works better
outdoors or in solid contaminants (Figure 8).
Figure 7
5
Close packed tread pattern
Open-toed shoes, sandals, flip-flops also don’t form part of “sensible footwear” group.
Figure 8
Open tread pattern
10
CAN SLIPPERINESS BE MEASURED?
Slipperiness is a function of frictional resistance of the surface material. As a
result, it is possible to measure the slipperiness of a surface.
10.1
Microroughness test
A surface needs to have enough microroughness to break through a
contaminant to have a low slip potential (Figure 3, Figure 4). Cadogans uses
Surtronic Duo to carry out microroughness measurements.
Photograph 1
Surface Roughness meter - Surtronic Duo
Surface Texture or roughness is the measurement of the fine frequencies on
the surface of a material. Because the surface is made up of a number of
frequencies it is important to remove the relevant part of the surface from
the profile. For example, if we imagine a desert with sand dunes – the
surface form would be the hills and valleys of sand, the waviness would be
the rippling in the surface, maybe caused by the effect of the wind and the
texture would be the grains of sand themselves. To extract the fine
frequency surface roughness we need to apply a filter. One cannot choose
the frequency as this has already been done back in the early days of
surface measurement and the global standards are now defined.
For the majority of surfaces to be checked a 0.8mm (0.030”) is used. This
means that only wavelengths that are shorter than 0.8mm pass through the
filter.
To make a reliable measurement we need to assess a number of these
0.8mm lengths – called the cut offs or assessment lengths. The industry
standard is 5. Therefore 0.8mm x 5 gives a measured length of 4mm (see
Figure 9).
Figure 9
Measurement of surface roughness
Due to the way that the filter works it is necessary to actually track across
the surface more than 4mm. For instance the globally approved Gaussian
filter employed on the Surtronic Duo requires a measurement length of
4.8mm as one of the 0.8mm cut offs is removed as a function of the filter
itself. A typical Surtronic Duo actually tracks about 5mm which allows the
instrument to get up to a constant measurement speed and complete a
4.8mm measurement.
Once the data is collected from the surface, the instrument will apply
parameter results to the modified/filtered surface. The Rz parameter
favoured by the HSE is calculated by looking at the individual 0.8mm cut
offs – each has a peak to valley reading.
The five readings are added
together and then divided by 5 to give the average peak to valley result.
The HSE has specified a Rz roughness value required for a surface according
to the type of contaminant. A rougher surface is required for for oils than
say water.
10.2
Co-efficient of friction test
Co-efficient of friction tests directly measure the slip resistance of a flooring
material. A variety of slip tests are available to measure the slip resistance.
Three most popular of these tests are:
i)
Pendulum test
ii)
Ramp test
iii)
SlipAlert test
Pendulum test is HSE’s preferred method of testing, because it is accurate,
portable and works in the condition that slip accidents happen.
Cadogans uses Pendulum test to assess the co-efficient of friction of the
concerned surface for the following reasons.
10.3
i)
It is HSE’s preferred method of testing
ii)
Pendulum can be used in real workplace conditions.
iii)
Allows for comparison between clean and contaminated floors.
iv)
Pendulum simulates frictional resistance of floor while people walk
v)
Pendulum’s ability to simulate shod and barefoot conditions
Pendulum Test
HSE and UK Slip Resistance Group (UKSRG) publish guidelines on carrying
out slip tests and assessing the test results. The guidelines on assessment
of test results follow the research work by Building Research Station (which
is now Building Research Establishment (BRE)) in 1950s.
The values of slip resistance are read directly from the pendulum scale.
They are called pendulum test values (PTV) and are about 100 times
greater than the coefficients of friction.
Following factors affect the accuracy of pendulum test results.
i)
Levelling of the pendulum
ii)
“Setting the zero”
iii)
Setting the slider’s contact length
iv)
Preparation/re-preparation of the slider
11
INTERPRETATION OF SLIP RESISTANCE TEST RESULTS
11.1
Microsurface Roughness test results
The microroughness meter measures about five millimetres of floor at a
time, making the area tested very small, so readings can vary widely as
they can be affected by, for example a crack in a tile. An average of ten
readings
is
therefore
taken
to
ensure
measurements
are
more
representative of the entire floor. Table 1 shows a relationship between the
potential for slip in the wet condition (with clean water) and average of Rz
parameter.
Table 1
Relationship between Rz parameter and Slip Potential
Average Surface Roughness
Rz (µm)
Below 10
10 to 20
20 +
11.2
Slip Potential
High
Moderate
Low
Pendulum test results
Figure 10 shows a relationship between slip potential for pedestrians
walking on a level surface and PTV. The values change for inclined surfaces.
Figure 10
Relationship between PTV and slip potential
The HSE classifies slip potential of pedestrians walking in a straight line on a
level surface in three categories, low, moderate and high, based on the
value of the PTV. This relationship is given in Table 2.
Table 2
Relationship between PTV and Slip Potential
PTV
Slip Potential
0 - 24
High
25 - 35
Moderate
36 +
Low
12
WHY IS SLIP SUCH A BIG DEAL
More than 30,000 people suffered slip and trip injury while at work during
2011/2012
(http://www.hse.gov.uk/STATISTICS/tables/index.htm).
amounts to about 30% of the employment related injuries.
Figure 11
HSE statistics based on employment related injuries
This
13
CERTIFICATION OF TEST EQUIPMENT
It is important to keep the test equipment calibrated to ascertain the
accuracy
of
the
test
results.
Cadogans’
equipment
and
associated
accessories are calibrated and certified either by BSI or UKAS accredited
laboratories.