Variable
Incidence Tribometer
Variable
Incidence Tribometer
(EnglishXL)
From the early days of
attempting to evaluate floor conditions with a fish scale and a bean bag, the
measurement of slip resistance or coefficient of friction has been an evolving
science. Over 50 years ago, a large bulky contraption called the James Machine
employed an articulated strut to determine the angle at which a weighted shoe
material specimen (usually leather) would start to slip on a test surface.
Adopted by Underwriter's Laboratories, this method quickly became the method of
choice of testing both shoe material and flooring materials and compounds.
There were a couple of drawbacks to the method however. First, testing could
not be done in the field or on actual floor surfaces. Second, there could be no
testing on wet or contaminated floors.
In 1947, Percy Sigler
at the National Bureau of Standards published a paper which described a
relatively new method of measuring slip resistance. With Sigler's instrument, a
pendulum was shod with a shoe material and the specimen was aligned with the
walkway surface. The pendulum was raised to a fixed height above the walkway
and allowed to swing freely across the surface. As the specimen passed across
the surface and a helical spring pressed the specimen on to the surface. The
specimen rubbed across the surface, and a portion of the energy of the swinging
pendulum was lost due to friction. This loss of energy could then be correlated
to the coefficient of friction. The test method was relatively simple with a
high degree of precision. Not only was the instrument portable, but tests could
be run using a variety of shoe materials and on surfaces which were either wet
or contaminated. The Sigler method was specified in Federal Test Method
Standard 501a, Method 7121. For nearly 20 years, this method of testing was
favored by both institutions and safety professionals. However, science is
never free of politics and self promotion, so by 1976, Robert Brungraber on
leave from Bucknell University prepared "An Overview of Floor Slip-Resistance
Research" (National Bureaus of Standards Technical Note 895) which prepared the
groundwork for a shift away from the dynamic coefficient of friction as
determined by Sigler to the development of devices which would measure the
static coefficient of friction.
The bean bag and fish scale was replaced by more accurate force measuring
devices and controlled surface specimen holders.
ASTM C-1028 and F-609 became popular for field work. This simple
technology, however, did not have the flash and dazzle many thought were
necessary for scientific testing. Brungraber developed a field version of the
articulated strut testing devices. The Brungraber device was very effective in
measuring the static coefficient of friction on dry, non-contaminated floors.
However, none of the devices could adequately measure the coefficient of
friction when the surface was wet with good precision and accuracy.
William English, P.E.
developed a device, which is shown above, that provides a high precision means
of measuring both the coefficient of friction on dry surfaces and the slip
resistance of wet ones.
The English XL is a light weight instrument that challenges all of the
previous methods of testing. Adopted as ASTM Standard Test Method F-1697-96,
the instrument has proven to be very accurate in field and laboratory testing.
I have personally compared field test data obtained by the English XL, the
Horizontal Pull Slipmeter (ASTM F-609), the Sigler device, and the Horizontal
Dynamometer Pull-Meter Method (ASTM C-1028). The English device was both more
accurate and precise and any of the other methods in determining the slip
resistance of a standard test tile when the surface was wet with water.
I have used the English
XL for over 5 years on a wide variety of flooring surfaces while conducting
field investigations in over 400 civil cases. During this time, the reputation
of the English XL has grown; it is now the instrument of choice of the majority
of safety professionals across the country. It has proven to be accurate and
reliable. However, the inventor of the instrument, William English, has
determined that without specific instruction, the results obtained by using the
methodology contained in ASTM F-1637 can be challenged. The additional
instructions and precautions contained in the instruction manual that
accompanies the instrument are necessary to prevent erroneous results. As of
October 2002, the manufacturer has revised the instruction manual to incorporate
all of the procedural changes. Copies of the revised instruction manual
are available through his website.
One would hope that
these necessary, if not critical, steps can be added to the standard method.
The purpose of standards methods is to provide the operator with adequate
instructions so that if all of the procedures are followed as prescribed, then
data obtained is both adequate and reliable. In any case, when used properly
and with adequate knowledge of both the method and the instrument, the English
XL is the instrument of choice.