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Human Factors

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Safety Subject Information:

SLIP TESTING

One primary variable to consider when evaluating a surface for safety is relative slip resistance.  This is often quantified as the coefficient of friction.  This is often expressed as the Greek letter "mu" (µ), "COF" or as "cf."  In its simplest form "mu" is the ratio of the force required to slide an object parallel to a surface, to the normal (vertical for a horizontal surface) force that the object exerts on that surface (Handbook of Physics & Chemistry).   Static "mu" utilizes the force required to start an object sliding.   Dynamic or kinetic (HPC) "mu" utilizes the force required to keep an object sliding at a constant velocity. 

November 11, 2007 found dozens of tribometrists on hand at the USC Kinesiology Lab in Los Angeles for a testing session.  Preston was there with his Brungraber Mark I.

For the past fifty years or so, controversy has been raging among academics about just how  "mu" should be measured on surfaces and how the data can be employed and applied to assess the safety of floors and quantify the danger of slip and fall injuries.

A growing number are now saying that "mu" is irrelevant and the "slip resistance factor" should be applied.  There is no ASTM value for slip resistance factor approved in a standard as a "safe," or slip resistant value, although .50 slip resistance factor measured with the Variable Incidence Tribometer (VIT, English XL) is fast becoming the de facto standard in the industry.

The simplest and most common method of measuring mu involves a horizontal pull slipmeter (high school physics, a drag sled).  There are two ASTM standard horizontal pull slipmeter (only one called HPS) devices, but both use the same principles to actually measure the sliding force and the normal force.  A weight is weighed to get the normal force (assuming a horizontal surface).  The weight is then slid over the surface to obtain the surface friction or drag, parallel, force.

The problem that arises is that none of the traditional (drag sled) methods are currently approved for measuring wet or lubricated surfaces where the overwhelming majority of slip and fall events occur.  Their value when wet is dubious.

I have at my disposal the Brungraber Mark I (standard for wet bathtubs) and Sigler Device (paperweight) as well as the Measurement Instruments version of the horizontal pull slipmeter (HPS - dry COF testing only).  The latest addition to my collection of slipmeters is the English XL Variable Incidence Tribometer (Wet or dry for slip resistance index).

The NBS Brungraber (Mark I) device is the ASTM F-462 standard device for measuring friction in bathtubs, shower pans, and bathing surfaces.  It is an articulated strut type of device, and it is functionally similar to the laboratory-sized "James" machine (it was even designed to use the same sensor foot).  It measures only the static "mu."  It is quite cumbersome to operate, and the standard requires attention to such details as using the "official" soap suds in testing.

My Brungraber Mark I device is ready and available for assessing the slip resistance of bathing surfaces or other surfaces which require it.  The F-462 standard is one of two ASTM Standards that actually express a numeric value as a pass-no pass point.  In the case of the F-462 Standard, the value is so low that the bathing surface may still be unsafe for bathers.  The Mark I was specifically designed by Robert Brungraber to be a portable James machine, and overcome many of the James machine's shortcomings.

The Brungraber Mark I is calibrated with a sheet of plate glass.  A bathing surface passes if it is less slippery than the glass.

The Brungraber Mark I is also the subject of the ASTM F-1678 standard which sets out its use wet or dry on ordinary walking surfaces.  The standard prescribes no actual value for safety.  It merely standardizes the method for using the device for these measurements.  ASTM calls it a Portable Articulated Strut Tribometer or PAST.

The Family of Brungraber devices all use the same sensor foot design to mimic the James machine. This is a 3" by 3" square of leather or other material.  The large pads are likely to hydroplane during wet testing, so modern usage requires a scoring of the pad in a tic-tac-toe pattern.

For the first time, here is a family portrait of all three of the Brungraber devices (from left to right)  Brungraber Mark III, Mark II, and Mark I.

The Sigler Device (of 1943) is a pendulum-type (Pendulum Impact Tester or PIT) device which is large and cumbersome to operate.  It  measures only dynamic "mu."  It is one NBS standard device.  Sigler was an ASTM (E-303) device, via the European standard for the British version of the device.  The Europeans continue to use it extensively and find it of value.  Its use in America has been largely discontinued.  I do not routinely use my Sigler device, but I have it available upon client request.

Roy Jablonsky attempted to miniaturize the Sigler device to make it more practical and portable.  The SPIT (Small PIT) was submitted to ASTM, but was not adopted as a standard device.  Jablonsky also made improvements to the James machine to make it more accurate and functional.  

A modern European Sigler device from Wessex (Wessex Universal Tester) is shown at right.

Another small pendulum device was to be featured at the Slipmeter testing November 11, 2007, but it was held up by airport security (it must have looked too much like a bomb).

The Europeans are compounding the problems of the Sigler Device by using the so-called TORTUS device.  This is a self-propelled unit with a sensor foot that it pushes onto the floor at intervals.  It measures the drag force by the applications of the sensor foot and automatically integrates the values to come up with what may (or may not) be a dynamic coefficient of friction (COF) value.  There are many versions of this device including one exclusively used by the Italians.  The TORTUS II (shown) has an integral printer that outputs the results.  The FSC-2000 is a "TORTUS II" type device that is sold in Germany and has been used in schools in British Columbia.

A revised version, the Tortus III, was on hand for the USC 2007 testing..

The BOT-3000 is another self-propelled device.  It is functionally similar to the Tortus.

A new entry called the "Slip Alert" comes from Australia.  I believe this unit is functionally similar to the Tortus devices.  The manufacturer states that the device correlates with the pendulum testers, which means that what is measured is the dynamic rather than static coefficient of friction.  The open wheels mean greater air resistance.

The James Machine (figure right), developed from the earlier Hunter machine is the prototype articulated strut device.  It is a laboratory machine, and is not suited for the field.  It was developed by Sidney James at the Underwriters Laboratories, and it is the basis of the old UL-501 standard for slip resistant floor finishes  as well as the ASTM D-2047 slip testing standard that was the genesis of Brungraber's research and Underwriters Laboratory's current Subject Note 410 (0.50 static coefficient of friction, tested clean and dry with leather sensor pad).  ASTM has two new standards which may breathe new life into the James Machine.  Both deal with a standard method for its use and calibration (F-489 and D-6205).

The results of James machine tests are tracings on a special chart which is ruled to yield a true static coefficient of friction value (a trigonometric tangent of the angle of attack) rather than the Slip Resistance factor of Brungraber Mark I (a trigonometric sine). There is a new manufacturer for the James Machine.  This manufacturer builds one that resembles a modern scientific instrument with automated positioning of the table and computer interfaces.  It also makes some safety claims by guarding pinch points and replacing lead weights with steel.

Michelman, a manufacturer of chemical emulsions and similar products is the only current manufacturer and seller of James Machines.  From the Michelman web site:

In the interest of making a more useful version of his Brungraber Mark-I (ASTM F-462, F-1678) and overcoming some of its operational difficulties, Brungraber developed the Brungraber Mark-II (shown at right).  This functions in a manner somewhat similar to a variable incidence tribometer.  ASTM calls it a Portable Inclinable Articulated Strut Tribometer or PIAST.  It applies its force on an incline and is, therefore, more similar to the English XL (below), than to the Brungraber Mark I.  The Mark-II is the subject of the ASTM F-1677 Standard (again the method only and no prescribed values).  The Mark II applies force to the test pad by gravity with a weight on a slider above the test pad.  The whole slider assembly can be inclined to apply a horizontal component to the application of the force to the pad.  However, unlike the Mark I, which has no horizontal velocity component at the  time of contact of the sensor pad with the surface, the Mark II applies some horizontal force during contact, making its measurement likely not a static value.  It uses the same sensor pads as the Brungraber Mark I. The Mark II is another ASTM Standard device.

Robert Brungraber worked up the spring actuated version of the Mark II machine.  This is expected to make the Mark III more widely accepted because it will reduce its weight.  An operational Mark III device is shown in the photo.  One was present for the 2007 testing at USC.

The Ultimate Weapon in the Slipmeter Wars

The English XL is a design to measure meaningful pedestrian slip resistance with a practical device of a convenient size.  This is called the Variable Incidence Tribometer or VIT.  The variable incidence tribometer  unit applies force to its sensor pad with gas pressure rather than gravity.  It measures SR or Slip Resistance factor.  This device is useful in the evaluation of floors for slipperiness.  Its sensor pad has horizontal velocity (the force of the cylinder is applied at an angle, less than normal, to the test surface).  At the time of contact with the test surface, therefore, the sensor has a horizontal component to its velocity.

Keith Vidal operates a VIT at the 2007 USC workshop:

The VIT has become an ASTM Standard device (ASTM F-1679 and D-5859). It is only its method and not the values which have been standardized by the ASTM.  The VIT is becoming the preferred device as it generally overcomes the drawbacks of the other devices.  It works wet or dry, can change sensor material, is small and relatively light, is simple to use, and does not use gravity as its driving force, so it can be used on inclines.  The principles employed in the VIT can be scaled up or down in size, as a large and heavy weight is not a limiting factor.  Currently, I believe that the English XL, VIT, F-1679, D-5859 device is the only ASTM device which has a program for operator certification, although some operators have not subscribed to certification.  The lack of an ASTM standard value for slip resistance may make operator certification of greater value.  The XL uses a smaller sensor pad than the James and Brungraber machines, which is said to correlate better to shoe contact area at time of slippage.

The English XL (VIT) is in my arsenal of slipmeters.  I have also passed the certification class for its use.

The KETT/Heidon 94i machine applies a force to a slider with a solenoid.  A force transducer horizontal and one vertical out-puts to an internal computer which integrates the values and gives a digital readout of SCF. It can do "wet," but the guts are not sealed. The deep soapsuds of the F-462 standard rule it out for this application, even though the small movements, uniform surfaces, and low friction in bathtubs would seem to suit the model 94 just fine.

An "Average" button allows the collection of multiple test data and the calculation of the mean.

The device has a standard test foot of 1" diameter chrome plated metal. This can be unscrewed with a special tool to allow the use of other sensor feet. The machine has a rubber foot which is a hemisphere. The machine comes with shims to raise the height of the unit to accommodate the height of sensors. The small movements of the sensor make this machine very tricky to use on all but the smoothest, most regular, and uniform surfaces.

This is all in a package about the size and shape of a small meatloaf.  The use of two force transducers and solenoid-applied sliding force seems to be at significant variance from either HPS methodologies. It has a Japanese patent.

This machine is probably fine for the lab or measuring the slip characteristics of a plastic film, but it will not do for real world floor surface evaluation device.

What price elegance?

The horizontal pull slipmeter (HPS, dynamometer or tribometer, hand pulled as ASTM C-1028) that I have on hand is most similar to the ASTM standard (F-609) "Irvine" device.  I use it because it is small and portable.  It is mechanically simple.  It measures both the static and dynamic "mu."  "Mu" is easily computed from the ratio of the two forces actually measured by the slipmeter, plugging the values into the µ=force[parallel]/force[normal] formula (HPC).  My slipmeter has three round feet with replaceable pads for using the standard leather material or other selections (ASTM F-609).  Testing is done dry, as this is the standard method.  Wet measurements are possible, but unreliable (and useless).  The Measurement Instruments version uses a separate weight rather than one affixed under the force meter as in a true Irvine device.

The standard F-609 device uses a winch to pull the drag sled.  Experienced and practiced operators can achieve results by pulling manually that are comparable to those obtained with the winch.

The ASTM C-1028 Device is often called "THE FIFTY-POUND MONSTER."  It is hand pulled, and it uses a large pad and fifty pounds of weight which makes it decidedly cumbersome to use in the field.  It specifies a standard tile.  The tiles are available.  Examples of tiles I have tested are significantly variable, and are therefore a questionable standard.

My self-built and laboratory-configured ASTM C-1028 device.

Some have ignored standards and made their own slip testers.  One consultant I have seen pulls a shoe weighted with a five-pound sugar sack to get a coefficient of friction value.

American Slipmeter has a unit with a digital force meter that would be comparable to a hand pulled HPS or fifty pound monster. Their web page says that they get superior results on wet surfaces.  The site makes no mention of the nature of their sensor pad(s) and does not show any image of the sensor pad(s).  If the pads are mounted on the force meter, it would be similar to the F-609.  If the pads are on a separate weight, it would be more similar to a C-1028.  Drag sleds have the advantage of being inexpensive.  However, since most slip and fall events occur on wet or lubricated surfaces, a drag sled is irrelevant.

This is a pulled device with substantial dwell time that would be unsuitable for wet measurements.  Their claim on the web site ignores current science that drag sled slipmeters are unsuitable for wet testing. 

There is a move afoot in the ASTM F-13 committee to de-standardize several of the devices.  This would include the F-609 and F-462 machines.  This would eliminate two devices while leaving sub-par standards (notably the C-1028) from other committees in place.

*These machines were employed at the ASTM slipmeter workshops held at the University of Southern California  Kinesiology Laboratory in Los Angeles in conjunction with two meetings of the F-13 committee and the Biomechanics of Slips and Falls Seminar in 2004

**These machines were employed at the slipmeter workshop held at the University of Southern California  Kinesiology Laboratory in Los Angeles on November 11, 2007.

Note: The pendulum machines use an edge of a square test foot for measurements.  The James and Brungraber machines use a standard 3" by 3" test foot flat on the surface.  The C-1028 machine uses a 6" by 6" test foot flat on the surface.  The F-609 machine uses three 5/8" round feet flat on the surface.  The VIT uses a 1 1/4" round test foot flat on the surface.  The Mark I and II and the VIT are standardized for wet use.

Recent studies have shown that the James machine-type sensor pads have a tendency to hydroplane on wet surfaces, rendering the values obtained  highly variable.  This problem has been solved by grooving the sensor pads to provide sipes for escape of the fluid.  The VIT (XL) and the two PIAST machines (Mark II and Mark III) now yield consistent and comparable results.

Standards are being written and/or proposed which require a value for COF or slip resistance factor.  Some of these specify the device.  Some require operators to be certified with the instruments.  Some specify the sensor material to be employed.  Some specify a lubricant to be used.

It should be pointed out that a standard of, say 0.5, is generally useless unless methods and materials are specified.

A consensus seems to be building to bypass establishing a number at all.  The choice may be ultimately to devise a series of standard tiles ranging from dangerously slippery to non-slip under  wet or dry conditions, and allow the use of any standard device that has been shown to be able to rank the sample tiles in the standard order.

Then one would test the subject surface with any of the devices and place it along the continuum of the standard tiles to determine whether or not the floor is satisfactory or not for its intended use.

A similar method has been used in mineralogy and gemology for testing hardness and color.  The standards are specimens.

So now there are the following standard devices:    ASTM: James (D-2047, D-6205, F-489), Irvine (F-609), Ceramic Tile Inst. (C-1028 [50# Monster] ), Brungraber Mark-I (F-462, F-1678), Brungraber Mark-II (F-1677), and English XL (F-1679, D-5859).  There are the non-standard TORTUS. Machines which are often used in Europe.

The only standards which actually call out a number are the F-462 (0.04 in soapsuds in bathtubs and silicone rubber [no longer widely available due to the DOW/Corning silicone imbroglio] sensor foot with the Brungraber Mark-I) and the D-2047 (0.50 with James Machine for non-slip floor finishes in the lab with leather sensor foot).

Liberty Mutual has been using the F-609 Irvine device and a value of 0.50 ( with leather sensor feet ) since at least 1969.  UL has been using the James machine as in D-2047.

The ADA regulations lifted a figure of "0.60 required coefficient of friction" from a research report using force plates to determine "required coefficient of friction" as a term of art and adopted it.  This value apparently lives on in an appendix, even though it is not part of the regulations.

Some jurisdictions have been requiring a coefficient of friction as high as .80, again without specifying method or materials.  Higher CF seems better, so .80 must better than .60, and .60 must be better than .50.  Unless the method and material of the sensor foot is specified, any values specified are meaningless.

The ASTM F 1166-07 Standard Practice for Human Engineering Design for Marine Systems, Equipment, and Facilities was updated in 2007.

Among other things, this latest update affirms many of the codes and generally recognized safe practices that I have advocated and/or employed during my involvement with maritime cases. It covers the whole gamut from control design to stairway and ladder dimensions.

Of particular interest is the adoption of a wet coefficient of friction value of 0.60 for decks, walkways, stair treads, and other walking and working surfaces. This is higher than the generally recognized safe standard of 0.50.

At this point, I do not know if this higher requirement is due to an appreciation of the need for greater friction in the marine environment or if this example of the .60 value is another artifact of the discredited ADA "Standard."

A standard value should be established with a specified machine and a specified sensor material or use of standard specimens.  The "standard tile" of the Ceramics Tile Institute is too irregular to serve as a reliable specimen standard.  ANSI 1264.2-2001 calls out a .50 slip resistance factor for safety in walking working surfaces, but this is in the explanatory information and specifically excluded from the standard.

The consensus is developing that .50 wet with the VIT or other inclinable strut machine and a small-area Neolite (tm) sensor pad is a safe floor.  I have no argument with that.

Q:  Was the wooden floor as slippery as butter?

A: No, parquet!

Copyright 2000 - 2011, Jay W. Preston.  Distribution permission granted when this notice is printed in full.  For questions or comments: contact prestonoidaol.com.  The J-P, Plus Design and SAFETYBIZ. are registered service marks of Jay William Preston. Permission for use of specific Safety Subject Information is only granted when this notice is printed in full and Preston has been contacted by phone, fax, or email prior to use.