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Slip Resistance Matrix: Keeping Occupational Safety on its Feet

Germany continues to lead the way on slip and fall research, however we in the US are beginning to catch up. Our friends at Wuppertal University in Northwestern Germany are developing an extensive floor / shoe matrix which will help make choosing workplace safety footwear easier. Click the link below to view the article: 

Wuppertal Slip Resistance Matrix.pdf

There are no less than 14 different phases of the human gait cycle, plus some five ranges of motion for the associated joints. Thus it would be impractical, if not impossible, to create a device capable of simulating the walking characteristics of most human beings. While slipping and falling on a surface during walking may seem like a simple thing to analyze, it involves much more than meets the eye. As long as the available friction of a walkway surface is greater than the required friction needed for the ambulation process, no slipping should occur. The problem is that different people have different requirements in different circumstances.

Experiment: Visit any large donation store that sells used footwear. Turn over half a dozen or so random pairs of shoes. You should observe a variety of wear patterns on the soles and heels. Some may even exhibit different wear patterns between the left and the right shoes! Take a close look at your own shoes. You may be surprised at your own wear patterns. These "contact patches" are the vital areas of the shoes where the most interaction with the walking surface occurs. As you can see in the photo below, they are much smaller than the entire sole. The heel of a worn shoe is usually slightly rounded.

It is our belief that trying to simulate human ambulation with a mechanical device is the wrong approach to tribometry (measurement of friction). We do believe however, that it is practical to correlate a test device to a method or model that includes humans in the measurement loop. The BOT-3000 has established a statistically significant wet DCOF correlation coefficient (0.879) to the German ramp (DIN 51130), and a 0.926 correlation coefficient to the GMG-100 (DIN 51131) using SBR rubber during a study performed at Wuppertal University (Germany) in 2008 using many types of flooring.

Some Questions and Answers Regarding the Current State of Meaningful Tribometry

·         Q: Is it true that only a VIT (variable incidence tribometer), VAT (variable angle tribometer),  or PIAST (portable inclineable articulated strut slip tester) can provide an accurate reading of floor surface traction levels because they better simulate human ambulation?

·         A: Most people don’t kick or stomp the floor surface as they stride along.  A human being traversing a flat walkway surface (at normal walking velocity) first plants the heel, and then gently rolls through the step until the toe finally lifts off. There is no mechanical tribometer or slip tester that can accurately and truthfully emulate the complex mechanics of human ambulation, despite what the articulated strut type device manufacturers may claim.

Note: Slow motion high speed video often reveals varying degrees of flexure and mechanical instability within articulated strut type devices that employ springs, actuators, or substantial weights which are rapidly activated. The sensor, or test foot may skip or bounce upon initial floor surface impact, resulting in a greater uncertainty of measurement.

·         Q: I have been informed that drag sled type devices don’t work because of “stiction” or excessive adhesion effects when surface dwell or residency times exceed 0.2 seconds. Is this really true?

·         A: The word “stiction” was coined from the two words “static” and “friction” according to many common dictionary definitions. Some have attempted to use a fabricated word “sticktion” (with a “k”) to describe any number of adhesive effects based upon the contact or “dwell” times of two intimately joined flat surfaces. This effect may be easily demonstrated with relatively soft rubber compounds, however soft rubber compounds are not normally used for COF testing. Both neolite and SBR sensor materials used in floor testing standards are exceedingly hard (greater than 90 on the shore A hardness scale). The BOT-3000 employs a unique curved sensor shape which further reduces the contact area, reducing any stiction effects. Currently, we are unaware of any documented scientific evidence as to the full effects of stiction as it specifically relates to wet SCOF testing. Our own laboratory experiments have shown that it usually requires 10 to 20 seconds of loaded dwell time to create any noticeable stiction or adhesion effects with curved rubber sensors. The BOT-3000 minimizes contact dwell time to less than 2 seconds.

·         Q: Can drag sled type devices measure wet surfaces correctly since much of the liquid is “squeezed” out from beneath the contact pad, as downward force is applied?

·         A: If you observe a person step onto a wet sheet of glass from below a platform, you will notice that the very same effect occurs.

Note: The German Government’s Workers Insurance Institute (similar to Workmans Comp. in the U.S.) uses portable drag-sled technology to assess risk levels of walkways in factories, office buildings, retail stores, restaurants, warehouses, commuter terminals, schools, etc. If this technology was questionable or unreliable, this highly productive country would surely be using something else to help protect their workers and fellow citizens!

·         Q: Can’t an analog or mechanical dial be just as good as a digital readout for reading and recording precision test results?

·         A: Since the 1990’s most modern scientific measurement instruments have converted to digital readouts in order to accurately and repeatedly produce readings beyond one decimal place. The centuries-old mechanical dials and pointers are typically fragile, require periodic calibrations or adjustments, and are subject to visual parallax effects by the observer.

·         Q: Is a hand-written set of data collection results just as valid as any other form of reporting?

·         A: A clear and legible test result embedded into a non-manipulatable time/date stamped hardcopy printout is a superior and preferred form of test result reporting, than is simply scribbling a result by hand on a note pad. Sometimes numbers can be confused, forgotten, or misread if the recorder’s handwriting isn’t perfect.

·         Q: Isn’t the term “slip index” just another name for coefficient of friction?

·         A: In the U.S., coefficient of friction is commonly measured and expressed in two recognized forms; static coefficient of friction (SCOF) or dynamic coefficient of friction (DCOF), sometimes referred to as kinetic COF. Anything else can be misleading or deceptive. Most articulated strut type tribometers measure a complex combination of both SCOF and DCOF, so they do not report readings in industry-standard COF terms. The word “index” simply means graduations on a scale and has no obvious relationship to SCOF or DCOF measurement. If a specification or standard test method calls for measurements to be expressed in terms of SCOF or DCOF, then a slip index reading should be converted using a formula based upon a true correlation to COF, and be exclusive to that device.

·         Q: Do you think it would still be acceptable to use one of the articulated strut tribometers that had their standards withdrawn several years ago?

·         A: In the fields of science, health, and law, standards are of utmost importance. To perform a test, and then attempt to persuade a court that your test is meaningful, depends entirely on data comparison to an industry accepted standard. Without a standard by which to compare your test results, you are left with nothing more than unjustified opinion. Those devices had their standards withdrawn by ASTM for legitimate reasons and no one should pretend that they are still current, valid or relevant. 

·         Q: Has the BOT-3000 completed the ASTM F2508-11 “calibration standard” yet?

·         A: Yes, last year and the results are posted on this website; it should be noted however, that ASTM has not proven that the four tiles supplied were indeed traceable to the same tiles employed in the particular USC study that the standard was based upon. In fact, there were no certificates of any kind provided with the purchased tile set from ASTM. It should also be noted that one of the tile types provided was uncoated VCT, which technically is a faulty walkway surface choice since VCT is normally coated with a finish in installed applications.

·         Q: If my tribometer can pass the criteria found in the ASTM 2508-11 calibration standard, doesn’t that qualify it to produce accurate and repeatable results in slip resistance measurement applications?

·         A:  The ability to correctly rank a set of four surfaces, even with a high degree of certainty, doesn’t fit the scientific definition of calibration. If I were to put on a blindfold and correctly rank the four tiles with my hand, would that ability make my hand a calibrated tribometer? It is very important to compare slip resistance measurements to a range of scientifically established reference values using well-proven, credible test methodologies.