The field of Tribometry as a scientific endeavor has a long and prestigious history that began with Leonardo di Vinci in the 15th century. However, our main concern here deals with friction and its precise measurement for the safety of pedestrian ambulation on a variety of walking surfaces.
Experiments have shown that, generally, there is an empirical relationship between the maximum friction (F) necessary for prevention of slipping versus the weight (W) of an individual.
i.e. F = µ W
Here, the value µ is known as the coefficient of friction. F can have any value until it reaches a maximum, at which point slipping occurs. This critical value of µ is called the static coefficient of friction (SCOF). Once slipping occurs, the µ value is generally lower and is known as the dynamic coefficient of friction (DCOF).
DCOF is never greater than SCOF, but can occasionally be equal in value. This is the simplest explanation. Thus, in order to determine µ, one has to measure both F and W (assuming a flat and not an inclined surface).
Unfortunately, the slip and fall phenomena accounts for approximately 20,000 deaths per year, making it the second leading cause of accidental death, just behind car accidents, which cause about 37,000 deaths per year.
Human gait analysis has determined that a minimum SCOF of about 0.3 is required to prevent slipping. A “consensus “ threshold value of SCOF = 0.5 could be considered a safe value.
What are today’s Tribometry’s challenges? The problems are manifold.
The nature and physical properties of surfaces in contact.
Environmental factors such as, wetness, dirt and grease, etc.
Measurement techniques and equipment.
The most serious of these multifaceted challenges is the fact that virtually all Tribometers in use today produce grossly divergent values for both SCOF and DCOF. Moreover, most of them measure solely DCOF. Some of them, in fact, measure neither SCOF nor DCOF. One enterprising entrepreneur even introduced his own non-scientific terminology (e.g. slip resistance).
Furthermore, there is one serious phenomena which none of the these dozen or so Tribometers have been able to eliminate. This phenomena is called “Stiction.” Whenever two surfaces are in contact for a finite amount of time, an increase of their bonding inevitably takes place. Whereas, a pedestrian’s shoe-surface contact is of a short, yet finite duration, and thus a smaller value of SCOF is involved.
How does the new GS-1 resolve this conundrum?
The GS-1 actually measures Stiction as its first measurement and then eliminates it.
The GS-1 takes 10 successive measurements and rejects the first. The subsequent equal SCOF and equal DCOF measurements are displayed on a computer screen. The visual representation authenticates the quality of these unique measurements. An automatic calculation displays the average values of SCOF and DCOF to three significant figures on a computer screen and allows for ready print-out.
The GS-1 provides absolute µ values, since both the friction (F) and the weight (W) are independently measured and recorded. These unique features allow the GS-1 to be designated as the Gold Standard of the industry.
Fred M. Johnson, Ph.D.
Professor of Physics Emeritus