Floor Testing Science (Tribology)

The Science of Slips on Floors

A scientific topic know as TRIBOLOGY is the study, in pedestrian walking, of friction, lubrication and interacting surfaces in relative motion (i.e., shoe sole and heel with a floor surface). It has been studied for hundreds of years going back to Leonardo de Vinci as well as Ancient Egyptians. People who work in the field of tribology are referred to as tribologists.

Two fundamental 'laws' of friction exist loosely stating: -

1.    The (dynamic) force of sliding (kinetic) friction between two surfaces sliding against each other is proportional to the load pressing the surfaces together – and – the sliding (kinetic) friction will depend on the sliding velocity, the force applied and the contact area 

2.    The force of the friction is independent of the area of contact between the two surfaces.

The problem of liquids and contaminants in Floor Slip Injury Cases

When 2 surfaces come into contact, if both are DRY then there is instant contact between one surface and another and depending upon the surface roughness / material of both shoe (or bare foot) and ground, a friction will occur that is acceptable to provide a safe floor surface. In Pendulum Testing, this minimum acceptable value for a safe surface is 36PTV (or higher) which will give an adequate minimum potential of slip.  
However, when a floor is WET or CONTAMINATED with Wet or Dry Contaminants, then the floor to foot interaction is sharply reduced; akin in some cases top walking on ball bearings if the friction is that poor! This problem is known as the SQUEEZE FILM LUBRICATION PHENOMENON also known as HYDRODYNAMIC FILM THEORY. It arises when the surfaces move towards each other but any liquid/­contaminant present between the two surfaces cannot be instantaneously squeezed out when the two surfaces collide and the value of friction available is reduced, often greatly so. As a result, the foot moves forwards uncontrolled and a slip commences; how far it continues depending on factors such as: -

-    the force applied; 
-    the instantaneous speed of slip;
-    the ongoing speed of slip;
-    the liquid / contaminant in question; 
-    the surface finish of floor and footwear/bare feet; 
-    the material the shoes soles / heels are made from;
-    the tread pattern of the sole / heel;
-    the contact area; 
-    the gait; 
-    the mass of the person;
-    the slope of the floor. 

Modelling studies have demonstrated that hydrodynamic pressures are typically located near the trailing edge of the shoe surface – i.e., the heel.

How is tribology measured?

Method 1 – The Pendulum Test Equipment

Tribometry is the measurement of friction and wear of tribological systems as performed by a tribometer. The pendulum friction test equipment is a Tribometer measuring the friction of a spring-loaded assembly consisting of a rubber slider mounted on a solid backing plate interacting with a floor surface as it is swung across the floor on a weighted pendulum arm (Hence Pendulum Test Equipment). The rubber of the slider is considered a representative of the heel of the shoe or the heel of a bare foot and the rubber will have different hardness values depending if the test is to represent bare feet (55 IRHD or 57 IRHD) or shod feet (96 IRHD). The frictional force is that force acting at a tangent to the floor surface; the Pendulum Test Equipment measures this force (loss of energy) known as the Coefficient of Friction (CoF) but often simply referred to the Pendulum Test Value or PTV (CoF x 100 = PTV)

Testing is conducted in 3 conditions, dry floor surface, wet floor surface and (if applicable) contaminated floor surface. The measuring method can be used for measurements in off-site laboratory conditions as well as for measurements on site

Method 2 – The Floor Surface Roughness Test

The floor surface roughness affects the tribological behavior of surfaces. Ra and Rz are different parameters of roughness measured in microns (in μm).Ra is the average roughness of a surface. Rz is the difference between the tallest ‘peak’ and the deepest ‘valley’ at microscopic level of the floor surface. FloorSlip only use Rz in Surface Roughness Testing

Non-Slip Floor Design

The design of any kind of tribological material must depend on the requirements of the particular application.

Non-Slip Shoe Design

In tests, shoes without tread were shown to be twice as likely to cause a slip than shoes with tread. Shoe tread is included to reduce / prevent the hydrodynamic pressures by providing channels where fluid can from the shoe to floor interface; Increased tread width and depth are believed to offer higher available friction values, but that is not always the case. The UK HSE in conjunction with Shoe Manufacturers have arrived at a Star Rated System to assist purchasers in selecting the best shoes

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