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Nelson & Associates :: Premises :: Coefficient of Friction Testing

The Causes and Prevention of Slip-Type Falls

Select Definition of Terms Pertaining to the Measurement of Coefficient of Friction


On wet surfaces, the slipperiness (slip index) of a walking surface is a function of the ability of the surface asperities to penetrate the hydrodynamic squeeze film. That is, the potential for heel-slid that produces the most common disabling fall on smooth surfaces is controlled by installing a walking surface having asperities sufficient to penetrate the hydrodynamic squeeze film and arrest the sliding tendency by engaging the shoe bottom. The sharpness of the asperities is the primary factor in slip resistance.

Asperity

n., pl. asperities

  • Roughness or harshness, as of surface, sound, or climate
  • A slight projection from a surface; a point or bump.
  • Middle English asperite, from Old French asprete, from Latin asperits, from asper, rough.
Source: The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2000 by Houghton Mifflin Company.

Hydrodynamic squeeze film

  • A technical term used in lubrication theory referring to the lubricant layer between two bodies.

Adhesion and Sticktion

  • Adhesion (as usually applied to dry testing of walking surfaces) and sticktion (as applied to wet testing of walking surfaces) arises as a function of the test slider’s residence time on the surface. If there is any delay between the instant of surface contact and the application of the horizontal force, sticktion or adhesion will occur.

  • Water is a very tenuous film, and any residence time of the shoe on the wet surface causes the water film to be squeezed out of the interface between show bottom and the walking surface so that the tendency to slip is greatly reduced. This is called sticktion. On surfaces wet with water, residence times shorter than 0.2 seconds (200 milliseconds) are known to produce significant sticktion that can result in slip meter readings that are higher under wet conditions than would be obtained on the same surface in a dry state.

  • On dry surfaces, the longer the residence time, the greater adhesion will be. This is because as the testfoot rests on the surface, its surface begins to conform to the microtopography of the test surface.

Static Coefficient of Friction (SCOF)

  • The force required to initiate relative motion between an object and a surface it is in intimate contact with. SCOF is the ratio of the force required to move the object to its mass. In variable incident strut instruments the SCOF is the tangent of the angle from the vertical at which slipping begins to occur.

Dynamic Coefficient of Friction (DCOF)

  • Defined in pedestrian tribometry as the force required to keep a sliding object in motion once slipping has begun – that is, DCOF is measured during the sliding stage after a slid begins. Special note: DCOF is sensitive to velocity, and therefore not possible to measure until it is known how fast a particular slip takes place. In strobe photography of pedestrian heel slides, it can be seen that the foot accelerates from the instant of slip initiation until the heel leaves the floor, so meters that move at a constant speed are not measuring true SCOF relative to a pedestrian foot slide.

Slip Resistance

  • Slip resistance is defined in ASTM F1637-95, Practice for Safe Walking Surfaces as "the relative force that resists the tendency of the shoe or foot to slide along the walking surface." Slip resistance is relative to a combination of factors including the walkway surface, the footwear bottom, and the presence of foreign material between them.

  • A walking surface is slip-resistant to the extent that surface asperities are sharp enough to penetrate the hydrodynamic squeeze film that otherwise provides a lubricant to promote sliding.

Slip Index

  • Slip Index is short for Slip Resistance Index. The output scale of the English XL VIT is denominated in terms of Slip Index. The output of the English XL VIT is SCOF if the surface is dry. If it is wet or otherwise contaminated, the output will be the slip resistance index.

Tribometry

  • The science of the measurement of friction – or in the case of pedestrian safety and the prevention of slip-type falls, it is the measurement of traction.


Reference: Unless otherwise indicated, the above material is cited from English, William, Pedestrian Slip Resistance – How to Measure It and How to Improve It, 2nd Edition, 2003.

See also: http://www.exceltribometers.com


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