INFLUENCE OF HYDRODYNAMIC FLUID PRESSURE AND SHOE TREAD DEPTH ON AVAILABLE COEFFICIENT OF FRICTION Gurjeet Singh/University of Wisconsin- Milwaukee Kurt Beschorner/ University of Wisconsin- Milwaukee ABSTRACT Slip and fall accidents are a major occupational health concern. Identifying the lubrication mechanisms affecting shoe- floor-contaminant friction under biofidelic (testing conditions that mimic human slipping) conditions is critical to identifying unsafe surfaces and designing a slip-resistant work environment. The purpose of this study is to measure the effects of varying tread design, tread depth and fluid viscosity on underfoot hydrodynamic pressure, the load supported by the fluid (i.e. load carrying capacity), and the coefficient of friction (COF) during a simulated slip. A single vinyl floor material and two shoe types (work shoe and sportswear shoe) with three different tread depths (no tread, half tread and full tread) were tested under two lubrication conditions: 1) 90% glycerol and 10% water (219 cP) and 2) 1.5% Detergent-98.5% (1.8cP) water solutions. Hydrodynamic pressures were measured with a fluid pressure sensor embedded in the floor and a forceplate was used to measure the friction and normal forces used to calculate coefficient of friction. The study showed that hydrodynamic pressure developed when high viscosity fluids were combined with no tread and resulted in a major reduction of COF (0.005). Peak hydrodynamic pressures (and load supported by the fluid) for the no tread-high viscous conditions were 234 kPa (200.5 N) and 87.63 kPa (113.3 N) for the work and sportswear shoe, respectively. Hydrodynamic pressures were negligible when at least half the tread was present or when a low viscosity fluid was used despite the fact that many of these conditions also resulted in dangerously low COF values. The study suggests that hydrodynamic lubrication is only relevant when high viscous fluids are combined with little or no tread and that other lubrication mechanisms besides hydrodynamic effects are relevant to slipping like boundary lubrication. INTRODUCTION Slip related fatal injuries are the second most fatal work related injury and 14 % of total fatal occupational injuries were due to falling events in 2009 [1-3]. Slipperiness is a major factor contributing between 40 % and 50 % to fall injuries [4]. A floor surface often becomes slippery when a fluid contaminant lubricates the shoe-floor interface. Boundary, mixed or elastohydrodynamic lubrication have all been suggested to be relevant to slipping but the exact mechanism(s) have not been verified experimentally [5]. Friction plays an important role in slipperiness and probability of a slip decreases when the difference between available and required coefficient of friction decreases [6]. Therefore available friction typically must be above the required value of friction for normal walking to avoid a slip [7]. The variation of coefficient of friction in different lubrication regimes as speed increases is explained by the Stribeck [8] curve. In boundary lubrication regime, the fluid contaminant thickness and hydrodynamic pressure is minimal but increases as the lubrication transitions to mixed and elastohydrodynamic lubrication [9]. During the transition from boundary to mixed and elastohydrodynamic lubrication, the asperity contact also decreases , causing a decrease in coefficient of friction [9]. The purpose of the study is to measure the effects of shoe tread design, tread depth, viscosity of contaminant on underfoot hydrodynamic pressure and the load supported by the fluid (i.e. load carrying capacity). Furthermore, the effect of hydrodynamic pressure development on the available coefficient of friction will be analyzed. by a custom-developed robotic slip simulator tribometer (SST). 1 Copyright © 2012 by ASME Proceedings of the ASME/STLE 2012 International Joint Tribology Conference IJTC2012 October 7-10, 2012, Denver, Colorado, USA IJTC2012-61173 Downloaded from http://asmedigitalcollection.asme.org/IJTC/proceedings-pdf/IJTC2012/45080/49/4227127/49_1.pdf by Siemens Energy, Inc user on 11 February 2023