Kurt Beschorner

Analysis of the Contribution of Adhesion and Hysteresis to Shoe–Floor Lubricated Friction in the Boundary Lubrication Regime

Slip and fall accidents cause frequent occupational injuries. Despite recent evidence that boundary lubrication is relevant to slipping, few studies have examined the mechanisms that contribute to shoe–floor friction in this lubrication regime. This study aims to identify the contributions of adhesion and hysteresis to friction in boundary lubrication. Three shoe materials (40 Shore A hardness polyurethane, 60 Shore A hardness rubber, and 70 Shore A hardness rubber), two floor materials (vinyl and marble), and six lubricants (water, 1.5xa0% detergent, 25xa0% glycerol–75xa0% water, 50xa0% glycerol–50xa0% water, 75xa0% glycerol–25xa0% water, and canola oil) were tested at a single sliding speed (0.01xa0mxa0s−1). Dry adhesion and hysteresis were quantified for each of the shoe–floor combinations and lubricated adhesion was quantified for all shoe–floor-fluid combinations. The contribution of adhesion and hysteresis to shoe–floor-lubricant friction was affected by both the shoe and floor material due to differences in hardness and roughness. Lubricated adhesion was complex and multifactorial with contributions from the shoe, fluid, shoe–floor interaction, floor-lubricant interaction, and shoe-lubricant interactions. A simple regression model including two fluid coefficients and the dry adhesion friction force was able to predict 49xa0% of the lubricated adhesion friction variability.

Fluid pressures at the shoe-floor-contaminant interface during slips: Effects of tread & implications on slip severity

Previous research on slip and fall accidents has suggested that pressurized fluid between the shoe and floor is responsible for initiating slips yet this effect has not been verified experimentally. This study aimed to (1) measure hydrodynamic pressures during slipping for treaded and untreaded conditions; (2) determine the effects of fluid pressure on slip severity; and (3) quantify how fluid pressures vary with instantaneous resultant slipping speed, position on the shoe surface, and throughout the progression of the slip. Eighteen subjects walked on known dry and unexpected slippery floors, while wearing treaded and untreaded shoes. Fluid pressure sensors, embedded in the floor, recorded hydrodynamic pressures during slipping. The maximum fluid pressures (mean+/-standard deviation) were significantly higher for the untreaded conditions (124+/-75 kPa) than the treaded conditions (1.1+/-0.29 kPa). Maximum fluid pressures were positively correlated with peak slipping speed (r=0.87), suggesting that higher fluid pressures, which are associated with untreaded conditions, resulted in more severe slips. Instantaneous resultant slipping speed and position of sensor relative to the shoe sole and walking direction explained 41% of the fluid pressure variability. Fluid pressures were primarily observed for untreaded conditions. This study confirms that fluid pressures are relevant to slipping events, consistent with fluid dynamics theory (i.e. the Reynolds equation), and can be modified with shoe tread design. The results suggest that the occurrence and severity of unexpected slips can be reduced by designing shoes/floors that reduce underfoot fluid pressures.

A Method for Measuring Fluid Pressures in the Shoe–Floor–Fluid Interface: Application to Shoe Tread Evaluation

OCCUPATIONAL APPLICATIONSu2003This study introduces a method for measuring fluid pressures in the shoe–floor interface. The novel method was then applied to shoes with varying tread depths. The rationale for this approach is that measuring fluid pressures can help to identify the reason for low friction and guide interventions for increasing slip resistance. High fluid pressures were observed in the absence of tread and the presence of high viscosity fluids. Fluid pressures were negligibly small when at least 1.5 mm of tread depth was present or when a low viscosity fluid was present. This study indicates that shoe tread is effective at channeling fluid out from the shoe–floor interface in the presence of highly viscous fluids. The presented methodology may be suitable for testing the performance of tread designs and establishing wear limits for shoe replacement. TECHNICAL ABSTRACT Background:u2003Fluid contaminants cause slipping accidents by reducing shoe–floor friction. Fluid pressures in the shoe–floor interface reduce contact between the surfaces and, thus, reduce friction between the surfaces. A technological gap for measuring fluid pressures, however, has impeded improved understanding of what factors influence these pressures. Purpose: This study aimed to introduce a technique for measuring fluid pressures under the shoe and to demonstrate the utility of the technique by quantifying the effects of tread depth and fluid viscosity on fluid pressures for two different shoes. Methods: A fluid pressure sensor embedded in the floor surface was used to measure fluid pressures, while a robotic slip-tester traversed the shoe over the floor surface. Multiple scans were collected to develop 2D fluid pressure maps across the shoe surface. Two shoe tread types (an athletic shoe and a work shoe), two fluids (high-viscosity diluted glycerol and a low-viscosity detergent solution), and three tread depths (full tread, half tread, and no tread) were tested, while fluid pressures were measured. Results: Untreaded shoes combined with a high-viscosity fluid resulted in high fluid pressures, while treaded shoes or low-viscosity fluids resulted in low fluid pressures. The increased fluid pressures that were observed for the untreaded shoes are consistent with tribology theory and evidence from human slipping studies. Conclusions: The methods described here successfully measured fluid pressures and yielded results consistent with tribological theory and human slipping experiments. This approach offers significant potential in evaluating the slip-resistance of tread designs and determining wear limits for replacing shoes.

Earliest Gait Deviations During Slips: Implications For Recovery

OCCUPATIONAL APPLICATIONS This study identified that deviations in vertical force and knee angle/angular velocity of the slipping leg occur earlier in stance and with greater magnitude than other lower-body motions when a person experiences an unexpected slip. Deviations in the ankle angle/angular velocity and hip angular velocity occurred soon after the knee angle and with smaller magnitudes. These results suggest that foot somatosensation and ankle/knee proprioception may play a role in sensing a slip. Therefore, workers with sensation loss in their foot, ankle, and/or knee may have an impaired ability to respond to slipping. Exposure to simulated slip perturbations may also be part of slip-and-fall prevention programs. To ensure biofidelity of such training perturbations, foot forces should initially be reduced, followed by extension deviations of the knee, and then plantarflexion deviations of the ankle. TECHNICAL ABSTRACT Rationale: Slip-and-fall accidents are a serious occupational and public health concern. The biomechanical deviations due to a slip occurring prior to the postural response onset are still not well understood. Understanding this period of the slip would provide insight into the sensory cues for slipping and may provide guidance in developing slip-training protocols. Purpose: This study examined the timing and magnitude of deviations in vertical force and lower-body joint angles and angular velocities of slips compared to unperturbed walking. Methods: Twenty-nine younger and 29 middle-aged participants walked under normal unperturbed conditions and during an unexpected slip. Joint angle and angular velocity trajectories and ground reaction forces were evaluated. Deviations occurring during the slipping trial that exceeded ±2.58 standard deviations (99% confidence interval) were identified as the onset of deviation from normal walking. Results: Deviation timing (and peak magnitude in the first 200 ms) of vertical force, knee angle, knee angular velocity, ankle angle, ankle angular velocity, and hip angular velocity of the slipping leg occurred at 58 ms (0.17 BW reduction), 116 ms (6.7° extension), 111 ms (87°/s extension), 156 ms (4.4° plantarflexion), 122 ms (86°/s plantarflexion), and 149 ms (18.9°/s flexion), respectively. Deviations normalized to baseline stride-to-stride standard deviation revealed largest deviations in vertical force and knee angle and then knee angular velocity and ankle angle and angular velocity. No age effects were found. Conclusions: These results suggest that foot somatosensation as well as ankle and knee proprioception from the slipping leg may be among the first sensory cues to slipping. Exposure to simulated slip perturbations may be part of slip-and-fall prevention programs. To ensure biofidelity of such perturbations, foot forces should initially be reduced, followed by extension deviations of the knee, and then plantarflexion deviations of the ankle.

A Novel Method for Evaluating the Effectiveness of Shoe-Tread Designs Relevant to Slip and Fall Accidents

Slip and falls account for a large share of occupational accidents. Slips are typically initiated when an insufficient amount of friction is present between the shoe and floor surfaces during walking. Shoe tread is thought to enhance the friction by channeling fluid contaminants away from the shoe and floor surface thus mitigating the fluid’s ability to lubricate the two surfaces and reduce friction. This study presents a novel method for evaluating the effectiveness of shoe tread by measuring fluid pressures during simulated slips. Sensors embedded into the floor measured fluid pressure while a robotic slip-tester simulated a human slip. A work shoe with three different tread depths (no, medium and full tread) was tested against a vinyl floor using a diluted (90%) glycerol and diluted detergent (2% detergent, 98% water) contaminant. Fluid pressures were high in the no tread condition but negligible in the other two tread depth conditions for the diluted glycerol and were negligible for all diluted detergent conditions. The no tread (COF: 0.005) also had lower friction coefficient values than treaded conditions (COF: 0.08-0.38). This study suggests that the effectiveness of tread to reduce the lubricating quality of the fluid can be directly measured using a robotic slip-tester and a fluid pressure sensor embedded in the floor. This method has the potential for developing tread depth recommendations and in evaluating the validity of slip-testers to simulate under-shoe conditions.

The Effects of Floor Roughness on Shoe-Floor Friction Adhesion and Hysteresis

Slip and fall accidents are a major source of occupational accidents. The coefficient of friction (CoF) that is required for gait is approximately 0.2. Floor roughness has been demonstrated to affect the available CoF. Building on this knowledge, this research aims to investigate the effect of changing floor roughness on two components of friction: adhesion and hysteresis. The experiments were carried out using a custom developed pin-on-disk type tribometer. Two common types of rubber shoe material, with Shore A hardness 50 and 95, were slid over ceramic tiles that were prepared to different roughness levels. The tiles were abraded using aluminum oxide media (commonly called “sand blasting”). Three levels of roughness were achieved, measured using the average peak height (Rz) with a stylus profilometer: 16.6 μm, 24.3 μm, and 34.6 μm. The experiments were conducted at 0.01 m sec-1 at a contact pressure of 266.1 kPa under ambient conditions to specifically examine the role of adhesion and hysteresis in the absence of hydrodynamic effects. The coefficient of friction was recorded without lubricant (dry) and lubricated with: 2% detergent solution, canola oil, and SAE 75W140 gear oil. Hysteresis was measured with SAE 75W140 because the high lubricity of the gear oil minimizes adhesion. Adhesion in dry and wet conditions was measured by subtracting the hysteresis from the coefficient of friction. Hysteresis was found to increase from 0.101 to 0.358 for the hard rubber and from 0.269 to 0.611 for the soft rubber when floor roughness was increased from 16.6 μm and 34.6 μm. Higher roughness was also associated with a decrease in dry adhesion from 0.651 to 0.277 for the hard rubber and from 0.435 to 0.041 for the soft rubber. Wet adhesion decreased from 0.285 to 0.049 for soft rubber on detergent. Canola oil, for both hard and soft, and detergent combined with hard rubber did not make a significant difference in the adhesion available. Hysteresis, which is a more robust form of friction in the presence of fluids, was found to be positively correlated with floor roughness while adhesion was negatively correlated with roughness. This indicates that increased floor friction is associated with better floor slip-resistance in the presence of fluids. Abrasively blasting floor tiles to increase the roughness of the floor surface, may lead to improved boundary lubrication friction, particularly when accompanied by soft shoe materials.Copyright

Potential to fall of bipeds using foot kinematics

This research compares normal to unexpected slipping gaits of healthy adults to detect potential to fall. Using various x, y, and z position analyses, including a Root Mean Squared Error (RMSE), significant differences are shown between normal and unexpected slipping gaits. Our results show that after heel strike of the slipping foot, the recovery foot rapidly changes position to restore balance and lower falling potential. We found RMSE of the recovery foot is significantly greater than the slipping foot, and that potential to fall is easily quantifiable through comparing normal to unexpected gaits. This research provides a solid foundations for a generalized understanding of fall potential for various gaits.

Analysis of the Contribution of Adhesion and Ploughing to Shoe-Floor Lubricated Friction in the Boundary Lubrication Regime

Slip and fall accidents cost billions of dollars each year. Shoe-floor-lubricant friction has been shown to follow the Stribeck effect, operating primarily in the boundary and mixed-lubrication regimes. Two of the most important factors believed to significantly contribute to shoe-floor-lubricant friction in the boundary lubrication regime are adhesion and ploughing. Experiments were conducted using a pin-on-disk tribometer to quantify adhesion and ploughing contributions to shoe-floor friction in dry and lubricated conditions. The coefficient of friction between three shoe materials and two floor materials of different hardness and roughness were considered. Experiments were conducted under six lubricants for a sliding speed of 0.01 m/sec at ambient conditions. It was found that the contribution of adhesion and ploughing to shoe-floor-lubricant friction was significantly affected by material hardness, roughness, and lubricant properties. Material hardness and roughness are known to affect adhesion, with increased hardness or increased roughness typically resulting in decreased adhesion. The smoothest shoe material, while also being the hardest, resulted in the greatest adhesional contribution to friction. The roughest material, while also being the softest, resulted in the lowest adhesional contributions under dry conditions. Canola oil consistently resulted in the lowest percent of full adhesion and water consistently resulted in the highest percent of full adhesion, presumably due to the thickness, of the boundary lubrication layer. Ploughing contribution was dependent upon the hardness of the shoe and floor materials. A positive correlation was found between the shoe and floor hardness ratio and ploughing coefficient of friction.Copyright

Biotribology and Human Tribology

In biotribology and human tribology, tribological theories are applied to biological systems and human interactions, respectively. This chapter focuses on the human tribology fields, slip and fall accidents and hand-object interaction, and the biotribology fields, ocular and oral tribology. Slip and fall accidents are caused by low friction between the shoe and floor surface, frequently due to a fluid contaminant. Experimental methods of evaluating shoe-floor friction are most relevant to human slips when mimicking the dynamics of human stepping and the environmental conditions (using common shoes, floors, and/or contaminants). The tribological mechanisms affecting shoe-floor friction are adhesion, hysteresis, boundary lubrication, and hydrodynamic lubrication. Modeling efforts have shown that certain floor roughness parameters correlate well with shoe-floor friction, that adhesion and hysteresis can be simulated with finite element analysis, and that models using Reynolds equation can simulate the hydrodynamic effects. Skin friction is essential for everyday activities such as gripping and manipulating objects. The friction of the outermost layer of the skin, the stratum corneum, is modulated by hydration allowing the body to optimize skin friction through perspiration. Increasing skin friction leads to improved performance by increasing grip strength and fine motor speed. The tribological interaction in the eye, with or without a contact lens, contributes significantly to comfort. Eye discomfort and the disorder dry eye syndrome occur when abnormalities occur to any of the three tear film layers that protect and lubricate the eye. Low friction is essential to the comfort of contact lens. Experiments and models have implicated that adhesion, hysteresis, boundary lubrication, and elastohydrodynamic lubrication may all contribute to eye lens friction. Tooth wear due to abrasion and corrosion is a major threat to healthy teeth and dental restorative surfaces. Tribological principles such as two-body wear, three-body wear, and corrosion along with innovative modeling and experimental techniques have revealed personal risk factors and the effects of behavior on dental wear. Another application of oral tribology is creaminess perception in the mouth. Creaminess of a food is largely dependent on its ability to lubricate the mouth surfaces. Replication of this sensation using low-fat alternatives to traditional high-fat creamy foods has the potential to achieve reductions in obesity. This chapter demonstrates the applications of tribology to biological systems.

USING A 3-DIMENSIONAL VISCOELASTIC FINITE ELEMENT MODEL TO ANALYZE THE EFFECTS OF FLOOR ROUGHNESS, SLIDING SPEED AND MATERIAL PROPERTIES ON SHOE-FLOOR FRICTION

Slip and fall accidents are a major occupational health concern. Important factors affecting shoe-floor friction is critical to identifying and resolving unsafe surfaces and designing. Experimental studies have indicated that several factors including floor roughness, sliding speed and shoe materials affect shoe-floor friction although the precise nature of the mechanism behind this phenomenon is not well understood. In addition, recent studies have suggested that boundary lubrication is highly relevant to slipping and that adhesion and hysteresis are the main contributing factors to boundary lubrication. The purpose of this study is to perform the numerical simulations to analyze the effects of floor roughness (asperity height), sliding speed and material properties on ratio of real area of contact and normal force (relevant to adhesion friction) and hysteresis friction for a viscoelastic shoe material interacting with a hard floor surface. A 3D shoe model and 3D vinyl floor model was simulated with speed 0.01 m/s, 0.5 m/s, 0.75 m/s and 1 m/s in three different floor surfaces. The material property was also varied in the numerical simulations. The study showed that roughness affects both the hysteresis and adhesion friction whereas sliding speed and material property affects the adhesion friction only. The depend ence of adhesion and hysteresis friction on roughness, sliding speed and material property is useful in understanding the shoe-floor friction phenomenon and development of slip resistant sports and work shoes.