Elsevier

Safety Science

Volume 40, Issues 7–8, October–November 2002, Pages 593-611
Safety Science

The effects of slip criterion and time on friction measurements

https://doi.org/10.1016/S0925-7535(01)00061-3Get rights and content

Abstract

Different institutions reported different results for friction measurements of identical material combination and surface conditions using identical slipmeters. The objective of this study was to evaluate three factors, slip criterion, sample, and time, that could contribute to such differences with two commonly used slipmeters, the Brungraber Mark II and the English XL. Friction between 16 commonly used footwear materials and three floor materials was measured under four surface conditions with two slip criteria at the interface. Some of the measurements were repeated at a different time. The results indicated that variations due to different samples were probably the smallest statistically among the three factors evaluated. The effect of slip criterion on friction coefficient could be quite significant compared with the effect of time for some material combinations and surface conditions. A more consistent slip criterion could potentially reduce the differences significantly in the results reported among different institutions.

Introduction

Slips and falls are a serious problem. The annual direct cost of occupational injuries due to slips and falls in the USA may be as high as seven billion dollars based on the information provided by an earlier study (Leamon and Murphy, 1995). Falls on the same level accounted for 65% of claim cases and, consequently, 55% of claim costs in the total direct workers' compensation for occupational injuries due to slips and falls (Leamon and Murphy, 1995).

Many slipmeters have been used to assess the slipperiness between footwear and floor surfaces which is a major factor in slips and falls accidents on the same level. These slipmeters can be quite different in their measurement characteristics. Friction coefficient, also called slip resistance, is the typical output of a slipmeter measurement. The Brungraber Mark II and the English XL as shown in Fig. 1, Fig. 2, respectively, are widely used in the USA and were used in this experiment. Therefore, the following literature review is limited to the relevant publications related to these slipmeters. The operating principle of these slipmeters is to simultaneously apply forces parallel and normal to a floor surface with an impact of a footwear sample on the floor at an inclined angle in order to eliminate the dwell time problem with the static friction measurement. The Brungraber Mark II and the English XL are also known as a portable inclinable articulated strut slip tester (PIAST) and a variable incidence tribometer (VIT), respectively. Standard test methods for using these slipmeters are published by the American Society for Testing and Materials (ASTM; American Society for Testing and Materials F-1677-96, 1996, American Society for Testing and Materials F-1679-96, 1996).

The Brungraber Mark II is an inclined-strut slipmeter driven by gravity. The footwear pad impacts floor surfaces at an inclined angle with the vertical direction. If a non-slip occurs at the interface, the inclined angle is increased. Conversely, the angle is decreased if a slip occurs. According to the standard (ASTM F-1677-96, 1996), the starting angle is supposed to be smaller than the angle that a slip is anticipated and it should be increased until a slip occurs. The tangent of the angle is the friction coefficient shown on the slipmeter. As indicated in the standard, it might be necessary to use the average of the maximum friction coefficient that a non-slip occurs and the minimum friction coefficient that a slip happens as the result of the measurement.

The English XL is an inclined-strut slipmeter driven by a pneumatic pressure of 172 kPa (25 psi). According to its standard (ASTM F-1679-96, 1996), the contact force should not be applied to the footwear pad for more than 1 s and a slip occurs when the strut kicks out in an arc with the pneumatic cylinder extending to its full stroke. The operating principle of the English XL is very similar to that of the Brungraber Mark II. A measurement should start from a small angle with a non-slip to a larger angle until a slip occurs and the friction coefficient obtained from the angle at which a non-slip is changed to a slip should be recorded.

The friction coefficient obtained directly with the Brungraber Mark II was compared with that calculated from the ground reaction forces, obtained with a force plate, produced by this slipmeter (Marpet, 1996, Marpet and Fleischer, 1997, Grönqvist et al., 1999, Powers et al., 1999). The results indicated that the friction coefficients obtained from these two methods were in good agreement over different floor surfaces with different surface contaminants for non-slip and barely slip conditions. The friction coefficient measured with the Brungraber Mark II was also shown to have a good correlation (r>0.954) with that measured with a dynamic apparatus to simulate a slip although the absolute values of friction coefficients from these two devices could be quite different (Grönqvist et al., 1999). Fendley et al. (1999) investigated the effect of sanding on the repeatability of friction measurement with the Brungraber Mark II. Their results indicated that the repeatability was better with sand paper of 180 grit than with that of 400 grit over three test surfaces.

The friction coefficient directly obtained with the English XL was shown to be in good agreement with that obtained with a force plate for non-slip conditions (Powers et al., 1999). Chang (1999) investigated the effect of floor tile surface roughness on the friction measured with five commonly used slipmeters. The friction coefficient measured with the English XL was shown to have the best correlation with certain surface roughness parameters under dry and wet surface conditions among the slipmeters evaluated, while the friction coefficient measured with the Brungraber Mark II was the second best.

Although the guidelines for these slipmeters are specified in their respective standards, the details of how these slipmeters ought to be used are somewhat ambiguous. One of the most critical elements in using these slipmeters is to determine whether a slip happens at the interface between footwear sample and floor surfaces. The manufacturers' user manuals for these slipmeters are not clear regarding the slip criterion and operators are left to develop their own criteria. Any movement at the interface can potentially be interpreted as a slip. However, experiments in biomechanics suggested that slips without any fall or injury occur quite often (Leamon and Li, 1990) and a slip will lead to a fall if the sliding speed exceeds 0.5 m/s (Strandberg, 1983). Therefore, some movements can be allowed at the footwear/floor interface without leading to an accident. This concept can be applied analogously to the slip criterion used in friction measurements between footwear and floor samples. Therefore, some movements at the interface can be considered as a non-slip from an accident prevention viewpoint. Different slip criteria could potentially lead to very different results.

Different results of friction measurements from identical material combination and surface conditions using identical slipmeters were reported from different institutions. The possible sources of discrepancy could include different slip criteria, different samples, measurements taken at different times, sample preparation, humidity, and temperature. It is not clear which factors contribute the most. While some of the factors such as measurements taken at different times and different samples are somewhat uncontrollable, slip criterion for the English XL and the Brungraber Mark II can be controlled through a more detailed and consistent measurement protocol.

In this experiment, two slip criteria were used to determine the friction coefficients obtained with these two slipmeters. These two slip criteria are two extreme conditions and could cover all possible criteria that the operators could use. Multiple samples were used for the identical material combinations and identical surface conditions to ensure consistent results across different samples. Some of the measurements with identical samples were taken at a different time. An analysis of variance (ANOVA) was performed to determine if the differences among different slip criteria, different samples and different times were statistically significant.

Section snippets

Test apparatus and design of experiment

All the friction measurements were performed at a temperature of 21±1.7 °C (70±3 °F) and a relative humidity of 50±5%. The 16 footwear sole materials used in this experiment, listed in Table 1, represent typical materials used as shoe soles by shoe industries. The abbreviations in parentheses following the full names of the materials shown in Table 1 will be used throughout this paper. For example, SBR solid with diamond tread will be referred to as SBRS. The full names of the material

Results

A three way ANOVA was applied to the friction coefficient results generated with the 106 different combinations of footwear and floor materials, and surface conditions to determine whether different slipmeters, slip criteria and samples resulted in a statistically significant difference. As expected, the results indicated that the differences in the friction coefficient obtained with different slipmeters were statistically significant. Out of 106, only 10 had non-significant differences with

Discussion

During the experiment, it was observed that significant amounts of footwear material residue accumulated on the stainless steel and vinyl floor surfaces whenever NS and Natural Rubber (NR) were used. These excessive residues affected the formation of water on the floor surfaces during the measurements. It became very time consuming and problematic to carry out friction measurements especially with the slipmeters that require repeated measurements. It certainly affected the consistency of

Conclusions

The effects of slip criterion, time and sample on the measurements of friction coefficient were evaluated for 16 commonly used footwear materials with the Brungraber Mark II and the English XL. The results indicated that the variations due to different samples were probably the least significant statistically among the three factors evaluated. The effect of slip criterion on friction coefficient could be quite significant compared with the effect of time for some material combinations and

Acknowledgements

The author likes to thank Mr. Richard Holihan, Mr. Simon Matz and Ms. Mary Jane Woiszwillo for their assistance during the course of this study. Fruitful discussions with Mr. Mike Wilson at Shoe and Allied Trades Research Association (SATRA), Ms. Ruth Payne at Artech Corporation and Mr. Philip Suraci at American Apparel and Footwear Association (AAFA), formerly known as Footwear Industries of America (FIA), were greatly appreciated. This work was partially supported by a funding from AAFA.

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