study on rubber tread block design for high slip ... · slip resistance is often measured using the...

Proceedings 19th Triennial Congress of the IEA, Melbourne 9-14 August 2015

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Study on Rubber Tread Block Design for High Slip Resistance on Liquid-Contaminated Condition

Takeshi Yamaguchi, Yu Katsurashima, Kei Shibata, Kazuo Hokkirigawa

Graduate School of Engineering, Tohoku University, Sendai, Miyagi, JAPAN

This study analyzes the design criteria for obtaining high slip-resistant shoe sole tread pattern the by adept control of various design parameters such as the height, orientation, and hardness of rubber strips. In the typical process, specimens of natural rubber (NR) strip with a width of 3 mm, length of 25 mm, and height of 1, 2, 3, 4, or 5 mm were affixed onto the base rubber sheet (25 mm × 25 mm × 2 mm) at an interval of 2 mm. The Shore A hardness values of the rubber strips were 37, 47, and 61. The specimens thus obtained were slid against a stainless steel plate that was lubricated with 90 wt.% glycerol solution. Slid tests were performed to analyze the effects of hardness, height of the rubber strip, and the orientation angle of the rubber strip with respect to the sliding direction on the static coefficient of friction (SCOF) and dynamic coefficient of friction (DCOF) values. Results indicated that soft (Hs37) and thin rubber strip (height: 2 mm) with an orientation angle of 0° (parallel to the sliding direction) provided the highest SCOF (1.39) and DCOF (1.18) values. For rubber hardness values corresponding to 47 and 61, a thin rubber strip (height: 2 mm) with an orientation angle of 30° showed the highest SCOF and DCOF values. The contact area observed using the total reflection of light demonstrated a larger contact area for the thin rubber strip corresponding to the orientation angle of 0°, thereby resulting in high SCOF and DCOF values. Softer rubber tread block with high stiffness toward frictional force in the sliding direction is effective in facilitating high slip resistance under lubricated surfaces. Practitioner Summary: This study will provide deeper insights on the design criteria for obtaining high slip-resistant shoe sole tread pattern. Keywords: slip, fall, slip resistance, tread pattern, stiffness

1. Introduction

Slipping is one of the frequent events leading to accidents by falling (Courtney, et al., 2001; Nagata and Kim, 2007). It has been reported that most slip and fall accidents in the workplace occur on wet floor (Grönqvist, 1995; Leclercq, et al., 1995; Manning and Jones, 2001). Slip resistance is often measured using the coefficient of friction (COF) between the footwear and the underfoot surface. In particular, while walking, it is imperative to have high static and dynamic coefficient of friction (SCOF and DCOF, respectively) values at the shoe–floor interface to prevent slip initiation and stop a slips.

Surface pattern designs of footwear soles, including the tread pattern (macroscopic pattern) and surface roughness (microscopic pattern), are often helpful to drain liquid from the shoe–floor interface, which in turn, will reduce the squeeze film effect and thus increase the slip resistance, i.e., COF (Chang, et al., 2001; Li and Chen, 2004, 2005; Yamaguchi, et al., 2012; Yamaguchi and Hokkirigawa, 2014). In terms of the tread groove design, the width and orientation of the groove significantly affect the COF between the shoe sole and floor under a wet contaminated condition (Li and Chen, 2004, 2005). Li and Chen (2005) indicated that the use of a wider groove results in higher COF. Besides, designing the groove perpendicular to the sliding direction provides higher COF than that designed parallel to the sliding direction. However, to the best of our knowledge, there are no studies on the effect of groove depth (height of tread block). In addition, the mechanism of high COF due to tread pattern design still remains unclear. Thus, further systematic study is needed to investigate the effect of tread pattern design of shoe sole on COF under wet contaminated surfaces in order to establish the design criteria of high slip-resistant tread pattern.

To this end, the objectives of the current study are as follows: 1) to investigate the effect of hardness, height, and orientation of rubber tread blocks on SCOF and DCOF values under lubrication with glycerol solution and 2) to establish design criteria for realizing a high slip-resistant tread pattern.


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2. Methods

2.1 Sliding friction test 2.1.1 Rubber block specimen

Natural rubber (NR) sheets of thicknesses 1, 2, 3, 4, or 5 mm were cut into strips of length 25 mm and width 3 mm. The Shore A hardness of NR was 37, 47, or 61. Five-pieces of NR strip were affixed with an adhesive onto the base NR rubber sheet (25 × 25 × 2 mm) with a Shore A hardness of 90, as shown in Fig. 1. The interval between each NR strip was 2 mm. 2.1.2 Experimental procedure

Sliding friction test was performed using a specially designed reciprocating linear sliding-type tribometer (SHINTO Scientific Co., Ltd.) The schematic of the experimental setup is shown in Fig. 2. In the typical process, a rubber block specimen affixed to the steel sample holder was slid in a linear manner against a stainless steel plate (500 mm × 60 mm × 1 mm) with a surface roughness Ra of 0.1 µm. The linear motion stage was driven using a ball screw servo motor. The specimen was applied a normal load of 66 N, including a dead weight (60 N) and jig (6 N). The frictional force was measured with a push–pull type digital force gauge (DS2-500N, IMADA CO., LTD.), and corresponding friction force data were recorded using a digital data logger. Other experimental parameters are as follows: sliding velocity was 0.2 m/s, sliding distance for a single stroke was 0.3 m, and the number of reciprocation was 10. A lubricating surface was created using 90 wt.% aqueous solution of glycerol (viscosity η: 0.224 Pa･s). The sliding velocity and lubrication conditions adopted in this study were chosen on the basis of the conditions of the evaluation test of slip-resistance for safety footwear in JIS T8101:2006 (Nagata, 2008). As shown in Fig. 3, the orientation of the longitudinal rubber strip with respect to the sliding direction was 0°(parallel to the sliding direction), 30°, 45°, 60°, or 90°(perpendicular to the sliding direction).

Figure 1. Schematic of the dimension of the rubber specimen; h is the height of the rubber strip (1, 2, 3, 4, or 5 mm)

Figure 2. Schematic of the experimental apparatus used in this study


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(a) θ = 0° (parallel) (b) θ = 30° (c) θ = 45° (d) θ = 60° (e) θ = 90° (perpendicular)

Figure 3. Orientation of the longitudinal rubber strip with respect to the sliding direction

Figure 4. Representative graph of variation in the coefficient of friction against time, hardness of rubber: Hs61; height of rubber strip: 5 mm; orientation angle of rubber strip: 90° (perpendicular to the sliding direction)

The coefficient of friction (COF) was calculated by dividing the frictional force by the normal load. The SCOF value was determined as the first peak of the COF after the onset of sliding. The DCOF value was determined as the mean value of the COF at a constant stage velocity (for 1.0 s). Figure 4 shows the representative graph of variation in the coefficient of friction against time. The mean value of 20 SCOF values (10 values each for both forward and backward sliding) and that of 20 DCOF values (10 values each for forward and backward sliding) were used for the analysis. The sampling frequency of friction force data was 100 Hz.

2.2 Observation of the contact area

Contact area was observed during the sliding test between the rubber block specimen and the glass plate (BK7, thickness: 20 mm) of surface roughness Ra < 0.01 µm. Other experimental parameters such as the normal load, sliding velocity, and lubrication condition were the same as that adopted for the sliding test on steel plate, as described in section 2.1. Fibre light was used to illuminate the contact interface between the


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rubber block sample and the glass plate. The contact interface was observed using a high-speed camera (VW-6000, Keyence Corporation) through a mirror mounted beneath the glass plate (Yamaguchi, et al., 2012). For the angle of incidence corresponding to 55.5°–59.7°, total reflection can occur only at the contact interface between the glycerol solution and the glass plate. Under such condition, the interface can be observed as a dark section. On the other hand, at these angles, the contact interface of the rubber block and the glass plate can be observed as a bright section. 3. Results and discussion

3.1 Effect of hardness, height, and orientation of rubber strip on SCOF and DCOF

Figures 5 and 6 show the effect of the orientation angle of the rubber strip on the mean SCOF values and DCOF values, respectively. The error bars indicate the standard deviation. When the Shore A hardness of the rubber strip was 37 (Figs. 5(a) and 6(a)), the mean SCOF and DCOF values decrease with an increase in the orientation angle, irrespective of the height of the rubber strip. For this hardness value, the orientation angle of 0° (parallel to the sliding direction) provided the highest SCOF values (0.99–1.39) and DCOF values (0.67–1.18), whereas that of 90° (perpendicular to the sliding direction) resulted in the lowest SCOF values (0.38–0.78) and DCOF values (0.28–0.74). On the other hand, for the Shore A hardness values of 47 and 61, the SCOF and DCOF values had a peak at the orientation angle of 30° (Fig. 5(b) and (c); Fig. 6(b) and (c)). Besides, for orientation angles greater than 30°, the SCOF and DCOF values corresponding to the Shore A hardness values of 47 and 61 are higher than those corresponding to the hardness value of 37.

(a) Hs37 (b) Hs47 (c) Hs61 Figure 5. Effect of the orientation of rubber strip on the mean SCOF values

(a) Hs37 (b) Hs47 (c) Hs61 Figure 6. Effect of the orientation of rubber strip on the mean DCOF values


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(a) Hs37 (b) Hs47 (c) Hs61 Figure 7. Effect of the height of rubber strip on the mean SCOF values

(a) Hs37 (b) Hs47 (c) Hs61 Figure 8. Effect of the height of rubber strip on the mean DCOF values

Figures 7 and 8 show the effect of the height of rubber strip on the mean SCOF and DCOF values, respectively. The error bars indicate standard deviation. For the Shore hardness values corresponding to 37 and 47, the SCOF and DCOF values decreased with an increase in the height of the rubber strip. Thus, for such hardness conditions, a thinner rubber strip is often preferred to obtain higher SCOF and DCOF values. However, the effect of the height is found to be less significant for the rubber block with hardness of 61 (Figs. 7(c) and 8(c)).

The results obtained in this study indicate that the soft (Hs37) and thin rubber strip (h = 2 mm) with an orientation angle of 0° provides the highest SCOF (1.39) and DCOF (1.18) values under lubrication in glycerol solution. For the hardness values corresponding to 47 and 61, a thin rubber strip (h = 2 mm) with an orientation angle of 30° demonstrated higher SCOF and DCOF values. 3.2 Observation of contact area

Figure 9 shows the snapshots of the contact area between rubber block specimen and glass plate before and during sliding. The bright part in the rubber strip is the contact area between the rubber strip and glass plate, where the glycerol solution was drained. The area enclosed by a white dashed line in the snapshot while sliding corresponds to the darker area, where the rubber is not in contact with the glass plate. As can be seen in the figures, the section near the posterior edge of each rubber strip is darker than that in other sections. This implies that the section near the posterior edge was detached from the glass plate. The detached area of the posterior part at the orientation angle of 0° (Fig. 9(a)) is small when compared to that at the orientation angle of 45° (Fig. 9(b)). As shown in Figs. 9(b) and (c), the detached area corresponding to the rubber strip height of 5 mm was larger than that corresponding to the strip height of 2 mm.

The results obtained in this study suggest that the thin rubber strip at the orientation angle of 0° was successful in securing a large contact area with the glass plate because of the smaller detachment of the


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posterior strip part. Therefore, these conditions could result in high SCOF and DCOF values. On the other hand, larger height of the strip and the orientation angle of 90° produced significant detachment of the posterior part of the strip, resulting in a low contact area. 3.3 Criteria for designing rubber tread block with high friction

The detachment of the posterior part of the strip from the mating surface could possibly be related to the stiffness of the rubber strip against the frictional force in the sliding direction. When the stiffness of the strip is high, the deflection of the strip due to frictional force tends to be small. Moreover, the detachment area of the posterior part of the strip will also be small, which could secure a larger contact area (Fig. 10(a)). On the other hand, with lower stiffness, the area of the detachment of the strip tends to be large and the contact area becomes small (Fig. 10(b)).

Stiffness K [N/m] with respect to the frictional force in the sliding direction can be expressed by the following equation:

, (1)

, (2),

where E [MPa] is the Young’s modulus of the rubber strip (Hs37: 5.4 MPa; Hs47: 6.6 MPa; Hs61: 10.7 MPa), I [m4] is the moment of inertia of the area of the rubber strip, b [m] is the width of the rubber strip: 3 mm, L [m] is the length of the strip: 25 mm, and θ is the orientation angle of the strip.

(a) h = 4 mm, θ = 0° (b) h = 4 mm, θ = 45°

(c) h = 2 mm, θ =90° (d) h = 5 mm, θ = 90° Figure 9. Snapshots of the contact area between the rubber specimen and the glass plate; the area enclosed by a white dashed line corresponds to the rubber strip that is not in contact with the glass plate; white arrows indicate the sliding direction of the rubber block specimen.


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(a) High stiffness (b) Low stiffness

Figure 10. Schematic illustrating the relationship between contact area and deflection of rubber strip due to frictional force

Figures 11 and 12 show the relationship between the stiffness of the rubber strip and the mean SCOF and DCOF values, respectively. As shown in the figures, the mean SCOF and DCOF values show positive correlation with the stiffness for Shore hardness values of 37 or 47. However, the correlation between the SCOF and DCOF values and the stiffness was relatively lower when the hardness of the rubber strip was 61. Thus, when using softer rubber as the rubber strip, it is possible to effectively increase the stiffness with respect to the frictional force by adept control of the shape design parameters such as height and orientation of the rubber strip, and thereby increases the SCOF and DCOF values. However, when using harder rubber as the rubber strip, increasing stiffness by controlling the shape design parameters does not have a significant impact on increasing the SCOF and DCOF values.

(a) Hs37 (b) Hs47 (c) Hs61 Figure 11. Relationship between stiffness of rubber strip and mean SCOF

(a) Hs37 (b) Hs47 (c) Hs61

Figure 12. Relationship between stiffness of rubber strip and mean DCOF


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4. Conclusions

The laboratory-scale investigation performed in this study demonstrated that the hardness, height, and orientation of the rubber strip significantly affected the SCOF and DOF values under lubrication with glycerol solution. Results indicated that the soft (Hs37) and thin rubber strip (h = 2 mm) with an orientation angle of 0° (orientation of the longitudinal direction of the rubber strip parallel to the sliding direction) provided the highest SCOF (1.39) and DCOF (1.18) values. For rubber hardness values of 47 and 61, the thin rubber strip (h = 2 mm) with an orientation angle of 30° showed higher SCOF and DCOF values. The contact area observed using the total reflection of light demonstrated that the posterior part of the rubber strip was not in contact with the mating glass surface and that the contact area of the thin rubber strip at the orientation angle of 0° was successful in securing a larger contact area, resulting in high COF values. A softer, rectangular-shaped rubber tread block with high stiffness with respect to frictional force is effective in facilitating high slip resistance in lubricated surfaces. Further studies, in particular those with other slip resistance evaluation tests, are needed to confirm the design criteria of the shoe tread block pattern. Acknowledgements

The authors would like to thank Kohshin Rubber Co., Ltd. for preparing and providing the rubber sheet. References Chang, W. R., I. J. Kim, D. P. Manning, Y. Bunterngchit. 2001. “The role of surface roughness in the measurement of

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study on rubber tread block design for high slip ... · slip resistance is often measured using the...

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