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The friction coefficient between footwear and floor surface has been used to assess floor slipperiness [1-3]. The friction coeffi-cient between the footwear pad and the floor is affected by the footwear material, floor, and contamination condition, [4-7]. For all the floor and footwear material con-ditions, the friction coefficient values under the oily conditions were in the range of 0 to 0.02 regardless of the groove width and groove orientation used in the study. The oil seemed to provide a significant lubricating effect on the floors for the oily conditions. This was consistent with the findings in the previous study, [8]. For the groove orienta-tion, the 0° and 45° groove conditions had significantly higher friction coefficient val-ues than that of the 90° condition. This im-plies that shoe soles with tread grooves parallel to the walking direction are unde-sirable as compared to the other two tread groove orientation designs because they resulted in lower friction coefficient when stepping on wet, water-detergent, and some oily contaminated floors.Tread groove designs are helpful in liquid drainage between the shoe sole and the floor, thus facilitating contact between the two surfaces, [9]. Both tread groove width and groove orientation significantly affect-ed the friction coefficient. The footwear pads with 0.9 cm wide grooves resulted in higher friction coefficient than the 0.3 cm grooved footwear pads. The 0° and 45° groove oriented footwear pads provided higher friction coefficient values than the 90° pads in most of the footwear materi-al/floor/contamination conditions. Tread grooves with the two widths tested in this study were ineffective in increasing friction on a floor contaminated by soybean oil. Tread grooves designed perpendicular and/or oblique to the walking direction should be open and wide enough to achieve better drainage capability on wet and water-de-tergent contaminated floors.

Very little attention has been directed to-ward the effect of the surface roughness of the shoe soling, [10]. For example, measure-ment of the friction coefficient of soles is usually undertaken on newly moulded or only lightly abraded samples. It was men-tioned that the microscopic roughness of the soling surface is a major determinant of slip-resistance on lubricated surfaces, [11]. Research revealed significant correlations between surface roughness of shoes and friction coefficient for a given floor surface. It was found that, [12, 13], abrasion of rub-ber soles in steps with increasingly coarse grit gradually raised the roughness in paral-lel with a rise in the friction coefficient on water wet surfaces and that both rough-ness and friction coefficient fell during sub-sequent polishing. Besides, a significant correlation between the rank order of fric-tion coefficient of footwear on water-wet, oily, and icy surfaces was shown. Any specification of flooring by measuring friction coefficient based on dry surfaces could lead to an increase in the number of injuries caused by slipping on the wet sur-faces. An experiment in which five pairs of shoes were soled with the same rubber compound was described, [14]. Four of the pairs were abraded by different grades of grit to produce a range of roughness values. The friction coefficient of the five soles was then measured repeatedly by the walking traction method on wet surfaces including glazed wall tiles, vinyl asbestos coated with the wax floor polish, and both sides of

Friction · Rubber · Sliding · Oil · Water · Ceramics

The static friction coefficient caused from sliding of rubber specimens against ceramic surfaces lubricated by oil and oil diluted by water was investi-gated on test specimens with V-grooves at 50, 100 and 150 N normal load.For oil lubricated ceramic, the friction coefficient decreased with increasing height of the grooves. The decrease may be from the well adherence of oil on the rubber surface and a film forma-tion. A mixture of oil and water dis-played values of friction much lower than that observed for oil lubricated condition. Friction coefficient de-creased as the height of the grooves increased and consequently the capac-ity of the groove increased to restore lubricant fluid and feed once again into the contact area as load decreases.

Reibung von Gummi beim Rut-schen auf Keramikoberflächen, Einfluss von Öl und Öl-Wasser Gemische

Reibung · Gummi · Rutschen · Öl · Wasser · Keramik

Der Einfluss von Öl und Öl-Wasser Gemischen auf den statischen Rei-bungskoeffizienten beim Rutschen von gekerbten Gummiproben auf kera-mischen Oberflächen wurde bei Nor-mallasten von 50 bis 150 N ermittelt. Auf den mit Öl belegten Keramikober-flächen nimmt der Reibungskoeffizient mit der Höhe der Kerben ab. Dieser Effekt kann durch die Bildung eines Ölfilms auf der Gummioberfläche verursacht sein. Wird jedoch ein Öl-Wasser Gemisch eingesetzt, so nimmt dieser Effekt deutlich zu. Der Reibungs-koeffizient nimmt ab wenn die Höhe der Kerben ansteigt und die Möglich-keit erhöht wird das Schmiermittel erneut auf die Kontaktfläche aufzubrin-gen, wenn die Normallast abnimmt.

AuthorsA. M. Samy, M. M. Mahmoud, M. I. Khashaba, W. Y. Ali, El-Minia (Egypt)

Corresponding author:Prof. Dr. Waheed Yosry AliEl-Minia UniversityFaculty of EngineeringP.N. 61111, El-Minia, EgyptE-mail: [email protected]

Friction of Rubber Sliding Against Ceramics Part II1

Influence of Oil and Oil-Water Lubrication

1 Part I is published in KGK 6, 2007, page: 322

ROHSTOFFE UND ANWENDUNGENRAW MATERIALS AND APPLICATIONS


694 KGK · Dezember 2007

a sheet of float glass. It was found that sol-ing roughness is a major factor in determin-ing the Friction coefficient of this rubber soling material. It was observed that, dry sliding of the rub-ber test specimens containing saw teeth grooves displayed the highest value of fric-tion coefficient (1.5) due to increased adhe-sion and deformation. As the height of the V-grooves increased friction increased. For water lubricated ceramics, the value of the friction coefficient dropped to 0.65 due to the easy leakage of the fluid away from the contact area to the grooves. Further de-crease in friction coefficient was observed when water was detergent by soap. For ce-ramic lubricated by water and soap and con-taminated by sand, the friction coefficient increased significantly compared to the slid-ing conditions of water and soap only. This behavior may be attributed to the increased contact between ceramic surface and sand particles. In the presence of oil and sand on the sliding surface, the friction slightly in-creased. The friction coefficient reached to 0.35. This behavior may be caused by sand embedment in rubber surface and conse-quently the contact became between ce-ramic and sand. At lubricated sliding surface by oil and water contaminated by sand, the friction presented higher value than that of oil and sand sliding conditions. In the present work, the effect of the height of the vee grooves, introduced in the rubber specimens, on the static friction coefficient when sliding against ceramics lubricated by oil and oil diluted by water is investigated.

ExperimentalThe test rig used in the present work was designed and manufactured to measure the friction coefficient between the rubber specimens and the ceramics through meas-uring the friction force and applied normal force.The ceramic surface is placed in a base supported by two sets of thin spring steel sheets, where strain gauges were adhered, the first can measure the horizontal force (friction force) and the second can measure the vertical force (applied load). Friction co-efficient is determined through the meas-urement of the friction force by strain gaug-es. The load is applied on the specimens by dead weights. The arrangement of test rig used in friction force measurement is shown in Figure 1.Friction test were carried out at 50, 100 and 150 N normal load, at oil (Paraffin oil, SAE 30) and oil diluted by water, (95 wt. % H2O), lubricated sliding conditions. The measurement of friction force was carried

Arrangement of the test rig11

1

Test specimen used in the experiments22

2 3

The sliding condition of the test specimens33

Friction coefficient of rubber sliding against oil lubricated ceramic, (T = 3 mm)

44

4 5


55


66

6 7


77

695KGK · Dezember 2007

out by means of the deflection of the strain gauges. The strain gauges were mounted and connected to each other so that they are sensitive to measure the variation in the frictional force generated between the rub-bing surfaces. The test specimens were pulled manually by a horizontal force and the maximum value of the force was re-corded when the test specimen began to move. Friction coefficient was determined by the ratio of the horizontal force to the normal force applied.Rubber test specimens were prepared in the form of square cross section of 50 50 mm and 1.5 mm thickness. Test specimens were loaded against counterface of dry and wa-ter lubricated ceramic surfaces. The sliding surfaces were lubricated by, water as well as water and soap. V-grooves of different height (T) and constant width (t) of 2.0 mm were introduced in rubber test specimens, Figure 2. Then the rubber specimens were adhered on wood blocks.

Results and discussionThe sliding condition of the test specimen is shown in Figure 3, where the contact be-tween the rubber and ceramics is partially separated by the oil film. The results of the

experiments of rubber test specimens slid-ing against oil lubricated ceramics are shown in Figures 4-8. Figure 4 shows the friction coefficient for rubber specimen of T = 3 mm. In the presence of oil as lubricant, the smooth rubber specimens displayed the lowest friction values which were ranging from 0.12 and 0.18. Friction coefficient in-creased as the number of grooves increased. At 50 N applied load the increase in friction coefficient was more significant. As the load increased the friction coefficient de-creased due to the relatively strong adhe-sion of oil in the rubber surface where the removal of oil from surface was much dif-ficult because the oil was trapped in the contact area. The maximum value of fric-tion coefficient (0.58) was observed at 50 Nnormal load and number of grooves 8, where the minimum value of friction coef-ficient (0.11) was observed at smooth rub-ber specimens and 150 N normal load. For rubber specimen of T = 4 mm, Figure 5,the increasing height of grooves showed significant reduction in friction coefficient. The maximum value of friction coefficient (0.48) was observed at 50 N normal load and number of grooves of 8, while the min-imum value of friction coefficient was ob-

served at smooth rubber specimen and 150 N normal load. Figure 6 shows the fric-tion coefficient for rubber specimen of T = 5 mm. Increasing the height of the grooves showed insignificant decrease where the friction coefficient increased with increasing number of grooves. The maximum value of friction coefficient (0.46) was observed at 50 N normal load and number of grooves 8. For rubber specimen of T = 6 mm, Figure 7, as the dimension of grooves increased friction coefficient de-creased due to the increased capacity of the groove to restore oil and feed it back to the contact area. For rubber specimen of T = 7 mm, Figure 8, it is observed that, friction coefficient de-creased to a value of 0.4 at 50 N normal load and number of grooves 5. The followings are the results of experi-ments of the sliding of rubber test speci-mens against ceramics lubricated by oil di-luted by water. Figure 9 shows the friction coefficient for rubber specimen of T = 3 mm. It can be noticed that the emulsion of oil in water showed significant friction decrease due to the relatively lower viscosity of the emulsion, which was able to lubricate the contact area. As the number of grooves in-


88

8

Friction coefficient of rubber sliding against oil and water lubricated ceramic, (T = 3 mm)

99

9

Friction coefficient of rubber sliding against oil and water lubricated ce-ramic, (T = 4 mm)

1010

10


1111

11


1212

12

Friction coefficient of rubber sliding against oil and water lubricated ceramic, (T = 7 mm )

1313

13


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creased friction coefficient increased due to leakage of the emulsion to the groove. The highest load displayed the highest friction. The maximum value of friction coefficient (0.26) was observed at 150 N normal load and number of grooves of 8, while the min-imum value of friction coefficient (0.11) was observed at smooth rubber specimen and 100, 150 N normal load. For rubber specimen of T = 4 mm, Figure 10,the increased dimension of the grooves caused significant friction reduction due to the decreased in contact area. Friction coef-ficient increased with increasing number of V-grooves. The highest load displayed high friction value. At number of grooves 6, 7 and 8 the lower load displayed high friction coefficient. The maximum value of friction coefficient (0.25) was observed at 50 N nor-mal load and number of grooves 8. Friction coefficient for rubber specimen of T = 5 mm is shown in Figure 11. Friction co-efficient increased with increasing number of grooves. The maximum value of friction coefficient (0.27) was observed at 50 N nor-mal load and number of grooves 8. Figure 12 shows the friction coefficient for rubber specimen of T = 6 mm. Increasing the height of the grooves caused significant re-duction in friction coefficient. As the

number of grooves increased friction coef-ficient increased and the highest load dis-played the highest friction value. The maxi-mum value of friction coefficient (0.23) was observed at 150 N normal load and number of grooves 6. Further friction decrease was observed as the height of the grooves increased to T = 7 mm, Figure 13, where the maximum value of friction coefficient (0.21) was ob-served at 150 N normal load and number of grooves 5. The minimum value of friction coefficient was observed at smooth rubber specimen and 100, 150 N normal load.

ConclusionsFor oil lubricated ceramic, friction coeffi-cient decreased with increasing height of the grooves. It seems that height increase enables the oil to be well distributed on the contact surface and form a film which is re-sponsible for the friction decrease.Friction coefficient decreased as the height of the grooves increased and consequently the capacity of the groove increased to re-store lubricant fluid and feed once again into the contact area as load decreases.Diluting oil by water displayed values of friction much lower than that observed for oil lubricated condition.

References

[1] L. Strandberg, Ergonomics 28 (1985) 131.[2] M. Tisserand, Ergonomics 28 (1985) 1027.[3] W. R. Chang, Safety Sci. 40 (2002) 593.[4] R. Grönqvist, J. Roine, E. Korhonen,

A. J. Rahikainen, Occup. Accid. 13 (1990) 291.[5] R. Grönqvist, Ergonomics 38 (1995) 224.[6] C. M. Powers, K. Kulig, J. Flynn, J. R. Brault, J. Test.

Eval. 27 (1999) 368.[7] K. W. Li, W.-R. Chang, T. B. Leamon, Chen, C. J.,

Safety Sci. 42 (2004) 547.[8] K. W. Li, C. J. Chen, Appl. Ergon. 35 (2004) 499.[9] K. W. Li, C. J. Chen, Safety Sci. 43 (2005) 391.

[10] F. J. Rowland, C. Jones, D. P. Manning, J. Test. Eval. 24 (1995) 368.

[11] D. P. Manning, C. Jones, M. Bruce, Rubber Chem. Technol. 56 (1983) 703.

[12] D. P. Manning, et al. J. Royal Soc. Med. 83 (1990) 686.

[13] D. P. Manning, C. Jones, M. Bruce, Safety Sci. 14(1991) 1.

[14] P. Derek, et al. J. Safety Res. 29 (1998) 275.

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