DETERMINATION OF THE SOIL FRICTION COEFFICIENT AND SPECIFIC ADHESION

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1 TEKA Kom. Mot. Energ. Roln., 5, 5, 1 16 DETERMINATION OF THE SOIL FRICTION COEFFICIENT AND SPECIFIC ADHESION Arvids Vilde,Wojciech Tanaś Research Institute o Agricultural Machinery, Latvia University o Agriculture Deartment o Vehicles and Engines, Agricultural University o Lublin Summary. A method is worked out to determine the soil riction coeicient and seciic adhesion. To determine the coeicient o riction and the seciic adhesion, the soil sliing resistance should be assessed at several dierent values o the seciic ressure between the sliing suraces. Alteration o soil sliing coeicient has an alternative hyerbolic regress. On the basis o this oinion, by the method o least squares, one can determine the coeicients o soil riction and seciic adhesion orce. Key words: soil sliing resistance, soil riction coeicient, seciic adhesion, studying methods INTRODUCTION In most cases the movement o soil during its tillage roceeds along the suraces o the oerating arts o tilling tools and machines. Sliing resistance o soil signiicantly aects the drat resistance o these tools. For instance, the sliing riction between steel and soil may exceed 35-5% o the total drat resistance in loughing [Vilde and Rucins 4]. Thereore the roblems o reducing the sliing resistance o soil along the oerating arts o the tillage tools always attracts great attention both in designing new structures and the use o the existing machines. In order to tackle these roblems, one should know well the relationshis, which determine the value o the sliing resistance o soil along steel. Investigations by a number o authors show that the soil sliing resistance deends on many actors: its tye, its mechanical comosition, moisture, structure, adhesion, the humus content, seciic ressure o soil uon the surace, hysical and chemical roerties o the rubbing materials, the state o the surace, duration o the contact, the riction velocity, the value o the internal riction o soil, and others. The mathematical relationshis between them are not yet clariied to a suicient degree. Most o the existing investigations discuss the inluence uon sliing resistance o one, two, or seldom three and more actors. Variations in the sliing resistance as deendent on these actors are usually resented in a grahical way without any mathematical aroximation.

2 DETERMINATION OF SOIL FRICTION COEFFICIENT 13 General evolution o science in the ield o sliing riction during the revious decades had a great imact uon the theory o soil sliing along steel and other materials, too. So, in contrast to the revious investigations o the soil sliing resistance using the mechanical theories o riction (the Amonton Law), the recent studies ind increasing alication o molecular and molecular-mechanical theories o riction allowing or a better exlanation o the henomena roceeding when soil is sliing along the oerating arts o the tools [Vilde 1]. But in site o all this, u to the resent time there are no generalised mathematical relationshis that would allow considering the imact o various actors and the use o the above-mentioned theories in articular drat resistance calculations o tools and otimum designs o their oerating arts deending on oerating conditions. The urose o investigations. On the basis o the research o our own data as well as those rovided by various other authors, the aim o the resent study is to ind out the mathematical regularities existing between the resistance o soil sliing along steel, and its hysical and mechanical roerties. APROACH It is acceted to evaluate the rictional roerties o materials, including soil, by the sliing resistance coeicient. The latter deends on the riction coeicient and soil adhesion, and it varies with seciic ressure between the sliing suraces [Vilde 1]. The review o the literature devoted to soil roerties showed that investigations carried out only by ew authors more or less meet the requirements ut orward in the revious section. Among them are Ya.Ya. Nagla, P.U. Bakhtin, N.V. Shchuchkin, A.T. Yakovenko, M.I. Bredun, M.L. Nichols and H.G. Riek [Vilde 1]. The weak oint o the studies conducted by most other researchers is that they lack data concerning any o the basic actors, articularly the comosition o soil. Thereore they can serve only as individual examles or testing the useulness o the deduced general relationshis. As a rule, all the sources rovide sliing resistance coeicients o soil. On the basis o these data, by the method o least squares, we have determined the coeicients o riction and seciic adhesion orce, ater these deendencies were deduced, between them, the mechanical comosition and the moisture o soil. RESULTS On the bases o the revious investigations a method has been worked out how to determine the soil riction coeicient and its seciic adhesion using B.V. Derjagin s modiied two-art ormula: = (1 + a -1 ), (1) where: the resistance coeicient o soil sliing along a surace; the riction coeicient o soil along a surace; the seciic ressure o the layer (soil) uon the surace; a the seciic soil adhesion orce to the surace.

3 14 Arvids Vilde, Wojciech Tanaś In order to determine the coeicient o riction and the seciic adhesion, the soil sliing resistance is assessed at several dierent values o the seciic ressure between the sliing suraces. For examle, data are given urther (Table 1) which show the changing values in diagram (Fig. 1) o the soil sliing resistance coeicient along a steel surace at dierent seciic ressures along the suraces. Table 1. The resistance coeicient o wet clay soil sliing along a steel surace [kg. cm - ] P [kg. cm - ] Fig. 1. Alteration o soil sliing resistances coeicient along steel surace deending on the seciic ressure on the suraces It is evident rom Table 1 and the grah that the variation in the soil sliing coeicient has an alternative hyerbolic regress. By means o the method o least squares, one can determine the coeicients o soil riction and seciic adhesion orce a, using ormulas: y x x ( x) x xy =, () n n xy x n x ( x) a = y, (3)

4 DETERMINATION OF SOIL FRICTION COEFFICIENT 15 a a =, (4) where: x = -1 ; y = ; n the number o measurements (at least 5). Using this method reveals the riction coeicients and seciic adhesion or certain soils. For the clay soil, given as an examle above: the riction coeicient =.79 and the seciic adhesion a =.43 kg/cm. A comuterised laboratory device is develoed or the determination o these soil characteristics or several suraces and ranges o sliing seeds. The basic actors aecting the rictional roerties o soil are its comosition and moisture. Individual actors the coeicient o riction, adhesion, lasticity, internal riction etc. are more or less deendent on the irst ones. Thereore the evaluation o the rictional roerties o soil involves, irst o all, the assessment o the varying riction coeicient and seciic adhesion orce o soil deending on its comosition and moisture. It is evident that the answer to these questions can be ound only in exeriments in which the rictional roerties o soils are determined taking into account their comosition and moisture. The studies o mathematical correlations between the rictional roerties and the other arameters o soils should be comlex and all-embracing, covering as many actors as ossible; the riction coeicient and soil adhesion should be determined at a dierent seciic ressure, mechanical comosition, moisture, chemical comosition, density, structure, sliing seed and other arameters. As a rule, all the sources rovide sliing resistance coeicients o soil. On the basis o these data, by the method o least squares, we can determine the coeicients o riction and seciic adhesion orce, ater these deendencies were deduced between them and the mechanical comosition, and moisture o soil [Vilde 1], as well as drat resistance o soil tillage machines [Vilde and Rucins 4, Rucins and Vilde 4]. CONCLUSIONS 1. On the basis o investigations a method is worked out to determine the soil riction coeicient and seciic adhesion.. To determine the coeicient o riction and the seciic adhesion the soil sliing resistance should be assessed at several dierent values o the seciic ressure between the sliing suraces. 3. Variations in the soil sliing coeicient have an alternative hyerbolic regress. On the basis o this assumtion, using the method o least squares, one can determine the coeicients o soil riction and seciic adhesion orce.

5 16 Arvids Vilde, Wojciech Tanaś REFERENCES Rucins A., Vilde A. 4: Mathematical modelling o the oeration o lough bodies to determine their drat resistance and otimum arameters. Polish Academy o Sciences Branch in Lublin. TEKA Commision o Motorization and Power Industry in Agriculture, IV, Vilde A. 1: Physical and mechanical roerties o soil connected with its tillage. - Physical Methods in Agriculture, Aroach to Precision and Quality. Czech University o Agriculture. Prague, Czech Reublic, Vilde A., Rucins A. 4: The imact o soil hysical and mechanical roerties on drat resistance o loughs. - Polish Academy o Sciences Branch in Lublin. TEKA Commision o Motorization and Power Industry in Agriculture, IV,