عنوان مقاله [English]
The wheel is one of the simple and important components of the tractor because it must bear the weight of the car and also communicate the car with the ground. The tire pressure determines the tire stiffness, which has a significant effect on the tire contact surface and ground pressure distribution. Adjusting the air pressure inside the tire as a possibility to reduce soil compaction and improve the tensile efficiency of agricultural tractors. The shape of the contact surface of a wheel with the ground has a certain complexity due to the curvature of the wheel and the flexibility caused by the load on the wheel and the internal wind pressure of the wheel. Hence, several models have been proposed by researchers to estimate this parameter in accordance with wheel and surface conditions. Checking the contact surface of the wheel with the soil is important in two ways: Energy loss and Negative impact on product growth and production.
Wheel and soil tests are mainly performed under controlled conditions and are usually performed in the form of single-wheel testers in the soil storage environment. In these environments, it is possible to have more control over the wheel and soil variables. A typical agricultural tire (Barez Co., Iran, 8.25-16) with the following specifications was used in the experiments: HLFS flower tire specifications, rim width 175 mm, outer diameter 840 mm and cross section width 220 mm. Two soils were used in this study: sand which was clay loam clay with 13.8% sand, 79.31% sand and 6.89% of the tested soil, from a sieve score of 200 which is the size of Its holes, about 0.075 mm, had passed. The experiments were performed after preparing the soil storage and the single-wheel tester set. Prior to each experiment, the soil inside the canal was completely shaken by nail pruning to a depth of 20 cm. Then, with the help of a timber installed to the wheel carrier, the soil surface was leveled well. By placing the tester set on the track, a dynamic load was applied through a power screw. Dynamic loads on the wheel were considered 1814.85, 2207.25, 2599.65, 290.05 and 3384.45 kN at five levels. The amount of load is measured by the load cell and read on the screen and finally stored. The tire pressure was applied at three levels of 100, 200 and 300 kPa by a 600 liter compressor, 8 bar pressure and 5.5 hp. Wind pressure was measured and controlled by a Borden Gauge sphygmomanometer. After placing the tire on the ground (under a certain wind pressure and load), some gypsum powder was sprayed around it. The tire was then lifted off the ground and photographed from a fixed distance by the wheel on the ground with a digital camera. All images are processed using AutoCAD 2015 software (Autodesk, Inc., USA) to calculate the numerical value of the contact area (figures 2, 3). Equation 1 was used to determine the contact pressure between the tire and the soil. The experiments were performed factorially in a completely randomized design with three replications. Independent variables were: soil type (in two levels of clay and sand), vertical load (in five levels of 1814.85, 2207.25, 2599.65, 295.05, and 3384.45 kN) and tire pressure (in three levels of 100, 200 and 300 kPa). The dependent variables were: the contact surface between the wheel and soil and the contact pressure between them. Data were analyzed using SAS 9.1 software (SAS Institute, USA) and Duncan's multiple range test was used to compare the means.
In this study, the effects of tire pressure and vertical load on the wheel in two clay and sand substrates on the contact surface and contact pressure between soil and tire were investigated. Table 1 shows the analysis of variance of the test data obtained from the tests. The results showed that the simple effects of soil type, tire pressure and vertical load on the wheel on the tire contact surface with soil were significant at 1% level. Regarding the contact pressure between the tire and the soil, except for the simple effect of soil type which was significant at the 5% level, the other simple effects were significant at the 1% level. In addition, dual and triple interactions were also significant on the contact surface and contact pressure between the tire and the soil at the 1% level. Figures 4 and 5 show the trend of changes in the tire contact surface with the soil. As can be seen, the contact surface area increases with increasing vertical load on the wheel and decreases with increasing tire pressure. Figures 6 and 7 also show the trend of contact pressure changes between the tire and the ground. As it is known, the contact pressure between the tire and the ground has increased with increasing vertical load on the wheel as well as the tire pressure. Based on the findings of this study, it was found that with increasing tire pressure at different loads on the wheel, the contact pressure between the wheel and the soil increased. The changes in contact pressure were linear in terms of changes in tire pressure in sandy soil and at loads of 2207.257 and 2599.6599 kN in clay soil. However, in clay at loads above 2599.65 and below 2207.225, contact pressure changes did not show linear behavior. Also, similarly, the contact surface of the wheel with the soil changed under the influence of these factors, but in reverse. The contact surface in sandy soil had an inverse linear relationship between tire pressure and in clay, except for the mentioned loads, it had the same inverse linear relationship. In at all, it can be stated that with the change of tire pressure and vertical load on the wheel, changes in contact surface and contact pressure in sandy soil were almost linear and in clay soil were only linear in some conditions. It seems that because sandy soil had a more uniform texture composition, this linear relationship occurred, but since clay soil didn’t have a more uniform composition, a linear relationship did not occur.