Correct calibration of tractor tires

The subsoiling operation occurs when there is a need to decompress subsurface layers of the soil, whose high resistance to penetration prevents the full root development of crops, a frequent circumstance in areas of direct planting and intense traffic of agricultural machinery. Failure to disturb the soil and the increasing weight of machines due to the need for greater power and operational capacity are proven contributing factors to soil compaction.

For tractors to achieve better operational performance, they must be correctly adapted to the job. Especially subsoiling, as it is an operation that requires greater availability of power and traction force from tractors, with the direct consequence of high fuel consumption and operational costs. In a scenario where production costs need to be carefully considered and, if possible, reduced, adjusting the calibration of tractor tires can contribute to fuel savings and greater operational capacity.

Tire inflation pressure directly affects soil compaction, operational capacity and dynamic balance of tractors, as it provides a greater or lesser area of ​​pressure on the ground, tread adhesion, slippage and kinematic advancement of the wheelsets. Reducing or increasing the internal pressure of tires can better distribute their weight over the ground, optimizing traction force and travel time (converted into operational capacity).

A team from the Agricultural Mechanization Laboratory at the University of Brasília – Lamagri carried out work to evaluate the operational performance of a tractor/subsoiler set under field conditions using different tire inflation pressures and find a suitable calibration for the subsoiling operation.

The work was carried out at the Água Limpa Experimental Farm, also belonging to the University of Brasília. The region's soil is classified as a Red-Yellow Oxisol and the history of use of the experimental area was to grow corn for silage in direct planting six years ago, with the soil surface covered by corn harvest residues and spontaneous vegetation, accounting for 2.677,6kg/h of dry straw mass.

In a tractor model TM7020 4x2 TDA, with 149 hp of engine power, three tire inflation pressures were used: 10, 20 and 30 psi, using a subsoiler model SPCR, equipped with five rods spaced 0,4 m apart, tips without 80mm wide wing, 18” straw cutting discs and declogging roller, and worked at a depth of 0,35m. 

The tires used on the tractor were diagonal, with R1 on the front, measuring 18.4-26, 12 plies and 43mm high claws; and at the rear R1, measurements 20.8-38, 14 plies and claws 52mm high. The four tires were inflated with 50% water and the respective pressures were studied.

The variables evaluated were hourly fuel consumption, slippage and wheel advancement, in plots measuring 80 meters in length per inflation pressure studied.

Fuel consumption was obtained from the installation of two 2.000ml beakers, one installed to supply the engine with diesel oil and the other to collect the return of unused fuel in the cylinders, with the hourly consumption (L/h) given by volumetric difference of diesel oil between the two test tubes due to the travel time in the plot.

The slippage was determined based on the number of revolutions of the tractor's rear tire needed to cover the plot, with the implement lowered and then raised, according to the methodology described by Corrêa et al. (1999). The kinematic advance of the front wheels was determined by the number of revolutions of the front tire to cover the plot, with the auxiliary front traction (TDA) turned on and then turned off, according to the methodology described by Silveira (2018).

The data obtained were subjected to analysis of variance and the means compared using the Tukey test at a 5% probability of error.

The results of slippage, advancement and fuel consumption are shown in Figures 1, 2 and 3.

The slippage of the rear wheels and the advancement of the front wheels showed significant differences for the three tire inflation pressures, with the highest rates being obtained at a pressure of 10psi.

Slippage at 10psi was 16% greater than at 20psi and 34,1% greater than at 30psi. Between 30psi and 20psi slippage was 21,5% greater at 20psi. The result indicates that lower pressures, even lower than those indicated by the tire manufacturer, resulted in less adhesion between the tread and the ground, as well as greater tire deformation when the tractor's traction force is required for operation. According to a recommendation already consolidated and established by the scientific literature on the subject, the ideal slipping interval for tractors with diagonal tires and on firm terrain is 8% to 15%, therefore, for the results obtained, pressures of 30psi and 20psi fit. as ideal for skating. 

The kinematic advance at 10psi was 38,7% greater than at 20psi and 68,8% greater than at 30psi. Between 30psi and 20psi, the advance was 49,1% greater at 20psi. The results were compatible with those obtained in the skating variable. For the front tires, their manufacturer indicates inflation pressure varying between 24psi and 32psi, showing that low pressure affects tire grip, front axle traction and consequently the operational capacity of the tractor. According to the scientific literature, the recommended ideal range of kinematic advance for 4 x 2 TDA tractors is between 1,2% and 1,8%, therefore the results obtained indicate that the pressure of 30psi is adequate for the conditions studied. It is worth mentioning that this pressure is in line with that recommended by the front tire manufacturer.

When it comes to hourly fuel consumption, the lowest was observed with 30psi tire pressure, with consumption being 10% lower than at 20psi and 12,4% lower than at 10psi. This means consumption of 2,88L/h and 3,66L/h less, and considering the cost of diesel oil at R$3,80/L, the savings are R$10,94/h and R$13,90/ h, or R$87,52 and R$111,20 per day (eight-hour working day), and so on. Between 20psi and 10psi the consumption did not differ between them.

It is concluded that for the conditions under which the experiment was carried out, the higher pressure (30psi) provides better operational performance conditions for the tractor, with slipping and advancement rates within the ideal and lower hourly fuel consumption, enabling a subsoiling operation with greater operational capacity of the set and economy.

Arthur Gabriel C. Lopes, Gabriel Pastor de B. Lima, Wesley Matheus C. F. Taveira, Isabela Dias de Souza, Tiago Pereira da S. Correia and Francisco Faggion, UnB

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