Radial tires on agricultural machinery

Despite achieving greater durability and studies showing better load distribution and thus less soil compaction, the radial tire is still little used in Brazil due to the lack of measurements approved by manufacturers and available on the market, its price and lack of knowledge of the technology.

Tires, over time, stopped using only natural rubber and cotton and started to have new materials derived from petroleum such as synthetic rubber, nylon fibers, etc. There was also an evolution in the way the tire's constituent layers were arranged, with the main construction forms being: diagonal (A); diagonal waist better known as Twin or BPAF (low pressure high fluctuation) (B); and radial (C) (Figure 1).

Basic knowledge about the construction characteristics of tires helps a lot when choosing, as, depending on the tractor model, there are several tire options. This variety of dimensions and construction types is no coincidence, as the type of operation, soil or culture, will have a more appropriate tire to be used.

Another problem is that many consumers have difficulty identifying construction types. A simple way to identify would be by the unit in which the tire measurement is displayed, as most manufacturers describe the measurement of diagonal tires in inches (18.4-34) and radial and diagonal waist tires in millimeters (460/85R34). Another difference is in the load capacity: the bias tire is identified as “tarps” or by the letters (PR), example: 10 PR and the belted radials and diagonals are identified by the so-called “load index” or the letters (LI), example: 146 A6.

In practice, the user does not care about the construction type and dimensions of the tires, the first concern is the initial cost and they often end up choosing the wrong tire, compromising the machine's performance. In general, the durability of radial tires is greater, due to the quality of the compounds, and they normally use the tread with the R1W claw type, which is around 25% larger than the R1 claws (figure 2) that are used on tires diagonals, which contributed to its greater durability. 

It is clear that the performance of tractor wheelsets depends on the physical characteristics of the soil, type and geometry of the tire, loads applied to the wheelsets and their inflation pressure, the correct interaction between these variables can improve the operational efficiency of the tractors, increasing the useful life of wheelsets and consume fuel rationally, increasing economic profitability for the producer.

Faced with doubts about the construction types of tires, with the purpose of evaluating the performance of tires with diagonal and radial constructions, a test was carried out at the Agroforestry Machines and Tires Test Center (Nempa), of the Department of Rural Engineering of the Faculty of Agricultural Sciences - Unesp, Botucatu/SP Campus. The tire models tested were one with a diagonal construction measuring 18.4-34 12PR and another with a radial construction 460/85R34 147 A8, model TM600, brand Trelleborg (figure 3).

The tires were fixed to the horizontal axis of the hydraulic press, which simulates the tractor's axis, according to the loading procedures adopted, pressing the agricultural wheelset on a rigid surface, obtaining the stamp of the tire's tread on a white cardboard, with the tire previously painted with Indian ink.

The tires were inflated to the nominal pressures recommended by the manufacturers, based on an inflation pressure of 220,48kPa (32psi) for the bias tire and 151,58kPa (22psi) for the radial tire.

The loads were applied to the tires in an increasing manner, starting with 1.500kg and gradually reaching 2.500kg and 3.000kg.

After each pressing on the cardboard, using a digital camera, photos were taken of each contact area using a fixed and elevated platform on the structure of the hydraulic press.

In a final step, the digital photos were transferred to a microcomputer and, finally, graphic readings of these contact areas were obtained using the Autocad computer program.^®.

To determine the elastic deformations, a graduated scale was attached to the structure of the hydraulic press and a cursor to the tire axis, allowing the relative displacement between them to be measured. The value at the initial contact of the tire with the rigid surface (without load) was read on the graduated scale and then measured due to the maximum load applied to the pneumatic wheelset axle, for each tire, the elastic deformation being obtained by the difference between the two measures.

The values ​​for the contact areas and elastic deformations on the rigid surface for the two tires are shown in Tables 1 and 2.

With the increase in the load applied to the tires (diagonal and radial), there was an increase in the contact area of ​​both. The radial tire had an average contact area of ​​27% greater than that of the diagonal tire, considering all loads. At a load of 3.000kg, the radial tire had the greatest difference in contact area compared to the diagonal tire, 43% larger.

The radial construction tire obtained larger contact areas and also greater elastic deformations, which means in practice that it deforms more, becoming rectangular in shape, distributing the load better, helping to minimize soil compaction. In the automobile industry (cars and trucks) most vehicles come off the line with radial construction tires, while in the tractor, machinery and agricultural implements industry, adoption of tires with radial technology is still low. Several factors contribute to the small demand in the agricultural sector for radial technology, such as the high price, lack of measures available on the market, especially for light and medium tractors, lack of measures approved by manufacturers of tractors and agricultural machinery, and users' lack of knowledge of radial technology. In developed countries, radial tires on medium and low power tractors have been sold for a long time, but for now in Brazil this technology is restricted to heavy and extra-heavy tractors.

Thiago Martins Machado, UFMT – Sinop - MT; Kléber Pereiraanças, Unesp – Botucatu; Mauro Oliveira, José Artioli, Trelleborg; Indiamara Marasca, Unesp - Botucatu - SP

Article published in issue 150 of Cultivar Máquinas.