Assessment of skidding on tractors

Although it seems like a simple detail, tire pressure affects the slipping, sliding and operational performance of tractors.

Traction devices are essential components in agricultural tractors, as they are responsible for transferring power from the drive axle to the surface traveled by the machine. The ability to transfer power to the ground depends on the type of traction device, and metal mats, rubber mats and rubber tires can be used. The traction devices most used on agricultural tractors are rubber tires, which can be found in different types of construction: diagonal, radial and BPAF – low pressure and high fluctuation.

Slippage can be understood as the failure to convert the total perimeter of the wheelset into the real displacement of the machine, being expressed as the percentage difference between the peripheral tangential speed and the translational speed of the wheelsets. In other words, it is the movement of the wheelsets (tires) that is not converted into displacement of the motor-mechanized assembly.

Excessive amounts of slippage can eventually lead to increased fuel consumption, premature wear of tires and other tractor traction mechanisms, in addition to reducing operational capacity and reducing the efficiency of the machine's energy use.

The ideal slippage for bias tires (maximum traction efficiency) is between 8% and 12% and for radial tires, 10% and 15%.

In general, diagonal tires have higher slippage values ​​than radial tires, for the same load levels and soil conditions, this is due to the lower tire/ground contact occurring in diagonal tires. The characteristics of tires and their interactions with the soil are fundamental to evaluating a tractor's performance.

Numerous technical and operational factors affect the interaction between tire and soil and thus the wheelspin, such as type of soil, presence of vegetation cover, type of operation, ballast level, etc.

SKATING EVALUATED ON THE FIELD

The most practical way to determine slippage is the difference between the rotation of the tractor's tires, with and without traction load. It is a percentage value between the tractor's paths pulling an implement and the one that would be obtained under the same conditions, after the implement has been uncoupled.

INFLUENCE OF INTERNAL TIRE PRESSURE

In order to evaluate the slippage of the wheelsets of a 4x2 tractor with auxiliary front-wheel drive (TDA), depending on the speed of the tractor, the internal pressure and the type of tire construction, an experiment was conducted at the Federal University of Viçosa, located in municipality of Viçosa (MG), under a dystrophic Red Yellow Argisol, with an average slope of 1%.

The soil was classified as clayey in texture; at the time of work, it had a water content of 0,19kg/kg. A mechanized set was used, consisting of a John Deere tractor, model 5705 4x2 TDA, with a power of 62,56kW (85hp) in the engine at 2.400rpm, and a double-action plow harrow produced by Tatu Marchesan, model ATCR with 14 discs. 24”, coupled to the tractor by the traction bar, with the discs spaced 0,23 m apart. At the time of the tests, the opening between the sections was maintained in the intermediate position, which provided a working depth of 0,3m.

USED ​​EQUIPMENTS

The tractor was equipped with two types of tire construction, diagonal and radial. The bias tires used were Goodyear Dyna Torque II 12.4-24 on the front axle and Pirelli TM 95 18.4-30 on the rear axle. The radials were the 320/85R24 models on the front axle and 460/85R30 on the rear axle, both from Goodyear's Optitrac line.

Seeking greater data reliability, a set of electronic sensors was used to measure slippage, with the aid of a Hottinger Baldwin Messtechnik (HBM) data acquisition system, model Spider 8, managed by the HBM Catman 2.2 software installed on a computer. laptop mounted on the tractor. The data acquired by the computer was stored for later processing. During the execution of the tests, the system was managed at a sampling rate of 50Hz.

The speed developed by the mechanized set during the operation was obtained with the aid of a Doppler effect radar, made by Dickey John, model Radar II.

To measure the rotational speed of the tractor's driving wheels, inductive transducers from the Autonics brand, model PRCM 18, were used, positioned together with a crown arranged with equidistant fins around it, mounted on an encoder-type system.

To measure the internal pressure of the tires, pressure transducers from the brand Sensata Technologies were used.^®, model 100CP7-1, coupled to each tractor tire using a kinematic rotor.

The sliding of the wheelsets was obtained through the relationship between translational and rotational speed for each of the machine's wheelsets, according to Equation 1. The tractor used was ballasted with 75% water in the diagonal tires and 40% in the radials, with that in all tests the auxiliary front traction (TDA) was kept activated, seeking to achieve the maximum possible traction of the evaluated tractor.

Each experimental unit was 40m long and 2m wide, with a usable area of ​​80m², demarcating 15m between them in the longitudinal direction for maneuvers, implement traffic and stabilization of the set before data acquisition.

An experiment was set up for each type of tire construction. The experiments were set up using the central composite rotational design (DCCR), a 2 factorial³, including six axial points and five repetitions at the central point. For diagonal tires, the internal tire pressure varied between 68,95kPa and 137,90kPa (10psi - 20psi) and for radial tires 137,90kPa and 206,84kPa (20psi - 30psi), in order to guarantee the same rolling radius as rotated.

RESULTS

The slippage of the tractor's wheelsets using radial tires was not influenced by the travel speed or even by the internal pressure of the front and rear wheelsets.

For this reason, it was represented by the equation of a straight line consisting of a constant, whose value corresponded to the arithmetic mean of the slip values, obtained experimentally in all tests (Tables 1 and 2).

This behavior can be understood because it is a constructive form, which results in tires that are more malleable to irregularities in the ground. However, this lack of rigidity, when compared to the diagonals, allows the tire to move laterally, which may have interfered with the characterization of the sliding behavior in relation to the established treatments.

The front slip (Table 1) was considerably altered by the internal pressure of the wheelsets. This occurred due to the influence of the tires' internal pressure on the rolling radius; It is noted that the influence of internal pressure on sliding is positive, that is, for each unit of internal pressure (kPa) in the front tires, for the same travel speed, an increase of 0,0259% in the front wheels is promoted. Slipping.

Greater influence is promoted by the displacement speed, increasing 3,2107% of slip with the increase of one unit of displacement speed. By increasing travel speed, the tire's grip on the ground is reduced, causing greater levels of slippage, this effect was also evidenced by Coelho et al (2012), where they evaluated the effect of operational speed on different forms of soil preparation. Regardless of the mode of operation, the increase in travel speed contributed to the increase in wheel slippage.

As observed in the front wheels, the greatest effect on the slippage of the rear wheels (Table 2) was related to the travel speed, 2,9238% for each unit of travel speed, followed by the internal pressure of the front wheels (0,0252. XNUMX%), the increase in internal pressure in the front tires, which changes the rolling radius, and thus the kinematic advance, which in turn causes the front wheels to exert greater traction, promoting the drag of the rear wheels.

It is also observed that the internal pressure of the rear tires negatively influences slippage, that is, as the internal pressure levels of these increase, the percentage of slippage of the wheelsets decreases. This effect occurs due to the increased stiffness of the tires which, when forced on the ground, favors tangential displacement.

CONCLUSIONS

The slippage of the rear diagonal wheelsets was affected by all the variables analyzed, while the front wheels were only affected by the travel speed and internal pressure of the front tires. When the tractor was equipped with radial tires, the slippage of the wheelsets was not influenced by the factors evaluated.

Equation I

on what,

δ - Wheel slippage, %;

V_r - Rotational speed, m s^-1; and,

V_t - Translational speed, m s^-1.

How to measure skating

Slippage can be measured in a certain space covered by the tractor, demarcated by two markers, using the following equation. The tractor must start from a minimum distance that allows it to reach the first goal under normal working conditions (at least 30m), the engine speed must be exactly the same with load and without load.

Where:

P = skating (%)

Ncc = number of tire revolutions with load

Nsc = number of tire revolutions without load

PROCEDURES

  Note: it is recommended that these procedures be repeated to ensure greater reliability of the result.

EXAMPLE

When determining the slippage of a tractor and implement set, the following results were obtained.

To determine the slip level, we substitute the observed values ​​into the equation, as shown below.

Daniel Mariano Leite, Univasf; Marconi Ribeiro Furtado Júnior, Haroldo Carlos Fernandes, Anderson Candido da Silva and Paulo Roberto Forastiere, UFV

Article published in issue 164 of Cultivar Máquinas.