The rolling radius of a tire can be understood as the value that effectively contributes to its displacement, that is, it is the distance covered by a tire in one complete revolution of the axle divided by 2π, obtained in zero traction conditions, that is , self-propelled displacement on an indeformable surface.
The value of the rolling radius may vary depending on wear, internal tire pressure and the weight on the wheelset. This value is used to calibrate the tractors' travel speed indicator. In this way, something common can be observed: the speed recorded on the speedometer is not always the real one.
The advent and consolidation of computing and electronics in agriculture occurred by reducing costs and complexity, ensuring that such systems have favorable results in agriculture.
Tire internal pressure has a strong influence on the performance of mechanized sets, especially regarding the loss of efficiency of agricultural tractors caused by tire slippage. In addition to slippage, the effect of tire and soil deformation on reducing translational speed is also considered, with these factors acting concomitantly.
Tire slippage on an agricultural tractor can be obtained with the help of electronic instrumentation, where pulse counters are installed next to each tire. This operational parameter is defined by comparing the tractor's current speed with the peripheral tangential speed of the tires, which is given by the product of the angular velocity and the tire's rolling radius.
As the rolling radius is an important parameter in determining wheel slip, the objective of this work was to determine the influence of the auxiliary front traction drive (TDA) on the rolling radius for three internal tire pressures.
MATERIAL AND METHODS
The work was conducted at the Agricultural Mechanization Laboratory, at the Federal University of Viçosa, on the Viçosa Campus (MG). A John Deere tractor was used, model 5705 4x2 with auxiliary front-wheel drive (TDA) and with a power of 62,56kW (85hp) in the engine at 2.400rpm, which was instrumented to conduct the work. The tractor was equipped with Pirelli TM 95 18.4-30 tires on the rear axle and Goodyear Dyna Torque II 12.4-24 tires on the front axle, both of diagonal construction.
The rotation of the tractor's driving wheels was monitored with the aid of tubular inductive transducers associated with each wheel via supports. The change in the magnetic field (induction) of the transducers occurred through the passage of equidistant fins arranged on the periphery of a circular crown affixed concentrically within the hubcap of the wheelsets, thus serving as a reference system. All sensors were connected to the Spider 8 data acquisition system and configured using Catman 2.2 software, both available from HBM.
The treatments consisted of three internal pressures 82,74, 96,53 and 110,32kPa (12, 14 and 16psi, respectively) combined with the conditions of auxiliary front traction on and off, in a completely randomized design, with three replications. In all treatments of the experiment, gear 1st B was used at 2.400rpm in the engine, which provides an average theoretical operating speed of 1,48m s-1 (5,33km/h). The work was carried out on a concrete track with a flat topography, 30 meters long and an additional 15 meters to stabilize the movement of the set.
The speed developed by the tractor during operation, which corresponds to the translational speed of the tires, was obtained using a Doppler effect radar unit coupled to the tractor chassis. The rolling radius was obtained using a specific equation (see Box).
The data obtained were subjected to linear regression analysis, with the models selected based on the coefficient of determination, the behavior of the phenomenon and the significance of the regression coefficients using the t test. The traction effect was analyzed using the Tukey test at a 5% probability level.
RESULTS
The rolling radius determined with traction on and off showed no statistical difference for any of the internal pressures used in the tires (Table 1).
Table 1 - Average values for the rolling radius (m) of the tires used for the combinations between TDA on and off, internal pressure of the tires and axle
| Pressure (kPa) | TDA on | TDA off | |||
| Rear | Front | Rear | Front | ||
| 82,74 | 0,7092 A | 0,5279 B | 0,7127 A | 0,5348 B | |
| 96,53 | 0,7216 A | 0,5366 B | 0,7189 A | 0,5400 B | |
| 110,32 | 0,7291 A | 0,5431 B | 0,7257 A | 0,5473 B | |
Pressure (kPa)
TDA on
TDA off
Rear
Front
Rear
Front
82,74
0,7092 A
0,5279 B
0,7127 A
0,5348 B
96,53
0,7216 A
0,5366 B
0,7189 A
0,5400 B
110,32
0,7291 A
0,5431 B
0,7257 A
0,5473 B
The means followed by at least the same letter, capitalized in the line and for the same TDA activation condition, do not differ statistically from each other, using the Tukey test at the 5% probability level. Three repetitions were performed.
The rolling radii of the tires on the same axle did not show any significant difference between them, therefore, the analysis was based on the average rolling radius of the front and rear tires. Tire internal pressure had a significant, linear and positive effect on the average rolling radius of the front and rear tires (Figure 1).
Figure 1 - Rolling radius as a function of internal tire pressure. ** - Significant at 1% probability level
CONCLUSIONS
Increasing internal tire pressure increased the rolling radius of tractor tires. The adjusted model satisfactorily explains the behavior of tire rolling radius as a function of internal pressure.
BOX 1
EQUATION
On what,
rr – Rolling radius (m);
Vop – Real displacement speed obtained by the radar (m s-1); It is,
n – Rotation of the driving axle (rps).
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