Traction and energy efficiency tests of oligomeric tires for category 3 tractors

The wheeled running gear of an agricultural tractor must have sufficient traction and coupling properties to ensure that the tractor fulfills its main function. The solution to these problems depends to a large extent on the search and implementation of measures for improving and developing new tire designs, which are the main element of the wheel drive that meets modern requirements. The developing of new pneumatic tires requires creating a new formulation of rubber and materials, the use of rubber-oligomeric compositions that allow implementing new, advanced technologies in the manufacture of tires. Currently, tires based on rubber-oligomeric compositions are being created. In particular, the Austrian company Lim developed a range of wide-section tires based on oligomers. As a result of tests of tires for class 3 tractors, it was determined that the oligomer tire 66×43.00-25 has the best traction and energy performance on all supporting foundations, compared to the serial tire 21.3R24.


Introduction
Improving the productivity of an agricultural tractor is possible due to an increase in operating speeds, which leads to the appearance of increased dynamic loads in the "engine-transmission-running gearground" system. Dynamic loads, in turn, degrade tractor performance. Also, with an increase in the movement speed, the smooth running of the tractor deteriorates, which negatively affects the working conditions of the driver and the dynamic loading of the tractor's frame, as well as the agrotechnical indicators of the entire machine-tractor unit, which may include a tractor.
There are many constructive solutions aimed at reducing dynamic loads in the transmission of an agricultural tractor and improving its working conditions [1,2,3].
One of the factors affecting the performance of an agricultural tractor is the fineness of running gears, of which wheeled ones are dominant.
A wheeled running gear of an agricultural tractor must have high traction and coupling properties [2] in order to ensure the tractor fulfills its main function as a mobile power tool. The solution to these problems largely depends on the search and implementation of measures to improve and develop new tire designs, which are the main element of the wheeled running gear that satisfy modern requirements [4,5,6]. In the practice of tractor engineering, two following main trends prevail in improving the traction and energy, as well as agrotechnical properties of wheeled running gears [4,5]: the use of tires of a larger size and the improvement of tire designs.
Without denying the usefulness of research in the first direction, it should be noted that improving the performance of wheeled gear drivers by using tires of increased standard size has now practically exhausted its capabilities due to the limited dimensions of the tractor [2,5].
The experience of designing and testing agricultural tractors accumulated in the world shows that the most promising direction is the developing new highly elastic pneumatic tires and improving existing designs of tires, which can operate at low pressures without reducing their resource [6][7][8][9][10][11][12][13][14][15].
The creation of new highly elastic pneumatic tires with modern designs requires developing a new formulation of rubber and materials, the use of rubber-oligomer compositions that allow implementing new, advanced technologies in the manufacture of tires [4,6,15].
At present, both in our country and abroad, tires are being created on the basis of rubber-oligomer compositions. In particular, the Austrian company Lim created a range of wide-section tires based on oligomers.

Materials and Methods
The goal of research and testing was to determine the traction and energy indicators of an oligomeric tire of size 66×43.00-25 and 21.3R24 tire of the FD-14A model on a field prepared for sowing (background 1), winter wheat stubble (background 2) and concrete (background 3).
Field tests of oligomeric tires of standard size 66×43.00-25 and 21.3R24 tires were carried out in agrotechnical period on a field prepared for sowing and stubble of winter wheat, satisfying the basic requirements of Russian State Standard GOST 7057-2001 "Agricultural tractors. Test methods" and local standards. The practice grounds were flat, the inclination angle did not exceed 10 in any direction. The practice grounds were at a distance of more than 50 m from the field edges, and had dimensions of 600×1000 m. The characteristics of the practice ground are given in table 1. Values of normal wheel load and tire pressure when testing 66×43.00-25 oligomer tires and 21.3R24 tires are shown in Table 2. In order to assess the traction and energy indicators of tires, the following indicators were adopted [1,3,4,5]: k  − wheel traction efficiency;  -slip coefficient; Рf − tire rolling resistance; f  ,  components of wheel traction efficiency, which consider the loss of power on rolling and slipping, respectively.
The determination of these indicators was carried out on the basis of measuring the following values: Мk − torque on the axis of the wheel; Р1, Р2 − longitudinal components of the forces acting on the wheel; α − inclination angle of the frame; nk, nnk − the number of revolutions of the tested and track measuring wheels.
Tests of tires were carried out on a "tire tester" [2,4,5] at nominal values of the normal load on the wheel N, which corresponds to the maximum loaded wheel of the traction class 3 tractor, internal air pressure in the tire (see table 2) in neutral, free and leading mode [2]. The length of the test section of the track for each loading level was 60...80 m.
Indicators of traction and energy efficiency qualities of the tested tires are calculated using the following formulas: − tractive effort: where N is the normal response reaction of the wheel; mk is the mass of the wheel and drive parts fixed on its axis; g is the acceleration of gravity. − rolling radius: where Sop is the length of the test section, determined by the speed of the track measuring (passing) wheel; nk r − rolling radius of the measuring (passing) wheel. − slip coefficient: where 0 k r is the rolling radius in the free rolling mode ( − power loss on slipping: − wheel rolling power loss:

Results
As a result of processing the experimental data [2,4,5], the basic characteristics of tire with sizes 66×43.00-25 and tires 21.3R24 were obtained, based on which their traction characteristics were calculated and constructed (Figures 1, 2, 3, 4, 5, 6). a b    The maximum values of traction efficiency 0.73...0.81 for the oligomeric tire 66×43,00-25 were obtained by a changing the traction load from 6.0 to 14.0 kN, slipping in this case varies from 2.5 to 19% (see Figure 2). On concrete (see Figures 3, 6), the oligomer tire 66×43.00-25 has the maximum traction efficiency of 0.915, while the tire 21.3R24 reaches a value of 0.79.
The test tires are rolling on concrete with little slipping. Thus, for a 66 × 43.00-25 oligomeric tire, slippage when changing the hook force from 0 to 18 kN reaches only 3%. A further increase in hook effort to 19 kN causes sharp increase of slipping, a decrease in traction can also be detected, which indicates a loss of traction properties of the tire.
The 21.3R24 tire has more slippage on concrete than the 66×43.00-25 oligomeric tire: slipping reaches 10% with a hook load of 16 kN. A sharp slippage increase is registered with an increase in the hook load of more than 16 kN.
According to the basic and traction characteristics of the tires, the rolling torque f M and the free rolling radius of the wheel 0 k k r r  at Рkp = 0, as well as the traction efficiency w  were determined. Also, slipping coefficient  was determined, as well as k М torque, which is applied to the wheel for realizing the nominal traction force with a nominal traction force of a class 3 tractor wheel equal to 7.5 kN.
The resistance force in the free rolling mode was determined according to the following formula: The tire performance indicators defined using this method are summarized in table 3. The calculation results showed that the rolling resistance force Pf at Pkp = 0 for the tire 21.3R24 is 2.92 times higher on concrete (background 3), 1.5 times on winter wheat stub (background 2), and 1.07 times lower on the field prepared for sowing than the same of the oligomeric tire. A significant increase in the resistance force on the stubble and the field prepared for sowing compared to concrete for the oligomeric tire 66×43.00-25 is obviously explained by the large width of the tire. And the increase in rolling resistance with an increase in hook load is associated with the rutting process. Traction efficiency with a nominal traction force Pkp = 7.5 kN is lower for a 21.3R24 tire than for an oligomeric 66×43.00-25tire: by 16.85% on concrete, 17.72% on stubble and 11.43% on the field prepared for sowing (table 3). It should be taken into account that the load on the 66×43.00-25 tire is almost 9% higher than on the 21.3R24 tire. The oligomeric tire develops maximum traction efficiency at higher hook loads than the 21.3R24 tire (table 4). Analyzing the power loss components on slipping and rolling of a wheel with various tires (Figure 7) shows that self-propulsion losses are predominant in both cases. However, it should be noted that the rolling power losses on the field prepared for sowing are almost the same for the compared tires. However, the rolling losses on the 21.3R24 tire are higher with a nominal traction force Рkp = 7.5 kN on concrete and stubble by 2.3 and 1.5 times, respectively. a b Figure 7. Components of power losses on winter wheat stub (a), on concrete (b).