Transmissions used in tractors

Companies have made a greater variety of tractors available each year, with different specificities. Observation of the type of transmission is one of the most important items to be taken into consideration so that activities on the property are carried out efficiently.

Through the modernization of agriculture and the cultivation methods adopted, it has become essential to use mechanisms that allow a high level of efficiency to be achieved in the most different activities that may be developed. These mechanisms, in some cases, aim to meet the demand for road transport vehicles and, subsequently, began to equip agricultural machinery seeking to meet a variety of tasks. As an example of this, there are some of the transmission systems that currently equip agricultural tractors, obtaining, in recent years, a great advance in relation to the technological contribution of these mechanisms.

Since agricultural tractors must operate on different transport routes and also carry out a series of activities, such as pulling machines and implements, it is essential that the transmission system is able to receive, transform and transmit engine power to the driving wheelsets and to the different agricultural implement coupling systems (power take-off and hydraulic lifting system, for example). Thus, unlike automobiles, this mechanism in agricultural machinery needs to enable a wide variation of speeds, given the immense variation in power demand in their coupling systems.

Therefore, a theoretically ideal transmission would be one that provides an infinite number of combinations of speed and traction force, under the power conditions provided by the engine in question. Along with this, they must present a variation in speed in which the maximum limit of the previous gear must be equal to or slightly greater than the minimum limit of the subsequent gear, that is, if the previous gear has a maximum limit of 10km/h, the subsequent gear it must have a minimum limit of 10km/h or less, thus, there will be better energy use with lower specific fuel consumption (kW/h), which is equivalent to carrying out the same work with lower hourly consumption (L/h).

Furthermore, they must have sufficient ratios to carry out at least three types of activities: slow work with a high demand for power; works that use a large part of the available power and are carried out at speeds of up to 12km/h; and transport work, where the travel speed is high and the power requirement is very low.

In Brazilian agricultural tractors there are basically two types of transmissions: mechanical and hydromechanical. The first are subdivided into three categories: sliding, partially synchronized and synchronized. In these, the engine's power is transmitted through friction between mechanical elements, that is, through direct contact between the gears, present in the gearbox, and the clutch. However, in the case of partially synchronized and synchronized transmissions, there are rings that regulate the rotation speed of the gears that will be coupled, making it possible to change gears while the tractor is moving, which is not possible in the previous ones. In the case of partially synchronized gears, these rings are only present in some gears, usually the highest ones.

The hydromechanical transmission is the association of hydraulic transmission components (uses the energy of fluid movement to transmit power) and mechanics, and the gear shifting occurs through the oil flow, making gear changing more precise and smooth, in addition to being able to be carried out with the tractor in motion.

With regard to maintenance, mechanical transmissions have a lower cost and are less likely to fail, while hydromechanical transmissions have a higher cost due to their greater number of components.

By obtaining a transmission system that offers adequate gear scaling, enabling an increase in the diversity of operations that can be carried out, agricultural tractors will be able to achieve travel speed and power availability appropriate to the agricultural operation carried out. This fact may result in fuel savings and, consequently, an increase in operational efficiency. However, a mechanism with greater technological support cannot always provide such gains, since a mechanical transmission, for example, can adequately meet certain demands.

From this, a survey was made of the different transmissions present in the motor power classes of Brazilian agricultural tractors. By searching technical catalogues, manuals, leaflets and direct contact with manufacturers, it was possible to create a digital database. In total, four manufacturers were obtained that produce 113 tractor models in the most different motor power classes, according to the methodology proposed by Anfavea (Table 1).

After data analysis and classification according to motor power class, the distribution of the number of tractors per class was obtained as shown in Figure 1.

Motor power class I presented 13,30% of agricultural tractors, which represents 15 tractors. Class II, 52,20%, representing 59 tractors. Class III presented 23% of tractors, 26. Finally, class IV of motor power presented 11,50%, which represents 13 tractors.

The fact that classes II and III concentrate the majority of tractors is justified because they are equipment with greater versatility, as they have sufficient mass and power to carry out different activities. These range from sowing, to cultural treatments, on an appropriate scale. Tractors above 147kW (199,8hp), class IV, can be used for all these activities, however, their use is recommended when they are carried out on a large scale, such as in a sowing operation using a large machine, for example. In other words, they are more recommended in activities that require a greater energy demand and weight/power ratio.

Along with this, there is a growing increase in technology in the transmission system as the motor power classes increase, that is, tractors in classes III and IV tend to be equipped with more advanced transmission systems than those in classes I. and II, as can be seen in Figure 2.

The predominant type of transmission in agricultural tractors is synchronized mechanics, which makes gear changing easier, as it allows it to occur while the tractor is in motion, and can be found in tractors of all power classes. Sliding mechanical transmissions, whose technological contribution is the lowest among all transmissions that equip tractors, are only found in classes I and II, while partially synchronized mechanical transmissions, a cheaper option compared to fully synchronized ones, where Manufacturers seek to have synchronization only in the gears in which it normally presents the greatest demand for carrying out agricultural tasks, they can only be found in power class II tractors. On the other hand, hydromechanical transmissions, which present the greatest technological contribution, provide the tractor with a greater number of gears and make it possible to obtain a wide variety of speeds and power while the tractor is in motion. However, these can only be found on class III and IV tractors.

Therefore, the technological contribution with regard to agricultural tractor transmissions is increasing depending on their motor power. With the need for tractors with lower engine power, aimed only at light or low-scale work, requiring reduced cost in relation to others so that their acquisition becomes viable, companies choose to use simpler technology in relation to higher power tractors. , but which still offer the possibility of carrying out a series of tasks, with energy expenditure and at appropriate speeds.

Final considerations

Currently, there is an increasing modernization of the transmission systems used, however, for models with low motor power, simpler systems are still used, with the aim of offering producers equipment with a lower acquisition cost.

Advances in agriculture are making automakers make a greater diversity of tractors available for producers to purchase, with the most different specifications. Therefore, it is extremely important to analyze and evaluate the information about each product made available by manufacturers and whether this equipment will be able to efficiently carry out activities on the property, always taking into account its particularities.

Pablo do Amaral Alonço, Airton dos Santos Alonço, Tiago Rodrigo Francetto, Dauto Pivetta Carpes, Laserg/UFSM

Article published in issue 171 of Cultivar Máquinas