Ideal transmission type for tractors

When buying a tractor, knowing what operations will be carried out by it is essential because based on this information it is possible to choose a model with the ideal type of transmission to carry out the tasks.

 Although many farmers, when purchasing, take into account the engine of agricultural tractors as a reference for comparisons, the power transmission is, without a doubt, the component that best characterizes the functionality of these machines, varying from one model to another, in different power ranges.

The transmission system is made up of a set of elements whose function is to ensure the transfer of torque, originating from the operation of the engine, to the different points where it is required, such as driving wheels, power take-off (TDP) and hydraulic system. The transmission system is simultaneously an engine speed reducer and a torque amplifier. The combination of these two factors makes up power.

GEARBOXES

The gearbox, after the clutch, constitutes the first part of the kinematic chain between the engine and the driving wheels. It is made up of a series of gears responsible for changing gears, that is, for achieving different transmission ratios (different torques and travel speeds). Furthermore, it has a “neutral” position and, at least, a ratio that allows travel in reverse gear.

The greater the number of gears, the greater the possibilities of correctly selecting the speed and torque to carry out a given agricultural operation, which translates into an excellent “working point” of the engine (power and fuel consumption). .

TYPES OF GEARBOXES

Fundamentally, depending on the way in which the transmission ratio is varied, there are two types of gearboxes, mechanical and hydraulic, the latter of which can be subdivided into hydrostatic (high pressure and low oil speed) and hydrodynamic (low pressure and high oil speed).

Hydrodynamic transmission has the principle of transferring power through the kinetic energy of a hydraulic fluid; however, this type of transmission is currently not equipped on any agricultural tractor.

MECHANICAL TRANSMISSIONS

In this transmission, the gearbox can be classified, depending on the gear, into: sliding and synchronized gear.

The mechanical transmission known as sliding gears is the simplest and has a high degree of efficiency, however the teeth are subject to greater wear and synchronization is not possible. Therefore, its use is limited to the simplest transmissions, as it forces the operator to completely stop the tractor before changing gear, or to select it before starting work.

The movement, between each two successive axles, in this transmission, is carried out by pairs of gears, which move on shafts with slots, to fit together. Thus, the output speed depends on the number of meshing pairs and the number of gear teeth.

In synchronized mechanical transmission there are synchronizer rings, which rotate continuously with the shaft, which connect the gears, and rotate freely until coupling occurs. This makes it easier to change gears, as it makes it possible to approximate the rotation speeds of the gears that will be coupled, giving rise to what is known as synchronization. This transmission allows you to change speed without stopping the gears, but the transmission of power between the engine and the wheels is interrupted due to the need to activate the clutch.

POWERSHIFT TRANSMISSIONS

Known as load gear, these are clutches made up of a set of several discs, which direct the movement of power, thanks to the principles of hydrostatic pressure. The discs on both axles come together when oil arrives under pressure, promoting gear shifting with torque transmission.

This system makes it possible to reduce speed and, consequently, increase torque, without the need to stop the tractor or operate the clutch pedal to change gears. This allows work to be carried out at the speeds recommended for each type of operation, providing increased operational efficiency and fuel savings.

However, you have to be careful about one detail. There are tractor models that, to change some gears, there is no need to press the clutch pedal, but rather to press a button. In general, these are not “on-load gearshifts”, but rather synchronized gears independent of the clutch pedal. They are useful for light work, because the clutch actuation is electro-hydraulic, and so fast that the tractor's speed does not drop sharply.

Currently in Brazil, the supply of tractors with gearbox type is increasing. Powershift, in relation to synchronized ones. With this type of transmission, without pressing the clutch pedal, it is possible to change all gears (Full Powershift) or just the gears of each group (Semi Powershift), that is, the shifting of each group is synchronized (you have to press the clutch pedal), and the gear changes are carried out by moving just one lever. In some cases there are three or four gears, and in others just two (Hi and Lo: High and Low), which allows, within the same gear, to go from a slower gear to a faster gear, and vice versa.

CONTINUOUSLY VARIABLE TRANSMISSIONS (CVT)

Contrary to popular belief, CVT transmissions had already been studied in 1968, when James H. Kress, an engineer at John Deere, published a work on the theoretical basis of branched CVT transmissions applicable to tractors. In this work, he described the types of branched transmissions and calculated the power distribution and efficiency in each of them.

The incessant increase in the number of gears in tractors has reached infinity, with the offer of CVT transmissions. This happened in 1995, when Fendt, from the AGCO Group, launched the tractor Fendt Favorite 926, first tractor standard produced in series, from 1996, equipped with the Vario transmission.

CVT transmissions, in agricultural tractors, have two branches, through which the power generated by the engine circulates to the driving wheels. This implies two nodes, one at the arrival of the CVT unit and another at the exit. In one of the branches a variator is introduced (hydrostatic branch) and the other has a fixed transmission ratio (mechanical branch). Of the two nodes, one becomes a planetary system.

Based on the CVT unit scheme, two types of CVT transmission were developed. The first tractor with a CVT transmission was the Fendt Vario, which employs the planetary divider system, in which the input node is the planetary system. Then came the S-MATIC transmissions from Case IH, AUTOPOWR from John Deere, TTV from Deutz-Fahr and TVT from New Holland, which are of the planetary summing type, since the planetary system is located at the output node of the CVT unit. . More recently, Massey Ferguson adapted the Vario transmission in some of its tractors, known as Dyna-VT, also of the planetary divider type.

The presence of the variator in the CVT unit is the main feature of this transmission. The variator means that there are infinite transmission ratios, that is, infinite gear changes and, consequently, infinite travel speeds. To achieve this, at least one of the hydraulic units, which can act as a motor or pump, must be of variable displacement, whereby continuous variation is achieved.

In order to obtain the range of speeds necessary for the work and movement of the tractor, after the CVT unit there is a gearbox, which can work in two ways: in series or in parallel with the CVT unit. In parallel, like Fendt, in each of the two groups a continuous variation between the minimum and maximum travel speed is achieved. The series arrangement does not contradict the continuity of transmission, as its design means that the maximum speed of each group coincides exactly with the minimum of the next group, so that the exchange between one group and another occurs without delays.

CVT transmissions make the engine and transmission act independently of each other. To achieve this, there is a joint management system for the engine and transmission, which, depending on the versatility of the tractor, offers several working possibilities. These possibilities are known as driving strategies. Furthermore, this joint management, engine and transmission, allows the engine to work at a point of greater efficiency and lower fuel consumption.

FINAL CONSIDERATIONS

Selecting between one type of gearbox and another when purchasing an agricultural tractor is not an easy task. This choice, like any other, depends on the needs of each user, the reliability of the product and, mainly, the cost of the technology used.

Currently, the appearance of CVT transmissions in agricultural tractors was caused by the incessant increase in the number of gears on the tractor; due to the versatility of operations it carries out; for the constant search for improvements in its efficiency and convenience; and smooth handling during the workday.

No transmission is “better” or “worse” than another, but it will depend on the circumstances in which it will be used, so that the advantages offered outweigh its drawbacks.

Marcelo Silveira de Farias, José Fernando Schlosser, NEMA – UFSM; Pilar Linares, Escuela Técnica Superior de Ingenieros Agrónomos, Universidad Politécnica de Madrid

Article published in issue 149 of Cultivar Máquinas.