Expanding the technological capabilities of the integral tractor chassis by reversing the transmission

. The aim of the work is to find technical solutions that provide a complete reversal of the transmission on the example of an integral tractor, while allowing maximum use of commercially available units or installed as an option for a serial machine. The objects of study are mainly the chassis of a wheeled integral tractor designed for agriculture. The relevance of creating fully reversible transmissions for integral tractors and other agricultural, road-building and transport machines is shown. The variants of installation places for the reverse gear are considered, the features of loading and the principles of designing the reverse gear for specific placement points are analyzed. The most promising options for the case of an integral tractor based on a serially produced chassis were chosen - placement of a controlled power distribution mechanism in the rear axle gear housing or instead of wheel gears. Kinematic schemes and a technique for determining gear ratios and kinematic parameters are proposed. The reverse gear control technologies and the technological base for the design and manufacture of such mechanisms are indicated: when designing an integral tractor in accordance with the principle of maximum unification with a serial chassis, it is advisable to place the reverse gear in the drive axle housing; when developing an original reverse transmission and a new carrier system, the reverse gear should be located in front of the gearbox; in the design of the reverse gear, it is preferable not to use a blocking clutch, replacing it with a planetary gear set and a brake

The interest of manufacturers in integral tractors is quite persistent, and projects are being considered, both original machines of various traction classes, and objects based on serial chassis.Integral tractors are characterized by a tendency to provide technological capabilities for performing several operations in one pass across the field, and therefore the design of the body kit becomes more complicated (an additional, front, hitch appears), the requirements for visibility from the operator's seat, maneuverability, etc.
For an integral tractor, the task of a complete transmission reversal is relevant -this simplifies maneuvering in the field and reduces the field area required for a turn.
As a rule, an integral tractor is based on a wheeled two-axle all-wheel drive chassis.Light tractors are characterized by steering control due to steered wheels (the composition of a typical transmission of such a tractor is illustrated in Fig. 1).For machines of 3-5 traction class, the use of an articulated chassis is more typical.A typical articulated machine transmission structure will differ from that shown in Fig. 1 by the absence of constant velocity joints in the design of the drive of the front drive wheels.Such trends are described in relation to agricultural, industrial, forestry wheeled vehicles (see, for example, sources [1][2][3][4][5][6][7][8][9], etc.).
A reciprocating internal combustion engine is usually used as an energy source on a modern integral tractor.However, the transmission shown in Fig. 2 is in principle suitable for operation with all types of hybrid power plant and with a central traction electric motor (although variants using wheel motors are also discussed, despite the fact that in this case the traction motor will operate under conditions of thermal and mechanical overloads, and its power and, consequently, the size will be overestimated).
Thus, the aim of the work is to find technical solutions that provide a complete reversal of the transmission on the example of an integral tractor, while allowing maximum use of commercially available units or installed as an option for a serial machine.
Tasks to be solved:  analysis of possible options for placement in the transmission of the unit responsible for the full reverse;  selection of the preferred installation location for such a unit for the case of using a serial chassis;  proposal of a kinematic diagram of the gear part of such a unit, a method for controlling it, a technological base for design and manufacture.

Materials and methods of research
The objects of study are mainly the chassis of a wheeled integral tractor designed for agriculture.The considered principles and approaches can be used for other wheeled and tracked transport-technological machines.
The methodological basis of the research is the basic approaches of the theory of a tractor and a car, the theory of machines and mechanisms; calculation methods used in the design of systems, units and transmission units of mobile machines.

Research results
Reversing transmissions in principle is not a new direction.A fairly rich selection of proven solutions for the creation and use of reverse gears with a gear ratio of ±1 has been accumulated in foreign tank building.In [4], an extensive selection of simplified kinematic schemes of reverse gears is given.These schemes provide for the mandatory use of a blocking clutch.
However, the locking clutch is a large, complex, less reliable assembly that operates at higher loads compared to the brake.Multi-disc locking clutches are prone to disc flutter (self-oscillations that cause unloaded wear of the disc pack and additional unloaded power losses).
The blocking clutch can be replaced by a brake and a planetary gear set (see sources [10][11][12][13][14][15][22][23] and others).In this case, it is not important that the gear ratio of such a gearbox be equal to ±1.
These theses are developed by us in reports [16][17]24] in relation to the transmission of a high-speed tracked vehicle.However, the choice of installation location for the reverse gear for an all-wheel drive wheeled tractor and the design of the gear itself may differ significantly from the cases considered in publications [16,17,[25][26][27].
Consider the options for installing a reverse gear as an independent unit (see Fig. 1): a) in front of the connecting mechanism (sub-options: as part of or as an alternative to this mechanism; it is preferable that the gearbox has a gear ratio of ±1, because otherwise adjustments to the gear ratios of the gearbox will be inevitable or it is necessary to provide compensation for changes in the gear ratio at the input to the gearbox); b) in front of the gearbox (or integrated into the gearbox; similarly, to the previous option, it is preferable that the gearbox has a gear ratio of ± 1); c) in front of the transfer case (a sub-option is included in it; the gear ratio of the gearbox can be different from ± 1); d) in front of the rear axle gearbox (a sub-option is included in it; the gear ratio of the gearbox can be different from ± 1; potentially it becomes possible to abandon wheel gearboxes); e) an individual reverse gear that replaces the wheel gear.
Obviously, the smallest dimensions of the reverse gear can be achieved by placing it at point a or b, since in this case it will work with the smallest torques.However, it is most often simply not possible to place such a node on a serial chassis without making major changes to the carrier system.In addition, if the gear ratio of the gearbox differs from ±1, the load on all subsequent transmission elements will change, which is unacceptable.
For a new tractor, it is possible to place a reverse gear with a gear ratio of ±p (for p ≠ 1) at point c by adjusting the gear ratio range of either the gearbox or the transfer mechanism.Obviously, this comes at a high cost if not built into the concept of the tractor from the outset.
The greatest prospects are opened by the "central reverse" of the drive axle.In place of the rear axle gearbox, in fact, an inter-wheel controlled power distribution mechanism is installed.As an example, in Fig. 2 shows a simplified kinematic diagram using a simple planetary gear set with kinematic parameter k0=(+2) as an analogue of a symmetrical differential.
If, when the brake T1 is applied, a forward stroke is realized, then when using TR, we get complete reverse of the transmission.A differential connection is maintained between the wheels of the axle in all modes of operation of the transmission.The gearbox can be simplified by eliminating reverse gears.To reduce the load on the axle shaft, it is enough to change the gear ratio of the main pair.
When the brakes T1 and TR are applied at the same time, the mechanism is blocked.
As controls, it is advisable to use disc brakes driven by tear mechanisms (a description of the operating principles and a design technique for such mechanisms are given in books [12,14]).The forward brake should be permanently on, with shutdown by means of a hydromechanical or electromechanical actuator (see [8,[18][19][20][21], etc.), and the reverse brake should be permanently off, with a traditional engagement drive.In the simplest designs for light vehicles, disc brakes can be replaced with synchronized splined clutches, prohibiting (to increase the life of the synchronizers) switching while driving.
Methods for designing the main elements of a reverse gear of a similar design have been developed and tested (see, for example, publications [1,3] and others).
The disadvantages of the considered option are the complexity of the design, significant weight (more than 50 kg), the need to install a gearbox on each drive axle.Installing a reverse gear at point e is advisable if there is a wheel gear that can be excluded from the transmission.In this case, the gear ratio p will be equal in absolute value to the gear ratio of the excluded wheel gear and there is no need to revise the design of other transmission units.
But at point e, the gearbox is loaded with large torques, which makes it cumbersome and the operation of the controls less reliable.An individual gearbox will need to be installed on each drive wheel (although their design will be unified for the chassis family).On Fig. 3 shows one of the variants of the kinematic diagram of such an "individual" gearbox.
The control elements and their drive will be similar in design to those considered for the case of "central reverse".In the gear part, it is proposed to use a complex four-link mechanism.
There is a positive experience in the manufacture and operation of a complex four-link mechanism in highly loaded transmissions, so you can refer, for example, to sources [11][12][13][14][15].When conducting a kinematic analysis, the SSM is equivalent to two simple planetary mechanisms and is characterized by two kinematic parameters.
If the input link is assigned index 0, and the output link is x, then we can write: In this case, 0 R k  , and 1 0 k  .The gear ratios of the gearbox in two modes of operation can be calculated from the dependencies: In this case, it is desirable to provide When designing an integral tractor in accordance with the principle of maximum unification with a serial chassis, it is advisable to place the reverse gear in the drive axle housing.
2. When developing an original reverse transmission and a new carrier system, the reverse gear should be located in front of the gearbox.
3. In the design of the reverse gear, it is preferable not to use a blocking clutch, replacing it with a planetary gear set and a brake.

Conclusion
The design and manufacture of a reverse gear can be carried out on a methodological and technological basis, tested in the production of high-speed tracked vehicles.The design and production of the reverse gear can be completely localized in the Russian Federation.

Fig. 2 .
Fig. 2. Simplified kinematic diagram of the version of the reverse gearbox installed in place of the drive axle gearbox: T1, R -brakes; k0,1,R are the kinematic parameters of the planetary gear sets.