Hydraulic Mechanical Gearbox

Hydraulic Mechanical Gearbox
The invention relates, on the one hand, to a hydro mechanical gearbox with a hydraulic pump, a hydraulic motor and a set of planetary gears, whereby the pressure line of the hydraulic pump is conducted back, via the inserted hydraulic motor, to the suction line of the hydraulic pump, thereby forming a circuit, and whereby either the sun wheel or the planet-carrier of the set of planetary gears is coupled in a rotationally fixed manner to the driven shaft of the gearbox, and the other one of these two elements of the planetary gear is coupled in the same manner to the output shaft of the gearbox.

A series of gearbox designs, which also include the hydraulic gearbox in which a hydraulic pump drives a hydraulic motor via an oil circuit, have become known from the prior art. In hydraulic gearboxes of this kind, however, the efficiency declines rapidly with rising rotational speeds, since flow resistances make themselves felt in a disadvantageous manner as the speed of flow of the hydraulic oil increases. Consequently, a particularly large amount of energy is wasted precisely when the maximum output is required of the gearbox, with the result that the internal-combustion motor providing the drive has to be overdimensioned to a relatively major extent.

These disadvantages of the arrangements known from the prior art result in the problem, which is initiating the present invention, of designing an infinitely variable gearbox which develops its optimum efficiency precisely when the maximum throughput of energy occurs.

This problem is successfully solved in a gearbox of the above described type when one rotational unit of the hydraulic motor, preferably its rotor, is coupled in a rotationally fixed manner to the driven shaft of the gearbox, and the other unit of the hydraulic motor, preferably its static part, is coupled either to the frame/housing of the gearbox or to its output shaft. As will be further explained later on, in an arrangement of this type, the delivery quantity of the hydraulic pump declines as the rotational speed of the output shaft rises, until it finally becomes absolutely zero at a maximum rotational speed.

This means that, at high rotational speeds at which the greatest output is taken off at the output shaft, no flow losses occur in the hydraulics and therefore the efficiency reaches its maximum. Because of its favorable efficiency, a gearbox of this type permits a reduction in the structural size of the driving motor. As one rotational part of the hydraulic motor is coupled in a rotationally fixed manner to the driven shaft of the gearbox, it is possible to transfer rotational energy from the output shaft of the gearbox through the hydraulic oil circuit to the driven shaft, so that in the case of a pushing load the stabilizing characteristics of a combustion engine can be used to limit the rotational speed of the gearbox elements.

The inventive hydro mechanical gearbox offers the possibility of controlling the speed of a vehicle through the gear ratio, while the combustion engine runs at a nearly constant speed, where the torque is as high as possible or its efficiency has its best value. The dynamical characteristics of the inventive gearbox are superior, because at a standstill of the vehicle the hydraulic pump runs at a high speed, so that a great amount of rotational energy is stored within its rotating masses. When the vehicle starts, these rotating masses are decelerated, whereby the stored amount of energy is delivered at the output shaft additionally to the amount of energy, which is directly transmitted from the combustion engine. So at the moment, when the vehicle starts, the inventive hydro mechanical gearbox may have an efficiency of more than 100%. This aspect is very important for tractors for agricultural purposes, which operate often at a rough or steep landscape. If the tractor has to start from an inclined position, at the moment of disconnecting the brake the gearbox has to deliver the full output at its output shaft.

One possible way of adjusting the gearing ratio of the gearbox according to the invention consists in the fact that the delivery quantity of the hydraulic pump is adjustable. With the intake volume of the hydraulic motor unchanged, it is possible, by adjusting the delivery quantity of the hydraulic pump, to change the braking action of the latter and in this way achieve an adjustment of the gearing of the gearbox. Axial piston pumps, for example, are suitable for this purpose since, in the case of these pumps, the inclination of the oblique disc actuating the axial piston is adjustable during operation. Axial control pistons, which are connected to the pump casing in a rotationally fixed manner, are used for this purpose.

A hydraulic motor offers a greater or lesser working resistance, depending upon the mechanical load, to the hydraulic oil taken in, and can therefore be used for throttling the flow of oil. One particular advantage of such an arrangement lies in the fact that the energy drawn off as a result of the throttling-down of the hydraulic pump providing the drive is not lost but is re-introduced to the driving shaft of the gearbox in the form of an additional torque, so that the combustion engine is unburdened. Since it is also possible, by varying the throttling, to influence the braking action of the hydraulic pump and, with it, the gearing ratio of the gearbox according to the invention, the said invention provides for the possibility of adjusting the intake volume of the hydraulic motor.

If, therefore, the intake volume of the hydraulic motor is set lower than the delivery volume of the hydraulic pump, the delivery flow of the latter is supported on the hydraulic motor, so that the hydraulic pump is braked. The use of an axial piston motor as the hydraulic motor suggests itself since, in the case of the said hydraulic piston motor, adjustment of the intake volume is possible by changing the inclination of the oblique disc actuated by the axial control piston. A hydraulic motor of this kind is based on a reversal of the operation of the hydraulic pump principle, so that the hydraulic motor can be constructed, at least in principle, so as to be structurally identical to the hydraulic pump coupled to the set of planetary gears.

It lies within the scope of the invention that the control pistons for the hydraulic pump, and also, optionally, for the hydraulic motor, are hydraulically actuated. In this event, the hydraulic pressure necessary for this purpose can be taken off, in a particularly advantageous manner, from a gas pressure/spring reservoir which is fed by the hydraulic pumps of the gearbox according to the invention.