The invention relates, on the 1 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 performed back, through 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 components of the planetary gear is coupled in the exact same manner to the output shaft of the gearbox.
A series of gearbox styles, which also contain the hydraulic gearbox in which a hydraulic pump drives a hydraulic motor through an oil circuit, have turn out to be recognized from the prior art. In hydraulic gearboxes of this sort, nevertheless, the efficiency declines swiftly with rising rotational speeds, considering that flow resistances make themselves felt in a disadvantageous manner as the speed of flow of the hydraulic oil increases. Consequently, a especially big quantity of power is wasted precisely when the maximum output is needed of the gearbox, with the outcome that the internal-combustion motor providing the drive has to be overdimensioned to a relatively main extent.
These disadvantages of the arrangements known from the prior art outcome in the difficulty, which is initiating the present invention, of designing an infinitely variable gearbox which develops its optimum efficiency precisely when the maximum throughput of power occurs.
This difficulty is effectively 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 portion, 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 implies that, at high rotational speeds at which the greatest output is taken off at the output shaft, no flow losses happen in the hydraulics and therefore the efficiency reaches its maximum. Simply because of its favorable efficiency, a gearbox of this variety permits a reduction in the structural size of the driving motor. As 1 rotational part of the hydraulic motor is coupled in a rotationally fixed manner to the driven shaft of the gearbox, it is attainable to transfer rotational power from the output shaft of the gearbox by means of the hydraulic oil circuit to the driven shaft, so that in the case of a pushing load the stabilizing traits of a combustion engine can be used to limit the rotational speed of the gearbox components.
The inventive hydro mechanical gearbox offers the possibility of controlling the speed of a car via the gear ratio, even though the combustion engine runs at a practically continuous speed, exactly where the torque is as high as attainable or its efficiency has its very best value. The dynamical qualities of the inventive gearbox are superior, due to the fact at a standstill of the car the hydraulic pump runs at a higher speed, so that a wonderful quantity of rotational power is stored within its rotating masses. When the car begins, these rotating masses are decelerated, whereby the stored quantity of power is delivered at the output shaft in addition to the amount of power, which is straight transmitted from the combustion engine. So at the moment, when the car begins, the inventive hydro mechanical gearbox may have an efficiency of much more than 100%. This aspect is quite essential for tractors for agricultural purposes, which operate usually at a rough or steep landscape. If the tractor has to commence from an inclined position, at the moment of disconnecting the brake the gearbox has to deliver the full output at its output shaft.
A single feasible way of adjusting the gearing ratio of the gearbox according to the invention consists in the reality that the delivery quantity of the hydraulic pump is adjustable. With the intake volume of the hydraulic motor unchanged, it is attainable, by adjusting the delivery quantity of the hydraulic pump, to adjust the braking action of the latter and in this way accomplish an adjustment of the gearing of the gearbox. Axial piston pumps, for instance, are appropriate for this objective given that, in the case of these pumps, the inclination of the oblique disc actuating the axial piston is adjustable during operation. Axial manage pistons, which are connected to the pump casing in a rotationally fixed manner, are utilised for this purpose.
A hydraulic motor delivers a greater or lesser operating resistance, based upon the mechanical load, to the hydraulic oil taken in, and can for that reason be used for throttling the flow of oil. A single distinct benefit of such an arrangement lies in the truth that the energy drawn off as a result of the throttling-down of the hydraulic pump delivering the drive is not lost but is re-introduced to the driving shaft of the gearbox in the kind of an extra torque, so that the combustion engine is unburdened. Considering that it is also achievable, 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 mentioned invention offers for the possibility of adjusting the intake volume of the hydraulic motor.
If, as a result, the intake volume of the hydraulic motor is set reduced 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 given that, in the case of the said hydraulic piston motor, adjustment of the intake volume is feasible by altering the inclination of the oblique disc actuated by the axial handle piston. A hydraulic motor of this sort is primarily 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 handle pistons for the hydraulic pump, and also, optionally, for the hydraulic motor, are hydraulically actuated. In this occasion, the hydraulic pressure required for this goal 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.
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