Hydrostatic transmission for motor vehicles

Abstract: A transmission particularly for motor vehicles such as tractors or the like which comprises a hydrostatic unit (11) associated with an internal-combustion engine. The hydrostatic unit is constituted by a first variable-delivery pump (12) which is connected, in a closed circuit, to a hydraulic motor (14) adapted to provide vehicle propulsion. In the hydrostatic unit (11) there is a second boost pump (15), to compensate for the volumetric losses of the circuit; overpressure control means; actuators; and cooling elements. The second pump (15) is connected, through the interposition of a distributor unit (17) actuated by a control pedal (18), to first actuators (23) which are adapted to vary the displacement of the first pump (11) and to second actuators (27,28) which are adapted to actuate a pedal crank (19) of the accelerator of the internal-combustion engine.

Description: The present invention relates to a transmission, particularly for motor vehicles such as tractors or the like. It is known that hydrostatic transmissions, owing to their functional features, are currently used successfully, particularly in the field of the manufacture of motor vehicles such as tractors or the like. Commercially available transmissions are currently of the closed type and comprise a hydrostatic unit, associated with an internal-combustion engine, which is constituted by a first variable-delivery pump connected, in a closed circuit, to a hydraulic motor which is adapted to provide the propulsion of the motor vehicle. The hydrostatic units also comprise a second boost pump to compensate for the volumetric losses of the circuit, and overpressure control means, actuation means and cooling means. In conventional transmissions it is currently possible to provide an operating condition which is commonly known in the field as "automotive" and in which the ratio between the rpm rate of the motor and the rate of flow of the hydraulic transmission remains constant. In particular, this occurs by adjusting the displacement of the first variable-displacement pump. In particular, in conventional transmissions the displacement of the first pump is controlled directly by the accelerator control of the internal-combustion engine, while selection of the advancement direction of the second pump and therefore of the motor is provided by means of an electric valve. However, the configuration of conventional transmissions as regards the controls that operate them forces the operator to perform continuous adjustment and complex management of said controls. GB-A-2 128 718 discloses a drive unit consisting of an internal combustion engine and a steplessly adjustable hydrostatic transmission which comprises a combination of elements as defined in the precharacterizing portion of the appended claim 1. The aim of the present invention is to provide a transmission which solves the drawbacks noted above in conventional transmissions, particularly, without altering the functional characteristics for which hydrostatic transmissions are appreciated, by greatly simplifying the management of the various functional and operating modes in any condition of application. Within this aim, an object of the present invention is to provide a transmission which is particularly reliable and sturdy and is adapted to be applied to various types of vehicle, optionally to vehicles that have already been prepared or manufactured. Another object of the present invention is to provide a transmission which has competitive costs with respect to conventional transmissions. Another object of the present invention is to provide a transmission which can be manufactured with conventional technologies and facilities. In accordance with the invention, there is provided a transmission, particularly for motor vehicles such as tractors or the like, as defined in the appended claims. The particular technical characteristics and advantages of the present invention will become apparent from the description of an embodiment thereof, illustrated only by way of non-limitative example in the accompanying drawings, wherein: Figure 1 is a schematic view of a transmission according to the invention; Figure 2 is a circuit diagram of the transmission of Figure 1. With reference to Figures 1 and 2, a hydrostatic transmission, particularly for motor vehicles such as tractors or the like, according to the invention, is generally designated by the reference numeral 10. The transmission 10 comprises a hydrostatic unit, generally designated by the reference numeral 11, which is associated with a conventional internal-combustion engine which is shown schematically in Figure 1 and is designated by the reference numeral 11a. The hydrostatic unit 11 is constituted by a first variable-delivery pump 12 with two directions of rotation which is connected, by means of a closed circuit 13, to a hydraulic motor 14 of the type with two directions of rotation and which is adapted to provide propulsion for the motor vehicle, not shown. The hydrostatic unit 11 also comprises: a second boost pump 15 for compensating for the volumetric losses of the circuit; means for controlling overpressures, described hereinafter; actuation means, also described hereinafter; and conventional cooling means, generally designated by the reference numeral 16, which are parallel-connected to the motor 14, drawing the cooling flow alternatively from the branch that is at lower pressure. The second pump 15 is connected, through the interposition of a distributor unit 17 actuated by a control pedal 18, to first actuation means, described in greater detail hereinafter, which are adapted to vary the displacement of the first pump 12, and to second actuation means, also described in greater detail hereinafter, which are adapted to drive a pedal crank 19 of the accelerator of said internal-combustion engine. In particular, the distributor unit 17 comprises two diverting distributors 20 associated with an equal number of ducts 21 which are connected to the first actuation means and are connected to a reservoir 22 at their discharge. The two diverting distributors can be activated alternatively under the control of the control pedal 18. The first actuation means, in this case, are constituted by two cylinders with a single-acting piston and with a return spring, generally designated by the reference numeral 23. The second actuation means are connected to the delivery of the first actuation means through the interposition of a first bistable valve 24 and of a second cartridge valve 25 which is also of the two-way normally-closed type. The second valve 25 is actuated so as to be opened and closed by a solenoid 26, while the second actuation means, in this case, are constituted by a single-acting hydraulic actuator 27 in which return is actuated by a spring 28. In particular, the stem of the actuator 27 is associated with the pedal crank 19 by means of a pivot 29. The overpressure control means comprise, in this case, a branch 30 with an overpressure limiting valve 31 which is connected, at its discharge, to a reservoir 32 and to the second pump 15 at its delivery. The overpressure control means also comprise, in this case, another branch 33 which has two check valves 34, each of which is parallel-coupled to a corresponding pressure limiting valve 35; said check valves are connected to the reservoir 32 at their discharge and, at their delivery, to a corresponding branch for mutually connecting the first pump 12 and the motor 14. Figure 2 furthermore illustrates a hand throttle 36 for manually operating the pedal crank 19, which however operates independently of the transmission 10. In practice it has been found that the present invention has achieved the intended aim and objects. In particular, it should be noted that by means of the control pedal the operator is capable of controlling both the acceleration of the internal-combustion engine 11a and the variation in the displacement of the second variable-delivery pump 15. Furthermore, again by means of the control pedal 18, it is possible to reverse the rotation direction, further simplifying the operation of the machine. It should also be noted that all this has been achieved without altering the functional characteristics of the transmission according to the invention, which besides, by virtue of its constructive characteristics, can be manufactured at competitive costs with respect to conventional transmissions. The present invention is susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept. The materials and the dimensions may be any according to requirements. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims: A transmission (10), particularly for motor vehicles such as tractors or the like, comprising: a hydrostatic unit (11) associated with an internal-combustion engine (11a), said hydrostatic unit (11) being constituted by a first variable-delivery pump (12) which is connected, in a closed circuit (13), to a hydraulic motor (14) so as to provide vehicle propulsion; a second boost pump (15) to compensate for the volumetric losses of the circuit (13); control means (30-35) and first and second actuation means (23;27-28); said second pump (15) being connected, by virtue of the interposition of a distributor unit (17) actuated by a control pedal (18), to said first actuation means (23) which are adapted to vary the displacement of said first pump (12) and to said second actuation means (27-28) which are adapted to actuate the accelerator of said internal-combustion engine (11a), said second actuation means comprise a single-acting hydraulic actuator (27) in which return is actuated by a helical spring(28), the stem of said actuator being kinematically connected to the accelerator of said internal-combustion engine (11a), said second actuation means (27-28) are connected to the delivery of said first actuation means (23) by virtue of the interposition of valves (24,25), characterised in that said stem of said actuator is kinematically connected to a pedal crank (19) of the accelerator of said internal-combustion engine (11a), and said single-acting hydraulic actuator (27) of said second actuation means (27-28) is connected to the delivery of said first actuation means (23) by virtue of the interposition of a first bistable valve (24) and of a second two-way normally-closed cartridge valve (25), said second valve (25) is opened by a solenoid (26). A transmission according to claim 1, characterized in that said distributor unit (17) is constituted by two diverting distributors (20) which are associated with an equal number of ducts (21) connected to said first actuation means (23) and, at the discharge, to a reservoir (22), said two diverting distributors (20) being activatable alternatively under the control of said control pedal (18). A transmission according to claim 1, characterized in that said first actuation means comprise two cylinders with a single-acting piston with return spring (23). A transmission according to claim 1, characterized in that said control means comprise overpressure control means constituted by at least one branch (30) with a pressure limiting valve (31), which is connected to a corresponding reservoir (32) at its discharge and to said second pump (15) at its delivery. A transmission according to claim 4, characterized in that said overpressure control means comprise at least one branch (33) with two check valves (34), each of which is coupled to a corresponding pressure limiting valve (35), each check valve (34), together with the corresponding pressure limiting valve (35), being connected to a corresponding reservoir at its discharge and, at its delivery, to a branch for connecting said first pump (12) to said hydraulic motor (14).

Inventors: MARTIGNON GIROLAMO, MARTIGNON, GIROLAMO
Applicants: ANTONIO CARRARO S P A, ANTONIO CARRARO S.P.A.
Application year: 1998
Main classification: B60K41
IPC codes: F16H61, B60W10