[0002] A speed change drive, generally speaking, is a mechanism apt to transfer motion from an input element to an output element with different velocity ratios, so that the relative speed between the input element and the output element may be varied as required.

[0003] Traditionally, driving mechanisms wherein a speed change may be performed are manual change gears, well known in the car industry.

[0004] In manual change gears, an input element, typically a gear with a suitable diameter (or, which amounts to the same, with a suitable number of teeth), is alternately coupled with a series of intermediate elements of different sizes transmitting motion to an output shaft. Depending on which intermediate element engages the input gear in a given moment, a specific velocity ratio is obtained.

[0005] It is also known that manual change gears allow stepping velocity ratio changes, while, in many applications, stepless changes would be desirable: this allows to better exploit the driving torque, thus resulting in better performances and lower consumptions.

[0006] Automatic variable pitch pulleys, i. e. devices wherein the pitch line diameter of two driving pulleys-connected with each other by a trapezoidal belt-may be steplessly varied, have already been manufactured. However, for various reasons, these devices may only transfer low torque values, thus low power values. This is a serious drawback, since the greatest advantages of stepless change would be precisely in high power appliances, for example in powerful motor vehicles and in lorries.

[0007] One object of the present invention is that of solving the drawbacks of the prior art, providing a stepless speed change drive that does not imply any specific limits on the transmittable torque, that allows to obtain a wide speed change range and an easy drive engagement/disengagement.

[0008] Such an object is obtained, according to the invention, with a change drive as described in its characterising features in claim 9), i. e. a change drive wherein the driving device includes a fluidmechanical variable displacement pump/engine pair, reciprocally interconnected so as to obtain the desired stepless change.

[0009] Furthermore, a fluid mechanical pump that is efficient and guarantees excellent flow continuity, so as to allow smooth functioning, is particularly desirable for this use and other uses too. At present, the market does not offer such a device.

[0010] Therefore, another object of the invention is that of providing a continuous-flow pump that may be particularly yet not exclusively used in a change drive as described above. Such an object is reached by means of a pump as described in its characterising features in claims 1) to 8).

[0011] Further features and advantages of the device according to the invention will however result from the following detailed description of a preferred embodiment thereof, given by way of example and illustrated by the appended drawings, wherein: [0012] Fig. 1A is a longitudinal sectional view of a constant- flow, fixed-displacement reversible pump; [0013] Fig. 1B is a transversal sectional view taken along line I-I of fig. 1A ; [0014] Fig. 2 is a cylindrical sectional view developed on the plan of the pump of figs. 1A and 1B ; [0015] Fig. 3A is a longitudinal sectional view of a constant- flow, variable-displacement pump according to the invention; [0016] Fig. 3B is a transversal sectional view along the line III-III of fig. 3A ; [0017] Fig. 4 is a view similar to that of fig. 2, but referring to the pump in figs. 3A and 3B; [0018] Fig. 5 is a longitudinal sectional view of an embodiment of a change drive according to the invention ; [0019] Fig. 6 is a transversal sectional view taken along the line VI-VI of fig. 5; and [0020] Fig. 7 is a view similar to that of fig. 2, referring to the central portion of the change drive in fig. 5.

[0021] For reasons of coherence and clarity, the operating principle of a constant-flow, fixed-displacement hydraulic pump according to the invention will be described first.

[0022] With reference to fig. 1A, a preferred pump has a casing 1 within which a driving gear or wheel 2, engaging an input/output gear 3, is rotatingly mounted.

[0023] The driving gear 2 is rotatingly mounted, through suitable bearings 2a, onto a supporting shaft 4 longitudinally traversing the pump.

[0024] The base surface 2b of the gear 2, the casing 1 and the shaft 4 define a working chamber C (fig. 2). The working chamber C communicates with the outside through an inlet port 5c and an outlet port 5s.

[0025] On the ceiling of the working chamber, adjacent to the plane surface of the casing, there is a shim 6. The shim 6, according to this embodiment, may also be integral with the casing 1. As can be seen in fig. 2, the shim 6 has a substantially uniform thickness for a certain portion of the chamber C, i. e. the portion defined by the letter d-a'. As can be seen in fig. 1B, such a uniform portion of the shim 6 is in the shape of a circular sector, the angle of which is dependent on the number of pump vanes, according to what will be described later.

[0026] Between the shim 6 and the base 2a of the driving gear 2, there is a space that is, precisely, the working chamber C.

In the portion defined by the angle a'-b', ending at the outlet port 5s, the thickness of the shim increases (in the direction indicated by the arrow R), for example at a constant rate, becoming thickest at b' (fig. 2): here the thickness is such that the shim 6 seal-contacts the base surface of the gear 2.

All along portion b'-c', the chamber C is thus blocked by an occlusion 6a: this means that a flow between the inlet and outlet ports 5c and 5s may only occur within the arc c-b' subtended in the direction of the arrow R.

[0027] Preferably, the inlet/outlet ports are connected to the chamber C through connecting grooves 5s'and 5c', provided in the body of the shim 6 (fig. 1B).

[0028] Furthermore, at least two movable vanes 7a and 7b are set into the driving gear body. Better still, the minimum number of vanes is the one ensuring at least one vane on the"working" arc d-a' (i. e. the portion where, between each vane and the shim 6, the chamber is totally sealed in order to prevent communication between the ports). In the embodiment shown, there are four vanes 7a-7d.

[0029] Vanes 7a-7d are axially movable within suitable seats of the gear 2 and are mounted so as to be always pushed towards the shim 6, where they should establish a sliding seal-contact.

In the drawings, elastic means 8a-8d, apt to push the vanes 7a- 7d against the shim 6, are schematically represented, but many other means may be provided, including, for example, cam means.

[0030] During the rotation of the driving gear 2-driven, for instance, by the input gear 3-the vanes 7a-7d are made to rotate in the direction of the arrow R, always keeping in contact with the shim, thus progressively entering and exiting their seat only at the area corresponding to the input and output ports, to overcome the occlusion in the portion b-c, b'- c',..., and so forth.

[0031] At this point, the operation is intuitive. Be it assumed that the condition illustrated in fig. 2 subsists.

Between the front vane 7b and the rear vane 7a there is a constant-volume chamber C: the fluid contained therein (for example, oil), during the rotation of the gear 2 is simply made to shift in the direction of the arrow R. The volume behind the rear vane 7a, i. e. defined by the rear side of the vane 7a and the occlusion 6a (the vane 7d, in fact, does not completely block the passage, as the shim 6 is partially opened by the groove 5c'), progressively increases as the gear 2 rotates, producing a suction effect on the inlet port 5c. The opposite occurs with the volume in the front of the vane 7b, thus causing a compressing effect on the outlet port 5s.

[0032] When the front vane 7b reaches the outlet port 5s, the inclined plane of the shim 6 (or other driving means) pushes said vane against the spring 8, making it re-enter its seat, until the portion b'-c'is reached, wherein the vane has re- entered completely and a seal-contact is exhibited between the base 2b of the gear 2 and the occlusion 6a.

[0033] While the front vane 7b progressively re-enters its seat, the fluid contained in the chamber C communicates with the port 5s: in this condition, the compressing action is delegated to the rear vane 7a. Provided that there is always at least one vane hermetically placed between the two inlet and outlet ports 5c and 5s, the fluid compression and suction action occurs continuously.

[0034] In order to improve the operational evenness, further connecting passages (7b') are provided in the vanes 7a-7d, such that the working fluid may flow from the inner end of the vane (i. e. of the volume of the sliding seat thereof) to the outer end thereof, so as to fill up the volume of chamber C left empty by the vane when it enters the driving gear 2, and the opposite when the vane exits.

[0035] The number of vanes, as well as the amplitude of the arc subtended by the constant portion of the shim 6 (the arc d- a'), may be varied depending on the design conditions, through assessments within the range of an expert in the art.

[0036] In this way, a substantially reversible, constant-flow, fixed-displacement pump is provided. Indeed, by operating the input gear 3 through an engine, it is possible to supply fluid to the pump from the inlet port 5c and pump it out through the outlet port 5s, while, using a compressor to pump a fluid from the inlet port 5c to the outlet port 5s, it is possible to operate the device as an engine, i. e. to make the output gear 3 rotate.

[0037] The illustrated pump, furthermore, due to its arrangement, is particularly fit to become a variable- displacement pump.

[0038] In figs. 3A, 3B and 4 another embodiment of the above- described pump is indeed shown.

[0039] In this case, the shim 6 in the portion defining the chamber C, is mounted on a guiding and moving system G which allows to move it axially relative to the casing 1.

[0040] In fig. 3a, the guiding and moving system is schematically illustrated by a mount 100 onto which a threaded pin 101 is rotatingly mounted, apt to move a slide 102 which is integral, by means of through studs 103, with the shim block 6.

[0041] As well shown in fig. 4, the displacement of the movable portion of the shim 6 allows to increase or decrease the volume of the chamber C, thus varying the displacement of the pump according to the invention.

[0042] At this point it is possible to describe in detail also the arrangement of the speed change drive according to the invention that exploits the above-described variable- displacement pump.

[0043] The speed change drive according to the invention (figs. 5 and 6), is indeed made of two pumps of the above indicated type, reciprocally connected through the respective working chambers, and the one, P', operating as a pump (the one upstream relative to the motion direction) and the other, P' as an engine (the one downstream).

[0044] Fig. 5 clearly shows a first driving gear 2'and a first set of vanes 7'of the first pump P' (defining a first working chamber C'), a second set of vanes 7"and a second driving gear 2"of a second pump P" (defining a second working chamber C"), all of which are inserted in a common casing 1.

[0045] Gears 2'and 2"are coupled, through suitable gearings, to respective input/output shafts Al and A2.

[0046] A shim 6 common to both chambers C'and C''is then provided. The occlusion portion 6a, at the area corresponding to the input/output ports, is fixed, while the remaining portion 6 (illustrated in fig. 7 with a squared hatching) is displaceable along the longitudinal axis of the change drive. In this way the two coupled pumps have an opposite variable displacement, i. e. to an increase in the displacement of one pump corresponds a decrease in the displacement of the other and vice versa.

[0047] In the illustrated exemplary embodiment, the movable shim 6 may be driven from the outside through an operating or servo control system. In fig. 5, this operating system is schematically represented by studs mounted integrally with a slide 102 that is axially moveable on a worm 12 made to rotate by an electric engine 13.

[0048] According to the invention, the outlet port of the first working chamber C'communicates with the inlet port of the second working chamber C", while the outlet port of the latter communicates with the inlet port of the former.

[0049] As seen from fig. 7, a closed circuit for the fluid is obtained, such that the rotation of gear 2'in a direction D1 causes the rotation of the gear 2''in the opposite direction D2. This condition is besides advantageous also to balance the inertial forces due to the various rotating masses.

[0050] Preferably, the connection between the inlet/outlet ports occurs by means of a body la provided with a stop valve V.

This valve V has substantially two operating positions, one according to which inlet/outlet ports communicate crossing each other as described in the previous paragraph, and the other in which the corresponding inlet/outlet ports, i. e. those belonging to the same working chamber, communicate. The utility of the latter condition will be discussed further on.

[0051] It is now to be noted that the hydraulic pump/engine arrangement as described above actually represents a speed change drive. Indeed, since the fluid flow-rate between the chambers C'and C''must be the same (this being a closed circuit), if either of them has a smaller volume than the other, the vane speed, hence of the respective driving gear, will correspondingly have to be higher than the speed of the vanes in the other chamber.

[0052] This condition is used to regulate, the power entering the shaft Al being constant, the output speed of the shaft A2.

Indeed, when the displacement of P'increases, the revolutions of gear 2'being constant, the flow rate of the fluid circulating therein also increases, causing an amplified effect in P"due to the simultaneous decrease of the displacement of the chamber C'', thus obtaining an increase of the speed of vanes 7''and consequently of the output gear 2''and the shaft A2.

[0053] When, on the other hand, the displacements of the two systems P'and P''are equal, the two shafts Al and A2 (in ideal conditions, without any leaks in the system), rotate at the same speed.

[0054] From the above-said, it may be appreciated that through the motor 13 it is possible to drive the velocity ratio continuously and according to a desired law, by displacing the shim 6 and thus varying the displacement of the two coupled systems P'and P''. Advantageously, the motor 13 may also be automatically driven by an electronic unit according to signals and parameters that an expert in the art will be able to suitably choose.

[0055] It must however be noted that, even without actively using the actuator 13, the system has an independent self- regulating capacity.

[0056] Be it assumed, in fact, that the moveable shim 6 is free to move in the absence of friction and that the system is under substantial balanced steady conditions. If, in a certain instant, the load torque increases, the output shaft A2 tends to slow down: the inertia of the fluidmechanical system causes a temporary increase in the pressure within the chamber C''which results in an increase of the volume of C'and thus of the displacement of P" ; accordingly, the displacement of chamber C' tends to decrease: this makes the overall velocity ratio, which is proportional to C'/C", decrease temporarily, thus the change drive is able to automatically oppose the new condition of greater load.

[0057] The design of the change drive according to the invention has a further important advantage. Through the valve V it is possible to block the circulating fluid, interconnecting the corresponding inlet/outlet ports, i. e."short-circuiting" each of the devices P'and P".

[0058] In these conditions, all the power entering in the system through the shaft Al is exclusively taken up by the device P', simply causing the gear 2'to rotate, while the gear 2''is completely idle. As can be deducted, this operating state coincides with the"idle"condition of the change drive. The great advantage, relative to the prior art, is the very low power required to operate the valve V in order to reach the "idle"condition, as compared with what is traditionally required to operate a big friction clutch apt to disengage a driving system. The fact that the clutch is no longer necessary is a further great simplification, resulting in clear economic advantages.

[0059] As can be well understood, the speed change drive according to the invention fully achieves the objects expressed above. Indeed, it allows to obtain a steplessly variable velocity ratio, with no particular constraints as regards the transmissible power, using a minimum power quantity for the servo control and eliminating the need for a friction clutch.

[0060] It is however understood that the invention is not limited to the above-illustrated specific embodiments, which are only non-limiting examples of the scope of the invention, but many alternatives are possible, all within the reach of an expert in the art, without thereby departing from the protective scope of said invention.

[0061] For instance, there does not necessarily have to be a common shim for the two portions of the change drive P'and P", as it is sufficient that the displacement of the two portions tend to change accordingly and alternately: for example, two separate variable displacement pumps may be simply connected (one functioning as a pump, the other as an engine), and the two shims conveniently connected so that they move in opposite directions.

[0062] Furthermore, it is not strictly necessary for both the coupled pumps to be perfectly reversible, as it is conceivable that one of them always functions as a pump (the one upstream of the motion direction), while the other always functions as an engine. Therefore, suitable design features may be provided, improving the overall performance of the change drive and making the two coupled devices definitely different.

[0063] Finally, the operational evenness and efficiency may also be enhanced by suitably choosing the working fluid, which will have to be preferably incompressible, and by suitably designing the various channels connecting the inlet/outlet ports.