Reversible hydraulic machine with helical teeth gear provided with bilateral hydraulic system for the balancing of axial forces

Abstract

A hydraulic machine (100) of reversible type comprises a first toothed wheel (1) and a second toothed wheel (2) with helical teeth and bushings (3, 4) for revolvingly supporting the shafts (10, 20) of the toothed wheels; the rear bushing (3) has two grooves (8, 82) in communication with a first zone (Z1) inside the casing, and the front bushing (3) has two grooves (9, 92) in communication with a second zone (Z2) inside the casing in order to balance axial forces (A, C) on the first toothed wheel (1) and axial forces (B, D) on the second toothed wheel (2), respectively.

Description

[0001] REVERSIBLE HYDRAULIC MACHINE WITH HELICAL TEETH GEAR PROVIDED WITH BILATERAL HYDRAULIC SYSTEM FOR THE BALANCING OF AXIAL FORCES

[0002] DESCRIPTION

[0003] The present invention relates to a reversible hydraulic machine with gear comprising two toothed wheels with helical teeth, provided with a bilateral hydraulic system to balance the axial thrusts on the toothed wheels.

[0004] A hydraulic machine is a pump or a hydraulic motor. Although specific reference will be made below to reversible gear pumps, the present invention also refers to reversible hydraulic gear motors. Hydraulic gear motors are constructively the same as pumps; however, they differ from them in their principle of operation: whereas pumps convert mechanical energy (torque applied to the driving shaft) into hydraulic energy (pressurized oil), conversely, motors convert hydraulic energy into mechanical energy. The pressurized oil that is conveyed into the hydraulic motor from one of the ports provided on the motor body acts on the toothed wheels, putting them into rotation; the torque thus produced represents the output available on the shaft on which a load is applied.

[0005] External gear pumps are commonly used in many industries, such as automotive, earthmoving machinery, automation and control.

[0006] Straight teeth gear pumps are known, in which the gear comprises toothed wheels with straight teeth. In such a case, during the operation of the pump, on the toothed wheels the forces transmitted in the coupling between contacting teeth have no axial component. Therefore, straight teeth gear pumps do not need to balance the axial forces.

[0007] Pumps with helical teeth gear are known, in which the gear comprises two toothed wheels with helical teeth as described, for example, in W02010063705A1 , in the name of the same applicant, or in US2159744 (Maglott) or US3164099 (Hitosi). Helical gears provide a significant reduction in the noise and pulsation induced by the pump in the hydraulic circuit. It should be noted that in order to properly mesh two helical toothed wheels with the same geometrical characteristics, said toothed wheels must have a helix angle with discordant sense.

[0008] However, during operation, the use of toothed wheels with helical teeth originates axial loads or stresses on the toothed wheels, which increase when the helix angle increases.

[0009] Figs. 1 and 2 show a gear pump comprising a first helical toothed wheel (1 ) that meshes into a second helical toothed wheel (2). The toothed wheels have respective shafts (10, 20) rotoidally coupled to a casing (5) by means of a rear bushing (3) and a front bushing (4). The bushings (3, 4) are arranged in a casing (5) closed by a front flange (6) and a rear cover (7).

[0010] Looking at the pump from the front flange (6), as shown in Fig. 2, the casing (5) has a first duct (51 ) on the left side and a second duct (52) on the right side.

[0011] The shaft (10) of the first toothed wheel crosses the front flange (6) and protrudes from the front flange in order to be connected to a motor. So the shaft (10) of the first toothed wheel is the driving shaft, the first toothed wheel (1 ) is the driving toothed wheel, and the second toothed wheel (2) is the driven toothed wheel. Looking at the pump from the front flange (6), the driving shaft (10) rotates counterclockwise. In such a case, the first duct (51 ) is the delivery duct, and the second duct (52) is the suction duct.

[0012] Figs. 3 and 4 show the same helical gear pump as Figs. 1 and 2, wherein the drive shaft (10) rotates clockwise. In such a case, looking at the pump from the side of the front flange (6), the second duct (52) is the delivery duct, whereas the first duct (51 ) is the suction duct.

[0013] Referring to Figs. 1 and 2, in this case, axial forces (A, B) resulting from the forces respectively acting on the helical toothed wheels (1 , 2) are directed toward the rear cover (7) of the pump. If not counteracted, the occurrence of the axial forces (A, B) greatly increases the specific pressure that is discharged on the surface of the rear bushing (3) of the pump, thus reducing the mechanical efficiency of the pump due to frictional losses, as well as reducing the reliability and the maximum pressure of the pump. The resultant of the axial forces on each toothed wheel is given by a transmission force axial component (Fa) and by a pressure force axial component (Pa), acting on the tooth profiles of the toothed wheels along the axial direction. The sense of the two axial components depends on the sense of the helix given to the toothed wheels. In order to achieve meshing with parallel axes, the sense of the helix of the driving toothed wheel (1 ) and that of the driven toothed wheel (2) must be opposite. The images refer to a pump having a driving toothed wheel (1 ) with a left helix and a driven toothed wheel (2) with a right helix. However, the same principles will apply also to the symmetrical case.

[0014] On the driving toothed wheel (1 ), the transmission force axial component (Fa) has the same sense as the pressure force axial component (Pa). Therefore, on the driving toothed wheel (1 ), the resulting axial force is given by the following relationship:

[0015] A = Pa + Fa.

[0016] On the driven toothed wheel (2), the axial component (Fa) has opposite sense relative to the axial component (Pa). Therefore, on the driven toothed wheel (2), the resulting axial force is given by the following relationship:

[0017] B = Pa - Fa

[0018] Various systems are known for balancing the axial forces on toothed wheels.

[0019] W02006090652A1 (Shimadzu) describes a pump having a gear with bihelical toothed wheels, in such a case the problem of balancing the axial forces does not exist since they are directly balanced on the toothed wheels. However, such a solution is impaired by several disadvantages. In fact, the greater constructive complexity of the bi-helical toothed wheels, together with the precision required for the construction of high-pressure gear pumps or motors make this solution uneconomical.

[0020] US3658452 (Kita) discloses a system for balancing the thrusts acting on the pump gears, both of which are directed toward the rear cover of the pump, through hydraulic pistons located in the cover and acting at the ends of the shafts of the toothed wheels. The hydraulic pistons are powered through conduits that connect the hydraulic pistons with the delivery area of the pump. The use of compensating pistons is a rather inexpensive and easily implemented solution, as the machining and the components required to achieve it are simple and reliable. Of course, for balancing to work, the pump configuration must be such that the resulting forces A and B are directed toward the cover, and not in the opposite sense.

[0021] WO201 0102722 (Bosch) discloses a system for balancing the axial forces directed toward a rear bushing through two pressurized cavities formed on the surface of the rear bushing. The two pressurized cavities are fed through conduits that put them into communication with the delivery area of the pump. The areas of the two pressurized cavities must be properly dimensioned to balance the axial forces. As in the previous case, such a solution is valid for balancing the resultant forces A and B directed toward the cover, and not in the opposite sense.

[0022] All the solutions according to the prior art as described above are limited to solving the problem of compensating the axial forces only in the case of oneway hydraulic pumps or motors, which are constructed to operate only in a predefined clockwise or counterclockwise rotation direction of the driving toothed wheel, wherein the axial forces (A, B) are directed toward the rear cover (7), as shown in Figs. 1 and 2. Such a fact forces the manufacturer to make rather constraining technical choices. In the case of a counterclockwise rotating pump, the driving toothed wheel (1 ) must have a left helix and the driven toothed wheel (2) must have a right helix; conversely, in the case of a clockwise rotating pump, the driving toothed wheel must have a right helix and the driven toothed wheel must have a left helix, otherwise no compensation of the axial forces can be realized.

[0023] However, the use of reversible hydraulic pumps or motors is necessary in some hydraulic applications. The use of reversible pumps makes it possible to reverse the motion of the driving toothed wheel, thus changing the direction of the oil flow and reversing the low and high pressure zones (suction and delivery), allowing, for example, for reversing the motion of hydraulic actuators. Similarly, the use of reversible hydraulic motors is useful in applications where it is necessary to change the direction of the torque available at the output shaft of the hydraulic motor.

[0024] Fig. 3 illustrates the distribution of the axial forces (C, D) in the case where the pump of Fig. 1 is a reversible pump, in the case where the driving toothed wheel (1 ) rotates clockwise. In such a condition, the axial forces (C, D) are directed toward the front bushing (4), so:

[0025] C = - A

[0026] D = - B

[0027] In this case, the solutions of the prior art for the compensation of the axial forces, e.g., US3658452 (Kita) and WO2010102722 (Bosch), are not applicable, because the reversing of the motion and of the suction side with the delivery side results in the reversing of the axial forces acting on the toothed wheels (1 , 2), as shown in Fig. 3. Under this operating condition, the axial forces (C, D) are directed toward the front flange (6) and not toward the rear cover (7) as shown in Fig. 1 .

[0028] Due to an unavoidable protruding part of the shaft (10) of the driving toothed wheel that projects from the front flange (6), the axial force (C) on the driving toothed wheel (1 ) can no longer be compensated with the solutions of the prior art described in US3658452A and W02010102722A2. The only applicable solution would be the use of the bi-helical toothed wheels described in W02006090652A1 , but as mentioned earlier, the greater constructive complexity of the bi-helical toothed wheels, together with the precision required for the construction of high-pressure gear pumps or motors, make this solution uneconomical.

[0029] The same situation described for a pump occurs in a hydraulic motor, where there is a high-pressure fluid inlet side and a low-pressure fluid outlet side. In such a case, there is not a driving toothed wheel and a driven toothed wheel, but simply a first toothed wheel (1 ) and a second toothed wheel (2); in such a case, the protruding part of the shaft (10) of the first toothed wheel is not suitable for being connected to a motor, but is suitable for being connected to a resisting load. EP2154372 (Lavezzi) discloses a compensation system for a partial balancing of a reversible helical gear pump. Such a compensation system is characterized by the fact that it is one-sided because the proposed solution axially compensates alternately only one of the two resultants A or B acting on the toothed wheels (1 , 2), depending on the rotational direction of the pump. Consequently, such a solution only provides a partial balancing of the axial stresses, and only of the axial stresses directed toward the rear cover; also in this case, the solution is not applicable to compensate C or D, i.e. forces directed toward the front flange (6) due to the fact that the driving shaft passes through and protrudes from the front flange (6).

[0030] DE2315630A1 describes a reversible gear machine that can operate as a pump or as a hydraulic motor.

[0031] US2012 / 156080A1 describes a gear pump having a balancing system of axial thrusts that is substantially similar to that described in WO201 0102722A2.

[0032] US2014 / 086779A1 describes a gear machine that can operate as a pump or as a hydraulic motor, having a low-pressure duct with non-circular cross section.

[0033] The purpose of the present invention is to overcome the drawbacks of the prior art by providing a hydraulic machine of reversible type (operating with either clockwise or counterclockwise rotating shaft), with helical tooth gear, having a bilateral hydraulic system for a complete balancing of the axial forces that can be used as a pump or as a motor.

[0034] Another purpose is to provide a hydraulic machine that is versatile, efficient, reliable, economical, and easy to manufacture.

[0035] These purposes are achieved in accordance with the invention with the features of the attached independent claim 1 .

[0036] Advantageous realizations of the invention appear from the dependent claims.

[0037] Further features of the invention will appear clearer in the detailed description that follows, referring to a purely illustrative and therefore nonlimiting embodiment, illustrated in the accompanying drawings, wherein: Fig. 1 is a sectional view of a gear pump with helical toothed wheels according to the prior art, in counterclockwise operation;

[0038] Fig. 2 is a cross-sectional view taken along the plane ll-ll of Fig. 1 ;

[0039] Fig. 3 is an axial sectional view of the same pump as Fig. 1 , in clockwise operation;

[0040] Fig. 4 is a cross-sectional view taken along the plane IV- IV of Fig. 3;

[0041] Fig. 5 is an axial sectional view of a hydraulic machine according to the invention;

[0042] Fig. 6 is a cross-sectional view taken along the plane IV- IV of Fig. 5;

[0043] Fig. 7 is an exploded axial view of the hydraulic machine of Fig. 5;

[0044] Fig. 8 is a detail of Fig. 5, illustrating only the toothed wheels and the bushings;

[0045] Fig. 8A is an enlarged detail enclosed in circle A of Fig. 8, illustrating a groove formed in the rear bushing;

[0046] Fig. 8B is a variation of the groove shown in Fig. 8A;

[0047] Fig. 9 is a cross-sectional view taken along the plane IX- IX plane of Fig. 8;

[0048] Fig. 10 is a cross-sectional view taken along the plane X-X of Fig. 8;

[0049] Fig. 1 1 is an exploded perspective view of the detail of Fig. 8;

[0050] Fig. 12 is a view as Fig. 8 when the shaft rotates with counterclockwise direction;

[0051] Fig. 13 is an exploded perspective view of the detail of Fig. 12;

[0052] Fig. 14 is a view as Fig. 8 when the shaft rotates with clockwise direction;

[0053] Fig. 15 is an exploded perspective view of the detail of Fig. 14; and

[0054] Fig. 16 is a schematic view showing the two toothed wheels of the hydraulic machine according to the invention and the forces and moments acting on them.

[0055] With the help of Figs. 5 to 15, the hydraulic machine according to the invention is described, comprehensively denoted with reference numeral 100.

[0056] Referring to Figs. 5, 6 and 7, the hydraulic machine (100) comprises a first toothed wheel (1 ) and a second toothed wheel (2). The first toothed wheel (1 ) has helical teeth with a left helix. The first toothed wheel (1 ) comprises a shaft (10). The shaft (10) has a front end portion suitable for being coupled to an electric motor or a power take-off, in the case where the hydraulic machine (100) is a pump, or to a resisting load in the case where the hydraulic machine (100) is a hydraulic motor.

[0057] The second toothed wheel (2) has helical teeth with a right helix. The second toothed wheel (2) comprises a shaft (20). In the case where the hydraulic machine (100) is a pump, the first toothed wheel (1 ) is the driving toothed wheel and the second toothed wheel (2) is the driven toothed wheel.

[0058] The teeth of the first toothed wheel (1 ) and of the second toothed wheel (2) can have a profile like the one described in W02010063705A1 , which is incorporated herein by reference.

[0059] The shafts (10, 20) of the toothed wheels are rotoidally supported by a rear bushing (3) and by a front bushing (4) arranged posteriorly and anteriorly with respect to the toothed wheels (10, 20).

[0060] The rear bushing (3) has two bores (30, 31 ) that accommodate and rotoidally support the shafts (10, 20) of the toothed wheels. The front bushing (4) has two bores (40, 41 ) that accommodate and rotatably support the shafts (10, 20) of the toothed wheels.

[0061] In this way, the first toothed wheel (1 ) meshes with the second toothed wheel (2), forming a gear.

[0062] The assembly comprising the toothed wheels (1 , 2) and the bushings (3, 4) is contained in a casing (5) that is open at the front and at the back.

[0063] With reference to Fig.6, the assembly comprising the toothed wheels (1 , 2) and the bushings (3, 4) housed inside the casing (5) divides the internal volume into a first zone (Z1 ) communicating with a first duct (51 ) of the casing, and a second zone (Z2) communicating with a second duct (52) of the casing. Depending on the type of operation of the hydraulic machine (100), the conduits (51 , 52) of the hydraulic machine can be an inlet conduit and an outlet conduit of the fluid or vice versa. Consequently, during the operation of the hydraulic machine, the first zone (Z1 ) is a high pressure zone and the second zone (Z2) is a low pressure zone or vice versa, depending on the operating conditions (pump or motor) and on the direction of rotation of the shaft (10).

[0064] The casing (5) is closed at the front and at the back by a front flange (6) and a rear cover (7), respectively. The front flange (6) has a bore (60) for the passage of the front end portion of the shaft (10) of the first toothed wheel (1 ), which protrudes anteriorly from the front flange in order to be connected to an electric motor or to a power take-off.

[0065] Referring to Fig. 7, the rear bushing (3) has a rear face (32) facing the rear cover (7) and a front face (33) facing the teeth of the toothed wheels (1 , 2).

[0066] The toothed wheels (1 , 2) have a rear face (14, 24) coupled to the front face (33) of the rear bushing and a front face (15, 25) coupled to the front bushing (4).

[0067] The front bushing (4) has a rear face (42) coupled to the front face (15, 25) of the teeth of the toothed wheels (1 , 2) and a front face (43) coupled to the front flange (6).

[0068] Referring to Figs. 7, 8 and 9, on the front face (33) of the rear bushing there is a first groove (8) around the first bore (30) of the rear bushing and a second groove (82) around the second bore (31 ) of the rear bushing.

[0069] The grooves (8, 82) of the rear bushing communicate with the first zone (Z1 ) inside the casing, respectively, by means of a first communication duct (80) and a second communication duct (81 ) formed in the rear bushing (3) and facing the first zone (Z1 ) of the casing (5). By way of example, in the view of Fig. 9, the communication ducts (80, 81 ) exit at the left side of the rear bushing.

[0070] The cross section of the grooves (8, 82) and of the communication ducts (8, 81 ) can be semicircular as shown in Fig. 8A, or rectangular as shown in Fig. 8B.

[0071] With reference to Fig. 9, the grooves (8, 82) of the rear bushing formed on the front face (33) of the rear bushing have respective front surfaces with areas (Wi , W2 ) that are dimensioned according to the formulas F9 and F1 1 in such a way to balance the rear-facing axial forces acting on the toothed wheels (1 , 2), respectively. The dimensioning of the areas (Wi, W2 ) of the front surfaces of the grooves will be illustrated later.

[0072] The first groove (8) has an annulus shape concentric to said first bore

[0073] (30) of the rear bushing.

[0074] Since in the operating mode shown in Fig. 12 the axial force (B) acting on the second toothed wheel (2) is lower than the axial force (A) acting on the first toothed wheel (1 ), the area (W2) of the front surface of the second groove (82) will be lower than the area (W1) of the front surface of the first groove (8).

[0075] For this reason, the second groove (82) can extend only as an annulus concentric to the second bore (31 ) of the rear bushing. Specifically, the second groove (82) can extend as an annulus sector concentric to the second bore

[0076] (31 ) subtended by an angle at the center of 60°-180°.

[0077] With reference to Figs. 7, 8, 10 and 1 1 , on the rear face (42) of the front bushing, a first groove (9) and a second groove (92) are provided around the first bore (40) and the second bore (41 ) of the front bushing, respectively.

[0078] The grooves (9, 92) of the front bushing communicate with the second zone (Z2) inside the casing (5), respectively by means of a first communication duct (90) and a second communication duct (91 ) formed in the front bushing (4) and facing the second zone (Z2) inside the casing. The cross section of the grooves (9, 92) and of the communication ducts (90, 91 ) of the front bushing can be semicircular or rectangular.

[0079] To ensure a proper reversible operation of the hydraulic machine, as clearly shown in Fig. 1 1 , the communication ducts (90, 91 ) of the front bushing are directed toward the second zone (Z2) with opposite direction relative to the one of the communication ducts (80, 81 ) of the rear bushing.

[0080] Referring to Fig. 10, the grooves (9, 92) of the front bushing have respective front surfaces with areas (W, W2 ) dimensioned according to formulas F9 and F1 1 in order to balance the forward-facing axial forces acting on the toothed wheels (1 , 2), respectively. The dimensioning of the areas (W1, W2) of the front surfaces of the of the grooves of the front bushing will be illustrated hereinafter. The first groove (9) extends as an annulus concentric to the first bore (40) of the front bushing.

[0081] Since the axial force (D) acting on the second toothed wheel (2) is lower than the axial force (C) acting on the first toothed wheel (1 ), the area (W2) of the front surface of the second groove (92) will be lower than the area (W1) of the front surface of the first groove.

[0082] For this reason, the second groove (92) can extend as an annulus concentric to the second bore (41 ) of the front bushing. Specifically, the second groove (92) can extend as an arc of an annulus arc subtended by an angle at the center of 60°-180°.

[0083] In the case where the first zone (Z1 ) inside the casing (5) is at high pressure, pressurized fluid flows into the grooves (8, 82) of the rear bushing, pushing the toothed wheels (1 , 2) toward the front flange (6), whereas in this case, the second zone (Z2) inside the casing (5) is at low pressure, and thus the grooves (9, 92) of the front bushing do not contribute to the balancing of the axial forces.

[0084] In the opposite case, on the other hand, if the second zone (Z2) inside the casing (5) is at high pressure, pressurized fluid flows into the grooves (9, 92) of the front bushing, pushing the toothed wheels (1 , 2) toward the rear cover (7), whereas in this case, the first zone (Z1 ) inside the casing (5) is at low pressure and thus the grooves (8, 82) of the rear bushing do not contribute to the balancing of the axial forces.

[0085] With reference to Figs. 12 and 13, if the hydraulic machine (100) is operated as a pump with counterclockwise rotation, in this case the resulting axial forces (A, B) on the driving wheel and on the driven wheel (1 , 2), respectively, are directed toward the rear bushing (3). The axial forces (A, B) are hydraulically balanced by the pressurized fluid contained in the grooves (8, 82) of the rear bushing, which generates balancing forces (A', B') in the opposite sense relative to the axial forces (A, B) resulting on the toothed wheels.

[0086] In fact, as shown in Fig. 13, the grooves (8, 82) of the rear bushing are connected via the communication ducts (80, 81 ) with the first zone (Z1 ) that corresponds to the pump delivery, i.e. with the zone inside the casing (5) of the pump that is at high pressure. The dimensioning of the areas (Wi, W2) of the front surfaces of the grooves (8, 82) of the rear bushing performed according to F1 and F2 is such that the axial forces (A, B) are balanced. In this case, the grooves (9, 92) of the front bushing (4) are irrelevant, since the grooves (9, 92) of the front bushing (4) communicate via the communication ducts (90, 92) with the second zone (Z2) that corresponds to the pump suction, i.e. with the zone inside the casing (5) of the pump that is at low pressure, i.e. a pressure substantially equal to or lower than the atmospheric pressure.

[0087] Referring to Figs. 14 and 15, if the hydraulic machine (100) is operated as a pump with clockwise rotation, in this case the resulting axial forces (C, D) on the toothed wheels (1 , 2) are directed toward the front bushing (3). The axial forces (C, D) are hydraulically balanced by the pressurized fluid contained in the grooves (9, 92) of the front bushing, which generates two balancing forces (C, D') in the opposite sense relative to the axial forces (C, D).

[0088] In fact, as shown in Fig. 15, the grooves (9, 92) of the front bushing are connected via the communication ducts (90, 91 ) with the second zone (Z2), which in this case corresponds to the pump delivery, i.e. the area inside the casing (5) of the pump that is at high pressure. The dimensioning of the areas (W1, W2) of the front surface of the grooves (9, 92) of the front bushing performed according to F1 and F2 is such that the axial forces (C, D) are balanced, respectively. In this case, the grooves (8, 81 ) of the rear bushing (3) are irrelevant, since the grooves (8, 81 ) of the rear bushing (3) communicate via the communication ducts (90, 91 ) with the first zone (Z1 ), which in this case corresponds with the pump suction, i.e. with the zone inside the casing (5) that is at low pressure, i.e. a pressure substantially equal to or lower than the atmospheric pressure.

[0089] A similar reasoning can be developed in the operation of the hydraulic machine (100) as a hydraulic motor. Unlike the prior art, such a configuration of the balancing system of the axial forces allows for the construction of reversible hydraulic machines that can operate both as a pump and as a hydraulic motor. Hereinafter, with reference to Fig. 16, some formulas are given for the dimensioning of the area (Wi) of the front surfaces of the grooves (8, 9) and of the area (W2) of the front surfaces of the grooves (82, 92) provided on the front face (33) of the rear bushing (3) and on the rear face (42) of the front bushing (4).

[0090] The resulting axial force (A) on the conducting toothed wheel (1 ) is given by the sum of the pressure force axial component (Pa) and of transmission force axial component (Fa), according to the formula:

[0091] A = Pa + Fa [F1]

[0092] The resulting axial force (B) on the driven toothed wheel (2) is given by the difference of the pressure force axial component (Pa) and the transmission force axial component (Fa), according to the formula:

[0093] B = Pa - Fa [F2]

[0094] Pa is the axial component of the resultant of the pressure forces of the fluid, so that, once the geometric parameters of the toothed wheels are defined, it varies linearly with the pressure:

[0095] Pa= Cp■ P [F3]

[0096] Cp= Constant of geometric proportionality

[0097] P = Pressure difference between suction and delivery

[0098] Based on the assumptions made, the axial component Pa will have the same intensity and the same sense on both toothed wheels.

[0099] If we consider a gear pump with left helical toothed wheels with known characteristics, in left rotation (driving toothed wheel (1 ) rotating counterclockwise), at a known operating speed, the torque absorbed by the driving toothed wheel (1 ) is:

[0100] ] / = Displacement

[0101] P = Pressure difference between suction and delivery m = Hydromechanical efficiency (experimentally obtainable value) Referring to Fig. 16, assuming that the toothed profiles of the toothed wheels (1 , 2) are equally dimensioned, the resistant moment Mf of the fluid transmitted to the toothing of the toothed wheels (1 , 2) is the same:

[0102] Mf= Mr / 2 [F5]

[0103] The axial component of the resultant of the actions transmitted in the helical toothing is: d = Primitive diameter of toothed wheels Helix inclination angle Pressure difference between suction and delivery

[0104] The transmission force axial component (Fa) will act on the driving toothed wheel (1 ) and on the driven toothed wheel (2) with the same intensity and the same direction but with opposite sense.

[0105] From formula [F6] it can be seen that once the geometry of the toothed wheels is defined, we have:

[0106] Fa= C P [F7]

[0107] Cf = Constant of geometric proportionality

[0108] The front areas (Wi) of the grooves (8, 9) and (W2) of the grooves (82, 92) can be dimensioned in such a way to respectively balance the axial forces (A, B) when directed toward the rear cover (7), or the axial forces (C, D) when directed toward the front flange (6) being C = - A and D = - B; hence:

[0109] The dimensioning of the areas (W1, W2) of the front surfaces of the grooves of the bushings depends essentially on the geometry of the toothing, consequently the shape and the geometric dimensions of the first grooves (8, 9) of the two bushings are the same, and the shape and geometric dimensions of the second grooves (82, 92) of the two bushings are the same. The bilateral balancing system is therefore independent of the operating pressure.

Claims

CLAIMS1. Hydraulic machine (100) of reversible type, comprising:- a first toothed wheel (1 ) with helical teeth comprising a shaft (10),- a second toothed wheel (2) with helical teeth comprising a shaft (20),- a rear bushing (3) arranged to the rear of the toothed wheels (1 , 2) and having a first bore (30) and a second bore (31 ) for revolvingly supporting the shafts (10, 20) of the first and the second toothed wheel,- a front bushing (4) arranged in front of the toothed wheels (1 , 2) and having a first bore (40) and a second bore (41 ) for revolvingly supporting the shafts (10, 20) of the toothed wheels,- a casing (5) wherein said rear and front bushings (3, 4) are arranged, together with the helical teeth of the toothed wheels meshing with each other, so as to define a first zone (Z1 ) communicating with a first duct (51 ) of the casing and a second zone (Z2) communicating with a second duct (52) of the casing, said first duct (51 ) and said second duct (52) being respectively the suction duct or the delivery duct, depending on the operation of the hydraulic machine of reversible type,- a front flange (6) that closes the casing (5) frontally; said front flange having a bore (60) for the insertion of an end portion of the shaft (10) of the first toothed wheel,- a rear cover (7) that closes the casing (5) at the rear; wherein the rear bushing (3) has a rear face (32) facing the rear cover (7), a front face (33) facing rear faces (14, 24) of the teeth of the toothed wheels (1 , 2) and a first groove (8) in its front face (33), around the first bore (30) of the rear bushing; said first groove (8) of the rear bushing communicates with said first zone (Z1 ) inside the casing (5) via a first communication duct (80) of the rear bushing; wherein the front bushing (4) has a front face (43) facing the front flange (6), a rear face (42) facing rear faces (15, 25) of the teeth of the toothed wheels (1 , 2) and a first groove (9) in its rear face (42), around the first bore (40) of the front bushing; said first groove (9) of the front bushing communicates with saidsecond zone (Z2) inside the casing (5) via a first communication duct (90) of the front bushing; wherein the groove (8) on the front face (33) of the rear bushing (3) and the groove (9) on the rear face (42) of the front bushing (4) have a frontal surface having an area (Wi) suitably dimensioned to balance the forces acting on the first toothed wheel (1 ) so that: when there is a pressurized fluid in the first zone (Z1 ) inside the casing, pressurized fluid flows into the groove (8) of the rear bushing, respectively generating a balancing force (A') that balances an axial force (A) resulting on the first toothed wheel (1 ); and when there is a pressurized fluid in the second zone (Z2) inside the casing, pressurized fluid flows into the groove (9) of the front bushing, respectively generating a balancing force (C) that respectively balances an axial force (C) resulting on the first toothed wheel (1 ).

2. The hydraulic machine (100) according to claim 1 , wherein a second groove (82) is provided on the front face (33) of the rear bushing, around the second bore (31 ) of the rear bushing; said second groove (82) of the rear bushing communicates with said first zone (Z1 ) inside the casing (5) via a second communication duct (81 ) of the rear bushing; wherein a second groove (92) is provided on the rear face (42) of the front bushing, around the second bore (41 ) of the front bushing; said second groove (92) of the front bushing communicates with said second zone (Z2) inside the casing (5) via a second communication duct (91 ) of the front bushing; wherein the groove (82) on the front face (33) of the rear bushing (3) and the groove (92) on the rear face (42) of the front bushing (4) have a front surface having an area (W2) suitably dimensioned to balance the axial forces acting on the second toothed wheel (2) so that: when there is a pressurized fluid in the first zone (Z1 ) inside the casing, pressurized fluid flows into the groove (82) of the rear bushing, respectively generating a balancing force (B') that balances an axial force (B) resulting on the second toothed wheel (2); andwhen there is pressurized fluid in the second zone (Z2) inside the casing, pressurized fluid flows into the groove (92) of the front bushing, respectively generating a balancing force (D') that respectively balances an axial force (D) resulting on the second toothed wheel (2).

3. The hydraulic machine (100) according to any one of the preceding claims, wherein the grooves (8, 82) on the front face (33) of the rear bushing (3) and the grooves (9, 92) on the rear face (42) of the front bushing (4) have a semicircular or rectangular cross-section.

4. The hydraulic machine (100) according to any one of the preceding claims, wherein the communication ducts (80, 81 ) on the front face (33) of the rear bushing (3) and the grooves (90, 91 ) on the rear face (42) of the front bushing (4) have a semicircular or rectangular cross-section.

5. The hydraulic machine (100) according to any one of the preceding claims, wherein said first groove (8) of the rear bushing (3) has an annulus shape concentric to said first bore (30) of the rear bushing, and said first groove (9) of the front bushing (4) has an annulus shape concentric to said first bore (40) of the front bushing.

6. The hydraulic machine (100) according to any one of the preceding claims, wherein said second groove (82) of the rear bushing (3) has an annulus shape concentric to said second bore (31 ) of the rear bushing and said second groove (92) of the front bushing (4) has an annulus shape concentric to said second bore (41 ) of the front bushing.

7. The hydraulic machine (100) according to any one of claims 2 to 6, wherein said second groove (82) of the rear bushing (3) has an annulus sector shape concentric to said second bore (31 ) of the rear bushing, and said second groove (92) of the front bushing (4) has an annulus sector shape concentric to said second bore (41 ) of the front bushing.

8. The hydraulic machine (100) according to claim 7, wherein said second groove (82) of the rear bushing (3) is shaped like an arc of annulus subtended by an angle at center of 60°-180°, and said second groove (92) of the front bushing (4) is shaped like an arc of annulus subtended by an angle at center of 60°-180°.

9. The hydraulic machine (100) according to any one of the preceding claims, wherein the profile of the teeth of the toothed wheels (1 , 2) is realized as disclosed in WO2010063705A1 .

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