Reversible hydraulic machine provided with a bidirectional

Abstract

A reversible hydraulic machine (100) comprising: a first gear (1) and a second gear (2) having helical teeth; and bushings (3, 4) for rotatably supporting the shafts (10, 20) of the gears; the rear bushing (3) has two grooves (8, 82) in communication with a first region (Z1) in the housing and the front bushing (3) has two grooves (9, 92) in communication with a second region (Z2) in the housing in order to balance axial forces (A, C) on the first gear (1) and axial forces (B, D) on the second gear (2), respectively.

Description

Technical Field

[0001] The invention relates to a reversible hydraulic machine with gears, comprising two gears with helical teeth, with a double-sided hydraulic system to balance the axial thrust on the gears. Background Art

[0002] A hydraulic machine is either a pump or a hydraulic motor. Although particular reference will be made below to reversible gear pumps, the present invention also relates to reversible hydraulic gear motors. Hydraulic gear motors are structurally identical to pumps. However, their operating principles differ: pumps convert mechanical energy (torque applied to a drive shaft) into hydraulic energy (pressurized oil). Conversely, motors convert hydraulic energy into mechanical energy. Pressurized oil delivered to a hydraulic motor from a port on the motor body acts on the gears, causing them to rotate; the resulting torque represents an output available on the shaft to which the load is applied.

[0003] External gear pumps are widely used in many industries such as automotive, earth-moving machinery, automation and control.

[0004] Spur gear pumps are known in which the gears comprise gears with spur teeth. In this case, during operation of the pump, the forces transmitted in the coupling between the contacting teeth on the gears have no axial component. Spur gear pumps therefore do not require balancing of axial forces.

[0005] Pumps with helical gears are known, wherein the gears comprise two gears with helical teeth, for example in WO2010063705A1 of the same applicant, or in US2159744 (Maglott) or US3164099 (Hitosi). Helical gears significantly reduce the noise and pulsation caused by the pump in the hydraulic circuit. It should be noted that in order for two helical gears with the same geometrical features to mesh correctly, the gears must have helix angles of different directions.

[0006] However, using helical gears creates axial loads or stresses on the gears during operation. The greater the helix angle, the greater the load or stress.

[0007] Figures 1 and 2 show a gear pump comprising a first helical gear 1 meshing with a second helical gear 2. The gears have respective shafts 10, 20 which are rotationally coupled to a housing 5 via a rear bushing 3 and a front bushing 4. The bushings 3, 4 are arranged in the housing 5 which is closed by a front flange 6 and a rear cover 7.

[0008] Looking at the pump from the front flange 6 , as shown in FIG. 2 , there is a first pipeline 51 on the left side of the housing 5 and a second pipeline 52 on the right side.

[0009] The shaft 10 of the first gear passes through the front flange 6 and extends from the front flange so as to be connected to the motor. Therefore, the shaft 10 of the first gear is the drive shaft, the first gear 1 is the driving gear, and the second gear 2 is the driven gear. When the pump is viewed from the front flange 6, the drive shaft 10 rotates counterclockwise. In this case, the first pipe 51 is the delivery pipe and the second pipe 52 is the suction pipe.

[0010] Figures 3 and 4 show the same helical gear pump as Figures 1 and 2, wherein the drive shaft 10 rotates clockwise. In this case, looking at the pump from the front flange 6 side, the second conduit 52 is the delivery conduit, and the first conduit 51 is the suction conduit.

[0011] Referring to Figures 1 and 2, in this case, the forces acting on the helical gears 1, 2 generate axial forces A, B directed toward the pump rear cover 7. If not counteracted, the presence of axial forces A, B will greatly increase the specific pressure discharged on the surface of the pump rear bushing 3, thereby reducing the mechanical efficiency of the pump due to friction losses, and reducing the reliability and maximum pressure of the pump.

[0012] The resultant axial force on each gear is given by the transmission force axial component Fa and the pressure axial component Pa, which act axially on the tooth profile of the gear. The direction of the two axial components depends on the helical direction of the gears. In order to achieve meshing with parallel axes, the helical directions of the drive gear 1 and the driven gear 2 must be opposite. The image refers to a pump with a left-handed drive gear 1 and a right-handed driven gear 2. However, the same principle also applies to the symmetrical case.

[0013] On the drive gear 1, the transmission force axial component Fa has the same meaning as the pressure axial component Pa. Therefore, on the drive gear 1, the axial force generated is given by the following relationship:

[0014] A=Pa+Fa.

[0015] On driven gear 2, the axial component Fa is opposite to the axial component Pa. Therefore, on driven gear 2, the axial force generated is given by the following relationship:

[0016] B=Pa-Fa

[0017] Various systems are known for balancing the axial forces in gears.

[0018] WO2006090652A1 (Shimadzu) describes a pump having a gear with bihelical gears. In this case, the problem of balancing the axial forces does not exist, since they are balanced directly on the gears. However, this solution is hampered by several disadvantages. In fact, the greater construction complexity of the bihelical gears, together with the precision required for the construction of high-pressure gear pumps or motors, makes this solution uneconomical.

[0019] US3658452 (Kita) discloses a system for balancing the thrust forces acting on the pump gear, both directed towards the rear cover of the pump, by means of a hydraulic piston located inside the cover and acting on the end of the gear shaft. The hydraulic piston is powered by a pipe connecting the hydraulic piston to the delivery area of ​​the pump. The use of a compensating piston is a cheaper and easier to implement solution, as the machining and components required to achieve the balancing are simple and reliable. Of course, in order for the balancing to work, the pump must be configured so that the forces A and B generated are directed towards the cover, and not in the opposite direction.

[0020] WO2010102722 (Bosch) discloses a system for balancing the axial forces directed towards the rear bushing by means of two pressurized chambers formed on the surface of the rear bushing. The two pressurized chambers are fed by conduits which connect them to the delivery area of ​​the pump. The areas of the two pressurized chambers must be appropriately determined to balance the axial forces. As in the previous case, this solution is effective for balancing the resultant forces A and B directed towards the cover (but not in the opposite direction).

[0021] All the solutions of the prior art described above are limited to solving the problem of axial force compensation of unidirectional hydraulic pumps or motors, which are designed to operate only in a predetermined clockwise or counterclockwise rotation direction of the drive gear, with the axial forces A, B directed towards the rear cover 7, as shown in Figures 1 and 2. This fact forces the manufacturer to make a rather strict technical choice. In the case of a pump that rotates counterclockwise, the drive gear 1 must have a left-hand helix and the driven gear 2 must have a right-hand helix; conversely, in the case of a pump that rotates clockwise, the drive gear must have a right-hand helix and the driven gear must have a left-hand helix, otherwise the compensation of the axial forces cannot be achieved.

[0022] However, in some hydraulic applications, the use of a reversible hydraulic pump or motor is necessary. Using a reversible pump makes it possible to reverse the motion of the drive gear, thereby changing the direction of oil flow and reversing the low-pressure and high-pressure areas (suction and delivery), for example, allowing the motion of a hydraulic actuator to be reversed. Similarly, the use of a reversible hydraulic motor is useful in applications where it is necessary to change the direction of the torque available on the hydraulic motor output shaft.

[0023] FIG3 shows the distribution of the axial forces C and D when the drive gear 1 rotates clockwise when the pump in FIG1 is a reversible pump. In this case, the axial forces C and D point to the front bushing 4, so:

[0024] C=-A

[0025] D=-B

[0026] In this case, the prior art solutions for compensating the axial forces, such as US3658452 (Kita) and WO2010102722 (Bosch), are not applicable, since the reversal of the movement and the reversal of the suction side and the delivery side leads to a reversal of the axial forces acting on the gears 1, 2, as shown in FIG3. Under such operating conditions, the axial forces C, D are directed towards the front flange 6, and not towards the rear cover 7, as shown in FIG1.

[0027] Since the shaft 10 of the drive gear inevitably protrudes from the front flange 6, the axial force C on the drive gear 1 can no longer be compensated by the prior art solutions described in US3658452A and WO2010102722A2. The only applicable solution is to use the herringbone helical gears described in WO2006090652A1, but as mentioned above, the higher structural complexity of the herringbone helical gears, together with the required precision of the construction of the high-pressure gear pump or motor, makes this solution uneconomical.

[0028] The pump is the same as a hydraulic motor, which has a high-pressure fluid inlet side and a low-pressure fluid outlet side. In this case, there is no driving gear and driven gear, but only a first gear 1 and a second gear 2; in this case, the protruding part of the shaft 10 of the first gear is not suitable for connection to the motor, but is suitable for connection to the resistance load.

[0029] EP2154372 (Lavezzi) discloses a partially balanced compensation system for a reversible helical gear pump. This compensation system is characterized by being one-sided, since the proposed solution alternately axially compensates only one of the two resultant forces A or B acting on the gears 1, 2, depending on the direction of rotation of the pump. This solution therefore provides only a partial compensation of the axial stresses, and only a partial compensation of the axial stresses directed toward the rear cover; also in this case, the solution is not suitable for compensating C or D, i.e. the forces directed toward the front flange 6 due to the drive shaft passing through it and protruding from it.

[0030] DE 23 15 630 A1 describes a reversible gear machine which can be operated as a pump or a hydraulic motor.

[0031] US 2012 / 156080 A1 describes a gear pump with an axial thrust balancing system, which is basically similar to the system described in WO 2010102722 A2.

[0032] US 2014 / 086779 A1 describes a gear machine which can be operated as a pump or a hydraulic motor and has a low-pressure conduit with a non-circular cross section. Summary of the invention

[0033] The object of the present invention is to overcome the disadvantages of the prior art and to provide a reversible hydraulic press (shaft can rotate clockwise or counterclockwise) with helical gears, having a bidirectional hydraulic system, which can fully balance the axial forces and can be used as a pump or a motor.

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

[0035] These objects are achieved according to the invention having the features of the attached independent claim 1 .

[0036] Advantageous results of the invention emerge from the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Other features of the invention will become more apparent from the following detailed description, which refers to the purely illustrative and therefore non-limiting embodiments shown in the accompanying drawings, in which:

[0038] FIG1 is a cross-sectional view of a gear pump having helical gears in the prior art when running counterclockwise;

[0039] FIG2 is a cross-sectional view taken along plane II-II of FIG1 ;

[0040] FIG. 3 is an axial cross-sectional view of the same pump as FIG. 1 when the pump is running clockwise.

[0041] FIG4 is a cross-sectional view taken along plane IV-IV of FIG3;

[0042] Figure 5 is an axial cross-sectional view of a hydraulic press according to the present invention;

[0043] Figure 6 is along Figure 5 A cross-sectional view taken along plane IV-IV;

[0044] Figure 7 yes Figure 5 Exploded axial view of a hydraulic press;

[0045] Figure 8 yes Figure 5 Details, showing only the gears and bushings;

[0046] Fig. 8A yes Figure 8 The enlarged detail view encircled in the center circle A shows the groove formed in the rear bushing;

[0047] Figure 8B yes Fig. 8A Variations of the grooves shown;

[0048] Fig. 9 is along Figure 8Cross-sectional view of plane IX-IX.

[0049] Fig.10 is along Figure 8 A cross-sectional view of plane XX;

[0050] Fig.11 yes Figure 8 Exploded perspective view of details;

[0051] Fig.12 yes Figure 8 The diagram when the axis rotates counterclockwise.

[0052] Fig.13 yes Fig.12 Exploded perspective view of details;

[0053] Fig.14 yes Figure 8 The diagram when the axis rotates clockwise;

[0054] Fig.15 yes Fig.14 an exploded perspective view of the details of

[0055] Fig.16 is a schematic diagram showing two gears of a hydraulic machine according to the invention and the forces and moments acting on them. DETAILED DESCRIPTION

[0056] Refer to the attached Figures 5 to 15 , a hydraulic machine according to the invention is described, which is generally indicated by the reference numeral 100 .

[0057] refer to Figure 5 , Figure 6 and Figure 7 The hydraulic press 100 includes a first gear 1 and a second gear 2 .

[0058] The first gear 1 has left-hand helical helical teeth. The first gear 1 comprises a shaft 10. The shaft 10 has a front end portion, which is suitable for connection to an electric motor or a power output device when the hydraulic machine 100 is a pump, or is suitable for connection to a resistance load when the hydraulic machine 100 is a hydraulic motor.

[0059] The second gear 2 has right-hand helical helical teeth. The second gear 2 includes a shaft 20. When the hydraulic machine 100 is a pump, the first gear 1 is a driving gear and the second gear 2 is a driven gear.

[0060] The teeth of the first gear 1 and the second gear 2 may have profiles similar to those described in WO2010063705A1, which is incorporated herein by reference.

[0061] The shafts 10 , 20 of the gears are rotatably supported by a rear bushing 3 and a front bushing 4 arranged rearwardly and forwardly with respect to the gears 10 , 20 .

[0062] The rear bushing 3 has two holes 30, 31 for receiving and rotating the shafts 10, 20 supporting the gears. The front bushing 4 has two holes 40, 41 for receiving and rotating the shafts 10, 20 supporting the gears.

[0063] In this way, the first gear 1 meshes with the second gear 2 to form a transmission system.

[0064] The assembly consisting of the gears 1, 2 and the bushings 3, 4 is accommodated in a housing 5 which is open at the front and rear.

[0065] refer to Figure 6 , the assembly including the gears 1, 2 and the bushings 3, 4 housed in the housing 5 divides the internal volume into a first zone Z1 communicating with the first duct 51 of the housing and a second zone Z2 communicating with the second duct 52 of the housing. Depending on the type of operation of the hydraulic machine 100, the ducts 51, 52 of the hydraulic machine can be inlet ducts and outlet ducts for the fluid, or vice versa. Therefore, during the operation of the hydraulic machine, the first zone Z1 is a high-pressure zone, while the second zone Z2 is a low-pressure zone, or vice versa, depending on the operating conditions (pump or motor) and the direction of rotation of the shaft 10.

[0066] The front and rear of the housing 5 are respectively closed by a front flange 6 and a rear cover 7. The front flange 6 has a hole 60 for the front end of the shaft 10 of the first gear 1 to pass through, the shaft protruding forward from the front flange for connection to an electric motor or a power take-off device.

[0067] refer to Figure 7 The rear bushing 3 has a rear end face 32 facing the rear cover 7 and a front end face 33 facing the teeth of the gears 1 and 2.

[0068] The gears 1 , 2 have rear faces 14 , 24 coupled to the front face 33 of the rear bushing and front faces 15 , 25 coupled to the front bushing 4 .

[0069] The front bushing 4 has a rear surface 42 coupled to the front end faces 15 , 25 of the teeth of the gears 1 , 2 and a front end face 43 coupled to the front flange 6 .

[0070] refer to Figure 7 , Figure 8 and Fig. 9 On the front end surface 33 of the rear bushing, a first groove 8 is provided around the first hole 30 of the rear bushing, and a second groove 82 is provided around the second hole 31 of the rear bushing.

[0071] The grooves 8, 82 of the rear bushing are connected to the first area Z1 inside the housing through the first communication conduit 80 and the second communication conduit 81 formed in the rear bushing 3 and facing the first area Z1 of the housing 5. Fig. 9 In the view of FIG. 8 , the communicating pipes 80 , 81 exit from the left side of the rear bushing.

[0072] The cross-sections of the grooves 8, 82 and the connecting pipes 8, 81 can be as follows: Fig. 8A The semicircle shown, or Figure 8B The rectangle shown.

[0073] refer to Fig. 9 The grooves 8, 82 of the rear bushing formed on the front end face 33 of the rear bushing have respective front surfaces, whose areas W1, W2 are respectively sized according to formulas F9 and F11, so as to balance the rearward axial forces acting on the gears 1, 2 respectively. The sizes of the areas W1, W2 of the front surfaces of the grooves will be described below.

[0074] The first groove 8 has an annular shape which is concentric with the first hole 30 of the rear bushing.

[0075] Because in Fig.12 In the operating mode shown, the axial force B acting on the second gear 2 is smaller than the axial force A acting on the first gear 1 , so the area W2 of the front surface of the second groove 82 will be smaller than the area W1 of the front surface of the first groove 8 .

[0076] Therefore, the second groove 82 can only extend into a ring shape concentric with the second hole 31 of the rear bushing. Specifically, the second groove 82 can extend into an annulus sector concentric with the second hole 31, and its central angle is 60° to 180°.

[0077] refer to Figure 7 , Figure 8 , Fig.10 and Fig.11 On the rear surface 42 of the front bushing, a first groove 9 and a second groove 92 are provided around the first hole 40 and the second hole 41 of the front bushing respectively.

[0078] The grooves 9, 92 of the front bushing are communicated with the second zone Z2 in the housing 5 through the first communication duct 90 and the second communication duct 91 formed in the front bushing 4 and facing the second zone Z2 in the housing. The cross-sections of the grooves 9, 92 of the front bushing and the communication ducts 90, 91 may be semicircular or rectangular.

[0079] To ensure that the hydraulic press can perform reversible operations correctly, such as Fig.11 As is clearly shown in FIG. 1 , the communicating ducts 90 , 91 of the front bushing point towards the second zone Z2 in the opposite direction relative to one of the communicating ducts 80 , 81 of the rear bushing.

[0080] refer to Fig.10 , the grooves 9, 92 of the front bushing have respective front surfaces, whose areas W, W2 are determined according to formulas F9 and F11, respectively, so as to balance the forward axial forces acting on gears 1, 2. The dimensions of the areas W1, W2 of the front surfaces of the grooves of the front bushing will be described below.

[0081] The first groove 9 extends as a ring concentric with the first hole 40 of the front bushing.

[0082] Since the axial force D acting on the second gear 2 is smaller than the axial force C acting on the first gear 1 , the area W2 of the front surface of the second groove 92 will be smaller than the area W1 of the front surface of the first groove.

[0083] To this end, the second groove 92 may extend into a ring that is concentric with the second hole 41 of the front bushing. Specifically, the second groove 92 may extend into an arc of a ring arc with a central angle of 60° to 180°.

[0084] When the first area Z1 in the housing 5 is at high pressure, the pressurized fluid flows into the grooves 8, 82 of the rear bushing, pushing the gears 1, 2 toward the front flange 6, while in this case, the second area Z2 in the housing 5 is at low pressure, so the grooves 9, 92 of the front bushing do not help to balance the axial force.

[0085] On the other hand, in the opposite situation, if the second area Z2 in the housing 5 is at high pressure, the pressurized fluid will flow into the grooves 9, 92 of the front bushing, pushing the gears 1, 2 toward the rear cover 7, while in this case, the first area Z1 in the housing 5 is at low pressure, so the grooves 8, 82 of the rear bushing do not help to balance the axial force.

[0086] refer to Fig.12 and Fig.13 , if the hydraulic machine 100 is operated as a pump in a counterclockwise rotation, then in this case the axial forces A, B acting on the driving and driven wheels 1, 2, respectively, are directed towards the rear bushing 3. The axial forces A, B are hydraulically balanced by the pressurized fluid in the grooves 8, 82 of the rear bushing, thereby generating balancing forces A', B' in the opposite direction to the axial forces A, B acting on the gears.

[0087] In fact, if Fig.13As shown, the grooves 8, 82 of the rear bushing are connected to the first area Z1 corresponding to the pump delivery, i.e., the high pressure area inside the housing 5 of the pump, through the communication pipes 80, 81. The size of the areas W1, W2 of the front surface of the grooves 8, 82 of the rear bushing is determined according to F1 and F2, so that the axial forces A, B are balanced. In this case, the grooves 9, 92 of the front bushing 4 are irrelevant, because the grooves 9, 92 of the front bushing 4 are connected to the second area Z2 corresponding to the pump suction, i.e., the low pressure (i.e., the pressure is substantially equal to or lower than the atmospheric pressure) area inside the housing 5 of the pump, through the communication pipes 90, 92.

[0088] refer to Fig.14 and Fig.15 , if the hydraulic machine 100 is operated as a pump in a clockwise rotation, in this case the axial forces C, D generated on the gears 1, 2 are directed towards the front bushing 3. The axial forces C, D are hydraulically balanced by the pressurized fluid in the grooves 9, 92 of the front bushing, thereby generating two balancing forces C', D' in the opposite direction to the axial forces C, D.

[0089] In fact, if Fig.15 As shown, the grooves 9, 92 of the front bushing are connected to the second zone Z2 via the communication ducts 90, 91, which in this case corresponds to the pump delivery, i.e. the high pressure zone inside the pump housing 5. The dimensioning of the areas W1, W2 of the front surface of the grooves 9, 92 of the front bushing according to F1 and F2 allows the axial forces C, D, respectively, to be balanced. In this case, the grooves 8, 81 of the rear bushing 3 are irrelevant, since the grooves 8, 81 of the rear bushing 3 are connected to the first zone Z1 via the communication ducts 90, 91, which in this case corresponds to the pump suction, i.e. to the low pressure zone inside the housing 5 (i.e. the pressure is substantially equal to or lower than the atmospheric pressure).

[0090] Similar reasoning applies to the operation of the hydraulic machine 100 as a hydraulic motor. Unlike the prior art, this configuration of the axial force balancing system allows for the construction of a reversible hydraulic machine that can operate both as a pump and as a hydraulic motor.

[0091] Reference below Fig.16 , the dimension calculation formulas for the front surface area W1 of the grooves 8 and 9 provided on the front end surface 33 of the rear bushing 3 and the rear end surface 42 of the front bushing 4 and the front surface area W2 of the grooves 82 and 92 are given.

[0092] The axial force A generated on the transmission gear 1 is equal to the sum of the pressure axial component Pa and the transmission force axial component Fa, and its formula is as follows:

[0093] A=Pa+Fa [F1]

[0094] The resultant axial force B acting on the driven gear 2 is equal to the difference between the pressure axial component Pa and the transmission force axial component Fa, and its formula is as follows:

[0095] B=Pa-Fa [F2]

[0096] Pa is the axial component of the fluid pressure resultant, so once the gear geometry is defined, it varies linearly with pressure:

[0097] P a =C p ·P [F3]

[0098] C p = geometric proportional constant

[0099] P = pressure difference between suction and delivery

[0100] Based on the assumptions made, the axial force component Pa will have the same magnitude and the same direction on both gears.

[0101] If we consider a gear pump with left-hand helical gears of known characteristics, the torque absorbed by drive gear 1 during left rotation (drive gear 1 rotates counterclockwise) at a known operating speed is:

[0102]

[0103] V = displacement

[0104] P = pressure difference between suction and delivery

[0105] η m = Fluid mechanical efficiency (value that can be obtained experimentally)

[0106] refer to Fig.16 , assuming that the tooth profiles of gears 1 and 2 are the same, the resistance torque Mf of the fluid transmitted to the teeth of gears 1 and 2 is the same:

[0107] M f =M r / 2[F5]

[0108] The axial component of the resultant force of the helical gear transmission is:

[0109]

[0110] d = original diameter of the gear

[0111] β = helical inclination

[0112] P = pressure difference between suction and delivery

[0113] The axial component of the transmission force Fa will act on the driving gear 1 and the driven gear 2 with the same strength and the same direction but in opposite directions.

[0114] From formula [F6] we can see that once the geometry of the gear is defined, we have:

[0115] F a =C f ·P [F7]

[0116] C f = geometric proportional constant

[0117] The front areas W1 of the grooves 8, 9 and the front areas W2 of the grooves 82, 92 can be dimensioned to balance the axial forces A, B towards the rear cover 7 or the axial forces C, D towards the front flange 6, respectively, where C = -A and D = -B; therefore:

[0118] A=Pa+Fa=C p ·P+C f ·P=W1·P [F8]

[0119] W1=C p +C f [F9]

[0120] B=Pa-Fa=C p PC f ·P=W2·P [F10]

[0121] W2=C p -C f [F11]

[0122] The size of the area W1, W2 of the front surface of the bushing grooves mainly depends on the geometry of the teeth, so the shape and geometry of the first grooves 8, 9 of both bushings are the same, and the shape and geometry of the second grooves 82, 92 of both bushings are the same.

[0123] Therefore, the bidirectional balanced system is independent of the operating pressure.

Claims

1. A reversible hydraulic press (100), comprising: - a first gear wheel (1) with helical teeth, comprising a shaft (10), - a second gear wheel (2) with helical teeth, comprising a shaft (20), - a rear bushing (3) arranged at the rear of the gears (1, 2) and having a first hole (30) and a second hole (31) for rotatably supporting the shafts (10, 20) of the first and second gears, - a front bushing (4) arranged in front of the gear (1, 2) and having a first hole (40) and a second hole (41) for rotatably supporting the shaft (10, 20) of the gear, - a housing (5), wherein the rear and front bushings (3, 4) are arranged with the helical teeth of gears meshing with each other so as to define a first zone (Z1) communicating with a first duct (51) of the housing and a second zone (Z2) communicating with a second duct (52) of the housing, the first duct (51) and the second duct (52) being respectively a suction duct or a delivery duct, depending on the operation of the reversible hydraulic machine, a front flange (6) closing the housing (5) from the front, said front flange having a hole (60) for inserting the end of the shaft (10) of the first gear, - a rear cover (7) which closes the housing (5) from behind, The rear bushing (3) comprises: a rear end face (32) facing the rear cover (7); a front end face (33) facing the rear end faces (14, 24) of the teeth of the gears (1, 2); and a first groove (8) located in the front end face (33) and surrounding the first hole (30) of the rear bushing; the first groove (8) of the rear bushing is connected to the first area (Z1) in the housing (5) through a first connecting pipe (80) of the rear bushing; The front bushing (4) comprises: a front end face (43) facing the front flange (6); a rear end face (42) facing the rear end faces (15, 25) of the teeth of the gears (1, 2); and a first groove (9) located in the rear end face (42) and surrounding the first hole (40) of the front bushing; the first groove (9) of the front bushing is connected to the second area (Z2) in the housing (5) through a first connecting pipe (90) of the front bushing; The groove (8) on the front end face (33) of the rear bushing (3) and the groove (9) on the rear end face (42) of the front bushing (4) have a front surface, and the front surface has an area (W1) of a suitable size so as to balance the force acting on the first gear (1), so that: When there is pressurized fluid in the first area (Z1) in the housing, the pressurized fluid flows into the groove (8) of the rear bushing, respectively generating a balancing force (A'), wherein the balancing force (A') balances the axial force (A) acting on the first gear (1); When there is pressurized fluid in the second area (Z2) in the housing, the pressurized fluid flows into the groove (9) of the front bushing, generating balancing forces (C') respectively, and the balancing forces (C') respectively balance the axial force (C) acting on the first gear (1).

2. The hydraulic press (100) according to claim 1, wherein: A second groove (82) is provided on the front end surface (33) of the rear bushing and surrounds the second hole (31) of the rear bushing; the second groove (82) of the rear bushing is connected to the first area (Z1) in the housing (5) through a second connecting pipe (81) of the rear bushing; The second groove (92) is arranged on the rear end surface (42) of the front bushing and surrounds the second hole (41) of the front bushing; the second groove (92) of the front bushing is connected to the second area (Z2) in the housing (5) through the second connecting pipe (91) of the front bushing; The groove (82) on the front end face (33) of the rear bushing (3) and the groove (92) on the rear end face (42) of the front bushing (4) have a front surface, and the front surface has an area (W2) of appropriate size, so as to balance the axial force acting on the second gear (2), so that: When there is pressurized fluid in the first area (Z1) in the housing, the pressurized fluid flows into the groove (82) of the rear bushing to generate balancing forces (B'), and the balancing forces (B') respectively balance the axial forces (B) acting on the second gear (2); When there is pressurized fluid in the second area (Z2) in the housing, the pressurized fluid flows into the groove (92) of the front bushing, generating balancing forces (D') respectively, and the balancing forces (D') respectively balance the axial forces (D) acting on the second gear (2).

3. The hydraulic machine (100) according to any one of the preceding claims, wherein: The groove (8, 82) on the front end surface (33) of the rear bushing (3) and the groove (9, 92) on the rear end surface (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 communicating ducts (80, 81) on the front end surface (33) of the rear bushing (3) and the grooves (90, 91) on the rear end surface (42) of the front bushing (4) have semicircular or rectangular cross sections.

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

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

7. The hydraulic press (100) according to any one of claims 2 to 6, wherein: The second groove (82) of the rear bushing (3) has an annular sector shape concentric with the second hole (31) of the rear bushing, and the second groove (92) of the front bushing (4) has an annular sector shape concentric with the second hole (41) of the front bushing.

8. The hydraulic press (100) according to claim 7, wherein: The second groove (82) of the rear bushing (3) is in the shape of a circular arc with a central angle of 60° to 180°, and the second groove (92) of the front bushing (4) is in the shape of a circular arc with a central angle of 60° to 180°.

9. The hydraulic machine (100) according to any one of the preceding claims, wherein: The profile of the teeth of the gears (1, 2) is presented in the manner disclosed in WO2010063705A1.

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