BACKGROUND

[0001] The present invention relates to axial piston pumps. More particularly, the invention relates to tandem axial piston pumps. Such hydraulic pumps can be found in the traction drive systems of skid steer construction vehicles and the like. The tandem axial piston pump will include two independently operable axial piston pump assemblies enclosed within a common housing.

SUMMARY

[0002] In one aspect, the invention provides a tandem axial piston pump. A first axial piston pump includes a first set of pistons arranged in a parallel, circular array and configured to reciprocate in response to rotation about a central axis. A second axial piston pump includes a second set of pistons arranged in a parallel, circular array and configured to reciprocate as they rotate about the central axis. A common drive shaft extends along the central axis, both the first and second sets of pistons being configured to rotate with the common drive shaft. Both the first and second sets of pistons are configured to reciprocate within corresponding separate first and second sets of cylinder bores of a common cylinder block.

[0003] In another aspect, the invention provides a tandem axial piston pump. A drive shaft defines an axis. A first set of pistons are arranged in parallel with the axis in a circular array and rotatable about the axis by the drive shaft. A second set of pistons are arranged in parallel with the axis in a circular array and rotatable about the axis by the drive shaft. A first swash plate is coupled to a base end of each of the pistons of the first set. The first swash plate defines a cam angle with respect to the axis, and the cam angle defines a stoke distance through which each of the pistons of the first set travels back-and- forth upon a rotation about the axis. A second swash plate is coupled to a base end of each of the pistons of the second set. The second swash plate defines a cam angle with respect to the axis, and the cam angle defines a stoke distance through which each of the pistons of the second set travels back-and- forth upon a rotation about the axis. A cylinder block has a first set of cylinder bores receiving the first set of pistons from a first end of the cylinder block and a second set of cylinder bores receiving the second set of pistons from a second end of the cylinder block, opposite the first end. [0004] Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Fig. 1 is a perspective view of a tandem axial piston pump.

[0006] Fig. 2 is an exploded assembly view of the tandem axial piston pump of Fig. 1.

[0007] Fig. 3 is a cross-section view of the tandem axial piston pump, taken along line 3- 3 of Fig. 1.

[0008] Fig. 4 is a perspective view of a tandem axial piston pump according to a second embodiment.

[0009] Fig. 5 is an exploded assembly view of the tandem axial piston pump of Fig. 4.

[0010] Fig. 6 is a cross-section view of the tandem axial piston pump, taken along line 6- 6 of Fig. 4.

DETAILED DESCRIPTION

[0011] Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways.

[0012] Figs. 1-3 illustrate a tandem axial piston pump 20 according to one embodiment. The tandem axial piston pump 20 is a type of variable displacement fluid pump in which two pump units are provided. The two pump units are separately controllable. In other words, the displacement or flow rate of each pump unit can be varied independently. Each pump unit includes one set of pistons 24 arranged in a parallel, circular array and configured to reciprocate with a variable stroke in response to rotation about a central axis A. The two sets of pistons 24 are arranged so that one set is positioned on each side of a central plane P. The pistons 24 of the two sets can all be identical as shown. In other constructions, some or all of the pistons 24 can have different characteristics from those shown. A cylinder block 28 is provided with separate first and second sets of cylinder bores 30 corresponding to the two sets of pistons 24. The two sets of cylinder bores 30 are provided in two opposing axial ends of the cylinder block 28, with one set of cylinder bores 30 positioned on each side of the central plane P. The cylinder bores 30 of the first set can be arranged on a circle of common diameter with the cylinder bores 30 of the second set. Additionally, the arrangement or pattern of the cylinder bores 30 along the circumferential direction is common between the two sets such that each cylinder bore 30 of the first set is aligned with a corresponding cylinder bore 30 of the second set as shown in Fig. 3. However, other relative orientations are optional. The cylinder block 28 is a singular unit, such that the tandem axial piston pump 20 is not provided with two separate cylinder blocks for the two separate pump units and is not provided with any valve plate positioned adjacent the inward ends of the cylinder bores 30. In some constructions, the cylinder block 28 is integrally formed as a single, monolithic piece. A common drive shaft 34 extends along the central axis A and is drivingly coupled to the cylinder block 28 to rotate both the cylinder block 28 and both sets of pistons 24 about the axis A. The drive shaft 34 can be supported for rotation by suitable bearings 36. The bearings 36 are positioned adjacent opposing ends of the drive shaft 34 to support the drive shaft 34 relative to a pair of axially opposed end flanges 40, 42. Each of the end flanges 40, 42 is secured to a corresponding housing portion 44, 46 of the tandem axial piston pump 20. As discussed further below, a port block 48 is sandwiched between the two housing portions 44, 46. The cylinder block 28 extends through a bore 50 in the port block 48. The bore 50 is positioned generally at the central plane P (e.g., the bore 50 may be centered on, or overlapping with, the central plane P).

[0013] Each piston 24 has an inner working end facing the central plane P and an outer drive end. The drive end of each piston 24 has a rounded shape received in a corresponding piston shoe 60. The piston shoes 60 of each set of pistons 24 are collectively retained by a shoe plate 62 to maintain the parallel spaced arrangement of the pistons 24 within each set. Directly outside the piston shoes 60 in the axial direction on each pump unit is a swash plate 66. The swash plates 66 are movable to tilt relative to the central axis A in order to vary the stroke of the corresponding pistons 24. The swash plates 66 are illustrated in a zero or neutral state in which the swash plates 66 are each arranged exactly perpendicular to the central axis A such that the pistons 24 do not stroke at all when the pistons 24 and the cylinder block 28 are rotated about the central axis A. When one of the swash plates 66 is angled away from the neutral state, each piston 24 of the corresponding pump unit reciprocates to complete one back-and-forth pumping stroke within the corresponding bore 30 upon each full rotation of the cylinder block 28 about the central axis A. The linear stroke of the pistons 24 of a particular set is varied by varying the angle of the corresponding swash plate 66. The independence of the displacement between the two pump units, which rotate together, is achieved by independent control of the two swash plates 66.

[0014] A radial bore 70 or connector port is provided through an outer cylindrical surface 72 of the cylinder block 28 for each of the cylinder bores 30 of each of the two sets. Each radial bore 70 extends inward from the outer cylindrical surface 72 to intersect with the corresponding cylinder bore 30 to establish fluid communication therewith. The radial bores 70 provide fluid communication with the port block 48 via one or more distribution rings 76 as discussed further below. Although shown as two separate components in the illustrated construction, the features of the two distribution rings 76 can alternately be incorporated into a single distribution ring. Each of the distribution rings 76 is fixed to the bore 50 in the port block 48. For example, the distribution rings 76 can be press-fit into the bore 50. The cylinder block 28, particularly the outer cylindrical surface 72, is provided in tight sliding relationship with the interior of each of the distribution rings 76. Thus, as the cylinder block 28 rotates within the distribution rings 76, the radial bores 70 are brought into and out of fluid communication with a pair of slots 80, 82 provided in each distribution ring 76. The two slots 80, 82 in each distribution ring 76 are circumferentially spaced apart, and may be diametrically opposed. Each of the slots 80, 82 may be about 90 arc degrees long, or more. The first slot 80 is a pickup or inlet slot which is in permanent fluid communication with a corresponding inlet port 86 of the port block 48, via a connecting passage through the port block 48. The second slot 82 is a distribution or outlet slot which is in permanent fluid communication with a corresponding outlet port 88 of the port block 48, via a connecting passage through the port block 48. Because the pump 20 includes two pump units, the port block 48 includes two separate pairs of the inlet and outlet ports 86, 88 corresponding to the two pump units.

[0015] When the swash plate 66 of one of the pump units is angled from the illustrated neutral position, each piston 24 of the corresponding pump unit moves in a retracting direction with respect to the corresponding cylinder bore 30 over a rotational range in which the corresponding radial bore 70 is in communication with the corresponding inlet slot 80, and thereby, the inlet port 86. As such, fluid is drawn into the cylinder bore 30 through the inlet port 86, the inlet slot 80, and the radial bore 70. Further rotation of the cylinder block 28 causes fluid communication to cease between the cylinder bore 30 and the inlet slot 80, and even further rotation of the cylinder block 28 causes fluid communication to be established between the cylinder bore 30 and the outlet slot 82. Due to the angle of the swash plate 66, the piston 24 extends into the cylinder bore 30 over a rotational range in which the corresponding radial bore 70 is in communication with the outlet slot 82. As such, the fluid previously drawn into the cylinder bore 30 from the inlet port 86 is pumped out of the pump 20 from the cylinder bore 30 through the radial bore 70, the outlet slot 82, and the outlet port 88. Each sequential piston 24 of the given pump unit follows the same sequence during rotation of the cylinder block 28, and if the swash plate 66 of the other pump unit is angled, a similar pumping method occurs with the other set of pistons 24. Although not shown in the drawings, a separate mechanism (e.g., a servo motor) may be coupled to the respective swash plates 66 to control the swash angles and thus, the pump displacement. While not necessarily limited to one fixed speed, the drive shaft 34 may be rotated by a separate drive element (e.g., an electric motor) at a constant speed during operation of the pump 20.

[0016] Figs. 4-6 illustrate a tandem axial piston pump 120 according to a second embodiment. Components and functions of the pump 120 of Figs. 4-6 are similar to those of the pump 20 of Figs. 1-3 and thus, are not reiterated below. Similar components are given similar reference numbers, incremented by a leading "1", and the below description focuses primarily on the features of the pump 120 which are different from the pump 20 of Figs. 1-3.

[0017] Rather than having separate housings and end flanges on each end, the pump 120 is provided with a two-piece case consisting of a first case portion 143A and a second case portion 143B. The case portions 143A, 143B can be mated to one another (e.g., by a plurality of fasteners or other means, not shown) in a direction parallel to the axis A. Both case portions 143A, 143B generally have a cup shape which surrounds at least a portion of the pump internals both radially and on one axial end. Furthermore, each case portion 143 A, 143B includes a control bore 145 A, 145B for receiving a servo piston or stroking piston (not shown) which manipulates the corresponding swash plate 166A, 166B, a drive portion 167 of which is positioned in the corresponding control bore 145 A, 145B. Similar to the pump 20 of Figs. 1-3, the swash plates 166A, 166B are movable to different angles independently of one another to vary the displacement of each pump unit of the tandem axial piston pump 120. [0018] The tandem axial piston pump 120 also does without a separate central port block, instead providing both the inlet ports 186 and the outlet ports 188 for both pump units in the second case portion 143B. The inlet ports 186 for both pump units can be located on a first side of the second case portion 143B and the outlet ports 188 for both pump units can be located on a second opposite side of the second case portion 143B, although other arrangements are optional.

[0019] In order to further limit an axial length of the cylinder block 128 to reduce the overall packaging size of the tandem axial piston pump 120, the arrangement of the cylinder bores 130 (and thus, the corresponding pistons 124) is altered from that of the pump 20 of Figs. 1-3. Rather than being entirely positioned on one side of the central plane P that bisects the cylinder block 128, the cylinder bores 130 of the two sets are positioned to overlap axially across each other. As shown, each of the cylinder bores 130 crosses over the central plane P. In order to facilitate the overlap, the cylinder bores 130 of the two sets are not aligned with each other. Instead, the cylinder bores 130 of one set can be arranged on a circle of a first diameter, with the cylinder bores 30 of the second set arranged on a circle of a second diameter that is smaller than or greater than the first diameter such that the cylinder bores 130 of one set are effectively arranged radially inside or outside the cylinder bores 130 of the second set. Alternately or additionally, the arrangement or pattern of the cylinder bores 130 along the circumferential direction can be offset from one set to the other. In the illustrated construction, the cylinder bores 130 of the two sets are arranged on circles of different diameter and also offset in the circumferential direction. This makes an efficient use of the volume defined by the cylinder block 128 and an axial length of the cylinder block 128 can be reduced. Although the respective swash plates 166A, 166B and shoe plates 162A, 162B may be different from one pump unit to the other due to the different diameters of the circles on which the piston sets are arranged, each and every piston 124 within the tandem axial piston pump 120 can be identical, although not required.

[0020] Similar to the pump 20 of Figs. 1-3, the cylinder block 128 rotates with one or more distribution rings 176 having respective inlet and outlet slots 180, 182 for the two pump units provided by the cylinder block 128. Although shown as two separate components in the illustrated construction, the features of the two distribution rings 176 can alternately be incorporated into a single distribution ring. The distribution ring(s) can be fixed to a bore 150 provided in the second case portion 143B. For example, the distribution rings 176 can be press-fit into the bore 150. The above description of the tandem axial piston pump 20 is hereby referenced with respect to all additional structural and operational details, and alternatives thereof, the full repetition of which would be redundant.

[0021] Various features and advantages of the invention are set forth in the following claims.