Hydraulic axial piston unit with central fixed hold down device

Abstract

Hydraulic axial piston unit (1) of the swashplate construction type having a drive shaft (2) adapted to drive or be driven by a cylinder block (4). The cylinder block comprises a plurality of cylinder bores (6), in which several pistons (8) are moveable in general along the rotational axis (10) of the drive shaft (2) and relative to the cylinder bores (6). First ends of the pistons (8) protrude outside of the cylinder bores (6) and are slidable fixed by means of slippers (14) to a swashplate (16). The slippers (14) are hold down in a sliding manner on a sliding surface (18) of the swashplate (16) by means of a slipper hold down ring (20) arranged parallel to the sliding surface (18). The slipper hold down ring (20) is in a sliding contact with its radial inner surface (22) with a matching surface (26) on a guide ball (24) rotationally fixed on the drive shaft (2) and axially moveable in direction of the rotational axis (10) relative to the cylinder block (4) against resilient forces of springs (28) characterized in that a mounting ring (30) is attached to the cylinder block (4) in order to limit the axial movement of the guide ball (24) towards the cylinder block (4).

Claims

1. A hydraulic axial piston unit of the swashplate construction type having a drive shaft adapted to drive or be driven by a cylinder block having a plurality of cylinder bores, each cylinder bore having a piston moveable relative to the cylinder bore, wherein first ends of the pistons protrude outside of the cylinder bores and are slidably attached by means of slippers to a swashplate, wherein the slippers are held down in a sliding manner on a sliding surface of the swashplate by means of a slipper hold down ring arranged parallel to the sliding surface, and wherein the slipper hold down ring is in a sliding contact with its radial inner surface with a matching surface on a guide ball rotationally fixed on the drive shaft and axially moveable in direction of the rotational axis towards the cylinder block against resilient forces of springs; wherein a mounting ring is attached to the cylinder block by abutting against a shoulder or groove in order to limit the axial movement of the guide ball towards the cylinder block; wherein a gap between the mounting ring and the guide ball is present when the hydraulic axial piston unit is at a standstill and/or operating at low rotational speeds and/or at low working pressure.

2. The hydraulic axial piston unit according to claim 1, wherein the mounting ring is made of metal, rubber or plastic material or of a combination of these materials.

3. The hydraulic axial piston unit according to claim 2, wherein the mounting ring is resilient in the axial direction.

4. The hydraulic axial piston unit according to claim 1, wherein the mounting ring is resilient in the axial direction.

5. The hydraulic axial piston unit according to claim 1, wherein the mounting ring is of a general rectangular cross section.

6. The hydraulic axial piston unit according to claim 1, wherein the swashplate is suitable to be swiveled with respect to the rotational axis of the drive shaft, in order to adjust the stroke of the pistons.

7. The hydraulic axial piston unit according to claim 1, wherein the gap between the mounting ring and the guide ball is closed partially or completely during operation.

8. The hydraulic axial piston unit according to claim 1, wherein each piston is moveable in the direction of the rotational axis.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows schematically a cross section of a hydraulic axial piston unit; and

(2) FIG. 2 is an enlarged partial view of the cross section of FIG. 1.

DETAILED DESCRIPTION

(3) FIG. 1 shows schematically a cross section of a hydraulic axial piston unit 1 of the swashplate construction type. In a housing 3 of the hydraulic axial piston unit 1 a drive shaft 2 is mounted having a rotational axis 10. The rotational axis 10 defines the axial direction of the hydraulic axial piston pump 1. The drive shaft 2 is adapted to drive or to be driven by a cylinder block 4 having a plurality of cylinder bores 6. In the cylinder bores 6 oriented in general in direction of the rotational axis 10, pistons 8 are accommodated and are movable reciprocally in direction of the cylinder bores 6, i.e. reciprocally in general along the axial direction 10 of the hydraulic axial piston unit 1. First ends of the pistons 8, which protrude outside of the cylinder bores 6 are slide-able fixed by means of piston slippers 14 to a swashplate 16 which does not turn around the drive shaft 2. As the cylinder block 6 turns with the drive shaft 2 the slippers 14 slide on the sliding surface 18 of the swashplate 16. A slipper hold down ring 20 is mounted to prevent the lifting-off of the slippers 14 from the swashplate 16. The slipper hold down ring 20 itself is held in place by a guide ball 24 rotationally secured to the drive shaft 2 and moveable in axial direction of the drive shaft 2. Thereby, the guide ball 24 overlaps partially with the cylinder block 4 and is mounted pre-stressed against the cylinder block 4 via guide ball springs 28 in order to provide elastic forces onto the slipper hold down ring 20 preventing lifting-off of the slippers 14 from the swashplate 16 at standstill and/or at low rotational speeds and/or at a low working pressure of the drive unit of the hydraulic axial piston unit. Near the same end of the guide ball 24 facing the cylinder block 4 a mounting ring 30 is allocated on cylinder block 4 with a distance/gap to the guide ball 24 at the aforementioned conditions of the hydraulic axial piston unit 1.

(4) In operation of the hydraulic axial piston unit 1working exemplarily as a hydraulic axial piston pump as shown in FIG. 1working fluid under pressure is guided into the cylinder bores 6 pressing pistons 8 out of cylinder bore 6, therewith initiating the rotation of cylinder block 4. Within the piston 8 a longitudinal bore 9 is provided for guiding hydraulic fluid from cylinder bores 6 to the slippers 14 each of which comprise a lubrication bore 19 for guiding the hydraulic fluid towards the sliding surface 18 on swashplate 16. It can be derived from FIG. 1 that when hydraulic fluid under pressure is guided via the longitudinal bore 9 in piston 8 and via the lubrication bore 19 in slipper 14 onto the sliding surface 18 of the swashplate 16 the slippers 14 are intended to be lifted-off from the sliding surface 18 of the swashplate 16. The slipper hold down ring 20 acts against these lifting forces as it abuts with a radial inner surface 22, against a matching surface 26 on guide ball 24. Guide ball 24 is mounted on drive shaft 2 and elastically abuts in the axial direction on cylinder block 4. In this manner a resilient axial movement of the guide ball 24 is permitted wherein the guide ball 24 is always in contact with its matching surface 26 with the inner radial surface 22 of the hold down ring 20. At least at standstill of the hydraulic axial piston unit 1 a gap between guide ball 24 and the mounting ring 30 is present.

(5) In operation conditions of the hydraulic axial piston unit 1 the gap between the guide ball 24 and the mounting ring 30 can be closed partially, or completely, thereby enabling a corresponding gap between the slippers 14 and the sliding surface 18 on swashplate 16. In this gap between the slippers 14 and the swashplate 16 a lubrication means can enter, preferably hydraulic working fluid under pressure. Mounting ring 30 limits the gap between the slippers 14 and the sliding surface 18 on swashplate 16 by forming a physical stop for the guide ball 24 movement in axial direction towards the cylinder block 4. Hydrostatic and rotational gyroscopic forces lift the slippers 14 from the sliding surface 18 of swashplate 16 and push the hold down ring 20 with its inner radial surface 22 on the matching surface 26 on guide ball 24 and push therewith guide ball 24 towards the mounting ring 30. Finally, in operational conditions of the hydraulic axial piston unit 1 the slippers 14, the slipper hold down ring 20, the guide ball 24, the mounting ring 30, the cylinder block 4 and a valve plate 32 at the opposite end of cylinder block 4 are in continuous physical contact to each other. Furthermore, the mounting ring 30 can be positioned adequately in axial direction of the cylinder block 4 such that the width of the gap between the slippers 14 and the swashplate 16 is optimized for forming optimum lubrication and operational conditions for the drive unit of the hydrostatic axial piston unit 1.

(6) In FIG. 2 an enlarged partial cross section of FIG. 1 is shown with the inventive hydraulic axial piston unit at standstill. From FIG. 2 it can be depicted that springs 28 push the guide ball 24 away from cylinder block 4 and that the slippers 14 are pressed against the sliding surface 18 of swashplate 16 via the matching surface 26 on guide ball 24 and the radial inner surface 22 of the hold down ring 20 by means of the resilient forces of springs 28. Next to the guide ball 24 the mounting ring 30 is fixed with a predefined gap on the cylinder block 4 providing a physical stopper for guide ball 24 when the same is moved in axial direction towards the cylinder block 4. This occurs when hydraulic fluid under pressure via the longitudinal bore 9 in piston 8 and via the lubrication bore 19 in slipper 14 creates a hydraulic force in order to separate the slipper 14 from swashplate 16. While the slipper hold down ring 20 physically engages with its inner radial surface 22 with the matching surface 26 on guide ball 24 the three elementsslipper 14, mounting ring 20 and guide ball 24are moved in axial direction towards the cylinder block 4 until guide ball 24 abuts against mounting ring 30. This situation occurs in working conditions of the hydraulic axial piston unit, wherein the gap between the slipper 14 and the swashplate 16 is at its maximum when the guide ball 24 touches the mounting ring 30. This maximum gap is maintained constant as long as the working pressure together with the rotational gyroscopic forces exceeds the forces of springs 28. This defined gap provides an optimum size for lubrication between the slippers 14 and sliding surface 18 of swashplate 16.

(7) It can be easily derived, in particular from FIG. 2, that the position of mounting ring 30 defines the size of the gap between the slippers 14 and swashplate 16. In consequence, if the position of mounting ring 30 is adjustable, the gap between slippers 14 and swashplate 16 is adjustable too. This provides for a compensation of production and assembly tolerances of the cylinder block 4, the guide ball 24, the mounting ring 30, the slippers 14 and the swashplate 16. In this tolerance compensation chain, valve plate 32 and end cap 34 on the other side of the cylinder block 4 participate as well, the latter building the opposite end of the drive unit of the hydraulic axial piston unit 1.

(8) In particular from FIG. 2 it can be derived further that an integration of springs 28 in mounting ring 30 forms part of the scope of design possibilities for a person with relevant skills in the art, in order to achieve a resilient abutment of the guide ball 24 on the mounting ring 30. However, it can be preferred for high precision units that the limit of the gap size respectively of the axial movement of the guide ball 24 is performed by a physical stopper, thereby physically limiting the width of the gap between the slippers 14 and the swashplate 16.

(9) Finally, a hydraulic axial piston unit 1 is provided in which optimum lubrication conditions between the slippers (piston slide shoes) and the swashplate 16 at any operational conditions of the hydraulic axial piston unit 1 is achieved in a simple, cost effective and robust manner.

(10) While the present disclosure has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this disclosure may be made without departing from the spirit and scope of the present disclosure.