The present invention relates to a hydraulic machine comprising a housing and a spool arranged in the housing rotatably about an axis of rotation, wherein the spool comprises a first circumferential groove, a second circumferential groove, a set of first axial grooves connected to the first circumferential groove and a set of second axial grooves connected to the second circumferential grooves, the axial grooves being arranged between the first circumferential groove and the second circumferential groove and the first axial grooves and the second axial grooves are alternately distributed in circumferential direction of the spool.

Such a hydraulic machine is known, for example, from US 4 082 480 A. The spool is used to supply hydraulic fluid under pressure to a working section and to return hydraulic fluid with low pressure to a tank port. The working section can be, for example, a gerotor machine.

The housing comprises a bore accommodating the spool. The inner diameter of the bore and the outer diameter of the spool have to fit to each other so that leakages can be kept small. It is, however, almost impossible to avoid completely leakages.

The object underlying the invention is to keep leakages as small as possible.

This object is solved with a hydraulic machine as described at the outset in that the first groove comprises a first bottom wall between two first transition areas at the axial ends of the bottom wall, the first bottom wall having a varying distance to the axis of rotation. In other words, the first bottom wall does not run parallel to the axis of rotation over the whole length of the bottom wall. This makes it possible to increase the thickness of the material of the spool in the region of the first circumferential groove. When the thickness of the material is increased, the spool becomes stiffer and has a higher resistance against deformation which can be caused by the pressure of the hydraulic fluid. When the deformation of the spool due to the high-pressure hydraulic fluid is reduced, leakages can also be reduced.

In an embodiment of the invention the distance between the first bottom wall and the axis rotation is larger near the first axial grooves than remote from the first axial grooves. The resistance against deformation of the spool is smallest in the region of the axial grooves. When the distance between the first bottom wall and the axis of rotation is larger near the first axial grooves, more material of the spool is present near the first axial grooves and the stiffness or resistance against deformation is increased, so that leakage can be kept small.

In an embodiment of the invention the first bottom wall is inclined with respect to the axis of rotation. The first bottom wall, i.e. the section between the first transition areas, can be flat which facilitates the production of the first circumferential groove. The inclination is a simple way to provide more material near the first axial grooves than remote the first axial grooves.

In an embodiment of the invention the first bottom wall and the axis of rotation enclose a first angle in a range from 5° to 15°. Although such an angle is rather small, it shows a satisfactory result in increasing the stiffness of the spool.

In an embodiment of the invention the first angle is in a range from 5° to 8°. In a preferred embodiment the angle is around 6°. In an embodiment of the invention the first axial grooves intersect the first circumferential groove over at least 35 % of the axial length of the first circumferential groove. When the distance between the first bottom wall and the axis of rotation is larger near the first axial grooves than remote from the axial grooves, there is a risk that the entrance or input opening of the first axial groove shows a too large throttling resistance. This risk can be avoided when an input area of the first axial groove is extended in axial direction.

In an embodiment of the invention the second circumferential groove comprises a second bottom wall between two second transition areas at the axial ends, the second bottom wall having a varying distance to the axis of rotation. The advantages described in connection with the first circumferential groove also apply here.

In an embodiment of the invention the distance between the second bottom wall and the axis of rotation is larger near the second axial grooves than remote from the second axial grooves. This means that more material can be retained near the second axial grooves so that the stiffness of the spool at the transition from the second axial grooves to the second circumferential grooves is improved which is positive for keeping the leakages small.

In an embodiment of the invention the second bottom wall is inclined with respect to the axis of rotation. In a sectional view the second bottom wall can be flat which makes production of the spool simple.

In an embodiment of the invention the second bottom wall and the axis of rotation enclose a second angle in a range from 1 ° to 15°. This corresponds to the range of angles of the first bottom wall. In an embodiment of the invention the second angle is in a range from 5° to 8°, in particular around 6°.

In an embodiment of the invention the second axial groove intersects the second circumferential groove over at least 35% of the axial length of the second circumferential groove. As with the first axial grooves, the entrance into the groove is made as large as possible.

In an embodiment of the invention the first angle and the second angle are equal. Thus, the behaviour of the hydraulic machine can be made similar or equal in both directions of operation.

In an embodiment of the invention the spool comprises a hollow and a spline in a spline region of the hollow, wherein one of the circumferential grooves having a bottom wall with varying distance to the axis of rotation is at least partly arranged around the spline region. In the spline region the circumferential wall of the spool is weakened. With the varying distance of the bottom wall it is possible to enforce the circumferential wall of the spool so that the resistance against deformation can be increased and the risk of leakages can be decreased.

A preferred embodiment of the invention will now be described with reference to the drawing, wherein:

Fig. 1 shows a sectional view of a hydraulic machine,

Fig. 2 shows an enlarged view of a spool,

Fig. 3 shows a sectional view of the spool and

Fig. 4 shows an enlarged detail of Fig. 3. Fig. 1 shows schematically in a sectional view a hydraulic machine 1 comprising a housing 2 and a spool 3 arranged in the housing 2 rotatably about an axis of rotation 4. The spool 2 is connected or integral with a spool shaft 5.

The hydraulic machine comprises a working section 6 in form of a gerotor arrangement. The gerotor arrangement comprises a star wheel 7 and a ring gear 8. The star wheel 7 is arranged eccentrically in the ring gear 8 and comprises a number of teeth which is one less than the number of teeth of the ring gear 8.

The star wheel 7 and the spool 3 are connected by means of a dog bone 9 or cardan shaft.

The spool 3 comprises a hollow 10 of cylindrical form. The hollow 10 comprises a spline region 11 at an end remote from the star wheel 7. The dog bone 9 comprises a spline geometry 12 which is in engagement with the spline region 11 . The dog bone comprises another spline geometry 13 at the other end which is in engagement with a corresponding spline geometry of the star wheel 7.

The spool 3 comprises a first circumferential groove 14 and a second circumferential groove 15. A number of first axial grooves 16 is connected to the first circumferential groove 14. A number of second axial grooves 17 is connected to the second circumferential groove 15. The first axial grooves 16 and the second axial grooves 17 are arranged between the two circumferential grooves 14, 15. The first axial grooves 16 and the second axial grooves 17 are arranged alternately in circumferential direction.

A distributer plate 18 is arranged between the spool 3 and the working section 6. As can be seen in Fig. 3, the first circumferential groove 14 comprises a first bottom wall 19. The first bottom wall extends between two transition areas 20, 21 which define a transition between the bottom wall and side walls of the first circumferential groove 14 in circumferential direction. The first bottom wall 19 does not run parallel to the axis of rotation 4, but has a varying distance to this axis of rotation 4.

In other words, the first bottom wall 19 is inclined with respect to the axis of rotation 4, so that the distance between the first bottom wall 19 and the axis of rotation 4 is larger near the first axial grooves 16 than remote from the first axial grooves 15.

The first bottom wall 19 and the axis of rotation 4 enclose a first angle in a range from 5° to 15°, in particular in a range from 5° to 8° and most preferably 6° or around 6°.

As can be seen in particular in Fig. 2, the first axial groove 16 intersects the first circumferential groove 14 over at least 35 % of the axial length of the first circumferential groove 14. This means that, although the first bottom wall 19 is not parallel to the axis of rotation 4, there is no dramatic increase in a throttling resistance at the entrance or input opening of the first axial grooves 16.

In principle, the same features can be used for the second groove 15 as well. The second groove 15 comprises a second bottom wall 22 between two transition regions 23, 24, wherein the transition regions 23, 24 form a transition between the second bottom wall 22 and the side walls of the second groove 15 in circumferential direction.

The second bottom wall 22 has a varying distance to the axis of rotation 4. In particular, the distance between the second bottom wall 22 and the axis of rotation 4 is larger near the second axial grooves 17 than remote from the second axial grooves 17. The second bottom wall 22 is inclined with respect to the axis of rotation 4. The second bottom wall and the axis of rotation enclose a second angle in a range from 5° to 15°, in particular from 5° to 8° and preferably 6° or around 6°.

The second axial groove 17 intersects the second circumferential groove 15 over at least 35 % of the axial length of the second circumferential groove 15.

In a preferred embodiment the first angle and the second angle are equal.

When the hydraulic machine is operated in one direction of rotation only it may be sufficient to have only one of the circumferential grooves 14, 15 embodied with the bottom wall having a varying distance to the axis of rotation 4. However, it is preferred that both circumferential grooves 14, 15 are formed in a kind of symmetry.

Due to the increase of thickness of the wall of the spool 3 the resistance against a deformation under higher pressures of the hydraulic fluid is increased. Thus, the tightness of the hydraulic machine can be kept large and the leakages can be kept low.