The present invention relates to a hydraulic machine comprising a housing, a working section and a control section, wherein the control section comprises a spool arranged in the housing rotatably about an axis of rotation and a distributer plate is arranged between the control section and the working section, wherein the spool comprises a hollow surrounded by a wall and a front face of the wall rests in a contact area against the distributer plate.

Such a hydraulic machine is known, for example, from US 5 407 336 A or DE 10 356 301 B3.

The working section can be, for example, a gerotor arrangement having a star wheel and a ring gear, wherein the ring gear has one tooth more than the star wheel. Pressure chambers are formed between the star wheel and the ring gear. The spool of the control section is used to supply hydraulic fluid to working chambers having an increasing volume and to return hydraulic fluid from working chambers having a decreasing volume when the working section is operated as a motor.

In order to avoid internal leakages in the hydraulic machine the spool must fit as close as possible in a bore of the housing. When the machine is used as a motor, it is common to use a part of the spool as an output shaft. During operation of the motor the output shaft must be able to absorb or take over forces. To this end the spool is supported in the housing by means of a bearing. This bearing can take radial forces and axial forces acting in a direction pulling the spool out of the housing. However, it is rather difficult to find a solution in which axial forces acting in the opposite direction are taken or compensated. This has as a consequence that the front face of the spool slides on the distributer plate causing friction. The forces acting on the spool or the output shaft connected to the spool or being integrated to the spool cause wear which is detrimental for the lifetime of the hydraulic machine.

The object underlying the invention is to keep wear small.

This object is solved with a hydraulic machine as described at the outset in that at least one groove is provided in the contact area, the groove connecting the hollow and an outer diameter of the spool.

The groove allows hydraulic fluid to enter the region between the front face of the spool and the distributer plate, so that hydraulic fluid can be used to lubricate the contact area and to reduce the friction between the spool and the distributer plate.

In an embodiment of the invention the groove extends in radial direction and in circumferential direction of the spool. The groove must extend in radial direction to connect the hollow and the outer diameter of the spool. Furthermore, the groove extends also in circumferential direction, so that the hydraulic fluid is distributed in circumferential direction when it flows through the groove.

In an embodiment of the invention the groove extends over a length in circumferential direction which length is larger than a largest width of the groove. The extension of the groove is not limited to the width of the groove. The width of the groove can be kept small, so that leakages through the groove are limited. Nevertheless, a reliable distribution of the hydraulic fluid over the contact area can be achieved. In an embodiment of the invention the groove extends over a length in circumferential direction which length is equal to or larger than a thickness of the wall of the spool. In other words, the groove covers at least an area which is defined by a square, wherein the length of the sides of the square corresponds to the thickness of the wall of the spool surrounding the hollow of the spool. Thus, the hydraulic fluid entering the groove is dragged over a quite long distance in the spool over the contact area.

In an embodiment of the invention the groove has a curved shape. This has two advantages. The first advantage is that the groove can be machined by turning. The second advantage is that the length of the groove is slightly increased so that a somewhat larger amount of hydraulic fluid can be transported in the groove to lubricate the contact area.

In an embodiment of the invention the groove has a radially inner opening at the hollow and a radially outer opening at the outer diameter of the spool, wherein the radially inner opening and the radially outer opening are offset to each other in circumferential direction. The offset between the two openings defines roughly the length of the groove in circumferential direction.

In an embodiment of the invention the groove has a single gradient. In other words, there is no reversal of the direction of the groove in the thickness of the wall. This keeps a pressure loss low.

In an embodiment of the invention the groove comprises a depth of 1 mm or less, preferably 0,3 mm or less. The section of the groove should not be too large to keep a flow through the groove in an acceptable magnitude. A depth of 1 mm or less, preferably 0,3 mm or less is suitable to fulfil this requirement. In an embodiment of the invention the groove is located in the front face of the spool. This is a simple way to produce the groove. The spool requires already a number of forming steps. It is no problem to add a single forming step producing the groove. This can be done, for example, at the end of turning off the hollow.

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

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

Fig. 2 shows a front view of a spool of the hydraulic machine and

Fig. 3 shows a perspective view of an end of the spool.

Same elements are denoted with the same reference numerals in all Figures.

Fig. 1 shows in a sectional view schematically a hydraulic machine 1 comprising a housing 2. The hydraulic machine comprises a working section 3 and a control section 4. The working section 3 is in form of a gerotor arrangement having a star wheel 5a and a ring gear 5b. The ring gear 5b comprises inner teeth and the star wheel 5a comprises outer teeth. The number of the outer teeth is one less than the number of the inner teeth of the ring gear 5b. Other embodiments of the working section 3 are possible.

The control section 4 comprises a spool 6 which is connected to a spool shaft 7 protruding out of the housing 2. The spool 6 is connected to the star wheel 5a by means of a cardan shaft 8 which is also called "dog bone". The cardan shaft 8 comprises a first spline arrangement 9 in engagement with the star wheel 5a and a second spline arrangement 10 in engagement with the spool 6. The star wheel 5a is arranged eccentrically in the ring gear 5b. The cardan shaft 8 makes it possible to transfer only the rotational movement of the star wheel 5a to the spool 6.

The spool 6 comprises a first circumferential groove 11 and a second circumferential groove 12. A number of first axial grooves 13 connected to the first circumferential grooves 11 and a number of second axial grooves (not shown) connected to the second circumferential groove 12 are arranged between the two circumferential grooves 11 , 12. The first axial grooves 13 and the second axial grooves are arranged alternately in circumferential direction.

The working section 3 has a number of working chambers which, during a rotation of the star wheel 5a, increase their volume and decrease their volume. When the spool 6 rotates, it connects a pressure area of the control section 4 to working chambers having an increasing volume and connects working chambers having a decreasing volume with a return section of the control area 4 (the pressure area and the return area of the control section 4 are not shown in detail). As mentioned above, the rotating movement of the star wheel 5a is transferred to the spool 6 and to the output shaft 7, so that the hydraulic machine can be operated as a motor.

The spool 6 is supported in the housing 2 by a radial bearing 14 and by an axial bearing 15. These two bearings 14, 15 are shown as separate elements. However, they can be combined in a single bearing.

The axial bearing 15 is able to support the spool 6 in the housing 2 against forces which are directed in a direction pulling the spool 6 out of the housing 2. However, the bearing 15 is not able to take over forces pushing the spool 6 into the housing 2. A distributer plate 16 is arranged between the control section 4 and the working section 3. The spool 6 is in contact with the distributer plate 16. When the spool 6 is rotated, there is a friction between the distributer plate 16 and the spool 6.

As it is shown in Fig. 2 and 3, the spool 6 comprises a hollow 17 which is surrounded by a wall 18. The hollow 17 is used to accommodate the cardan shaft 8. As mentioned above, the spool 6 rests against the distributer plate 16. In order to facilitate the following explanation, the region in which the spool 6 contacts the distributer plate 16 is termed "contact area" 19.

In order to reduce the friction between the spool 6 and the distributer plate 16 in the contact area 19, a groove 20 is provided. In the embodiment shown in Fig. 2 and 3 the groove 20 is located in a front face 21 of the spool 6. However, it can also be located in the distributer plate 16.

As can be seen in particular in Fig. 2, the groove extends not only in radial direction, i.e. from the hollow to the outer diameter of the spool 6, but also in circumferential direction of the spool 6. This is true even when the groove 20 is formed in the distributer plate 16.

The groove 20 extends over a length in circumferential direction which length is greater than a largest width of the groove 20 and more particular extends over a length in circumferential direction which length is equal to or larger than the thickness of the wall 18 of the spool 6.

The groove 20 has a curved shape. The curved shape is advantageous for the production of the spool 6, when parts of the spool 6 are formed by turning. The groove 20 can be formed by the same tool forming the hollow 17 or parts of the hollow 17. During operation the groove 20 fills with hydraulic fluid. The groove 20 transports or drags this hydraulic fluid over the whole contact area 19 and distributes it over the whole contact area 19. The extension of the groove 20 in circumferential direction has three advantages. One advantage is that an amount of hydraulic fluid can be transported which is larger than an amount or volume of hydraulic fluid which could be transported when the groove 20 would only be directed in radial direction. A second advantage is that the distribution of the hydraulic fluid in circumferential direction can be improved. Furthermore, the greater length of the groove 20 increases the throttling resistance of the groove 20 and keeps internal leakages of the machine small.

The groove 20 has a radially inner opening 22 at the hollow 17 and a radially outer opening 23 at the outer diameter of the spool 6, wherein the outer opening 23 opens into a rear journal bearing of the the spool 6. The radially inner opening 22 and the radially outer opening 23 are offset to each other in circumferential direction. Thus, there is only one direction of flow through the groove which direction is clockwise or counter clockwise. However, there is no reversal of the direction of the hydraulic fluid when the hydraulic fluid is flowing through the groove 20.

As can be seen in particular in Fig. 2 and 3 the groove has a single gradient.

The depth of the groove is preferably 1 mm or less, preferably 0,3 mm or less and in particular 0,2 mm or less. Although this is a quite small depth, it is sufficient to establish a sufficient lubrication of the contact area 19. On the other hand, this depth is of advantage in terms of internal leakages of the hydraulic machine. The drawing shows only a single groove 20. However, it is possible to use more than one groove, for example two or three grooves. However, in most cases a single groove 20 will be sufficient.

When the spool 6 is loaded axially, a situation can occur in which hydraulic fluid from the journal bearing of the spool 6 is trapped. This hydraulic fluid cannot escape to a drain chamber, so that no change of hydraulic fluid can take place leading to more heat and an increasing temperature of the hydraulic fluid. When oil is used as hydraulic fluid this can lead to an adverse effect on the viscosity with the consequence of a reduced radial load capacity and reduced life time of the rear journal bearing.

Furthermore, in such a situation there is a poor lubrication of the end surface of the spool with the consequence of more heat, rising temperature, and reduced life time of the axial bearing.

However, with help of to the groove 20 these problems can be overcome. The groove 20 connects the rear journal bearing with a drain chamber, so that there is an improoved lubrication for both bearings. The groove 20 will positively affect the performance of the machine, in particular, when the machine is used as motor.

The groove 20 allows a small, however sufficient, flow of hydraulic fluid independently of which ports of the machine is supplied with high pressure.

Compared to a flow over the front face of the spool 6 when the spool 6 is moved away from the distributor plate 16, the flow through the groove 20 in a situation when the spool 6 is pressed against the distributor plate 16 can be, for example, at least 50% of the first mentioned flow.