In the following, the connection of the chambers with the pressure connections will also be called main flow connec- tion, the valve being a main control. The connections be- tween the neighbouring chambers will in the following also be called secondary flow connections, which are acted upon by a secondary control.

At the moment, such hydraulic motors often have the so- called gerotors as gear sets. However, a problem with these gerotor-motors is that during operation the posi- tioning of the gear set in relation to the valve is in many cases exposed to certain inaccuracies. Thus, the flow connections from the pressure connections to the chambers are not released or blocked at the desired times, but sooner or later. This is called a faulty commutation.

Such faulty commutations occur, for example, with known types of gerotor-motors, in which the valve is arranged on the side of the gear set turning away from an output shaft. The reason is that here a mechanical connection be- tween gear set and valve is made separately by means of a

small cardan shaft, which causes a faulty commutation be- cause of slip. The slip is required when assembling these motors.

Also with motor types, in which the valve is arranged on the output shaft, a faulty commutation can appear because of a high load on the motor and the torsion of the output shaft caused by this.

EP 0 959 248 A2 shows a hydraulic motor as mentioned in the introduction with a gerotor, in which the movement of the gerotor is transmitted to an output element via a drive shaft inclined towards the gerotor. A spool valve is provided for controlling the main flow connection between the individual chambers of the gerotor and the high and low-pressure connections. The spool valve is arranged to be rotatable around the drive shaft in a housing bore, and is moved by the drive shaft. In order to avoid excess loading or damage to the gear set caused by a faulty com- mutation, each tooth of a surface of the gear wheel, which is in contact with the toothed ring during operation, is provided with two grooves. These grooves enable a secon- dary flow between two neighbouring chambers, also when, during operation, the gear wheel bears tightly on the toothed ring in the area between these two chambers.

This secondary flow ensures that no hydraulic fluid is trapped in the chamber in question on transition from an expansion phase to a contraction phase or vice versa. In this way, loads in the chamber caused by cavitation or ex- cess pressure at the beginning of an expansion or contrac- tion phase can be avoided. In this connection, the secon- dary flow is controlled in dependence of the position of

the gear wheel in relation to the toothed ring, which thus form the secondary control. Contrary to the main control formed by the valve, this secondary control is not influ- enced by the faulty commutation between the gear set and the valve. This procedure is also called second commuta- tion.

However, when creating the grooves, which are essential for the secondary control, the contact face between the gear wheel and the toothed ring is reduced. Consequently, the pressure in the remaining contact face and thus the load on the gear wheel during operation increases. This increased load on the gear set in the contact face will, at least partially, clear the reduction described above of the load on the hydraulic motor through reduced cavitation and excess pressure.

The invention is based on the task of reducing the loads occurring during operation of a hydraulic motor as men- tioned in the introduction.

According to the invention, this task is solved in that the secondary control is formed by the gear wheel and at least one motor element arranged next to the gear set.

In this way, the control and formation of the second flow connection between neighbouring chambers no longer occur exclusively inside the gear set. On the contrary, it is now possible to make the control of the secondary flows in dependence of the position of the gear wheel in relation to another element as the toothed ring. Further, the ar- rangement of the flow connections between the chambers can be selected more freely. For example, it is possible to

move parts of one of the flow connections into one or more other motor elements, which are exposed to less extreme loads during operation than the gear set. Such motor ele- ments can, for example, be formed by housing parts or car- rying elements. Thus, parts of the secondary control, which are required to produce the secondary flow, however, causing a weakening of the material, can also be made out- side the gear set. Thus, loads occurring during operation because of cavitation and excess pressure can be reduced without producing increased loads in carrying areas of the gear set through material recesses.

It is favourable that both gear wheel and motor element have flow areas, forming in dependence of the position of the gear wheel in relation to the motor element predeter- mined overlapping areas. Thus, the control function be- tween the gear wheel and the motor element is more easily realised. The control of the secondary flow occurs in de- pendence of the position of the flow areas of the gear wheel in relation to the flow areas of the neighbouring motor element. With this method, parts of the secondary control or the secondary flow connection, respectively, can be arranged outside the gear set. In this way, a me- chanical weakening of the gear wheel and thus of the com- plete gear set by the flow areas of the secondary control can be significantly reduced.

It is advantageous that the flow areas of the gear wheel are formed by recesses in at least one front side. Front sides are here the two base surfaces of the gear wheel, which, unlike a toothed surface that delimits the gear wheel radially outward, does not get in contact with the toothed ring. On the contrary, the. recesses are arranged

at a certain distance from the toothed surface. In this way, the flow areas of the gear wheel are moved to a less loaded area and a reduction of the contact face between the gear wheel and the toothed ring is avoided. The sur- face pressure existing between both elements during opera- tion thus remains the same, in spite of the recesses re- quired for the secondary control.

Preferably, the flow areas of the motor element are made as indentations in a surface facing the front side. Thus, the flow areas of the motor element are particularly eas- ily produced, which again reduces the costs of the hydrau- lic motor.

It is recommended that at least two different indentations and/or at least two different recesses be provided. The indentations or the recesses, respectively, can differ in size and in geometry or in both. Using such differently shaped flow areas make them better adaptable to their in- dividual functions during control of the secondary flow connection. This enables a very accurate control.

In a preferred embodiment it is provided that, in prede- termined positions of the gear wheel in relation to the motor element, one of the recesses has an overlapping area with two neighbouring indentations simultaneously. Even with simply shaped flow areas, this simultaneous connec- tion of the recess with two indentations enables a rela- tively accurate control of the secondary flow.

It is advantageous that, during operation, a position of the gear wheel in relation to the toothed ring, in which one of the chambers is a minimal chamber, is immediately

followed by a position, in which the minimal chamber has a secondary flow connection to one of the neighbouring cham- bers. Minimal chamber here means a chamber, which has, in the situation described, the smallest possible volume.

With this method, the connection of the neighbouring cham- bers occurs independently of the valve and thus also inde- pendently of a possible faulty commutation. Thus, it is constantly ensured by the secondary flow connection that, when shifting a chamber from a contraction phase to an ex- pansion phase, no hydraulic fluid is trapped in the cham- ber at the beginning of the expansion. Thus, a damaging cavitation in the chamber in question can be avoided.

Further, it is recommended that during operation, a posi- tion of the gear wheel in relation to the toothed ring, in which one of the chambers is a maximal chamber, is immedi- ately followed by a position, in which the maximal chamber has a secondary flow connection to one of the neighbouring chambers. Maximal chamber here means a chamber, which has, in the situation described, the largest possible volume.

The secondary flow connection ensures that, when shifting a chamber from an expansion phase to a contraction phase, also in case of a faulty commutation between the gear set and the valve, no hydraulic fluid is trapped in the cham- ber at the beginning of the contraction. Thus, damaging pressure peaks at the beginning of the contraction can be avoided.

Further, it is favourable that the motor element is formed by a housing cover. In this way the indentations can be made in an area of the motor, which has a relatively sim- ple embodiment and is easily replaceable. Thus, the plac- ing of the indentations is relatively free, and further it

is possible, by replacing the housing cover, to provide different arrangements of indentations for different modes of operation of the hydraulic motor.

Further, it is advantageous that the flow areas on the front side and on the surface are arranged to be radially symmetric around a centre. In this connection, the term "radially symmetric"means that the flow areas of the front side and the surface will be overlapping after each rotation of 360°/n around the centre, n being the number of teeth of the gear wheel or the number of uniform inden- tations in the motor element, respectively. Thus, it is possible to operate the hydraulic motor in both direc- tions, which again increases the application possibilities of the hydraulic motor.

In the following, the invention is described in detail on the basis of a preferred embodiment in connection with the drawings, showing: Fig. 1 a longitudinal section through a hydraulic motor according to the invention, Fig. 2 a section through a front side of the gear wheel Fig. 3 a section through a surface of a housing cover, Fig. 4 a schematic arrangement of the gear wheel oppo- site the housing cover when shifting a chamber from a contraction to an expansion

Fig. 5 a schematic arrangement of the gear wheel oppo- site the housing cover when shifting a chamber from an expansion to a contraction.

Fig. 1 shows a hydraulic motor 1 with a housing block 2, in which an output element 3, for example a shaft, is ar- ranged to be rotatable. In the housing block 2 is further provided a valve 4, which is arranged on the circumference of the output element 3 and is adjustable by means of the output element 3. The valve 4 is made as a disc valve. The output element 3 has an axial bore 5, in which is made a toothed area 6. An end of a drive shaft 7 with a first crowned toothing 8 projects into the axial bore. This first crowned toothing 8 engages with the toothed area 6 of the output element 3. A second crowned toothing 9 at the other end of the drive shaft 7 engages with a toothed inner ring 10 of a gear set 11.

The gear set 11 works according to the gerotor principle, and has a gear wheel 12, on which is formed the inner ring 10 as well as an outer toothing 13. The gear wheel 12 is arranged eccentrically in a toothed ring 14, which has an inner toothing 15 correlating to the outer toothing 13.

The tooth crests of the inner toothing 15 are formed by rotatably supported rollers 16 of the toothed ring 14. The toothed ring 14 has at least one tooth more than the outer toothing 13 of the gear wheel 12, so that during operation the gear wheel 12 is both rotated and orbited in relation to the toothed ring 14.

On a first front side 17 of the gear wheel 12 turning away from the housing block 2 is arranged a housing cover 18.

This bears with a surface 19 on the gear set 11. A second

front side 20 of the gear wheel 12 facing the housing block 2 bears on a port plate 21, which is arranged be- tween the gear set 11 and the housing block 2. By means of the housing cover 18, the gear set 11 and the port plate 21 are pressed against the housing block 2 via several bolts 22. Thus, main flow connections between the gear set 11 and a high-pressure connection 23 as well as a low- pressure connection (not shown) can be produced via the port plate 21 and the valve 3.

Fig. 2 shows a section (section A-A in Fig. 1) through the first front side 17 of the gear wheel 12. In this section, each tooth profile 26 of the outer toothing 13 of the gear wheel 12 has a recess 25. Each recess is arranged at a distance a to a toothing surface 27, which delimits the gear wheel in the radial direction. The recesses are ar- ranged to be radially symmetric around a centre MP1.

Fig. 3 shows a section (section B-B in Fig. 1) through the surface 19 of the housing cover 18 (without lateral de- limitation). On this surface 19 large indentations 28 and small indentations 29 are arranged to be radially symmet- ric around a centre MP2. Laterally, the indentations 28, 29 are delimited by a boundary 30. For a better view of the arrangement of the surface 19 in relation to the toothed ring 14, the rollers 16 of the toothed ring are also shown with dotted lines. The rollers 16 do not actu- ally project into the surface 19 and accordingly should not be visible in the sectional view. The large indenta- tions 28 are made so that they cover an area that extends from one of the rollers 16 of a neighbouring roller 16.

The large indentations 28 taper in the direction of the centre MP2. Each small indentation 29 is arranged between

the tapered areas of two neighbouring large indentations 28 on the side of one of the rollers 16 facing the centre MP2.

Fig. 4 shows schematically an arrangement during operation of the gear wheel 12 in relation to the indentations 28, 29 of the housing cover 18 as well as in relation to the rollers 16 of the toothed ring 14. For better clarity, the rollers 16 are still shown with dotted lines. Between any two neighbouring rollers 16, the toothed ring 14 has a tooth base 31, which in this section coincides with the radial outer part of the boundary 30 of the large indenta- tion 28 arranged here. In this way, any two neighbouring rollers 16 and the tooth base 31 between them, together with the large indentation 28 arranged here and the gear wheel, delimit a chamber 32.

In the shown positioning of the gear wheel 12 in relation to the toothed ring 14, one of the tooth profiles 26 (at the 9 o'clock position) of the gear wheel 12 is arranged to be as close to the tooth base 31 as possible. Thus, a minimal chamber 34 with a minimum chamber volume is formed. Through contact of the outer toothing 13 of the gear wheel 12 with the two rollers 16, which delimit the minimal chamber 34 laterally, a sealing line 33 is formed on each of the two rollers 16. Further, the recesses 25 have several overlapping areas 35 with the indentations 28,29.

Fig. 5 shows an additional positioning during operation of the gear wheel 12 in relation to the indentations of the housing cover 18 and the rollers 16 of the toothed ring 14. Here, the tooth base 31 at the 9 o'clock position is

arranged exactly opposite to a profile hollow 36 between two tooth profiles 16 of the gear wheel 12. This forms a maximal chamber 37 with a maximum chamber volume. Through contact of the outer toothing 13 of the gear wheel 12 with the two rollers 16, which delimit the maximal chamber 37 laterally, a sealing line 33 is again formed on each of the two rollers 16.

The basic mode of operation of a hydraulic motor 1 accord- ing to Fig. 1 is known, and is therefore only described briefly in the following.

During operation, each chamber 31 is cyclically connected with the high-pressure connection 23 and the low-pressure connection, passing alternatingly through an expansion phase, during which the chamber volume increases, and a contraction phase, during which the chamber volume de- creases. Through the valve 4, the expansion and contrac- tion phases of the individual chambers 31 are adapted to each other in such a way that the gear wheel 12 performs a continuous rotating and orbiting movement in relation to the toothed ring 14. Via the inner ring 10, this movement is transferred to the drive shaft 7, which again drives the output element 3 and the valve 4.

On the other hand, it is also possible to use the hydrau- lic motor according to Fig. 1 as a pump. For this purpose, the output element 3 merely has to be exposed to a torque, which ensures a transport of a fluid from the low pressure connection 24 to the high pressure connection 23 or vice versa.

According to the invention, it is now ensured that the flow areas 25,28, 29 are formed in the first front side 17 of the gear wheel and in the surface 19 of the housing cover 18, which are facing each other. As described, these flow areas 25,28, 29 can be recesses 25 and indentations 28,29 or have any other suited form. Other suited forms of the flow areas 25,28, 29 could be, for example, grooves, through holes, closed channels or all possible sorts of mixed forms. Further, it is also possible to make the recesses 25 exclusively or additionally in the second front side 20. In this case, the recesses 25 could cooper- ate with corresponding indentations 28,29 in the port plate 21.

In the embodiment shown, the recesses 25 are made to be guided along the indentations 28,29 of the housing cover 18 during operation. In this way, the overlapping areas 35 occur, which produce returning connections between the flow areas 25,28, 29 of the gear wheel 12 and the housing cover 18. In predetermined positionings of the gear wheel 12 in relation to the toothed ring 14, the flow areas 25, 28,29 are arranged to form secondary flow connections be- tween two neighbouring chambers 32. In the predetermined situations, these secondary flow connections enable a fluid flow from a chamber 32 also, when each of the two rollers 16, which delimit the chamber 31 in question, forms a sealing line with the gear wheel 12. Further, these secondary flow connections are formed independently of the valve 4. Thus, they are not influenced by a possi- ble faulty commutation between the gear wheel 12 and the valve 4.

Two cases, in which such a secondary flow connection be- tween two neighbouring chambers is provided, are described by means of the Figs. 4 and 5. In all other cases any neighbouring chambers 32, which are both acted upon by the high-pressure connection 23 or the low-pressure connec- tion, are connected with each other via the main flow con- nection.

In the positioning of the gear wheel 12 in relation to the toothed ring 14 shown in Fig. 4, the minimal chamber 34 is completely sealed in relation to the neighbouring chamber 32 by the sealing lines formed between gear wheel 12 and the rollers 16 in question. In this view, the boundary 30 of the large indentation 28 arranged at the minimal cham- ber 34 cuts the two rollers 16 exactly in the sealing lines 36. In this situation, the minimal chamber 34 changes from a contraction to an expansion.

At the beginning of the expansion, the gear wheel is now moved further to one side, so that the two sealing lines 36 are displaced in relation to the boundary 30 in ques- tion. One of the two sealing lines 36 is displaced into the boundary 30 of the large indentation 28, which enables a flow around the sealing line 33. Via the indentation 28, this flow occurs against the rotation direction of the gear wheel 12 into the neighbouring chamber 32. In this way, no fluid is trapped in the recent minimal chamber 34 at the beginning of the expansion, which prevents a cavi- tation.

In the positioning of the gear wheel 12 in relation to the toothed ring 14 shown in Fig. 5, the maximal chamber 37 is also sealed in relation to the neighbouring chambers 32 by

means of two sealing lines. At the same time, the maximal chamber 37 is already connected with the small indenta- tions 29 arranged at its rollers 16. Further, each of these two small indentations 29 has an overlapping area 35 with a recess 25. However, still no connections exist be- tween these recesses 25 and the large indentations 28 of the two neighbouring chambers 32 of the maximal chamber 37. In this situation, the maximal chamber 37 changes from an expansion to a contraction.

At the beginning of the contraction, the gear wheel 12 is moved further. The recess 25 at the maximal chamber 37 in the rotation direction now, additionally to the overlap- ping area 35 with one of the small indentations 29 at the maximal chamber 37, also forms an overlapping area with the large indentation 28 of the neighbouring chamber 32 in the rotation direction. In this way, a secondary flow con- nection occurs between the recent maximal chamber 37 and the neighbouring chamber 21 in the rotation direction.

Thus, no fluid is trapped in the recent maximal chamber 37 at the beginning of the contraction, and pressure peaks can be avoided.

The operating situations shown here for the 9 o'clock po- sition are merely meant as examples. Of course, these op- erating situations occur in connection with all chambers 32 of the gear set 11.

By means of the shown arrangement of the flow areas 25, 28,29 in the front side 27 of the gear wheel 12 and the surface of the housing cover 18, a secondary flow connec- tion can be established in the critical situations de- scribed above, without reducing the contact face between

the gear wheel 12 and the toothed ring 14 or weakening the stability of the gear set appreciably. In this way, the overall load of the hydraulic motor 1 during operation is substantially reduced.