Patent Description:
Furthermore, the invention relates to a hydraulic steering unit comprising such a measuring motor. Document <CIT> describes such a measuring motor and hydraulic steering unit and discloses the preamble of the independent claim.

A measuring motor is used to meter out the volume of a flow of fluid and to transform the volume of fluid passing the measuring motor into an angular displacement. In a hydraulic steering unit the measuring motor is connected to a valving means which usually comprises a spool and a sleeve which are rotatably arranged in a housing in which the measuring motor is also arranged. One of spool and sleeve is connected with a steering wheel and the other of spool and sleeve is connected to the star member of the measuring motor. When the steering wheel is rotated, spool and sleeve are rotated in relation to each other so that some orifices are opened, and other orifices are closed. Hydraulic fluid can then flow to the measuring motor driving the star member in relation to the ring member. The volume of fluid supplied to the measuring motor then flows to a steering motor, for example a steering cylinder. When the necessary amount of hydraulic fluid has been supplied to the steering motor, the star member has been rotated so far that the spool and the sleeve are restored back in their neutral position.

A problem arises that the sealing zones between the star member and the plates cannot be made tight. Thus, there is a leakage from all pressure chambers to the inner bore which is at a lower pressure than the pressure in the pressure chambers. Usually, the inner bore has a tank pressure. In a steering unit the pressure difference over the measuring motor is quite small and all pressure chambers use a relatively large pressure, so that leakage comes from the input side which is connected to the spool/sleeve set and from the output side connected to the steering motor.

The leakage on the output side which is connected to the steering motor will cause a drift of the steering wheel. The term "drift" means that cylinder and steering wheel or other steering input movement deviate from nominal. In other words, the direction indicated by the steering wheel and the direction of the steered wheel are not the same.

The object underlying the invention is to reduce misalignment between a steering member and steered wheels.

This object is solved with a measuring motor as described at the outset in that the star member comprises at least one ring groove in at least one of the first sealing zone and the second sealing zone, wherein the groove is connected to the inner bore via a leakage path and to at least one of the pressure chambers at a time via a connection arrangement.

As mentioned above, in a measuring motor the pressure difference over the measuring motor is usually rather low, i.e. in the magnitude of <NUM> bar or less. Thus, when the groove is connected to a pressure chamber via the connection arrangement, the pressure in the groove is approximately the same as the pressure in the pressure chamber connected to the groove via the connection arrangement. In this way it is possible to reduce the leakage around the measuring motor and to reduce the drift, i.e. the deviation between a steering wheel input and a wheel movement.

In an embodiment of the invention the connection arrangement comprises for each pressure chamber a connection. The connection is formed by one or more flow paths connecting the groove with the pressure chamber. The flow path may have a throttling resistance, so that the flow from the pressure chamber into the groove is kept low.

In an embodiment of the invention the star member opens and closes the connections upon rotation. Thus, the star member can be used for commutating the connections between the pressure chambers and the groove.

In an embodiment of the invention the measuring motor is connected to a commutating device supplying fluid to the pressure chambers in a controlled manner and the star member opens at least one connection of a pressure chamber which is supplied with fluid. Thus, the star member establishes a connection between one of the pressure chambers, for example, the pressure chamber having the highest pressure of all pressure chambers, and the groove. When the star member does not rotate, hydraulic fluid from this pressure chamber can then reach other pressure chambers having a lower pressure and can refill other pressure chambers which have lost hydraulic fluid due to a leakage. In particular, the pressure chamber or pressure chambers connected to a steering motor can be refilled to avoid the drift.

In an embodiment of the invention the star member opens a predetermined number of connections at a time and closes all other connections at the same time. Preferably, the star member opens only one connection at a time. Thus, only one chamber is connected to the groove at a time. However, it is also possible, that the star member opens more than one connections at a time.

In an embodiment of the invention the connection arrangement comprises for each pressure chamber a recess in one of the first plate and the second plate which plate is arranged on the side of the star member having the groove, wherein the groove is circular, and the recesses are arranged on a circle which is eccentric to the groove. The terms "circular" and "circle" are not meant in a mathematically exact sense. However, due to the orbiting movement of the star member in relation to the ring member such an arrangement is a simple way to make sure that one of the pressure chambers, for example, the pressure chamber having the highest pressure, is always connected to the groove and the other pressure chambers are not.

In an embodiment of the invention the circle is concentric with the ring member. This allows for a simple geometric relation.

In an embodiment of the invention the recess is elongated in a radial direction of the ring member. Thus, there is a tolerance range that facilitates the production of the measuring motor.

In an embodiment of the invention the recess is formed by at least two blind bores overlapping each other in radial direction of the ring member. This is a simple way to produce an elongated recess.

In an embodiment of the invention the leakage path rotates together with the star member. Thus, a connection between the groove and the inner bore is always available.

The object is solved with a hydraulic steering unit comprising a measuring motor as described above.

In an embodiment of the invention the steering unit comprises a steering valve, a supply port arrangement connected to the steering valve and a working port having a first working port connected to the steering valve by means of a first working line and a second working port connected to the steering valve by means of a second working line, wherein the measuring motor is arranged in one of the working lines. This has the advantage that the measuring motor is always placed in the same connection to the working ports.

The invention will now be described with reference to the drawing, wherein:.

<FIG> shows schematically a hydraulic steering unit <NUM> in a cross-sectional view. The hydraulic steering unit comprises a measuring motor <NUM>. The measuring motor <NUM> comprises a star member <NUM> and a ring member <NUM> which will be described in more detail with reference to <FIG>. The star member <NUM> and the ring member <NUM> are arranged between a first plate <NUM> and a second plate <NUM>. The first plate <NUM> and the second plate <NUM> are connected with a housing <NUM> of the steering unit <NUM>.

Pressure chambers <NUM> are formed between the star member <NUM>, the ring member <NUM> and the two plates <NUM>, <NUM>.

The hydraulic steering unit <NUM> furthermore comprises a spool <NUM> and a sleeve <NUM>. The sleeve <NUM> is rotatably arranged in the housing <NUM> and the spool <NUM> can be rotated with respect to the sleeve <NUM>. The spool <NUM> can be connected via a connection geometry <NUM> to a steering wheel. The sleeve <NUM> is connected by means of a dog bone <NUM> to the star member <NUM>. The dog bone <NUM> is connected to the sleeve <NUM> by means of a bolt <NUM>. Furthermore, a torsion spring <NUM> is arranged between the connection geometry <NUM> and the dog bone <NUM>.

The star member <NUM> comprises an inner bore <NUM> having an engagement geometry into which a corresponding engagement geometry <NUM> of the dog bone engages. Thus, a rotation of the star member <NUM> is transformed into a rotational movement of the spool <NUM>.

A first sealing zone is formed between a face of the star member <NUM> and the first plate <NUM> and a second sealing zone is formed between the opposite face of the star member <NUM> and the second plate <NUM>.

<FIG> shows a sectional view through the measuring motor <NUM>. The star member <NUM> comprises a first number of outer teeth <NUM> and the ring member <NUM> comprises a second number of inner teeth <NUM>. The second number is one greater than the first number. The pressure chambers <NUM> are formed between the star member <NUM> and the ring member <NUM>, as mentioned above.

When a steering wheel connected to the connection geometry <NUM> is rotated, this rotational movement causes a rotation of the spool <NUM> in relation to the sleeve <NUM>.

Spool <NUM> and sleeve <NUM> form a valve arrangement comprising a number of orifices. When the spool <NUM> is rotated in relation to the sleeve <NUM> some orifices are opened, and other orifices may be closed. The flow of hydraulic fluid can flow from a pressure port (not shown) to a pressure chamber <NUM> having an increasing volume. In <FIG> this is, for example, pressure chamber 8a. Hydraulic fluid of a pressure chamber 8b having a decreasing volume is supplied to a steering motor (not shown). A line <NUM> shows schematically a separation between an input side having pressure chambers <NUM> with an increasing volume and pressure chambers <NUM> having a decreasing volume. This line <NUM> rotates during operation of the hydraulic steering unit <NUM>.

The rotation of the star member <NUM> causes a corresponding rotation of the sleeve <NUM>, so that the sleeve <NUM> is restored to a neutral position in relation to spool <NUM> once the required amount of hydraulic fluid has been supplied to the steering motor.

A leakage in the sealing zones between the star member <NUM> and the plates <NUM>, <NUM> is unavoidable in practice. This is schematically shown in <FIG> wherein the inner bore <NUM> is schematically shown as a tank. Both sides of the measuring motor <NUM> are connected to the inner bore <NUM> via two leakage paths shown as orifices A, B which are schematically shown also in <FIG>. This is the situation in the prior art steering units. Such a situation has the risk that the leakage from the output side which is connected to the measuring motor leads to a drift of the steering wheel, so that the steering wheel moves without a corresponding movement of the steered wheels.

To avoid such a situation or at least reduce this risk, the star member <NUM> is provided with a groove <NUM> in at least one of the sealing zones, in the present embodiment on the side facing the first plate <NUM>. This groove <NUM> is connected to the inner bore <NUM> via a leakage path <NUM> schematically shown as orifice.

Furthermore, the first plate <NUM> comprises a number of elongated recesses <NUM> which can be formed, for example, by means of two or more blind bores overlapping in a radial direction of the first plate <NUM>. The recesses <NUM> are arranged on a circle which has the same center as the first plate <NUM>. The groove <NUM> is also of circular form and has the same center as the star member <NUM>.

When the star member <NUM> rotates, it performs at the same time an orbiting movement in relation to the ring member <NUM>. Thus, the elongated recesses <NUM> form an connection arrangement having a number of conncections <NUM> which corresponds to the number of pressure chambers <NUM>. However, the orbiting movement of the star member <NUM> in relation to the ring member <NUM> has the consequence that at a time only one connection <NUM> connected to one of the pressure chambers, for example a pressure chamber 8a having an increasing volume, is connected to the groove <NUM> while all other connections <NUM> are closed. Thus, for ecxample, the groove <NUM> is connected to the pressure chamber 8a having the highest pressure. It is, however, also possible that more than only one pressure chamber <NUM> is connected to the groove <NUM> at a time. Furthermore, depending on the operating conditions of the measuring motor, it is not always the pressure chamber <NUM> having the highest pressure which is connected to the groove <NUM>.

The function of this construction is schematically illustrated in connection with <FIG> is a schematic diagram showing the positions of the leakage path <NUM> and the connection <NUM> and <FIG> shows these elements in a view similar to <FIG>.

For example, a pressure in the pressure chamber 8a having an increasing volume is present in the groove <NUM>. Since the pressure difference between the input side (pressure chamber 8a) and the output side (pressure chamber 8b) is quite small, only a small flow of fluid can pass the connection <NUM>. Fluid in the groove <NUM> can escape to the inner bore <NUM> via the leakage path <NUM>. However, the leakage path <NUM> usually has a larger throttling resistance than the connection <NUM>. Thus, it is possible that the hydraulic fluid on the input side (pressure chamber 8a) refills the pressure chambers 8b on the output side, so that there is no further discrepancy between the angular position of a steering wheel and an angular position of the steered wheels.

The leakage path <NUM> rotates with the star member <NUM> and a connection between the groove <NUM> and the inner bore <NUM> is always established via the leakage path <NUM>.

In the present embodiment the recesses <NUM> are elongated in radial direction. This is, however, only an option having the advantage that the time of opening and closing of the connection <NUM> can be controlled precisely.

The measuring motor of the kind described can preferably be used in a steering unit that comprises a steering valve, a supply port arrangement connected to the steering valve and a working port having a first working port connected to the steering valve by means of a first working line and a second working port connected to the steering valve by means of a second working line, wherein the measuring motor is arranged in one of the working lines. A steering unit in which the measuring motor is placed in one of the working lines is known, for example, from <CIT>.

Claim 1:
Measuring motor (<NUM>) comprising a star member (<NUM>) having a first number of outer teeth (<NUM>) and an inner bore (<NUM>), a ring member (<NUM>) having a second number of inner teeth (<NUM>), wherein the first number is smaller than the second number, a first plate (<NUM>) and a second plate (<NUM>), wherein the star member (<NUM>) forms a first sealing zone with the first plate (<NUM>) and a second sealing zone with the second plate (<NUM>), and the star member (<NUM>) is arranged to rotate and orbit around an axis of the ring member (<NUM>) and together with the ring member (<NUM>) and the plates (<NUM>, <NUM>) forms pressure chambers (<NUM>), characterized in that the star member (<NUM>) comprises at least one ring groove (<NUM>) in at least one of the first sealing zone and the second sealing zone, wherein the groove (<NUM>) is connected to the inner bore (<NUM>) via a leakage path (<NUM>) and to at least one of the pressure chambers (<NUM>) at a time via a connection arrangement.