Patent Description:
Such a hydraulic transformer is known from <CIT>. Although the known hydraulic transformer is advantageous in terms of friction it has a relatively narrow operating range. The high-pressure connection and the low-pressure connection can be fluidly connected to a high-pressure line and a low-pressure line of a common pressure rail system, whereas the operating pressure connection can be fluidly connected to a load, such as a hydraulic cylinder or motor. A general advantage of a hydraulic transformer is that unlike throttling it operates in a non-dissipative manner. Energy can be recuperated and delivered to other loads or stored in hydropneumatic accumulators, for example.

Another hydraulic transformer is known from <CIT>.

An object of the invention is to provide an improved hydraulic transformer.

This object is accomplished with the hydraulic transformer according to the invention which is characterized in that one of the first to third passages has an opening at the back side of the face plate, one of the first to third passages has an opening at the circumferential outer wall of the face plate and one of the first to third passages has an opening at the circumferential inner wall of the face plate.

An advantage of the hydraulic transformer according to the invention is that the first to third passages have respective openings at different sides of the face plate, which provides the opportunity to rotate the face plate with respect to the housing through a relatively large angle without the risk of short circuiting between the respective openings. This means that the hydraulic transformer has a wide operating range.

In a particular embodiment the hydraulic transformer comprises a main shaft through which the barrel member is rotatable with respect to the housing, which main shaft is mounted in the housing and rotatable about a first axis of rotation, wherein the main shaft has a flange extending perpendicularly to the first axis of rotation, a plurality of pistons including respective spherical piston heads, which pistons are fixed to the flange at equiangular distance about the first axis of rotation and which have centrelines parallel to the first axis of rotation, wherein the barrel member comprises a plurality of separate sleeves within which the respective pistons are movable, thus forming the compression chambers, wherein a bearing surface of the barrel member opposite to its back side supports sleeve bottoms of the sleeves, wherein the barrel member including the sleeves are rotatable about a second axis of rotation which intersects the first axis of rotation by an acute angle such that upon rotating the main shaft and the barrel member including the sleeves each of the pistons moves with respect to the cooperating sleeve between a bottom dead centre and a top dead centre, wherein the barrel ports communicate with the respective compression chambers through barrel member passages in the barrel member and through-holes in the sleeve bottoms. In this embodiment the hydraulic transformer operates according to the so-called floating cup principle which has minimum friction.

The central hole in the face plate may be a through-hole, which allows the central hole to be fluidly connected to adjacent channels through openings at both the front side and the back side of the face plate.

In an embodiment the first passage has an opening at the back side of the face plate, the second passage has an opening at the circumferential outer wall of the face plate and the third passage has an opening at the circumferential inner wall of the face plate. This means that the opening at the back side of the face plate is fluidly connected with the high-pressure connection, the opening at the circumferential outer wall of the face plate is fluidly connected with the low-pressure connection and the opening at the circumferential inner wall of the face plate is fluidly connected with the operating pressure connection.

Preferably, the opening at the back side of the face plate is aligned with the cooperating face plate port and preferably has substantially the same shape and dimensions as the cooperating face plate port, since this minimizes a difference of pressure fields at opposite sides of the face plate, which in turn minimizes friction between the face plate and one of the barrel member and the housing. As a consequence, rotation of the face plate with respect to the housing requires relatively low power.

In an embodiment, the one of the first to third passages which has an opening at the circumferential outer wall of the face plate also has an opening at the back side of the face plate which is aligned with the cooperating face plate port and closed by an inner wall of the housing and which preferably has substantially the same shape and dimensions as the cooperating face plate port. This minimizes a difference of pressure fields at opposite sides of the face plate at the corresponding face plate port at the front side and the opening at the back side of the face plate. It is noted that under operating conditions hydraulic fluid does not flow, or only a very small leakage, through the opening at the back side of the face plate.

Similarly, in an embodiment, the one of the first to third passages which has an opening at the inner wall of the face plate also has an opening at the back side of the face plate which is aligned with the cooperating face plate port and closed by an inner wall of the housing and which preferably has substantially the same shape and dimensions as the cooperating face plate port. This minimizes a difference of pressure fields at opposite sides of the face plate at the corresponding face plate port at the front side and the opening at the back side of the face plate. It is noted that under operating conditions hydraulic fluid does not flow, or only a very small leakage, through the opening at the back side of the face plate.

The main shaft may be a hollow main shaft of which an internal space communicates via a central hole in the barrel member with the central hole of the face plate. Preferably, one of the central hole of the barrel member and the main shaft is provided with an inner sleeve including a spherical outer surface portion and the other one of the central hole of the barrel member and the main shaft is provided with an outer sleeve including a circular cylindrical inner surface which fits around the spherical outer surface portion of the inner sleeve. This allows the central hole to be fluidly connected to a channel at a distance from the face plate in a direction which is directed away from its front side. The cooperating inner and outer sleeves may create a sealed connection between the central hole in the barrel member and the internal space of the main shaft.

In an embodiment the arc length of the face plate port which communicates with the high-pressure connection is larger than the arc length of the face plate port which communicates with the low-pressure connection and larger than the arc length of the face plate port which communicates with the operating pressure connection. This provides the opportunity to create a wide operating range.

The arc length of the face plate port which communicates with the operating pressure connection may be smaller than the arc length of the face plate port which communicates with the low-pressure connection.

In an embodiment, in rotational direction about the second axis of rotation the face plate ports are separated by respective sealing lands, wherein at least one of the sealing lands is provided with a through-hole between the front side and the back side of the face plate which has a flow-through area that is smaller than <NUM>% of the flow-through area of the first passage and which widens in a direction from the front side to the back side of the face plate, wherein said one of the sealing lands preferably has a plurality of through-holes in circumferential direction of the face plate. Under operating conditions, the relatively small and widening through-hole generates a force on the face plate which counteracts a pressure field on the corresponding sealing land on the front side of the face plate. This leads to minimized friction between the face plate and the housing, hence facilitating rotation of the face plate. The flow-through area of the through-hole at the front side of the face plate may be smaller than the flow-through area of each of the barrel ports that passes the through-hole.

The flow-through area at the back side of the face plate may be formed by a pocket. It is noted that although using the term flow-through area, under operating conditions hydraulic fluid does not flow, or only a very small leakage, through the flow-through area at the back side of the face plate.

In a particular embodiment, the flange including the pistons is a first flange including first pistons and the main shaft is provided with a second flange extending perpendicularly to the first axis of rotation, wherein the hydraulic transformer is provided with a plurality of second pistons including respective spherical piston heads, which second pistons are fixed to the second flange at equiangular distance about the first axis of rotation and which have centrelines parallel to the first axis of rotation, wherein the barrel member including the sleeves is a first barrel member including first sleeves and the compression chambers are first compression chambers, wherein the hydraulic transformer is provided with a plurality of separate second sleeves within which the respective second pistons are movable, wherein the hydraulic transformer comprises a second barrel member which is rotatable with respect to the housing and comprises a plurality of separate second sleeves within which the respective second pistons are movable, thus forming respective second compression chambers, wherein a bearing surface of the second barrel member opposite to a back side thereof supports sleeve bottoms of the second sleeves, wherein the second barrel member including the second sleeves are rotatable about a third axis of rotation which intersects the first axis of rotation by an acute angle such that upon rotating the main shaft and the second barrel member including the second sleeves each of the second pistons moves with respect to the cooperating second sleeve between a bottom dead centre and a top dead centre, wherein the back side of the second barrel member includes second barrel ports which communicate through barrel member passages in the second barrel member and through-holes in the sleeve bottoms of the second sleeves with the respective second compression chambers, wherein the face plate is a first face plate and the hydraulic transformer is provided with a second face plate which is supported by the housing and provided with a front side that supports the back side of the second barrel member, a back side that faces the housing, a circumferential outer wall and a central hole that is surrounded by a circumferential inner wall, which second face plate is rotatable with respect to the housing about the third axis of rotation within a predetermined angle, wherein the face plate ports are first face plate ports and the front side of the second face plate is provided with three arcuate second face plate ports along which the second barrel ports travel upon rotating the main shaft and the second barrel member including the second sleeves, wherein the second face plate ports communicate with the high-pressure connection through a first passage in the second face plate, the low-pressure connection through a second passage in the second face plate and the operating pressure connection through a third passage in the second face plate, respectively, wherein one of the first to third passages in the second face plate has an opening at the back side of the second face plate, one of the first to third passages in the second face plate has an opening at the circumferential outer wall of the second face plate and one of the first to third passages in the second face plate has an opening at the circumferential inner wall of the second face plate. When the first pistons and the second pistons are projecting from the first and second flanges, respectively, in opposite directions, the forces on the first and second pistons counteract through the main shaft.

In case of a hollow main shaft, its internal space may communicate with the central hole of the second face plate, which means that it communicates with the central holes of both the first and second face plates.

The first and second face plates may be mechanically coupled to each other through an auxiliary shaft which extends through the internal space of the main shaft, which allows the first and second face plates to be rotated synchronously. The auxiliary shaft may also be a hollow shaft such that hydraulic fluid can also flow through the auxiliary shaft.

In the event that the main shaft is provided with the first and the second flange, the main shaft may be rotatably mounted in the housing through a bearing between the first and second flanges.

The invention will hereafter be elucidated with reference to very schematic drawings showing an embodiment of the invention by way of example.

<FIG> show an outer side of an embodiment of a hydraulic transformer <NUM> according to the invention. The hydraulic transformer <NUM> has a housing <NUM>, which is provided with a high-pressure connection <NUM>, a low-pressure connection <NUM> and two operating pressure connections <NUM>. The housing <NUM> forms an assembly of separate elements in order to fit parts inside the housing <NUM>. The hydraulic transformer <NUM> may be part of a hydraulic circuit including a high-pressure line which communicates with the high-pressure connection <NUM> and a low-pressure line which communicates with the low-pressure connection <NUM>. The operating pressure connections <NUM> may communicate with a hydraulic cylinder or a hydraulic motor which can be operated by the hydraulic transformer <NUM> through adjusting hydraulic pressure at the operating pressure connections <NUM>. The flow direction of hydraulic fluid from and to the operating pressure connections <NUM> is controlled by valves which will be described later with reference to <FIG>.

<FIG> show an interior side of the hydraulic transformer <NUM>. The hydraulic transformer <NUM> has a hollow main shaft <NUM> which is supported by the housing <NUM> through a pair of angular contact roller bearings <NUM>. The main shaft <NUM> is rotatable with respect to the housing <NUM> about a first axis of rotation <NUM>. The bearings <NUM> are located at a central portion of the housing <NUM> as seen in longitudinal direction of the first axis of rotation <NUM>. The bearings <NUM> have fixed positions with respect to the housing <NUM> in longitudinal direction of the first axis of rotation <NUM> by means of a collar <NUM> of the housing <NUM> and a sleeve <NUM> between which circumferential outer edges of the bearings <NUM> are sandwiched.

The main shaft <NUM> is provided with a first flange <NUM> and a second flange <NUM>, which extend perpendicularly to the first axis of rotation <NUM> at either side of the bearings <NUM>. Circumferential inner edges of the bearings <NUM> are sandwiched between the first and second flanges <NUM>, <NUM>. The second flange <NUM> is mounted as a separate element onto the main shaft <NUM> after placing the bearings <NUM> next to the first flange <NUM>. A key <NUM> locks the second flange <NUM> with respect to the main shaft <NUM> in rotational direction about the first axis of rotation <NUM>. The second flange <NUM> has a toothed circumference which is used for rotational speed detection, see <FIG>.

At each of the first and second flanges <NUM>, <NUM> a plurality of pistons <NUM> are fixed at equiangular distance about the first axis of rotation <NUM>, in this case fifteen pistons <NUM>. The pistons <NUM> project from the respective first and second flanges <NUM>, <NUM> away from each other. The pistons <NUM> have centre lines which extend parallel to the first axis of rotation <NUM>. In this case they are fixed to the respective first and second flanges <NUM>, <NUM> by screws, but alternative fixing means are conceivable.

Each of the pistons <NUM> cooperates with a separate sleeve <NUM> to form a compression chamber of variable volume. In this case the hydraulic transformer <NUM> has <NUM> compression chambers. Each of the sleeves <NUM> comprises a sleeve bottom <NUM> including a central through-hole <NUM> and a circular-cylindrical sleeve jacket <NUM>, see <FIG>. The sleeve jacket <NUM> extends from the sleeve bottom <NUM>. Each piston <NUM> is sealed directly to the inner wall of the cooperating sleeve jacket <NUM> through a piston head of the piston <NUM> which has a spherical outer side.

The sleeve bottoms <NUM> are supported by a bearing surface of a first barrel member <NUM> and a bearing surface of a second barrel member <NUM>. The sleeves <NUM> are slidable on the bearing surfaces of the first and second barrel members <NUM>, <NUM>, which is known as the floating cup principle. The first and second barrel members <NUM>, <NUM> are fitted around the main shaft <NUM> by means of respective ball hinges <NUM> and are coupled to the main shaft <NUM> by means of keys <NUM>, see <FIG>. Consequently, the first and second barrel members <NUM>, <NUM> rotate together with the main shaft <NUM> under operating conditions. The first and second barrel members <NUM>, <NUM> are the same.

<FIG> shows the first barrel member <NUM> in more detail. The first barrel member <NUM> has a front side <NUM> and a back side <NUM>. The back side <NUM> is opposite to the front side <NUM> and the bearing surface. The front side <NUM> is directed to the first flange <NUM> and the back side <NUM> is directed away from the first flange <NUM>. In order to keep the sleeves <NUM> in place during assembling the first barrel member <NUM> is provided with an inner ring <NUM> and an outer ring <NUM>. Each compression chamber communicates via the through-hole <NUM> in the sleeve bottom <NUM> and a barrel member passage with a barrel port <NUM> at the back side <NUM> of the first barrel member <NUM>. In this case each of the barrel ports <NUM> comprises a pair of openings at the back side <NUM> of the first barrel member <NUM>.

Referring to <FIG>, the hydraulic transformer <NUM> has a first face plate <NUM> which is sandwiched between the housing <NUM> and the first barrel member <NUM> as well as a second face plate <NUM> which is sandwiched between the housing <NUM> and the second barrel member <NUM>. The first and second face plates <NUM>, <NUM> are functionally the same, but mirrored with respect to a plane that extends perpendicularly to the first axis of rotation <NUM> and between the first and second face plates <NUM>, <NUM>. In the embodiment as shown in the figures, the first face plate <NUM> comprises a front part 28a and a back part 28b which have fixed positions with respect to each other. The first part 28a and the back part 28b may be made of different materials, but alternatively the first face plate <NUM> may be a single part. Similarly, the second face plate <NUM> has a front part 29a and a back part 29b. Hereinafter, the first and second face plates <NUM>, <NUM> will be described as if they are single parts, respectively.

<FIG> shows a front side <NUM> of the second face plate <NUM> which is directed to the back side <NUM> of the second barrel member <NUM>. <FIG> shows a back side <NUM> of the second face plate <NUM> which is directed to a wall of the housing <NUM>. The wall of the housing <NUM> supports the second face plate <NUM> which in turn supports the second barrel member <NUM>. Similarly, an opposite wall of the housing <NUM> supports the first face plate <NUM> which in turn supports the first barrel member <NUM>.

The housing <NUM> is adapted such that the first and second face plates <NUM>, <NUM> have an inclined orientation such that the first barrel member <NUM> and the second barrel member <NUM> are rotatable about a second axis of rotation 32a and a third axis of rotation 32b, respectively, which second axis of rotation 32a and third axis of rotation 32b are angled by acute angles with respect to the first axis of rotation <NUM>. As a consequence, the first and second barrel members <NUM>, <NUM> pivot about the respective ball hinges <NUM> during rotation with the main shaft <NUM>. The sleeves <NUM> rotate about the respective second and third axes of rotation 32a, 32b. Consequently, upon rotating the main shaft <NUM> the volumes of the compression chambers change. The angle between the first axis of rotation <NUM> and the second axes of rotation 32a and between the first axis of rotation <NUM> and the third axis of rotation 32b is approximately eight degrees in practice, but may be smaller or larger.

The first and second barrel members <NUM>, <NUM> are pressed against the first and second face plates <NUM>, <NUM>, respectively by means of springs <NUM> which are mounted in holes in the main shaft <NUM> and which press respective cheeks <NUM> against the first and second barrel members <NUM>, <NUM>, see <FIG>.

During rotation of the first and second barrel members <NUM>, <NUM> each sleeve <NUM> makes a combined translating and swivelling motion around the cooperating piston <NUM>. Therefore, the outer side of each piston head is spherical. The spherical shape creates a sealing line between the piston head and the sleeve jacket <NUM> which extends perpendicularly to a centre line of the cooperating sleeve <NUM>. The diameter of each piston <NUM> near the corresponding first or second flange <NUM>, <NUM> is smaller than at the piston head in order to allow the relative motion of the cooperating sleeves <NUM> about the pistons <NUM>. Under operating conditions each of the pistons <NUM> moves inside the cooperating sleeve <NUM> between a bottom dead centre and a top dead centre. In the drawing of <FIG> the upper pistons <NUM> at the first and second barrel members <NUM>, <NUM> are in top dead centre and the lower pistons <NUM> at the first and second barrel members <NUM>, <NUM> are in bottom dead centre.

Referring to <FIG>, the second face plate <NUM> is provided with a high-pressure passage <NUM>, a low-pressure passage <NUM> and an operating pressure passage <NUM>. The high-pressure passage <NUM> comprises eight through-holes and forms a high-pressure port at its front side <NUM> and a high-pressure port at its back side <NUM>. The respective high-pressure ports are partly circular and extend within an angle of about <NUM>° about the third axis of rotation 32b, but a different arc length is conceivable. The low-pressure passage <NUM> comprises four through-holes and forms a low-pressure port at its front side <NUM> and a low-pressure port at its back side <NUM>. The low-pressure port is partly circular and is shorter than the length of the high-pressure port, as measured in rotational direction about the third axis of rotation 32b, both at the front side <NUM> and the back side <NUM>. The operating pressure passage <NUM> comprises three through-holes and forms an operating pressure port at its front side <NUM> and an operating pressure port at its back side <NUM>. The operating pressure port is partly circular and is shorter than the length of the low-pressure port, as measured in rotational direction about the third axis of rotation 32b, both at the front side <NUM> and the back side <NUM>.

Upon rotating the main shaft <NUM> the barrel ports <NUM> of the second barrel member <NUM> travel along the high-pressure port, the low-pressure port and the operating pressure port at the front side <NUM> of the second face plate <NUM>. Similarly, upon rotating the main shaft <NUM> the barrel ports <NUM> of the first barrel member <NUM> travel along the high-pressure port, the low-pressure port and the operating pressure port at the front side <NUM> of the first face plate <NUM>.

The high-pressure passages <NUM> of the first and second face plates <NUM>, <NUM> communicate with the high-pressure connection <NUM> of the housing <NUM>, the low-pressure passages <NUM> of the first and second face plates <NUM>, <NUM> communicate with the low-pressure connection <NUM> of the housing <NUM> and the operating pressure passages <NUM> of the first and second face plates <NUM>, <NUM> communicate with the two operating pressure connections <NUM> of the housing <NUM>.

The first and second face plates <NUM>, <NUM> are rotatable with respect to the housing <NUM> through a predetermined angle α about the second and third axes of rotation 32a, 32b, respectively, for example <NUM>°. In the embodiment as shown the second face plate <NUM> is rotatable by a servo-actuator <NUM>, whereas the first face plate <NUM> is synchronously rotatable with the second face plate <NUM> by means of a hollow auxiliary shaft <NUM> which couples the first and second face plates <NUM>, <NUM> to each other through pins <NUM>, see <FIG>.

<FIG> shows an inner wall of the housing <NUM> which supports the back side <NUM> of the second face plate <NUM>. The inner wall is provided with a partly circular high-pressure port <NUM> which communicates with the high-pressure connection <NUM> of the housing <NUM>. <FIG> illustrates two extreme rotational positions of the second face plate <NUM> with respect to the housing <NUM> about the third axis of rotation 32b by means of a projection A of the high-pressure port of the high-pressure passages <NUM> at the back side <NUM> of the second face plate <NUM>, a projection T of the low-pressure port of the low-pressure passages <NUM> at the back side <NUM> of the second face plate <NUM> and a projection B of the operating pressure port of the operating pressure passages <NUM> at the back side <NUM> of the second face plate <NUM> by respective broken lines. In both extreme positions and between both extreme positions the high-pressure port <NUM> of the housing <NUM> communicates with the high-pressure port at the back side <NUM> of the second face plate <NUM>. It can be seen that the arc length of the high-pressure port <NUM> of the housing <NUM> is shorter than the arc length of the high-pressure port at the back side <NUM> of the second face plate <NUM>. This means that a part of the high-pressure port at the back side <NUM> of the second face plate <NUM> is always closed by the wall of the housing <NUM>. In order to create sufficient flow area between the second barrel member <NUM> and the high-pressure port <NUM> of the housing <NUM> the second face plate <NUM> can be provided with channels <NUM> between non-adjacent high-pressure passages <NUM>, see <FIG>. The channels <NUM> are drilled from a circumferential outer side of the second face plate <NUM>, whereas inserts <NUM> seal the respective channels <NUM>. The first face plate <NUM> and the wall of the housing <NUM> that supports the first face plate <NUM> are similar to the second face plate <NUM> and the wall of the housing <NUM> that supports the second face plate <NUM>.

The housing <NUM> is provided with a high-pressure channel <NUM> through which the high-pressure ports at the back sides <NUM> of the respective first and second face plates <NUM>, <NUM> communicate with the high-pressure connection <NUM> of the housing <NUM>. This is shown in <FIG> which illustrates by broken lines and arrows that hydraulic fluid can flow in opposite directions through the high-pressure channel <NUM>.

Referring to <FIG>, the low-pressure channels <NUM> are formed by through-holes between the front side <NUM> and the back side <NUM> of the second face plate <NUM> and radial channels <NUM> which are drilled from the circumferential outer side of the second face plate <NUM> to the respective through-holes. In assembled condition the through-holes are closed at the back side <NUM> of the second face plate <NUM> by the wall of the housing <NUM> as shown in <FIG> by projection T. This means that the low-pressure channels <NUM> form <NUM> degree bends in the first and second face plates <NUM>, <NUM>. The low-pressure channels <NUM> communicate with the low-pressure connection <NUM> through a low-pressure channel <NUM>, which is formed by an internal space of the housing <NUM>. <FIG> shows flow patterns of hydraulic fluid through the low-pressure channel <NUM> by broken lines and arrows, indicating that the hydraulic fluid can flow in opposite directions.

Referring again to <FIG>, the operating pressure channels <NUM> are formed by through-holes between the front side <NUM> and the back side <NUM> of the second face plate <NUM> and radial holes <NUM> which are drilled from the circumferential outer side of the second face plate <NUM> to a central hole <NUM> in the second face plate <NUM> whereas passing the respective through-holes. The central hole <NUM> is a through-hole. Inserts <NUM> seal the respective radial holes <NUM> at the circumference of the second face plate <NUM>. In assembled condition the through-holes of the operating pressure channels <NUM> are closed at the back side <NUM> of the second face plate <NUM> by the wall of the housing <NUM> as shown in <FIG> by projection B. This means that the operating pressure channels <NUM> form <NUM> degree bends in the first and second face plates <NUM>, <NUM>.

The operating pressure channels <NUM> communicate with the operating pressure connections <NUM> through an operating pressure channel <NUM>, which is formed by the central holes <NUM> of the first and second face plates <NUM>, <NUM>, central holes in the first and second barrel members <NUM>, <NUM> and the internal space of the main shaft <NUM>, see <FIG>. The operating pressure channel <NUM> is sealed with respect to the low-pressure channel <NUM>, i.e. the internal space of the housing <NUM>, by means of inner sleeves <NUM> and cooperating outer sleeves <NUM>, see <FIG>. The outer sleeves <NUM> are mounted to the first and second barrel members <NUM>, <NUM>, respectively. They may be slidable with respect to the first and second barrel members <NUM>, <NUM> along the second and third axes of rotation 32a, 32b, respectively. Each of the inner sleeves <NUM> is fixed to the main shaft <NUM> and has a spherical outer surface portion which contacts a circular cylindrical inner wall of the cooperating outer sleeve <NUM>. The spherical outer surface portions may have centre points which coincide with centre points of the respective ball hinges <NUM>. <FIG> shows flow patterns of hydraulic fluid through the operating pressure passages <NUM> and the operating pressure channel <NUM> by broken lines and arrows. Although not shown in <FIG>, the hydraulic fluid can also flow in opposite directions.

Referring to <FIG>, the high-pressure port at the front side <NUM> of the second face plate <NUM> and the high-pressure port at the back side <NUM> of the second face plate <NUM> are aligned and have substantially the same shape and dimensions. Similarly, the low-pressure port at the front side <NUM> of the second face plate <NUM> and the low-pressure port at the back side <NUM> of the second face plate <NUM> are aligned and have substantially the same shape and dimensions. Similarly, the operating pressure port at the front side <NUM> of the second face plate <NUM> and the operating pressure port at the back side <NUM> of the second face plate <NUM> are aligned and have substantially the same shape and dimensions.

<FIG> shows that the second face plate <NUM> is provided with series of small through-holes <NUM> at sealing lands between the high-pressure port and the low-pressure port, between the low-pressure port and the operating pressure port and between the high-pressure port and the operating pressure port, as seen in angular direction about the third axis of rotation 32b. The flow-through area of each through-hole <NUM> at the front side <NUM> of the second face plate <NUM> is smaller than <NUM>% of the flow-through area of the first passage <NUM> and also smaller than the flow-through area of each of the barrel ports <NUM> of the second barrel member <NUM> which travel along the through-holes <NUM>.

<FIG> shows that at the back side <NUM> of the second face plate <NUM> the through-holes <NUM> form pockets <NUM> which have a larger cross-sectional area than the through-holes <NUM> at the front side <NUM> of the second face plate <NUM>. Under operating conditions the barrel ports <NUM> of the second barrel member <NUM> travel along the through-holes <NUM>, hence generating pressure fields at the front side <NUM> of the second face plate <NUM> between the high-pressure port and the low-pressure port, between the low-pressure port and the operating pressure port and between the high-pressure port and the operating pressure port. Due to the presence of the through-holes <NUM> and the pockets <NUM> counteracting pressure fields are generated at the back side <NUM> of the second face plate <NUM>. This leads to minimized friction between the second face plate <NUM> and the housing <NUM>, which facilitates adjustment of the rotational position of the second face plate <NUM> with respect to the housing <NUM>. It is noted that the first face plate <NUM> is also provided with the through-holes <NUM> and the pockets <NUM>.

As already described hereinbefore the second face plate <NUM> is rotatable by a servo-actuator <NUM> which needs to overcome torque load of the first and second face plates <NUM>, <NUM>. Referring to <FIG>, the servo-actuator <NUM> is coupled to an electric servomotor <NUM> which controls a control shaft <NUM> including ports which communicate with the low-pressure channel <NUM> and the high-pressure channel <NUM>, respectively. The hydraulic servo-actuator <NUM> has a rotor and a stator, each having three ribs which form six displacement chambers. The pressures in these chambers are controlled by means of the rotational position of the control shaft <NUM>. The actual rotational position of the second face plate <NUM> with respect to the housing <NUM> is determined by a position sensor <NUM>.

The angle of rotation of the first and second face plates <NUM>, <NUM> is defined by respective arcuate grooves <NUM> in their back sides <NUM>, see <FIG> and <FIG>, in which grooves <NUM> pins <NUM> are received, see <FIG>. The pins <NUM> may be bolts which are screwed in the housing <NUM>.

<FIG> illustrates a hydraulic circuit in and around the hydraulic transformer <NUM>. The operating pressure connections <NUM> are coupled to a hydraulic cylinder which can be extended and retracted through control valves <NUM>, which are also indicated in <FIG> and <FIG>. The high-pressure connection <NUM> communicates with a high-pressure line HP and the low-pressure connection <NUM> communicates with a low-pressure line LP. The hydraulic transformer <NUM> also has two check valves <NUM> for avoiding cavitation, which check valves <NUM> are also indicated in <FIG> and <FIG>.

<FIG> and <FIG> show that the low-pressure channel <NUM> is fluidly connected with the check valves <NUM> and the control valves <NUM>. <FIG> shows that the operating pressure channel <NUM> is fluidly connected with the control valves <NUM>. The control valves <NUM> can be operated such that hydraulic fluid flows from the operating pressure channel <NUM> to a lower side of the hydraulic cylinder and from an upper side of the hydraulic cylinder to the low-pressure channel <NUM> for extending the hydraulic cylinder. Similarly, the control valves <NUM> can be operated such that hydraulic fluid flows from the lower side of the hydraulic cylinder to the operating pressure channel <NUM> and from the low-pressure channel <NUM> to the upper side of the hydraulic cylinder for retracting the hydraulic cylinder.

If, for example, an external load acts on the hydraulic cylinder during retracting it, the load may force the hydraulic fluid to flow through the operating pressure channel <NUM>, which results in transporting compressed hydraulic fluid via the high-pressure channel <NUM> and the high-pressure connection <NUM> to the high-pressure line HP. This means that the energy for retracting the cylinder by the external load is recuperated and converted to hydraulic pressure in the high-pressure line HP. It is noted that the high-pressure line HP and also the low-pressure line LP may be provided with pressure accumulators.

Claim 1:
A hydraulic transformer (<NUM>) comprising a housing (<NUM>) including a high-pressure connection (<NUM>), a low-pressure connection (<NUM>) and an operating pressure connection (<NUM>), a barrel member (<NUM>, <NUM>) which is rotatably mounted in the housing (<NUM>) and provided with a plurality of compression chambers of which the volume changes upon rotating the barrel member (<NUM>, <NUM>), wherein a back side (<NUM>) of the barrel member (<NUM>, <NUM>) includes barrel ports (<NUM>) which communicate with the respective compression chambers and a face plate (<NUM>, <NUM>) which is supported by the housing (<NUM>) and provided with a front side (<NUM>) that supports the back side (<NUM>) of the barrel member (<NUM>, <NUM>), a back side (<NUM>) that faces the housing (<NUM>), a circumferential outer wall and a central hole (<NUM>) that is surrounded by a circumferential inner wall, which face plate (<NUM>, <NUM>) is rotatable with respect to the housing (<NUM>) within a predetermined angle, wherein the front side (<NUM>) is provided with three arcuate face plate ports along which the barrel ports (<NUM>) travel upon rotating the barrel member (<NUM>, <NUM>), wherein the face plate ports communicate with the high-pressure connection (<NUM>) through a first passage (<NUM>) in the face plate (<NUM>, <NUM>), the low-pressure connection (<NUM>) through a second passage (<NUM>) in the face plate (<NUM>, <NUM>) and the operating pressure connection (<NUM>) through a third passage (<NUM>) in the face plate (<NUM>, <NUM>), respectively, and wherein one of the first to third passages (<NUM>, <NUM>, <NUM>) has an opening at the back side (<NUM>) of the face plate (<NUM>, <NUM>), characterized in that one of the first to third passages (<NUM>, <NUM>, <NUM>) has an opening at the circumferential outer wall of the face plate (<NUM>, <NUM>) and one of the first to third passages (<NUM>, <NUM>, <NUM>) has an opening at the circumferential inner wall of the face plate (<NUM>, <NUM>).