Patent Description:
More specifically, the invention concerns improvements to variable geometry assembly used in such turbomachines to adjust the flow of a fluid processed through the turbomachine.

<CIT> discloses a turbine with a variable inlet nozzle geometry.

Radial turboexpanders and turbines are commonly used turbomachines generating useful mechanical power from a flow of pressurized gas. Centripetal expanders and turbines are used to convert pressure energy and heat of a gaseous flow into mechanical energy to drive a load. Centripetal turbines are used for instance in turbocharges for internal combustion engines. Exhaust combustion gas from the combustion chamber of the internal combustion engine is expanded in the centripetal turbine, which in turn drives an intake air compressor. This latter boosts the pressure of intake air before it is mixed with fuel and the air-fuel mixture is finally combusted in the combustion chamber of the internal combustion engine. Turbochargers are nowadays commonly used in automotive applications, as well as in naval engines, to increase the power delivered by the internal combustion engine.

Radial turboexpanders and turbines are also used in applications other than supercharging of internal combustion engines. radial turboexpanders or turbines are used to drive electric generators or another rotary loads, such as industrial compressors or pumps.

It is often desirable to control the flow of the working gas processed through the radial turbine or turboexpander, to improve efficiency thereof or to control the operation of the turbomachine under variable, off-design operating conditions.

Variable geometry turboexpanders and turbines have been configured to address this need. These variable geometry turbomachines are usually provided with variable nozzle guide vanes, shortly termed variable NGV. The variable nozzle guide vanes are positioned annularly around a turbine inlet. Each vane is hinged around a respective pivoting axis, which is usually parallel to the rotation axis of the turbine impeller. Each pivoting vane is coupled by means of a lever to a rotating ring. The rotation of the ring by means of an actuator causes the vanes to simultaneously pivot around the respective pivoting axes, thus adjusting the throat area of flow passages formed between adjacent vanes, such as to control the working fluid flow through the turbine.

Centrifugal compressors are driven machines used to boost the pressure of a working gas from a suction pressure to a delivery pressure. One or more impellers are arranged for rotation in a casing and are driven into rotation by a driver, such as a gas turbine or an electric motor. Kinetic energy is delivered by the vanes of the rotating impeller to the gas flowing therethrough, such that the gas is accelerated through the impeller. The kinetic energy of the gas radially exiting the impeller is converted into pressure energy in a diffuser arranged annularly around the centrifugal outlet of the impeller. Some known centrifugal compressors are provided with vaned diffusers, i.e. diffusers wherein vanes are arranged, to improve efficiency of the turbomachine.

Variable diffuser vanes are sometimes used to improve efficiency of centrifugal compressor, which is required to operate under variable operating conditions. Similarly to variable nozzle guide vanes, variable diffuser vanes are mounted for rotation around respective pivoting axes. An actuating ring, whereto the variable diffuser vanes are linked through respective levers, causes the vanes to simultaneously pivot around the respective pivoting axes, thus controlling the gas flow passage and adjust the geometry thereof to variable operating conditions.

These known variable geometry mechanisms are complex to manufacture and difficult to assemble, due to the large number of components, they are formed of. Vanes, levers and other multiple connections linking the vanes to the actuator are prone to vibrations and failure. Link clearances cause backlash in the kinematic connections between the actuator and the vanes.

A need therefore exists for a more efficient variable geometry member suitable for adjusting the operating conditions of a turbomachine.

According to one aspect, a variable geometry assembly is disclosed, for modulating a fluid flow in a turbomachine such as a centripetal turboexpander or turbine or a centrifugal compressor. The variable geometry assembly comprises a first ring and a second ring. The first ring and the second ring are substantially coaxial to one another. The first ring comprises a plurality of first wedge-shaped elements. The second ring comprises a plurality of second wedge-shaped elements. The first ring and the second ring are angularly displaceable one with respect to the other. Moreover, the first ring and the second ring are configured to move axially with respect to one another when the first ring and the second ring are angularly displaced one with respect to the other. The first wedge-shaped elements and second wedge-shaped elements are configured and arranged to co-act with the one another.

Mutual co-action of the first wedge-shaped elements and second wedge-shaped elements can in general include mutual thrust in an axial direction, i.e. a direction substantially parallel to the ring axis, whereby the first wedge-shaped elements and the second wedge-shaped elements push the ones against the others in the axial direction. The first wedge-shaped elements and the second wedge-shaped elements can be configured to be maintained in mutual sliding contact relationship, e.g. by means of resilient members. The wedge-shaped elements have sliding surfaces, which can be inclined with respect to the rings axis and in the tangential direction, i.e. in a direction of extension of the rings around the axis. The inclination of the sliding surfaces causes the rings to be axially displaced one with respect to the other, i.e. to move closer or to be distanced from one another, as a consequence of the mutual angular displacement thereof.

Flow passages are defined between pairs of sequentially arranged first wedge-shaped elements and second wedge-shaped elements. a flow passage is defined between each pair consisting of one of the first wedge-shaped elements and one of the second wedge-shaped elements, arranged in sequence around the ring axis. The mutual axial and angular displacements of the first ring and second ring determine a variation of the cross-section of said flow passages, to modulate a fluid flow through the variable geometry assembly.

Flow modulation across the variable geometry assembly is thus obtained without the need for pivoting vanes. A simple, reliable and efficient flow modulating device is thus obtained.

The first sliding surfaces and the second sliding surfaces of the first wedge-shaped elements and second wedge-shaped elements are smooth, such that the first ring and the second ring slide continuously one over the other when the angular displacement therebetween occurs.

The first and second wedge-shaped elements are comprised of airfoil surfaces. Each flow passage is thus formed between a first airfoil surface formed on the respective first wedge-shaped element and a second airfoil surface formed on the respective second wedge-shaped element.

In some embodiments, the first airfoil surface and the second airfoil surface can be configured to match with one another such as to close the respective flow passage formed therebetween, such that fluid flow can be entirely prevented when the first ring and second ring are in a closure position.

The variable geometry assembly can be configured such that fluid flows through the flow passages according to a radially inwardly oriented direction or else in a radially outwardly oriented direction. A radially inwardly directed flow can for instance be generated in case the variable geometry assembly is used in a centripetal turboexpander or turbine. A radially outwardly directed flow can be established in case the variable geometry assembly is used in a centrifugal compressor, and more specifically at the outlet of a centrifugal impeller, forming a vaned diffuser.

An actuator can be functionally coupled to at least one of the first ring and second ring and can be configured for angularly displacing the first ring and the second ring with respect to one another around the axis thereof.

Resilient members can be further provided to elastically bias the first ring and the second ring one against the other, such as to maintain the sliding surfaces in mutual contact with one another, for instance.

According to a further aspect, a turbomachine such as a centripetal turboexpander or turbine or a centrifugal compressor is disclosed herein, which includes a variable geometry assembly as described above. Said turbomachine can comprise a casing and an impeller arranged in the casing for rotation around a rotation axis. Advantageously, the impeller is substantially coaxial to the first ring and second ring. A radially oriented fluid passage can be provided in fluid communication with the impeller. The first ring and the second ring can be arranged in the radially oriented fluid passage.

One of the first ring and second ring of the variable geometry assembly can be axially constrained to the casing and angularly displaceable with respect to the casing, around the rotation axis of the impeller. The other ring can be angularly constrained to the casing and axially displaceable with respect to the casing in a direction parallel to the rotation axis of the impeller. In this way the mutual angular and axial displacements are distributed such that one ring is displaceable only in the angular direction, while the other ring is displaceable only in the axial direction.

Embodiments disclosed herein also concern a variable geometry turbomachines such as a centripetal turboexpander or turbine or a centrifugal compressor, which comprises a casing, at least one impeller arranged in the casing for rotation around a rotation axis and a variable geometry member or variable geometry assembly, arranged in a radially oriented fluid passage, in fluid communication with the impeller. The variable geometry assembly comprises a first ring and a second ring arranged substantially coaxial to the impeller. The first ring can comprise a plurality of first wedge-shaped elements facing the second ring, and the second ring can comprise a plurality of second wedge-shaped elements facing the first ring. Each first wedge-shaped element can comprise a first sliding surface in sliding contact with a respective second sliding surface of a corresponding one of said second wedge-shaped elements. The first ring and the second ring are angularly and axially displaceable with respect to one another. As understood herein, an angular displacement is a rotation around the rotation axis of impeller of the turbomachine. As understood herein, an axial displacement is a displacement in a direction substantially parallel to rotation axis of the impeller.

Features and embodiments are disclosed here below and are further set forth in the appended claims, which form an integral part of the present description. The above brief description sets forth features of the various embodiments of the present invention in order that the detailed description that follows may be better understood and in order that the present contributions to the art may be better appreciated. There are, of course, other features of the invention that will be described hereinafter and which will be set forth in the appended claims.

The following detailed description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Additionally, the drawings are not necessarily drawn to scale. Also, the following detailed description does not limit the invention. Instead, the invention is defined by the appended claims.

<FIG> and <FIG> illustrate sectional views of an exemplary variable geometry turbomachine according to the present disclosure. In this exemplary embodiment the turbomachine is a centripetal turboexpander, forming part of a turboexpander and generator unit. Mechanical power generated by the turboexpander is used to rotate an electric generator, which converts the mechanical power into electric power.

In other embodiments, the turboexpander can be drivingly coupled to a different rotating load, e.g. to a compressor or a pump. In some embodiments the turboexpander can be used to drive a compressor of a turbocharger for an internal combustion engine. Referring now to <FIG> and <FIG>, a turboexpander-generator unit <NUM> is comprised of a turboexpander <NUM> and an electric generator <NUM>. The turboexpander-generator unit <NUM> comprises a turboexpander casing <NUM> and a generator casing <NUM>. The turboexpander casing <NUM> and the generator casing <NUM> can be rigidly coupled to one another to form a single body. The turboexpander-generator unit <NUM> can be used to convert stored energy of a process gas into electric energy. Turboexpander-generator units can be used in various applications, whenever compressed gas is available as a source of energy to drive the turboexpander. Possible applications of the turboexpander-generator unit <NUM> are in compressed air energy storage systems (CAES systems), waste gas energy recovery systems.

(WGER systems), pressure letdown stations (PLD stations), gas liquefaction systems, organic Rankine cycles (ORC), and the like.

The turboexpander casing <NUM> can comprise a gas inlet <NUM> and an axial gas outlet <NUM>. The gas inlet <NUM> is fluidly coupled to an inlet plenum <NUM>. The turboexpander <NUM> further comprises an impeller <NUM> arranged for rotation in the turboexpander casing <NUM> around a rotation axis A-A. The impeller <NUM> comprises a hub <NUM> and a plurality of blades or vanes <NUM> rigidly mounted on the hub <NUM> and extending therefrom. Gas flows (arrow F) through the impeller <NUM> in a centripetal direction, from a substantially radially oriented impeller inlet 15A, to a substantially axially oriented impeller outlet 15B (<FIG>). While flowing through the impeller <NUM>, the gas expands from an inlet pressure P1 to a discharge pressure P2. The enthalpy drop of the gas through the impeller <NUM> is converted into mechanical power, which drives the impeller <NUM> into rotation around rotation axis A-A.

The impeller <NUM> is mounted on a shaft <NUM>, which extends into the generator casing <NUM> and supports a rotor <NUM> of the electric generator <NUM>. The rotor <NUM> is arranged coaxially in a stator <NUM> and is driven into rotation by the mechanical power generated by the turboexpander <NUM>.

To control the operating conditions of the turboexpander <NUM>, a variable geometry assembly is provided. The variable geometry assembly is mainly comprised of a variable geometry member <NUM> arranged around the rotation axis A-A, between the inlet plenum <NUM> and the impeller inlet 15A. The variable geometry member <NUM> is configured to adjust a flow passage between the inlet plenum <NUM> and the impeller <NUM>, such as to adapt the gas flow rate flowing through the turboexpander <NUM> to variable operating conditions of the turboexpander.

With continuing reference to <FIG> and <FIG>, details of the variable geometry member <NUM> will now be described, reference being made to <FIG>.

The variable geometry member <NUM> can comprise a first ring <NUM> and a second ring <NUM>. The first ring <NUM> and the second ring <NUM> are arranged substantially coaxially to one another and to the impeller <NUM>, as shown in <FIG>. In <FIG>, <FIG>, <FIG> the first ring <NUM> and the second ring <NUM> are shown in a somewhat simplified fashion, limited to the main elements thereof, which are actually used to adjust the gas flow passage, while additional structural details are shown in <FIG> only.

The first ring <NUM> has opposite first side 33A and second side 33B. The second side 33B faces the second ring <NUM> (see <FIG>). The second ring <NUM> has in turn a first side 35A and a second side 35B, this latter facing the second side 33B of the first ring <NUM>.

As best show in <FIG> and <FIG>, the first ring <NUM> is provided with first wedge-shaped elements <NUM>, facing the second ring <NUM>. In the exemplary embodiment illustrated in the accompanying drawings, the first ring <NUM> is provided with five wedge-shaped elements <NUM>, but as will become clear from the following description, the number of wedge-shaped elements can be different. Each first wedge-shaped element <NUM> projects from the side 33B of the first ring <NUM> and faces the second ring <NUM>. Each first wedge-shaped element <NUM> is comprised of a respective first sliding surface 37A co-acting with the opposed second ring <NUM> in a manner to be described. Each first sliding surface 37A can be inclined with respect to a planar surface orthogonal to the rotation axis A-A both in the radial direction (as shown in the sectional view of <FIG>) and in the tangential direction (as shown in the axonometric view of <FIG> and in the side view of <FIG>).

Each first wedge-shaped element <NUM> is further comprised of side surfaces 37B and 37C. The side surface 37B is an airfoil surface which partly defines a gas flow passage as will be described later on. The side surface 37C is a substantially cylindrical surface coaxial to the rotation axis A-A of the impeller <NUM>. The side surfaces 37B, 37C converge towards a trailing edge 37D facing towards the interior of the first ring <NUM>.

The second ring <NUM> comprises a plurality of second wedge-shaped elements <NUM>. The number of second wedge-shaped elements <NUM> is equal to the number of first wedge-shaped elements <NUM>, i.e. five in the exemplary embodiment illustrated in <FIG>. Each second wedge-shaped element <NUM> is comprised of a respective second sliding surface 39A co-acting with the opposed first ring <NUM>. More specifically, as best shown in <FIG> and <FIG>, each first sliding surface 37A is in sliding contact with a corresponding second sliding surface 39A. Each second sliding surface 39A is inclined with respect to a planar surface orthogonal to the rotation axis A-A both in the radial direction (see <FIG>) and in the tangential direction (see <FIG> and <FIG>).

Each second wedge-shaped element <NUM> is further comprised of side surfaces 39B and 39C. The side surface 39B is an airfoil surface which partly defines a gas flow passage as will be described later on. The side surface 39C is a substantially cylindrical surface coaxial to the rotation axis A-A of the impeller <NUM>. The side surfaces 39B, 39C converge towards a rounded, outwardly oriented leading edge 39D of ring <NUM>.

As best shown in <FIG>, a respective gas flow passage <NUM> is formed between each pair of sequentially arranged first wedge-shaped element <NUM> and second wedge-shaped element <NUM>.

Each flow passage <NUM> is defined between airfoil surfaces 37B and 39B and portions of opposing first slide surface 37A and second slide surface 39A. The leading edges 39D are arranged at the inlet of each flow passage <NUM> and the trailing edges 37D are arranged at the outlet of each flow passage <NUM>. As will be explained in more detail later on, the cross-section of the flow passages <NUM> can be augmented or reduced, or the flow passages <NUM> can be completely closed, by angularly and axially displacing the first ring <NUM> and the second ring <NUM> one with respect to the other.

In the embodiment disclosed herein, the first ring <NUM> and the second ring <NUM> are mounted in the turboexpander casing <NUM> such that the first ring <NUM> is displaceable in an axial direction parallel to the rotation axis A-A, but angularly stationary with respect to the turboexpander casing <NUM>. Conversely, the second ring <NUM> is displaceable angularly around the rotation axis A-A, but is axially stationary with respect to the turboexpander casing <NUM>.

According to some embodiments, the first ring <NUM> is mounted around a stationary flange <NUM> integral with the turboexpander casing <NUM>. The first ring <NUM> can be provided with an outer annular ridge <NUM>, shown in <FIG> but omitted in <FIG> for the sake of simplicity. The annular ridge <NUM> surrounds the flange <NUM> and can slide with respect to the flange <NUM> in an axial direction, i.e. in a direction substantially parallel to the rotation axis A-A. A plurality of resilient biasing members <NUM> can be arranged between the first side 33A of the first ring <NUM> and the flange <NUM>. For instance helical compression springs can be used. In other embodiments, not shown, Belleville springs or other resilient members can be used instead of, or in combination with helical springs.

The resilient biasing members <NUM> push the first ring <NUM> against the second ring <NUM>, such that the first sliding surfaces 37A and the second sliding surfaces 39A are maintained in mutual pressure contact with one another. Guide rods <NUM> can be provided to allow an axial displacement of the first ring <NUM> in a direction parallel to rotation axis A-A, and to prevent any angular movement thereof around said axis. This latter function could be achieved, in other embodiments, via a reference pin in combination with guide pins. The guide rods can also be used for mounting and retaining the resilient biasing members <NUM> in their correct position between the first ring <NUM> and the flange <NUM>.

The second ring <NUM> can be rotationally supported on a stationary boss <NUM> integrally formed in the turboexpander casing <NUM>. A radial anti-friction bushing <NUM> and an axial anti-friction bushing <NUM> can rotationally and axially support the second ring <NUM> on the stationary boss <NUM>. The second ring <NUM> can be provided with an annular groove <NUM> (shown in <FIG> and omitted in the remaining figures for the sake of simplicity), wherein the stationary boss <NUM> projects. The second ring <NUM> can be angularly displaced around the rotation axis A-A under the control of an actuator <NUM>, which is connected to the second ring <NUM> by a connecting rod <NUM>.

Due to the first and second wedge-shaped elements <NUM> and <NUM>, which are in mutual sliding contact through the sliding surfaces 37A and 39A, when the second ring <NUM> is angularly displaced around the rotation axis A-A, the first ring <NUM> is forced to move axially away from the second ring <NUM> against the resilient force of the resilient biasing members <NUM>. The combined angular and axial displacement of the first ring <NUM> and second ring <NUM> one with respect to the other modifies the geometry of the variable geometry member <NUM> as can be best understood by comparing <FIG>. The displacement of the two rings causes a variation of the cross-sectional flow passages <NUM> defined by the variable geometry member <NUM>.

<FIG> illustrate views of the variable geometry member <NUM> in a first position, in which the flow passages <NUM> defined between the wedge-shaped elements <NUM> and <NUM> are closed. The airfoil surfaces 39B of second wedge-shaped elements <NUM> are in contact with the airfoil surfaces 37B of first wedge-shaped elements <NUM>, such that the flow passages <NUM> are closed.

<FIG> illustrate the same views of <FIG>, but with the two rings <NUM>, <NUM> in a slightly different mutual angular position. More specifically, the second ring <NUM> is displaced by <NUM>° with respect to the position of <FIG>. Mutually corresponding pairs of airfoil surfaces 37B, 39B are slightly distanced from one another such that the flow passages <NUM> formed between pairs of adjacent wedge-shaped elements <NUM>, <NUM> are slightly open. The wedge shape of the wedge-shaped elements <NUM>, <NUM> causes the two rings <NUM>, <NUM> to be slightly moved apart from one another as a consequence of their mutual angular displacement.

To further increase the total cross section of the flow passages <NUM>, the two rings <NUM>, <NUM> can be further displaced angularly one with respect to the other, e.g. by <NUM>°, as shown in <FIG>. A further rotation of the second ring <NUM> with respect to the first ring <NUM> will further open the flow passages <NUM> for increased flow rates. The additional angular displacement has caused a further axial movement between the rings <NUM>, <NUM>, which are further distanced from one another.

The operating condition of the turboexpander <NUM> can thus be adjusted by simply rotating one rigid member (ring <NUM>) and by slightly shifting another rigid member (ring <NUM>) in an axial direction.

In the above described embodiment the variable geometry member <NUM> is arranged at the inlet of a centripetal impeller <NUM> of a turboexpander <NUM>. A similar variable geometry member can be used in other turbomachine arrangements, where a similar need exists for adjusting flow passages as a function of the operating conditions of the turbomachine.

For instance, <FIG> schematically shows a centrifugal compressor <NUM>, with a vaned diffuser. The centrifugal compressor <NUM> comprises an impeller <NUM> mounted for rotation on a shaft <NUM> arranged in a casing <NUM>. Gas entering the impeller <NUM> (arrow F) is accelerated by the rotating blades of the impeller <NUM> and the kinetic energy thus imparted to the gas is converted into pressure energy in a diffuser <NUM>.

A variable geometry assembly comprised of a variable geometry member <NUM> is arranged around a radial outlet of the impeller <NUM>. The variable geometry member <NUM> can be configured in a manner similar to the above described variable geometry member <NUM>. For instance, the variable geometry member <NUM> can be comprised of a first ring <NUM> and a second ring <NUM>, each provided with respective first and second wedge-shaped elements <NUM>, <NUM>, similar to wedge-shaped elements <NUM> and <NUM>, and not shown in detail. The wedge-shaped elements <NUM>, <NUM> of first ring <NUM> and second ring <NUM> define flow passages through which the accelerated gas flows from the impeller outlet into a scroll <NUM>, wherefrom the gas flows in a delivery duct (not shown).

The position of the two rings <NUM>, <NUM> can be adjusted depending upon the operating conditions of the compressor <NUM>. The wedge-shaped elements <NUM>, <NUM> of the two rings <NUM>, <NUM> act in a way similar to variable diffuser vanes of centrifugal compressors of the current art. The wedge-shaped elements <NUM>, <NUM> may be different in shape from wedge-shaped elements <NUM>, <NUM>, in view of the different flow conditions through the flow passages defined between consecutively arranged wedge-shaped elements <NUM>, <NUM>. While in <FIG> the fluid flows in a centripetal direction through the variable geometry member <NUM>, in <FIG> the fluid flows in a centrifugal direction through the variable geometry member <NUM> and therefore the leading edges of the wedge-shaped elements are facing inwardly towards the rotation axis A-A of the impeller <NUM> and the trailing edges are oriented outwardly.

The compressor <NUM> can also be provided with variable inlet guide vanes <NUM> arranged in an axial inlet plenum positioned upstream of the impeller <NUM>. The angular position of the variable inlet guide vanes <NUM> can be adjusted in a way known to those skilled in the art, to adjust the gas flow conditions, in combination with an adjustment operated by the variable geometry member <NUM>. The use of a radial inlet plenum with radially arranged variable inlet vanes is not excluded, in which case a variable geometry member similar to member <NUM> or <NUM> can be used at the inlet of the compressor <NUM>.

Claim 1:
A variable geometry assembly for modulating a fluid flow in a turbomachine such as a centripetal turboexpander or turbine or a centrifugal compressor, the variable geometry assembly comprising:
a first ring (<NUM>; <NUM>) comprising a plurality of first wedge-shaped elements (<NUM>; <NUM>) and having an axis (A-A);
a second ring (<NUM>; <NUM>) comprising a plurality of second wedge-shaped elements (<NUM>; <NUM>) and having an axis (A-A), the second ring (<NUM>; <NUM>) being substantially coaxial to the first ring (<NUM>; <NUM>); the second wedge-shaped elements (<NUM>; <NUM>) co-acting with the first wedge-shaped elements (<NUM>; <NUM>);
wherein: flow passages (<NUM>) are defined between pairs of sequentially arranged first wedge-shaped elements (<NUM>; <NUM>) and second wedge-shaped elements (<NUM>; <NUM>), wherein each flow passage (<NUM>) is formed between a first airfoil surface (37B) formed on the respective first wedge-shaped element (<NUM>) and a second airfoil surface (39B) formed on the respective second wedge-shaped element (<NUM>);
the first ring (<NUM>; <NUM>) and the second ring (<NUM>; <NUM>) are angularly displaceable one with respect to the other; the first ring (<NUM>; <NUM>) and the second ring (<NUM>; <NUM>) are configured to move axially with respect to one another when the first ring and the second ring are angularly displaced one with respect to the other;
characterized in that the first wedge-shaped elements (<NUM>) are comprised of respective trailing edges (37D) and the second wedge-shaped elements (<NUM>) are comprised of respective leading edges (39D) wherein the trailing edges (37D) are facing radially inwardly towards the axis (A-A) of the first ring (<NUM>; <NUM>) and second ring (<NUM>; <NUM>) and the leading edges (39D) are facing radially outwardly away from said axis (A-A);
wherein each first wedge-shaped element (<NUM>; <NUM>) comprises a first sliding surface (37A) in sliding contact with a respective second sliding surface (39A) of a corresponding one of said second wedge-shaped elements (<NUM>; <NUM>) and wherein the first sliding surfaces (37A) and the second sliding surfaces (39A) are smooth such that the first ring (<NUM>; <NUM>) and the second ring (<NUM>; <NUM>) slide continuously one over the other when the angular displacement therebetween occurs.