PRIOR ART Volume screw machines of rotary type comprise conjugated screw elements, namely an enclosing (female) screw element and an enclosed (male) screw element. The enclosing (female) screw element has an inner profiled surface (female screw surface), and the enclosed (male) screw element has an outer profiled surface (male screw surface). The profiled surfaces (screw surfaces) are non-cylindrical and limit the elements radially. They are centred around respective axes which are parallel and which usually do not coincide, but are spaced apart by a length E (eccentricity).

A rotary screw machine of three-dimensional type of that kind is known from US 5,439, 359, wherein an enclosed element surrounded by a fixed enclosing element is in planetary motion relative to the enclosing element.

A first component of this planetary motion drives the axis of the male surface to make this axis describe a cylinder of revolution having a radius E around the axis of the female surface, which corresponds to an orbital revolution motion. In other words, the axis of the enclosed (male) element rotates around the axis of the enclosing (female) element, wherein the latter axis is the principal axis of the machine.

A second component of this planetary motion drives the male element to make it rotate around the axis of its screw surface. This second component (peripheral rotation) can also be called swivelling motion.

Instead of providing a planetary motion, a differential motion can be provided. Usually, synchronizing coupling links are used therefor.

However, the machines can also be self-synchronized by providing suitable screw surfaces.

In most cases, the screw surfaces of the rotary screw machines have cycloidal (trochoidal) shapes as it is for example known from French patent FR-A-997957 and US 3,975, 120.

Rotary screw machines of volume type of the kinds described above are known for transforming energy of a working substance (medium), gas or liquid, by expanding, displacing, and compressing the working medium into mechanical energy for engines or vice versa for compressors, pumps, etc. They are in particular used in downhole motors in petroleum, gas or geothermal drilling.

The transformation of a motion as used in motors has been described by V. Tiraspolskyi,"Hydraulical Downhole Motors in Drilling",, the course of drilling, p. 258-259, published by Edition TECHNIP, Paris.

The effectiveness of the method of transforming a motion in the screw machines of the prior art is determined by the intensity of the thermodynamic processes taking place in the machine, and is characterized by the generalized parameter"angular cycle". The cycle is equal to a turn angle of any rotating element (enclosing element, enclosed element or synchronizing link) chosen as an element with an independent degree of freedom.

The angular cycle is equal to a turn angle of a member with independent degree of freedom at which an overall period of variation of the cross section area (or overall opening and closing) of the working chamber, formed by the enclosing and enclosed elements, takes place, as well as an axial movement of the working chamber by one period Pm in the machines with an inner screw surface or by one period Pf in the machines with an outer screw surface, wherein Pm, Pf are pitches (periods) of a screw turn of the end sections around central axes of the respective elements.

The known methods of transforming a motion in volume screw machines of rotary type with conjugated elements of the curvilinear shape realized in the similar volume machines have the following drawbacks: - limited technical potential, because of imperfect process of organizing a motion, which fails to increase a quantity of angular cycles per one turn of the drive member with an independent degree of freedom; - limited specific power of similar screw machines; - limited efficiency ; - existence of reactive forces on the fixed body of the machine.

SUMMARY OF THE INVENTION It is an object of the invention to solve a problem of widening the technical and functional capabilities of the rotary screw machines, to help in increasing the specific power and efficiency of these machines and preferably to eliminate or reduce reactive forces on the fixed body of the machine.

In order to solve that problem, a rotary screw machine is provided which comprises an outer enclosing screw element having a profiled inner surface, an inner enclosed screw element having a profiled outer surface, and at least two intermediate screw elements which are both enclosing and enclosed and have both a profiled inner and a profiled outer surface. All screw elements form a series of elements of which each enclosed element is housed coaxially in the respective next enclosing element.

Such a rotary screw machine uses the volume as defined by its outer walls more effectively, due to the nested structure of enclosing and enclosed screw elements. The number of working chambers in which the working substance is displaced and expanded/compressed can thereby be increased. Furthermore, the specific power and efficiency of the machine is increased.

In order to optimize the motion of the medium, it is desirable that the working chambers are not defined independently. Rather, a coupled motion of several screw elements should take place.

In order to couple that motion, some screw elements have to be (mechanically) coupled to each other. Of course, not all screw elements can be coupled to each other. The screw elements which are coupled to each other have to be chosen in a well-defined manner. Regarding that choice, several criteria can be provided which correspond to three different aspects of the present invention.

According to a first aspect of the present invention, the elements are given ordinal numbers starting with the inner enclosed element to the next enclosing/enclosed screw element and thereon to the outer enclosing screw element which has the ordinal number which corresponds to the total number of screw elements. Either those elements having an even ordinal number are (mechanically) coupled to each other,

in particular by a means separate (different) from the elements having an uneven ordinal number, or it is just the other elements which are coupled, i. e. those elements having an uneven ordinal number are (mechanically) coupled to each other, in particular by a means separate (different) from the elements having an even ordinal number.

According to a preferred embodiment, the profiled surfaces are defined around an axis each, each such axis being coinciding with a principal axis of the machine or being parallel to the principal axis with eccentricity. The profiled surface or surfaces of an element having the ordinal number i in a cross section of the machine perpendicular to the axes has or have an order of symmetry ni, wherein the order of symmetry is increasing from the interior to the outside of the rotary screw machine, i. e. increasing with the ordinal number i the elements are given, nj+1 > ni.

Preferably, the order of symmetry is increasing by 1, nj+1=nj+1.

Providing surfaces having such a symmetry order relationship makes it possible to optimize the formation of working chambers and to increase the total number of working chambers which are simultaneously defined.

The screw elements can be formed in such a manner as to fit more or less ideally into each other. For example, the profiled surfaces of each element i can be defined as to be inscribed into a ring of mean radius Ri or ri with a radial extent of 2AI or 2ai. The profiled surface can in particular comprise undulations. The radial extent can also be fitted to the value of the eccentricity Ei. If a screw element is placed in the rotary screw machine in such a manner that the axis around which the profiled surface of the screw element is defined has eccentricity Ei with respect to the principal axis, then the radial extent can be 2EI. It is to be noted that in the intermediate elements, the inner and outer surfaces will sometimes not both have the same radial extent due to the shape of the wall of the screw element. In that case, of course, the radial extent of one of the surfaces can be a little bit higher than 2Ei, and the radial extent of the other of these surfaces can be a little bit smaller than 2Ei.

According to a preferred embodiment of the invention, those screw elements having an uneven ordinal number are provided with surfaces the symmetry order of which is also uneven, and those screw

elements having an even ordinal number are provided with surfaces the symmetry order of which is even.

In preferred embodiments, alternating screw elements are placed such that the axis around which the profiled surface or surfaces of the screw element is or are defined correspond to the principal axis, and the respective other one of these elements is placed such that axis around which its profiled surface or surfaces is or are defined has eccentricity with respect to the principal axis. In other words, there are two cases: Each screw element of ordinal number i with i being even is placed such that the axis around which its profiled surface or surfaces is or are defined has eccentricity with respect to the principal axis, and preferably these elements having ordinal number i with i being even are mechanically coupled. Alternatively, each screw element of ordinal number i with i being uneven can be placed such that the axis around which its profiled surface or surfaces is or are defined has eccentricity Ei with respect to the principal axis, and preferably these screw elements are also mechanically coupled to each other.

The coupling of those screw elements which are placed such that the axes around which their surfaces are defined have eccentricity with respect to the principal axis having the reason that those elements can cause the problem that the machine does not work dynamically balanced. The elements the surfaces of which are centred around the principal axis usually move very stably. The dynamical balance can be achieved if the corresponding mechanically coupled screw elements are coupled in such a manner that the distribution of the masses of the coupled screw elements is such that a statical mass balance of these elements is obtained and that upon motion of the elements in the machine the mass balance is maintained. Most simply, this mass balance can be maintained by maintaining the distance relationship of the axes having eccentricity Ei with respect to each other and to the principal axis during a motion of the machine. In other words, the mass distribution of the screw elements is not intended to change with time. In particular, it can be chosen such that the mass centre (centre of gravity of a slice of the elements) coincides with the principal axis.

Preferably, the mechanical coupled elements are coupled by a crank or a crank-like mechanism.

In a rotary screw machine, at least one of the elements is driven into rotation. Preferably, those elements coupled by the crank or crank-like mechanism are freely moving without being driven by any driving means, and an element which is not mechanically coupled to other elements is driven by such a driving means.

Preferably, such a driven element (rotor) is provided either in the very inner or in the very outer portion of the machine. If the first element is not taken to be a rotor, for example when the first element is placed with eccentricity to the principal axis, it can be the second element which is a rotor driven by a driving means. Alternatively, the outer enclosing element can be a rotor driven by a driving means.

In a configuration of screw elements in which alternating screw elements are placed around the principal axis and with eccentricity, respectively, the total number k should at best be even.

A preferred embodiment comprises only four elements, wherein for example the outer three screw elements define a planetary mechanism and the inner two screw elements define a differential mechanism.

In that embodiment, the inner enclosed element can have a profiled surface with a symmetry order of 3, the second element profiled surfaces with a symmetry order of 4, the third element profiled surfaces with a symmetry order of 5 and the outer enclosing element a profiled surface with a symmetry order of 6. The first element (inner enclosed element) and the third element can have eccentricity, and the second element and the fourth element (outer enclosing element) can be placed with their axes coinciding with the principal axis of the machine. Then, the inner enclosed element can be mechanically coupled to the third screw element by a crank mechanism.

Such an embodiment provides for a rotary screw machine having increased specific power and efficiency, and a process of organizing a motion is optimized whereas at the same time reactive forces on the fixed body of the machine are minimized by an optimal choice of the eccentricities, with those axes having eccentricity being mechanically coupled and moving in a well-defined relationship.

In a second aspect of the rotary screw machine according to the invention, the choice regarding the question which of the elements are coupled to each other (by means separate (different) from other screw

elements) is made such that at least two elements and preferably all elements the surfaces of which are defined around an axis having eccentricity to the principal axis are coupled. It is to be noted that this second aspect does not necessarily have to be present simultaneously to the first aspect as it was described above for a preferred embodiment of the invention. In other words, instead of coupling elements 1,3 and 5, one could couple elements 1 and 2 or elements 1 and 4 as long as these are placed with eccentricity with respect to the principal axis of the machine.

Once again, the coupling of those elements having eccentricity provides for improving the statical and dynamical balance of that system when the screw elements are moving because the coupling provides for a well-defined motion of the systems having eccentricity, wherein those elements which are placed such that their profiled surfaces are centred around the principal axis are usually not causing any balance problems when moving.

According to a preferred embodiment of the rotary screw machine according to that second aspect, those profiled surfaces which are defined around an axis having eccentricity to the principal axis have a symmetry in a plane perpendicular to said axis with uneven symmetry order all or with even symmetry order all. In other words, those screw elements which are coupled all have even or all have uneven symmetry order. Preferably, those elements being placed with eccentricity have uneven symmetry order.

Turning now to the third aspect of the rotary screw machine according to the present invention, the choice as to which screw elements are coupled is made in the following manner: Each profiled surface of a screw element i (or both surfaces thereof) comprises Ni undulations, i. e. Ni protrusions with cavities between these protrusions. Those screw elements the surfaces of which comprise an even number N, of undulations are coupled to each other (by a means separate (different) from other screw elements) or, alternatively, those screw elements the surfaces of which comprise an uneven number Ni of undulations are coupled to each other (by means separate (different) from other screw elements).

The rotary screw machine according to the third aspect of the invention does also provide for an optimized process of organizing a motion of the medium in working chambers and thereby to increase specific power and efficiency. It is clear that if profiles having an even number of undulations all or an uneven number of undulations all can be more easily coupled to another. For example, if profiles with four, six and eight undulations are coupled, the protrusions numbers 1 and 3 of the profile with four undulations, the protrusions 1 and 4 of the profile with six undulations and the profile 1 and 5 of the profile with eight undulations can be coupled to each other. If one wanted to couple for example elements having four, seven and nine undulations, it would be much more difficult to establish a relationship between respective protrusions of the undulations.

It is to be noted that in the definition of that third aspect, the screw elements do not necessarily have rotational symmetry. There may be screw elements having undulations of different size and/or shape. Of course, preferably the profiled surfaces of the screw elements do have rotational symmetry with symmetry order nj=Nj, and once again, in a preferred embodiment the symmetry order ni of each enclosed element is lower than the symmetry order ni+l of the respective enclosing element, nj<nj+1, i. e. the symmetry order is increasing from the inside to the outside of the screw element. Preferably, the symmetry order is increasing by 1 from one element to the next element, i. e. the symmetry order ni of each enclosed element is by 1 lower than the symmetry order nj+1 of the respective enclosing element, ni+1=nj+1.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will more easily be apparent from the following description of preferred embodiments thereof which is made exemplarily with respect to the accompanying drawings, in which: Fig. 1 shows a longitudinal section of the rotary screw machine according to the invention, and Fig. 2 shows a cross-section of the rotary screw machine according to the invention, in particular along the line II-II of fig. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE ROTARY SCREW MACHINE ACCORDING TO THE INVENTION A rotary screw machine according to the invention comprises a rigid stationary body 7. In that body 7, four different screw elements 1,2, 18 and 19 are provided. The outer enclosing screw element 1 which is shown in fig. 1 is a stator which is mechanically rigidly set in the body 7.

The intermediate screw element 2 is a two-sided planetary rotor-satellite.

The intermediate screw element 18 is a two-sided central rotor, and the inner enclosed screw element 19 is a one-sided planetary rotor-satellite.

The rotor 18 is driven by a shaft 8 into rotation.

The two planetary rotor-satellites are mechanically coupled by a two-throw crank 17 which corresponds to a link of synchronizing coupling, i. e. to the coupling means mentioned in the claims.

As shown in fig. 2, all profiled surfaces of the screw elements have rotational symmetry order. The outer surface 219 of the inner enclosed screw element 19 has a symmetry order of 3, the inner surface 118 and the outer surface 218 of the second screw element 18 (when counted starting with the inner enclosed screw element 19) have a symmetry order of 4, and the inner surface 102 and the outer surface 202 of the third screw element 2 have a symmetry order of 5, and finally the inner surface 101 of the outer enclosing screw element 1 has a symmetry order of 6. In other words, the symmetry of the surfaces of the respective elements is increasing by 1 from the interior to the outside of the rotary screw machine.

As shown in fig. 2, both the rotor 18 and the stator 1, i. e. those screw elements having an even symmetry order, are centred around the principal axis of the machine. The elements 19 and 2, i. e. the planetary rotor-satellites which have uneven symmetry order, are placed such that the centres around which their respective surfaces are defined are placed with eccentricity to the principal axis: The inner enclosed screw element 19 is centred around the axis Oi9, and the intermediate screw element 2 is centred around the axis 02. The axes 02 and °19 are disposed on opposite sides of the central axis. They are chosen in such a manner that the machine is statically balanced: It is clear that the mass centres of the elements 18 and 19 coincide with the principal axis of the rotary screw machine. If the eccentricities of the axes 02 and Oig are chosen in a well-

defined manner, the mass centre of the elements 19 and 2, when taken together, can also coincide with the principal axis of the machine.

The crank 17 provides for a coupled motion of the planetary- rotor elements 19 and 2, i. e. of those elements having uneven order of symmetry. The coupled motion is such that the axes oit and 02 maintain their distance relationship with respect to each other and with respect to the principal axis. In other words, when the machine is set into rotation, it is a line 019-principal axis X-O2 which is moving around the principal axis X. Thereby, the rotary screw machine is not only statically, but also dynamically balanced.

It is to be noted that the rotary screw machine shown in the figures comprises both a planetary and a differential mechanism, with the planetary mechanism being defined by the elements 1,2 and 18 (with outer surface 218), and the differential mechanism being defined by the elements 19 and 18 (with the inner surface 118).

In the machine of fig. 1, the stator 1 is moveless. If a relative angular velocity (A) ro (18) of the central rotor 18 is taken to be-1, the relative angular velocity (Dre (2, 19) of the line of the axis 02-principal axis- Oit of the rotors-satellites 2,19 around the axis X (angular velocity of the revolutions of the rotors-satellites 2,19) is given by (ore (2, 19) = -rO (18) nm (18)/2=+2, wherein nm (18) =4 is the symmetry order of the element 18. Regarding the swivelling motion, the relative angular velocity cos (2) of the rotor-satellite 2 around its axis 02 is given by cos (2) =-Mre (2, i9)/nm (2) =-0. 4, wherein nm (2) =5 is the symmetry order of the element 2. The relative angular velocity °s (l9) of the rotor-satellite 19 around the axis Oit is given by (s (i9) =4/3 [ (0ro (i8)- Mre (2, i9)] +Mre (2, i9) =-2.

Working chambers formed by the elements move due to the motion of conjugating contacts of the elements along the principal axis of the machine. It is to be noted that the rotor 18 is only driven into rotation via the shaft 8 if the machine operates as a compressor. If the machine operates as a detander, it is the rotors-satellites 2 and 19 which first move and which set the rotor 18 into rotary motion. The angular cycles of an axial movement of each of the six working chambers provided between the elements 1 and 2 in this machine are 180° angle of rotation of the output shaft 8 (two angular cycles per rotation). The angular cycles of an axial movement of the five working chambers between the elements 2 and

18 in this machine are 150° (2.4 cycles per rotation). And the angular cycles of each of the four working chambers between the elements 18 and 19 are 120° (3 cycles per rotation).

According to a second preferred embodiment of the rotary screw machine, which is not shown in the figures, the rotary screw machine is reversible : The central rotor 18 is moveless, and the outer enclosing element 1 is movable (motor-wheel). If then a relative angular velocity wro (l) is taken to be +1, the relative angular ve ! ocity Mre (2,19) of the line of the axis 02-principal axis-019 of the rotors-satellites 2 and 19 around the principal axis X is given by t0re (2, i9) = Mro (i) nm () /2=3, wherein im (i) =6 is the symmetry order of the element 1. The relative angular velocity of the rotors-satellites 2 and 19 around their axes 02, 019 is given by Cl) s (2) =+0. 6 and (Ds (i9) =-l.

The rotary screw machine according to the invention has a decreased angular extent of the thermodynamic cycles when compared to the prior art machines. Furthermore, the resultant momentum and reactive forces on the machine supports are decreased. The rotary screw machine according to the invention has a high specific power and also a very high efficiency.