EP0600807B1 - High pressure hydraulic generator-receiver for power transmission - Google Patents

Description

The present invention refers to improvements made to a hydraulic generator-receiver for the transmission of power of the kind described in European patents 165 884, 262 189 and 483 029.

In the first of these three documents, an apparatus has been described comprising two pinion gears with helical teeth coupled inside a stator, at least one of them being devoid of mechanical bearing. The stator is established in the form of a flexible envelope which moreover comprises at least one inlet opening for a liquid at low pressure and a discharge opening for a liquid at high pressure. The stator further comprises two identical flexible flanges and turned one relative to the other cooperating with the casing and enclosing the two pinions with which they provide lateral sealing by their identical inner face.

The flexible casing is subjected externally to a centripetal pressure which enables it to seal the tops of the teeth of pinions with helical teeth situated in the casing. The hydrostatic compensation forces on the flanges and on the casing originate on the one hand from the pressure of a permanent total pressure zone and on the other hand from the pressure prevailing in balancing hydrostatic compensation sectors, respectively of envelope and flanges. The sectors are fed by channels. Rigid covers cover the flanges, while an equally rigid body surrounds the casing.

pas pour un nombre de dents impair par l'intermédiaire des conduits des flasques et des canaux ménagés dans les pignons. Bien entendu, cette liaison n'existe pas dans la zone d'engrènement et dans les zones diamétralement opposées à cette zone d'engrènement par rapport aux pignons et dans lesquelles sont créés des paliers hydrauliques diamétralement opposés. Ainsi, au cours de la rotation des pignons, le bobinage met en relation les couples de dents opposées de façon à obtenir une même pression hydraulique dans les creux de dents pour des positions angulaires diamétralement opposées et à créer deux forces inverses sur les pignons en vue de provoquer leur engrènement sans jeu dans la zone d'engrènement.The internal hydraulic balancing is ensured by a hydraulic winding comprising rotor ducts in the pinions and stator in the flanges and the casing. Successive switches between the rotor and stator conduits are ensured by their ends running past one another on a switching circle. Balancing between the tooth hollows is ensured by the permanent connection between the opposite tooth hollows for an even number of teeth and the opposite tooth hollows with an offset of $\frac{\text{1}}{\text{2}}$

not for an odd number of teeth via the flange conduits and the channels formed in the pinions. Of course, this connection does not exist in the engagement zone and in the zones diametrically opposite to this engagement zone with respect to the pinions and in which hydraulic diametrically opposed bearings are created. So during the rotation of the pinions, the winding connects the pairs of opposite teeth so as to obtain the same hydraulic pressure in the hollow of teeth for diametrically opposite angular positions and to create two opposite forces on the pinions in order to cause their engagement without play in the mesh area.

For large displacements, the solution described in EP 483 029 consists in multiplying the number of satellite pinions driven by a main pinion, the latter having a number of sectors depending on the number of satellites, each satellite always having four sectors, two HP sectors and two BP sectors.

The embodiments according to EP 0165884, EP 0262189 and EP 0483029 do not provide satisfactory axial balancing of the driving pinion. Indeed, if the action of the radial hydraulic bearings ensures the radial balance, the axial balancing of the pinion leading by the hydrostatic compensations on the flanges at the point of engagement only ensures a local balancing of the axial component in this point causing greater local wear of the flange subjected to the axial component of the driving pinion and therefore a precarious endurance resistance. The axial general balancing of the driving pinion is not ensured due to the axial components, at each end of the assembly formed by each of the high pressure balancing sectors and by the two adjacent elements of the zone always at high pressure, resulting from hydrostatic compensations on the flanges.

On the other hand, the balance of the tangential components on the driving pinion is ensured by the reactions of the mechanical bearings. These reactions, through the wear they cause, are detrimental in the long term to the internal balance of the generator-receiver with a single driven satellite gear, generator-receiver with helical or chevron teeth, and it is possible to replace them. by a partial hydrostatic balancing in both directions of rotation and total for a single direction of rotation (the most frequent case for small displacements which requires only one satellite pinion). This device can therefore also be applied to generator-receivers with gears with helical herringbone teeth, this toothing being analogous to a simple helical toothing when the apparatus is with two gears.

Illustrated in fig. 1 to 5 the different hydraulic and mechanical forces acting on the driving face of the driving pinion referenced 9 and corresponding to the active part of the driving face at high pressure, the generator-receiver operating as a generator, the driving pinion comprising a propeller on the left and turning counterclockwise (SIH).

As for the driven pinion 10, it is totally balanced since the faces of each tooth recess are subjected to the same tangential, radial and axial mechanical and hydraulic forces.

• la force radiale FR (fig. 1)
• la force axiale FA (fig. 2)
• la force tangentielle FT (fig. 2)

On the pinion 9 are applied:

• the radial force FR (fig. 1)
• the axial force FA (fig. 2)
• the tangential force FT (fig. 2)

The tangential force FT is defined on the basis of the transmitted power and the speed of rotation, the forces FR and FA being deduced therefrom via FN and FX and the angle γ of real pressure and the angle β which is the complement of the helix angle α defined in EP 0 262 189.

In fig. 2 there is illustrated the tangential force FT which determines the axial force FA.

Illustrated in fig. 3 and 4 the orientation of the axial force FA to which the driving pinion 9 is subjected, while the torque MFA resulting from the force FA has been illustrated in FIG. 5.

In this figure is illustrated the balance of forces and torques to be balanced on the driving pinion 9 of a generator-receiver with a single satellite pinion 10, the pinions 9 and 10 being enclosed in the body 49, according to the direction of rotation SIH or SH. The illustrations on the right side of fig. 5 of course relate to the driving pinion 9. The axial force FA applying on it is perpendicular to the lateral face of this pinion, while the force FT acts in this same plane perpendicular to the axis 6 passing through the centers of the two gables.

This force acts from point 305 to point 306 diametrically opposite or vice versa depending on the direction of rotation. The MFA torque is balanced by the reactions of the bearings of the driving pinion.

The improvements which are the subject of the present invention aim to remedy these drawbacks and to allow axial and tangential hydrostatic balancing of the driving pinion.

To this end, in order to achieve axial balancing, the value of the surface of the zone is always varied at high pressure, so that an application of the high pressure is produced on a surface of one of the flanges, either larger or smaller, the other flange not being influenced by this surface variation, while in order to perform tangential balancing, the surface of the sectors of the envelope adjacent to the point of meshing is varied detriment to the area still under high pressure.

The appended drawing, given by way of example, will allow a better understanding of the invention, the characteristics which it presents and the advantages which it is capable of providing:

Fig. 1 illustrates the decomposition of the normal force FN into FR and FX perpendicular to FR at the point of engagement of the pinions 9 and 10.

Fig. 2 is a section developed along the original cylinder of the driving pinion. We have included in I-I the section plane of fig. 1.

Fig. 3 and 4 are sections developed according to the primitive cylinders of the driving pinions respectively and driven in gear position, illustrating the direction of the axial component of imbalance of the driving pinion, according to the clockwise (SH) and counterclockwise (SIH) directions respectively ) of the driving gear.

Fig. 5 illustrates the balance according to the direction of rotation of the forces and unbalance couples of the pinion driving a generator-receiver to a single driven pinion.

Fig. 6 is a longitudinal section of an apparatus according to the invention.

Fig. 7 is a cross section of an apparatus according to the invention produced along a plane passing through the point of engagement of the pinions.

Fig. 8 is a section along VIII-VIII (fig. 6) with an axial balancing device. There is shown in VI-VI the section plane of FIG. 6 and in VII-VII that of fig. 7.

Fig. 9 is a variant of the axial balancing device of FIG. 8.

Fig. 10 is a partial developed section of the envelope and of the body in the case of hydrostatic balancing on this envelope of the tangential component FT.

Fig. 11 is a partial section corresponding to FIG. 8 and 9, but illustrating the modifications of the flanges corresponding to the actual balancing on the envelope of the tangential component FT.

Fig. 12 is a section along XII-XII (fig. 10). There is shown in X-X the section plane of FIG. 10.

Fig. 13 to 16 illustrate the equilibrium polygon of the tangential components FT, of the torque vectors MFA and the axial components FA to be balanced in the case respectively of a receiver generator with chevron toothing and with 2, 3, 4 satellite pinions, and according to the direction of rotation.

Fig. 17 is a variant of the axial balancing device illustrated in FIG. 8 and comparable to the variant of FIG. 9.

We will not return to the demonstration carried out in the preamble following fig. 1 to 5 and according to which only the driving pinion must be balanced, the balancing of and the driven pinions (10) being carried out as demonstrated.

The elements corresponding to those of the earlier patents cited at the beginning of the present document have been referenced by the same numbers and the same clues, so there is no need to describe them again.

de valeur 1,5, donnant une saillie de dent légèrement supérieure à une fois le module apparent de denture M.The hydrostatic balancing of the force FR by the hydraulic bearings in zones 6 is obtained by a theoretical value of ½ angular pitch of toothing, ie Π / Z. This value is sufficient to balance the mechanical and hydrostatic radial forces in the engagement zone 3. This value of ½ step can vary according to the sealing conditions in 3 and the values of the pressures in the hollow of teeth. It is valid for a hydraulic recovery RE at mesh point 3 such that RE = $\frac{\text{Driving arc}}{\text{No basic}}$

with a value of 1.5, giving a tooth projection slightly greater than once the apparent modulus of toothing M.

The overpressure in the hollow of the tooth resulting from the irregularities of flow happens to be a natural functional safety, but if this was too high, it can be evacuated by a non-return towards zone 34, (zone in high pressure) or towards high pressure as provided in EP 0 165 884.

dentures hélicoïdales d'inclinaison opposées de ladite denture en chevrons.The hydrostatic balancing of the force FA must allow reversibility, that is to say the change of direction of rotation in generator or receiver of the apparatus according to the invention. The best solution for a device with a single driven sprocket is to adopt for the two sprockets a herringbone toothing thanks to which the axial forces FA naturally balance between the two $\frac{\text{1}}{\text{2}}$

helical teeth of opposite inclination of said herringbone toothing.

On the other hand, for a generator-receiver comprising two pinions with normal helical teeth, the balancing of the axial force FA must be carried out as described below.

We can first of all adopt a first solution illustrated in FIG. 8, according to which the diameter D of the permanent total pressure zone 34 is increased by a value ΔD to become D + ΔD at the sectors 60 ′ and 60 diametrically opposite to each other on the side of the driving pinion 9 , said sectors being arranged on the flanges 21, 22 while the dimensions of said sectors on the driven pinion side 10 remain unchanged. The increase in ΔD gives an additional surface ΔS of the zone 34 so that ΔS = FA / HP. This arrangement gives rise to an additional force on the flange where sectors 60 and 60 'are at low pressure. This force equal to FA, and in the opposite direction, balances the driving pinion 9 in the axial direction.

One can also use the solution illustrated in fig. 9 which consists in incorporating into the sector 60 "of the flanges 21, 22 an extension 301, to the detriment of the area 34 of high permanent pressure, of surface ΔS. The extension 301 is in the general form of a polygon surrounded by a 45 "joint deflection.

This structure results in the creation of a force perpendicular to the flange in which the sector 60 "is at high pressure, said force being equal to the axial force FA as a result of the choice of the surface ΔS and of opposite direction. The opposing force in cause -FA rebalances the driving pinion 9.

dans la solution illustrée en fig. 8 (dans laquele D_p est le diamètre primitif de la denture du pignon 9) est équilibré par les réactions des paliers mécaniques 123 du pignon 9.The torque resulting from the distance separating the antagonistic force -FA and the axial force FA and whose value is $${\text{M}}_{\text{FA}}\text{= FA ×}\frac{\text{DP}}{\text{2}}$$

in the solution illustrated in fig. 8 (in which D _p is the original diameter of the teeth of the pinion 9) is balanced by the reactions of the mechanical bearings 123 of the pinion 9.

In the case of an apparatus with several driven satellite gears n, the axial balancing of the driving pinion 9 is carried out according to one of the two solutions above and the moment vectors of the resulting couples cancel each other out. The additional rebalancing force is equal to the axial force FA per pair of pinions 9, 10 multiplied by the number of satellites n.

Illustrated in fig. 7 an apparatus according to the invention with a single driven pinion 10, the driving pinion 9 being a helix on the left. This pinion is therefore at the rear of the section plane of the figure, while the driven pinion 10 is located in front of said plane with propeller on the right.

Therefore, the axial force FA is directed downwards, so that the opposing force -FA must be directed upwards and in any case in the opposite direction to FA.

• si l'on augmente la zone 34, cette augmentation s'étend sur le secteur 60' et sur le secteur 60 opposé (fig. 8). Cette augmentation n'entraîne aucune force supplémentaire sur le flasque 21 sur lequel les secteurs 60 et 60' sont en haute pression, tandis qu'elle crée la force antagoniste recherchée -FA sur le flasque opposé 22 dont les secteurs 60 et 60' sont en basse pression.

Consequently :

• if the area 34 is increased, this increase extends over the sector 60 ′ and over the opposite sector 60 (FIG. 8). This increase does not cause any additional force on the flange 21 on which the sectors 60 and 60 'are under high pressure, while it creates the desired opposing force -FA on the opposite flange 22 whose sectors 60 and 60' are in low pressure.

Indeed, on the flange 21, the increase in the area of the area 34, corresponding to a decrease in the area of the sectors 60 and 60 ′, does not cause any variation in the axial hydrostatic compensation force.

• si l'on diminue la surface de la zone 34 en augmentant la surface du secteur 60'' de la valeur de l'extension 301 (fig. 7), on crée la force antagoniste -FA sur le flasque 22 dans lequel le secteur 60'' est en haute pression du fait que sur le flasque 21 dans lequel le secteur 60'' est en basse pression, l'extension 301 est en basse pression, de sorte qu'il y a équilibrage.

On the contrary, on the opposite flange 22, the surface of its sectors 60 and 60 'subjected to the low pressure having decreased and the surface of the zone 34 always in high pressure having increased, the opposing force -FA is created.

• if we decrease the surface of zone 34 by increasing the surface of sector 60 '' by the value of extension 301 (fig. 7), we create the opposing force -FA on the flange 22 in which sector 60 '' is at high pressure because on the flange 21 in which the sector 60 '' is at low pressure, the extension 301 is at low pressure, so that there is balancing.

Of course, if the pressures in the outlets 40 are reversed relative to the indications in FIG. 7, the reasoning is the same due to the symmetry with respect to the point of engagement of the pinions 9, 10.

The hydrostatic balancing of the tangential force FT is carried out by means illustrated in FIG. 10, 11 and 12.

The aforementioned means consist first of all, as shown in FIG. 10, to delete the zone 34 between the sectors 38 ′ and 38 adjacent to the point of engagement 3 of the pinions at the level of the spokes 305 and 306 of the pinion 9 perpendicular to the axis 6-6. The seals 37 'and 37 provided in the embodiment of fig. 9 of European patent 0483029 are replaced by a single joint 304 provided with a middle branch 304 a . This middle branch is located along departments 305 and 306 illustrated in fig. 11, as explained below.

The above modification requires deleting the part of the zone 34 between the sectors 60 ', 60 respectively 60 ", 60 of the flanges 21, 22 (fig. 11) at the level of the spokes 305 and 306, so that for each of the said rays, radial portions 45a, 45'a of the joints 45 and 45 '(fig. 8) and the radial portions 45 a and 45 "has seals 45 and 45' symmetrical to the previous relative to the geometric axis 6-6 are eliminated without affecting the axial balancing in any way.

As illustrated in fig. 11, the sectors 60 ′, 60 and 60 ″, 60 are surrounded by joints 302 and 303 in one piece which each have, respectively, along the spokes 305 and 306 a branch 302 a , respectively 303 a which separates the sectors considered .

de pas angulaire de denture de l'axe du "palier hydraulique" soit $\frac{\text{Π}}{\text{2Z}}$

. De ce fait, la composante FT est équilibrée par ce supplément de secteur 38' qui se trouve en haute pression pour une valeur un peu supérieure à $\frac{\text{Π}}{\text{Z}}$

+ ε. La valeur $\frac{\text{Π}}{\text{Z}}$

+ ε incorporée aux secteurs 38' du côté du pignon menant 9 doit être calculée pour équilibrer FT, en tenant compte du décalage introduit par la largeur du joint. Les valeurs sur les flasques 21, 22 doivent correspondre aux valeurs sur l'enveloppe 36 pour assurer l'étanchéité sur les faces. Ces dispositions permettent l'équilibrage partiel de la force FT (à cause de la largeur des joints) avec réversibilité totale en sens de rotation S.H. et S.I.H., en générateur et en récepteur. L'équilibrage parfait de la force tangentielle FT ne pourra être réalisé qu'avec un sens prioritaire de rotation S.H. ou S.I.H., en générateur et en récepteur du fait de la largeur des joints. Toutefois ce léger déséquilibre peut être considéré comme négligeable.The branch 304 has seal 304 on the casing 36 materializing the end of the part of the zone 34 incorporated into the sectors 38 ′ for balancing the force FT, is located at $\frac{\text{1}}{\text{4}}$

angular pitch pitch of the axis of the "hydraulic bearing" or $\frac{\text{Π}}{\text{2Z}}$

. Therefore, the FT component is balanced by this additional sector 38 'which is at high pressure for a value slightly higher than $\frac{\text{Π}}{\text{Z}}$

+ ε. The value $\frac{\text{Π}}{\text{Z}}$

+ ε incorporated into sectors 38 'on the side of the driving pinion 9 must be calculated to balance FT, taking into account the offset introduced by the width of the joint. The values on the flanges 21, 22 must correspond to the values on the casing 36 to ensure sealing on the faces. These provisions allow partial balancing of the FT force (because of the width of the joints) with total reversibility in the direction of rotation SH and SIH, in generator and in receiver. Perfect balancing of the tangential force FT can only be achieved with a priority direction of rotation SH or SIH, in generator and receiver due to the width of the joints. However, this slight imbalance can be considered negligible.

In herringbone toothing, the tangential force FT is balanced under the same conditions as for a helical toothing, the only difference being that the joint 304 then has, like the sectors 38 and 38 ', a herringbone shape.

In the case of a generator-receiver with n driven planet gears, the tangential forces FT naturally balance each other (closed force forces polygon).

The balancing arrangements above are illustrated with regard to herringbone toothing generators and receivers with a single satellite pinion driven in FIG. 13. We showed there the driving pinion 9 c and a satellite pinion 10 c in the body 49 c , the forces FT by branch of rafters to be balanced (hydrostatic balancing or reactions of the bearings 123), and the polygon of the vectors vectors MFA closed , we observe that the axial components by rafter branch cancel each other out (FA - FA = 0).

The same balancing arrangements above are illustrated with regard to the generator-receivers with n planet gears driven in FIGS. 14, 15 and 16 on which they are shown diagrammatically by the body 49 and the pinions 9 and 10.

In fig. 14, we are dealing with a pinion 9 'and two planet gears 10' in a body 49 '. The polygon of the forces FT is closed, the polygon of the torque vectors M _FA is closed under the same conditions. The additional axial component on the pinion 9 'is equal to twice FA per pair of pinions 9'-10'.

In fig. 15, a pinion driving 9 '' and three satellite pinions 10 '' have been illustrated in a body 49 ''. The polygon of forces FT is closed, as is the polygon of the torque vectors M _FA under the same conditions. The additional axial component on the pinion 9 '' is in this case equal to three times FA per pair of pinions 9 '' - 10 ''.

For fig. 16 which shows a pinion driving 9 '''and four satellite pinions 100''' in a body 49 ''', the same goes, the polygon of the forces FT is closed, the polygon of the torque vectors M _FA is closed in the same conditions and the additional axial component on the pinion 9 '''is equal to four times FA per pair of pinions 9''' - 10 '''.

• pour les grandes vitesses de rotation et en petites cylindrées un générateur-récepteur hydraulique avec un seul pignon satellite et une denture chevrons,
• pour les vitesses de rotation moyennes et faibles avec des moyennes et des grosses cylindées, un générateur hydraulique avec plusieurs pignons satellites menés n avec l'équilibrage de la force axiale du pignon menant 9.

In conclusion and taking into account the balancing possibilities above, we can adopt:

• for high rotational speeds and small displacement a hydraulic generator-receiver with a single satellite pinion and a herringbone toothing,
• for medium and low rotational speeds with medium and large displacements, a hydraulic generator with several satellite pinions driven n with the balancing of the axial force of the driving pinion 9.

A generator-receiver with a single driven satellite gear and helical teeth will only be used if this solution has certain economic advantages, since it is less rational than the previous two in terms of function. Axial balancing will be necessary, balancing the tangential force will depend on the conditions of use.

. Cette disposition entraîne une augmentation de force FA = HP x ΔS sur le flasque où le secteur 60'' est en haute pression qui compense FA car de sens opposé et équilibre le pignon dans le sens axial.It should moreover be understood that the foregoing description has been given only by way of example and that it in no way limits the field of the invention from which one would not depart by replacing the execution details described by all other equivalents. For example in the solution illustrated in fig. 17, the diameter D of the permanent total pressure zone 34 is reduced by a value ΔD to become D - ΔD at the sectors 60 '' and 60 opposite to each other on the side of the driving pinion 9, said sectors being arranged on the flanges 21, 22 while the dimensions of said sectors on the driven pinion side 10 remain unchanged The reduction in ΔD gives an area of the area 34 reduced by ΔS so that ΔS = $\frac{\text{FA}}{\text{HP}}$

. This arrangement results in an increase in force FA = HP x ΔS on the flange where the sector 60 '' is at high pressure which compensates FA because in the opposite direction and balances the pinion in the axial direction.

Claims (7)

1. A hydraulic generator-receiver having pinions with helicoidal teeth comprising a driving pinion and at least one driven pinion without any mechanical bearing, which is always balanced, the internal balancing being provided by a system of hydraulic windings which makes it possible to create hydraulic bearings and meshing without play, internal sealing being obtained by a flexible system comprising two flanges (21, 22) and an envelope (36) with hydrostatic compensation using hydrostatic compensation sectors (60-38), characterised in that the axial forces (FA) of the driving pinion (9) are balanced by varying the magnitude of the surface area of the zone (34) which is always at high pressure in such a way that this change brings about the application of high pressure to a surface of one of flanges (21, 22), which is either larger or smaller, this change in the magnitude of the zone (34) which is always at high pressure having no effect on the other flange (22 or 21), while in order to achieve tangential balancing (FT) of the driving pinion (9) the surface area of the sectors of the envelope (36) adjacent to the meshing point (3) is varied to the detriment of the zone (34) which is always at high pressure.
2. A generator-receiver according to claim 1, characterised in that axial hydrostatic balancing of the driving pinion (9) is obtained by creating a force (-FA) which is antagonistic to the axial component FA; the surface area of the zone (34) which is permanently at high pressure is increased on flanges (21, 22) by an amount ΔS is provided such that ΔS = FA/HP in the part of the zone (34) where this is adjacent to the hydrostatic compensation sectors 60' and 60 (which is diametrically opposite 60'), a force whose effect only appears on the side on which sectors 60' and 60 are at low pressure, thus permitting the system to be totally reversible.
3. A generator-receiver according to claim 1, characterised in that axial hydrostatic balancing of the driving pinion (9) is obtained by means of a force (-FA) which is antagonistic to the axial component FA created by reducing the zone (34) of permanent high pressure on flanges (21, 22) by increasing the surface area of the hydrostatic compensation sector (60") by means of an extension (301) having a surface area ΔS, a surface area such that ΔS = FA/HP providing a force whose effect is only felt on the side where the sector (60") is at high pressure, thus permitting the system to be wholly reversible.
4. A generator-receiver according to claim 1, characterised in that hydrostatic balancing of the tangential force FT acting on the driving pinion (9) is obtained by producing a force which is antagonistic to the tangential component FT and equal to the latter, by increasing the sectors (38') of the envelope (36) on the side of the driving pinion (9) and that of the hydrostatic compensating sectors (60' and 60") on the flanges (21, 22), an increase obtained by modifying the structure of the joints (304) of the envelope (36) and those (302, 303) of the said flanges, partial balancing for the directions of rotation SH and SIH and total balancing for a priority direction of rotation SH or SIH as a result of the width of the joints (302), (303), (304) in their radial parts (302a, 303a and 304a).
5. A generator-receiver according to claim 1, characterised in that the hydrostatic balancing is applied to generator-receivers comprising a plurality of driven satellite pinions (10) and in that the antagonistic force (-FA) balancing the driving pinion (9) is equal to the axial force FA per pair of driving/driven pinions (9-10) multiplied by the number of satellite pinions.
6. A generator-receiver according to claim 4, characterised in that the pinion teeth have helicoidal chevrons.
7. A hydraulic generator-receiver according to claim 1, characterised in that the axial hydrostatic balancing of the driving pinion (9) is obtained by means of a force (-FA) which is antagonistic to the axial component FA created by reducing the zone (34) of permanent high pressure on the flanges (21, 22) by increasing the surface areas of the corresponding hydrostatic compensation sectors (60") and (60) by reducing the diameter D by an amount ΔD, of surface area ΔS, a surface area such that ΔS = FA/HP provides a force whose effect is only felt on the side where the sector (60") is at high pressure, thus permitting the system to be wholly reversible.

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