Linear unit and system

Abstract

The present invention relates to a linear unit (10) and a system (100) comprising such a linear unit (10). The linear unit (10) comprises i) a housing (12), ii) a screw (14) rotatably mounted within said housing (12), iii) a nut (16) cooperating with said screw (14), and iv) two mounting assemblies (30) for mounting said housing (12) to an external structure (E). Said mounting assemblies (30) each comprise at least two trunnions (32) arranged transversely to said screw (14). Said trunnions (32) are preferably movably mounted in bearing shells (34) arranged on opposite sides of the screw (14).

Description

[0001] Linear unit and system

[0002] The present invention relates to a linear unit and a system comprising such a linear unit.

[0003] For moving and positioning components such as machine parts, linear units are known in the art. Such linear units convert rotational motion, e.g. of an electric motor, into translational motion. To this end, a rotatable screw is provided which cooperates with a nut. Upon rotating the screw by means of the motor, the nut translates along the screw. The movement of the nut is transferred to the machine part, e.g. via a push tube .

[0004] Longevity of such linear units strongly depends, amongst others, on the loads exerted on the screw. Particularly radial loads, which may increase friction or even slightly deform the screw, severely reduce service life of the screw and / or nut. Such loads may arise not only from insufficient (radial) guiding of the machine part, but also from thermal expansion of components of the linear unit itself. Further, when the linear unit is fixedly mounted to an external structure, deformation of the structure may cause deformation of the linear unit, i.e. components thereof, which also can lead to such loads.

[0005] It is thus an object of the invention to increase the service life of a linear unit, particularly to reduce loads acting on a linear unit due to deformation of an external structure to which the linear unit is mounted or due to thermal expansion of the linear unit.

[0006] This object is solved by a linear unit and a system comprising such a linear unit according to the independent claims. Preferred embodiments are subj ect to the dependent claims and the following description .

[0007] The linear unit for converting the rotational motion into translational motion, according to a first aspect o f the invention, comprises i ) a housing, ii ) a screw rotatably mounted within said housing, iii ) a nut cooperating with said screw, and iv) two mounting assemblies for mounting said housing to an external structure . Said mounting assemblies each comprise at least two trunnions arranged transversely to said screw, in particular to a rotational axis of the screw . Said trunnions are preferably movably mounted in, preferably annular, bearing shells arranged on opposite sides of the screw, in particular on opposite sides of the rotational axis of the screw .

[0008] An aspect of the invention is based on the approach to mount the linear unit to an external structure in a flexible, particularly articulated, manner such that deformation of said external structure is not imposed onto the linear unit, particularly onto the screw and / or a housing in which the screw is arranged . To this end, the linear unit preferably comprises a mounting system for mounting the linear unit to the external structure that allows for the linear unit , particularly the screw and / or the housing, to keep its original shape and / or orientation even when the external component deforms . The original shape preferably corresponds to the shape in absence of any load . For example , the mounting system may be configured to sel f-align the linear unit when the mounting points , at which the linear unit is mounted to the external structure , shi ft due to deformation of the external structure . The linear unit , particularly said mounting system, thus may at least partially accommodate any misalignments between the linear unit and the external structure caused by deformation of the external structure during operation . Also , the linear unit , particularly said mounting system, may at least partially accommodate any changes in the distance between mounting points at the linear unit caused by thermal expansion of the linear unit itsel f .

[0009] Preferably, the linear unit comprises two mounting assemblies for mounting the housing of the linear unit to the external structure , e . g . at four mounting points . Said mounting assemblies are advantageously configured to retain the l inear unit , particularly the nut and / or the housing, in an unloaded state , particularly independent of the position of the four mounting points relative to the linear unit . In other words , the four mounting points may, at least to a certain extend that is expected when considering thermal expansion or deformation of the external structure due to external loads , move relative to the linear unit without exerting any forces on the linear unit .

[0010] For example , the two mounting assemblies may each comprise two trunnions movably mounted in respective bearing shells . The bearing shells or trunnions thus may move with the external structure under deformation or with the linear unit under thermal expansion, respectively, without or with only partially trans ferring this movement to the respective counterpart . In other words , by movably mounting the trunnions in the bearing shells , the trunnions and the bearing shell s - and thereby the linear unit and the external structure - are preferably movably decoupled, at least to a certain extent . Such a trunnion movably mounted in a bearing shell may, for example , be rotatably, pivotably and / or translationally ( i . e . linearly movable ) mounted in said bearing shell . That means that the trunnion can rotate , pivot and / or translate relative to the bearing shell . To this end, the trunnion can be received by a corresponding seat within said bearing shell . Said seat may be, for example , rotatably and / or rotatably arranged in said bearing shell . Additionally or alternatively, said seat may allow for translational ( linear ) movement of the trunnion relative to the bearing shell .

[0011] By providing the linear unit with such a mounting system, particularly with such two mounting assemblies , functionality and reliability of the linear unit can be ensured, even on long timescales .

[0012] The nut and the screw preferably cooperate by means of a plurality of planetary rollers radially arranged between them .

[0013] The nut and the screw thus may form a planetary rol ler screw mechanism . Alternatively, the nut and the screw may cooperate by means of a plurality of balls , thus forming a ball screw mechanism .

[0014] Preferred embodiments of the invention and further aspects thereof are described below, each of which, unless expressly excluded, may be combined with each other and with the aspects of the invention described below as desired .

[0015] Said two mounting assemblies are preferably arranged at opposite ends of the linear unit , particularly the hous ing . Accordingly, the screw, particularly an outer threading of said screw, is arranged axially between said two mounting assemblies such that a sum of the axial distances between the nut and each of the mounting assemblies is constant . This allows for a particularly stable and robust attachment of the linear unit to the external structure .

[0016] Here and in the following, as long as not expressly indicated otherwise , "axial" or "axially" refers to a direction parallel to a longitudinal axis of the linear unit , i . e . parallel to the screw or its rotational axis , respectively .

[0017] In order to allow a decoupling of the linear unit from a deformation of the external structure that af fects the orientation of the mounting points , i . e . the components of the mounting assemblies fixedly attached to the external structure , the trunnions of at least one of said mounting assemblies are rotatably mounted in the respective bearing shells . The orientation of the mounting points is may be af fected e . g . by bending of the external structure in a plane perpendicular to the trunnions . Accordingly, it is preferred that the trunnions are rotatable about their longitudinal axis relative to the bearing shells .

[0018] In order to allow a decoupling from a bending of the external structure in a plane parallel to the trunnions , the trunnions of at least one of said mounting assemblies are pre ferably axially translatable relative to their respective bearing shell . To this end, the trunnions may have axial clearance with respect to their seat in the respective bearing shell . Here , "axial" or "axially" refers to the longitudinal axi s of the trunnions . Accordingly, the trunnions are advantageously further in and / or out of their respective seat .

[0019] In another preferred embodiment , one of said mounting assemblies is arranged at a headpiece of the linear unit , said headpiece being mounted at a front endplate of said housing . The front endplate may close said housing at its front, e . g . in a region adj acent to a free end of the screw . Having the headpiece mounted to the front endplate enables additional degrees of freedom for moving, particularly adapting orientation and / or axial position of , the mounting assembly arranged at the headpiece . For example , said headpiece may be pivotably mounted at said front endplate of said housing and / or axially trans latable with respect to said front endplate of said housing . By this means , the orientation and / or position of the corresponding mounting assembly may be altered without af fecting the orientation and / or position of the housing .

[0020] Such an adaptive connection between the front endplate and the headpiece may be achieved by providing a headpiece comprising a cylindrical bore in which an axial proj ection of said front endplate is received . In some embodiments , said proj ection has a lateral surface with an annular convex bulging . Said proj ection may be formed, for example , by a cylindrical trunnion having a spherical front end . The bulging of said proj ection advantageously contacts an inner wall of the central bore and may axially slide along said inner wall and / or pivot with respect to said wall .

[0021] Alternatively, the axial proj ection may be cylindrical , i . e . have a fully level lateral surface . In this case , the headpiece preferably comprises a cavity having an inner wall section formed complementary to a spherical surface section of a central bearing disc . Hence , said central bearing disc is pivotable within said cavity . The central bearing disc advantageously comprises a central bore in which the axial cylindrical proj ection of the front endplate is axially translatable . Hence , the headpiece and the central bearing disc may form a central spherical bearing which is axially translatable on the axial cylindrical proj ection of the front endplate .

[0022] Preferably, the trunnions of at least one of said mounting assemblies have a mounting base for fixedly attaching the trunnions to the external structure . In this case , the bearing shells of one of said mounting assemblies are advantageously arranged in the headpiece . The bearing shells are preferably at least partially formed by recesses in the headpiece, which receive the trunnions . Bushings can additionally be arranged within said recesses to facilitate rotation of the received trunnions relative to the headpiece .

[0023] Alternatively, the trunnions of one of said mounting assemblies extend from said headpiece on opposite sides . In this case , said trunnions are preferably integral with the headpiece , i . e . formed with the headpiece as a single piece . To improve adaptability of the orientation of the headpiece to a deformation of the external structure and / or thermal expansion of the linear unit , it is preferred that the trunnions are pivotably mounted in said bearing shells with respect to the bearing shell central axis . Particularly, the bearing shells may form spherical bearings for the trunnions .

[0024] Said bearing shell central axis preferably is paral lel to a surface normal of the opening of the bearing shell , i . e . an area enclosed by the bearing shell . When the bearing shells are mounted to the external structure at a mounting surface , the bearing shell central axes are preferably perpendicular to the mounting surface .

[0025] Another option for providing at least one additional degree of freedom for orienting and / or positioning the trunnions of the mounting assembly arranged at the headpiece relative to the external structure and / or the housing is to provide a headpiece comprising an eye in which a central spherical bearing is arranged . This central spherical bearing may be formed, in some embodiments , by bushings having spherical contact surfaces . Preferably, the trunnions extend from said central spherical bearing on both sides of the eye . In this case , the trunnions on both sides of the headpiece may be formed by a s ingle rod which is received by the central spherical bearing, particularly an inner bushing, arranged within the eye of the headpiece . Here , the headpiece is preferably formed integral with the front endplate of the housing .

[0026] In order to allow for axial translation of said trunnions relative to the housing, the trunnions preferably extend eccentrically from said central spherical bearing . To this end, the inner bushing may have a seat , e . g . a cylindrical bore, receiving the trunnions , particularly the single rod, which is eccentrically arranged with respect to a central longitudinal axis of said bushing . By means of this eccentric design of the central spherical bearing, a rotational movement of the trunnions about a central axis of the eye may have a translational component in the axial direction .

[0027] As the movements of the external structure relative to the linear unit due to deformation of the external structure and / or thermal expansion of the linear unit or at least some components thereof are small compared to the dimens ions of the linear unit , e . g . in the range of a few millimetres . Said central spherical bearing may be a deformable spherical bearing . Said deformable spherical bearing advantageously comprises an elastic material surrounding an inner bushing in which said trunnions are mounted . Particularly, the elastic material is arranged between an inner bushing and an outer bushing coaxial with the inner busing . Said bushings may have spherical surface sections facing each other, wherein said elastic material is arranged therebetween . By utili zing a deformable central spherical bearing, it is possible to absorb vibrations . Fur- ther, a certain amount of sti f fness can be provided in predefined direction as required, which is advantageous for standard operation of the linear unit .

[0028] As mentioned above , in order to allow for a pivoting movement of the trunnions in the bearing shells and thus further increase the degrees of freedom for movement and / or orientation of the linear unit relative to the external structure , said bearing shells may form spherical bearings . To this end, at least one of said mounting assemblies preferably comprises bearing discs rotationally mounted in the bearing shells of said mounting assembly . Preferably, said bearing di scs each have a socket receiving the respective trunnion . The bearing shells may each have a spherical inner surface section complementary with a spherical surface of the respective bearing disc . Hence , the bearing discs are pivotable within a cavity formed by the spherical inner surface of the bearing shells . When the bearing shells are fixedly attached to the external structure , the trunnions received by the sockets of the bearing discs may thus be pivotable relative to the external structure .

[0029] Alternatively or additionally, in order to compensate a compression or elongation of the external structure and / or the linear unit , the trunnions of at least one of said mounting assemblies are preferably translatable within said bearing shells in a direction perpendicular to a longitudinal axis of said trunnions , particularly parallel to the screw .

[0030] For example , at least one of said mounting assemblies may comprise bearing discs rotationally mounted in said bearing shells , wherein said bearing discs each have a socket receiving one of said trunnions . Preferably, said trunnions have two parallel flat surfaces contacting opposite inner surfaces of said sockets . Advantageously, said sockets are elongate such that the trunnions are translatable within said sockets . For example , said sockets may be formed by elongate holes in the bearing discs . Accordingly, each of said trunnions may independently translate within its bearing shell perpendicular to its longitudinal axis .

[0031] Alternatively, a translation of the trunnions within the bearing shells can be reali zed by providing at least one mounting assembly comprising bearing discs rotationally mounted in said bearing shells , wherein said bearing discs each have a socket receiving one of said trunnions . Advantageously, said sockets are eccentrically arranged at said bearing discs . A rotation of the trunnions about a central axis of said bearing discs or said bearing shells , respectively, also has a component in the axial direction, i . e . parallel to the screw .

[0032] The system, according to a second aspect of the invention, comprises a first component and a second component movable , particularly pivotable , relative to the first component . Preferably, a linear unit according to the first aspect of the invention is mounted to said first component by means of the mounting system, particularly the two mounting assemblies , and said second component is coupled to the nut of said linear unit .

[0033] In this configuration, the linear unit is preferably mounted to the first component in a free- floating manner . Particularly, this configuration may prevent or at least s igni ficantly reduce loads acting on the linear unit , particularly its screw, caused by deformation of the first component and / or thermal expansion of the linear unit . In other words , deformations of the first component such as bending, torsion and / or compression / elongation are not trans ferred the linear unit . This may signi ficantly increase service li fe of the linear unit , particularly its screw .

[0034] The properties , features and advantages of the invention described above , as well as the manner in which they are achieved, will be explained in more detail in connection with the figures in the following description of examples . Where appropriate , the same reference signs are used in the figures for the same or corresponding elements of the invention . The examples serve to explain the invention and do not limit the invention to the combinations of features indicated therein, even with respect to functional features . Moreover, any of the features disclosed in the above description as well as in the examples below may be considered in isolation and suitably combined with the features of any of the above embodiments and their further aspects . In particular, each of the features described above and below may be combined alone or in conj unction with others of the described features with the linear unit according to the first aspect of the invention and the system according to the second aspect of the invention .

[0035] It is shown, particularly schematically, in

[0036] Fig . 1 an example of a linear unit comprising two mounting ass e mb lies ;

[0037] Fig . 2 an example of a headpiece in a cross-sectional view;

[0038] Fig . 3 another example of a headpiece in a cross-sectional view;

[0039] Fig . 4 yet another example of a headpiece in a three-dimensional view; Fig . 5 an example of two mounting assemblies in a s ide view;

[0040] Fig . 6 another example of two mounting assemblies in a side view;

[0041] Fig . 7 an example of a flexible spherical bearing; and

[0042] Fig . 8 an example of a system comprising two components and a linear unit for pivoting the two components relative to each other .

[0043] Figure 1 shows an example of a linear unit 10 for converting rotational motion, e . g . of a motor 18 , into translational motion . Said linear unit 10 comprises a housing 12 , a screw 14 rotatably mounted within said housing 12 , a nut 16 cooperating with said screw 14 , and two mounting assemblies 30 for mounting the linear unit 10 , particularly said housing 12 , to an external structure (not shown) . The two mounting as semblies 30 may form a mounting system .

[0044] The motor 18 is operatively coupled to the screw 14 by means of a gear arranged within a gearbox 20 . The gearbox 20 is arranged at a distal or back end 12b of the housing 12 . Rotation of the screw 14 translates the nut 16 along the screw 14 . The nut 16 is coupled to a carriage 22 arranged outside of the housing 12 , such that upon movement of the nut 16 , the carriage 22 is taken along . Movement of the carriage 22 along the housing 12 is guided by guide rails 24 arranged on a top side of the housing 12 . The carriage 22 comprises an attachment structure 26 for attaching a component , for example a machine part , to the carriage 22 .

[0045] The mounting assemblies 30 each comprise two trunnions 32 mov- ably mounted in bearing shells (not visible in figure 1 ) . The two trunnions 32 of each mounting assembly 30 extend perpendicular to a longitudinal axis L of the linear unit 10 , particularly to the screw 14 . Advantageously, the two trunnions 32 of each mounting assembly 30 are arranged on opposite sides of the linear unit 10 , particularly the screw 14 . The linear unit 10 can thus be mounted to the external structure at four mounting points .

[0046] Each trunnion 32 comprises a mounting base 32a for fixedly attaching the respective trunnion 32 to the external structure . The mounting bases 32a may include , to this end, through holes 32c for screwing the trunnions 32 to the external structure ( for reasons of clarity, only one through hole 32c is indicated by a reference numeral ) .

[0047] One of the mounting assemblies 30 is arranged at the back end 12b of the housing 12 . The other mounting assembly 30 is arranged at a headpiece 40 of the linear unit 10 , wherein said headpiece 40 is mounted at a proximal or front end 12a of the housing 12 .

[0048] Preferably, the trunnions 32 of the mounting assembly 30 arranged at the back end 12b are only rotatably mounted in their respective bearing shells . In other words , the components of said mounting assembly 30 are all axially fixed . The trunnions 32 of the mounting assembly 30 arranged at the headpiece 40 , however, are preferably movably mounted in their respective bearing shells in a manner that allows movement of the headpiece 40 with respect to additional degrees of freedom, i . e . not only rotation, but also axial and radial movements . In the alternative or additionally, the headpiece 40 may be mounted to the housing 12 in a manner providing at least a part of said additional degrees of freedom . Accordingly, in the example of figure 1, the trunnions 32 follow any movement caused by a deformation of the external structure. Due to the trunnions 32 of the front mounting assembly 30 being movably mounted in their bearing shells and / or the headpiece 40 being movably mounted at the front end 12a, the linear unit 10 may retain its original orientation. In other words, movement of the trunnions 32 within the respective bearing shell and / or of the headpiece 40 relative to the housing 12 may compensate for any deformation, e.g. bending, torsion and / or compression / elongation, of the external structure. Likewise, movement of the trunnions 32 relative to the respective bearing shell may compensate for thermal expansion of the linear unit 10 or some of its components, particularly its screw 14. In this sense, the linear unit 10 may be attached to the external structure at it's front end 12a in a free-floating manner by means of the mounting assemblies 30. Due to the inherent stiffness of the linear unit's 10 components, particularly the housing 12, the mounting assemblies 30 can be regarded as being self-adjusting.

[0049] Figure 2 shows an example of such a headpiece 40 in a cross- sectional view. Here, said headpiece 40 is movably mounted to a front endplate 50 of the housing 12. The front endplate 50 closes the housing 12 at its front end 12a in the vicinity of a free end of the screw 14.

[0050] The headpiece 40 comprises a cylindrical bore 42 which receives an axial projection 52 of the front endplate 50. The axial projection 52, which preferably extends along the longitudinal axis L, is axially movable within said cylindrical bore 42. This may allow for compensating e.g. thermal expansion of the linear unit, or elongation / compression of the external structure E due to external loads. Additional degrees of freedom for positioning and / or orienting the external structure E relative to the linear unit may be enabled by providing the axial proj ection 52 with a convex, preferably spherical , annular surface section 54a . Said surface section 54a may be formed by a bulging 54 of the lateral surface 52a of the proj ection 52 . Preferably, under load, the surface section 54a contacts an inner wall of the cylindrical bore 42 in a point-like manner . This type of point- like contact between the axial proj ection 52 and the headpiece 40 allows the headpiece 40 to pivot about the axial proj ection 52 and also facilitates translation along the screw axis .

[0051] In the example shown in figure 2 , the trunnions 32 are fixedly attached via the mounting bases 32a to the external structure E . Particularly, the linear unit is arranged between two walls of the external structure E , wherein the trunnions 32 run through openings in said walls . The trunnions 32 are received in recesses 44 of the headpiece 40 . Here , the reces ses 44 form the bearing shells 34 of the mounting assembly 30 arranged at the headpiece 40 . In order to facilitate rotatability of the trunnions 32 in the bearing shells 34 , bushings 36 may optionally be arranged within the bearing shells 34 / reces ses 44 .

[0052] Preferably, the trunnions 32 have axial clearance D in the recesses 44 . Thus , the trunnions 32 can translate along their longitudinal axis A, enabling sel f-alignment of the mounting assembly 30 at the headpiece 40 upon bending of the external structure E in the figure plane of figure 2 .

[0053] Figure 3 shows another example of a headpiece 40 in a cross- sectional view . Similar to the example shown in figure 2 , the headpiece 40 is movably mounted at the front endplate 50 of the housing 12 . Here , however, the headpiece 40 comprises a, preferably spherical , cavity 46 in which a central bearing disc 48 is received . An inner surface section 46a of the cavity 46 is formed complementary to spherical surface section 48a of the central bearing disc 48 . The central bearing disc 48 is thus pivotable within said cavity 46 . In other words , the headpiece 40 and the central bearing disc 48 form a spherical bearing .

[0054] Preferably, the central bearing disc 48 comprises the cylindrical bore 42 in which the axial proj ection 52 of the front endplate 50 is received . Here , the axial proj ection 52 is cylindrical , i . e . has a level lateral surface 52a, such that it is only linearly translatable within the cylindrical bore 42 .

[0055] Further, in contrast to the example shown in figure 2 , the trunnions 32 of the mounting assembly 30 are fixedly attached to the headpiece 40 . Particularly, said trunnions 32 are integral with the headpiece 40 , i . e . formed with the headpiece 40 as a single piece . The free ends of said trunnions 32 are mov- ably mounted in the bearing shells 34 , wherein said bearing shells 34 may be fixedly attached to the external structure E .

[0056] In the present example , the trunnions 32 are not only rotatably, but also pivotably mounted in the bearing shel ls 34 , particularly with respect to a bearing shell central axis X ( indicated by a dashed line ) . To this end, the mounting assembly 30 comprises bearing discs 38 which provide a seat for the free ends of the trunnions 32 . The bearing discs 38 each comprise a spherical surface section 38a . An inner surface section 34a of the bearing shells 34 is formed as a complement , such that the bearing shells 34 and the bearing discs 38 together form a spherical bearing . The bearing shells 34 further comprise bores 38b receiving the trunnions 32 . The bores 38b thus form the seats for the trunnions 32 within the bearing shells 34 .

[0057] Preferably, the trunnions 32 , at least their free ends , are axially movable within the bores 38b, i . e . movable along their longitudinal axis A ( indicated by a dash-dotted line ) - and thus , with the bearing shells 34 attached to a mounting surface of the external structure E , perpendicular to said mounting surface .

[0058] Figure 4 shows yet another example of a headpiece 40 in a three-dimensional view . Here , the headpiece 40 is f ixedly attached to the front endplate 50 of the housing 12 . The headpiece 40 comprises an eye 64 in which a central spherical bearing 56 is arranged . Preferably, the trunnions 32 are extending from said central spherical bearing 56 on both sides of the eye 64 . The spherical bearing 56 may not only allow rotation of the trunnions 32 , but also pivoting about the central axis of said eye 64 .

[0059] In this example , the trunnions 32 may be formed by a single rod 58 , which runs through the eye 64 and the seat provided by the spherical bearing 56 .

[0060] Figure 5 shows an example of two mounting assemblies 30 in a side view . A linear unit 10 is mounted by means of said mounting assemblies 30 to an external structure E .

[0061] One of said mounting assemblies 30 is arranged at a back end 12b of a housing 12 of the linear unit 10 . Said mounting assembly 30 comprises two trunnions 32 ( of which only one is visible ) rotatably mounted in bearing shells 34 ( of which also only one is visible ) . For facilitating rotation, a bushing 36 is arranged radially between each trunnion 32 and the corresponding bearing shell 34 .

[0062] The other of said mounting assemblies 30 is arranged at a headpiece 40 of the linear unit 10 . The headpiece 40 is mounted to the housing 12 , particularly to a front endplate thereof (not indicated) , at a front end 12a of the housing 12 . Trunnions 32 of said mounting assembly 30 are movably mounted in bearing shells 34 as well .

[0063] In contrast to the mounting assembly 30 at the back end 12b, the trunnions 32 of the mounting assembly 30 at the headpiece 40 are not only rotatable within the respective bearing shell 34 , but also pivotable . To this end, the trunnions 32 are seated in bearing discs 38 . Said bearing discs 38 form spherical bearings with the corresponding bearing shells 34 , as has been elaborated in conj unction with figure 3 .

[0064] Additionally, the trunnions 32 are also translatable within their bearing shells 34 in a direction perpendicular to a longitudinal axis of the trunnions 32 (which is perpendicular to the figure plane in figure 5 ) . Particularly, the trunnions 32 may be translatable in a direction parallel to a longitudinal axis L of the linear unit 10 .

[0065] To this end, the trunnions 32 of the mounting assembly 30 at the headpiece 40 comprise two parallel flat surfaces 32b, which contact inner surfaces 60a of a socket 60 in the bearing discs 38 . Said sockets 60 thus define the seats of the trunnions 32 within the bearing shells 34 .

[0066] Advantageously, the sockets 60 are elongate , and the trunnions 32 have clearance within said elongate sockets 60transverse to their longitudinal axis . Figure 6 shows another example of two mounting assemblies 30 in a side view . As in figure 5 , a linear unit 10 is mounted by means of said mounting assemblies 30 to an external structure E .

[0067] The mounting assemblies 30 correspond to the mounting assemblies shown in figure 5 . The only di f ference is that translatability of the trunnions 32 of the mounting assembly 30 arranged at the headpiece 40 is not achieved by means of elongate sockets , but rather by an eccentric arrangement of the sockets 60 at the bearing discs 38 . In other words , the sockets 60 which receive the trunnions 32 are arranged at a distance to a centre of the bearing discs 38 . By this means , upon rotation of the bearing discs 38 within the bearing shells 34 , the trunnions 32 follow a curved path and translate perpendicular to their longitudinal axis . This translation along the curved path also has a component parallel to the longitudinal axis L of the linear unit 10 .

[0068] In the shown example , the center of the bearing shells 34 of the mounting assembly 30 at the headpiece 40 - and thus the pivot point or axis , respectively, of the bearing discs 38 within said bearing shells 34 - is radially of fset 0 with respect to the longitudinal axis L . The of fset 0 is preferably chosen such that in a neutral position of the linear unit 10 relative to the external structure E , i . e . without deformation of the external structure E and no thermal expansion of the linear unit 10 , the trunnions 32 are at the same height , i . e . arranged in a plane parallel to the longitudinal axis . Particularly, the longitudinal axis L and the center of said trunnions 32 , or at least their seats 60 , lie in the same plane . Figure 7 shows an example of a flexible spherical bearing in a cross section . This type of spherical bearing can be utilised, for example , as a central spherical bearing ( reference 56 in figure 4 ) . The flexible spherical bearing comprises two coaxially aligned bushings 62a, 62b, between which an elastic material 66 is arranged . The inner bushing 62a has an at least sectionally convex, preferably spherical , circumferential surface . The outer bushing 62b has a complementary, at least sectionally concave , preferably spherical , inner surface . However, in an alternative embodiment , the bushings 62 a, 62b may have level circumferential or inner surfaces , respectively . By utilising such a flexible spherical bearing, radial sti f fness required for standard operation may be provided, were radial , torsional , axial and conical degrees of freedom are achieved by the elastic material 66 in predefined directions as required .

[0069] Alternatively or additionally to utili zing the flexible spherical bearing as a central spherical bearing, such f lexible spherical bearings may also be used to movably mount trunnions at an external structure . Particularly, such flexible spherical bearings may in principle replace the spherical bearings formed by bearing discs and bearing shells shown e . g . in figure 3 . For example , the bearing discs and the bearing shells shown in figure 3 can be provided with the flexible material 66 arranged between them .

[0070] Figure 8 shows an example of a system 100 comprising a first component 102 and a second component 104 pivotably mounted at the first component 102 . The system 100 further comprises a linear unit 10 mounted to the first component 102 by means of two mounting assemblies 30 . A nut (not shown) of the linear unit 10 is coupled to the second component 104 , such that upon linear translation of the nut , the second component 104 pivots about a j oint 106 relative to the first component 102 . The connection between the nut and the second component 104 may be established by means of a carriage having an attachment structure 26 ( cf . Figure 1 ) and a connector 108 attached to the at- tachment structure 26 at one end and to the second component 104 at the opposite end .

[0071] By means of the two mounting assemblies 30 , the linear unit 10 can be decoupled from deformations of the first component 102 cause by e . g . load exerted by the second component 104 . Similarly, one of the mounting assemblies 30 may sel f-align during thermal expansion of the linear unit 10 such that no load is generated .

[0072] List of reference signs

[0073] 10 linear unit

[0074] 12 housing

[0075] 12a front end

[0076] 12b back end

[0077] 14 screw

[0078] 16 nut

[0079] 18 motor

[0080] 20 gearbox

[0081] 22 carriage

[0082] 24 guide rail

[0083] 26 attachment structure

[0084] 30 mounting assembly

[0085] 32 trunnion

[0086] 32a mounting base

[0087] 32b flat surface

[0088] 32c through holes

[0089] 34 bearing shell

[0090] 34a inner surface section

[0091] 36 bushing

[0092] 38 bearing disc

[0093] 38a spherical surface section

[0094] 38b bore

[0095] 40 headpiece

[0096] 42 bore

[0097] 44 recess

[0098] 46 cavity

[0099] 46a inner surface section

[0100] 48 central bearing disc

[0101] 48a spherical surface section

[0102] 50 front endplate 52 proj ection

[0103] 52a lateral surface

[0104] 54 bulging

[0105] 54a convex annular surface section

[0106] 56 central spherical bearing

[0107] 58 rod

[0108] 60 socket

[0109] 60a inner surface

[0110] 62a inner bushing

[0111] 62b outer bushing

[0112] 64 eye

[0113] 66 elastic material

[0114] 100 system

[0115] 102 first component

[0116] 104 second component

[0117] 106 j oint

[0118] 108 connector

[0119] E external structure

[0120] L longitudinal axis

[0121] A longitudinal axis

[0122] X bearing shell central axis

[0123] D clearance

[0124] 0 of fset

Claims

Claims1. A linear unit (10) for converting rotational motion into translational motion, comprising- a housing ( 12 ) ,- a screw (14) rotatably mounted within said housing (12) ,- a nut (16) cooperating with said screw (14) , and- two mounting assemblies (30) for mounting said housing (12) to an external structure (E) , wherein said mounting assemblies (30) each comprise at least two trunnions (32) arranged transversely to said screw (14) , said trunnions (32) being movably mounted in bearing shells (34) arranged on opposite sides of the screw (14) .

2. The linear unit (10) according to claim 1, wherein the trunnions (32) of at least one of said mounting assemblies (30) are rotationally mounted in their respective bearing shells (34) .

3. The linear unit (10) according to claim 1 or 2, wherein the trunnions (32) of at least one of said mounting assemblies (30) are axially translatable relative to their respective bearing shells (34) .

4. The linear unit (10) according to any one of the preceding claims, wherein one of said mounting assemblies (30) is arranged at a headpiece (40) of the linear unit (10) , said headpiece (40) being mounted at a front endplate (50) of said housing (12) .

5. The linear unit (10) according to claim 4, wherein said headpiece (40) is pivotably mounted at said front endplate (50) of said housing (12) and / or axially translatablewith respect to said front endplate (50) of said housing ( 12 ) .

6. The linear unit (10) according to claim 5, wherein said headpiece (40) comprises a cylindrical bore (42) in which an axial projection (52) of said front endplate (50) is received, said projection (52) having a lateral surface (52a) with an annular convex bulging (54) .

7. The linear unit (10) according to claim 5, wherein said headpiece (40) comprises a cavity (46) having an inner surface section (46a) formed complementary to a spherical surface section (48a) of a central bearing disc (48) such that said central bearing disc (48) is pivotable within said cavity (46) , and the central bearing disc (48) comprises a cylindrical bore (42) in which an axial cylindrical projection (52) of said front endplate (50) is axially translatable .

8. The linear unit (10) according to any one of claims 4 to 7, wherein the trunnions (32) of at least one of said mounting assemblies (30) have a mounting base (32a) for fixedly attaching the trunnions (32) to the external structure (E) , and the bearing shells (34) of one of said mounting assemblies (30) are arranged in said headpiece (40) .

9. The linear unit (10) according to any one of claims 4 to 7, wherein the trunnions (32) of one of said mounting assemblies (30) extend from said headpiece (40) on opposite sides and are pivotably mounted in said bearing shells (34) with respect to a bearing shell central axis (X) .

10. The linear unit (10) according to claim 9, wherein said headpiece (40) comprises an eye (64) in which a central spherical bearing (56) is arranged, said trunnions (32) extending from said central spherical bearing (56) on both sides of the eye (64) .

11. The linear unit (10) according to claim 10, wherein said trunnions (32) extend eccentrically from said central spherical bearing (56) .

12. The linear unit (10) according to any one of claims 10 or 11, wherein said central spherical bearing (56) is a deformable spherical bearing comprising an elastic material (66) surrounding an inner bushing (62a) in which said trunnions (32) are mounted.

13. The linear unit (10) according to any one of the preceding claims, wherein at least one of said mounting assemblies (30) comprises bearing discs (38) rotationally mounted in the bearing shells (34) of said mounting assembly (30) , said bearing discs (38) each having a socket (60) receiving the respective trunnion (32) , and said bearing shells (34) each having a spherical inner surface section (34a) complementary with a spherical surface section (38a) of the respective bearing disc (38) such that spherical bearings are formed.

14. The linear unit (10) according to any one of the preceding claims, wherein the trunnions (32) of at least one of said mounting assemblies (30) are translatable within said bearing shells (34) in a direction perpendicular to a longitudinal axis (A) of said trunnions (32) .

15. The linear unit (10) according to claim 14, wherein at least one of said mounting assemblies (30) comprises bearing discs (38) rotationally mounted in said bearing shells (34) , said bearing discs (38) each having a socket (60) receiving one of said trunnions (32) , and said trunnions (32) having two parallel flat surfaces (32b) contacting opposite inner surfaces (60a) of said sockets (60) , and said sockets (60) are elongate such that said trunnions (32) are translatable within said sockets (60) .

16. The linear unit (10) according to claim 14, wherein at least one of said mounting assemblies (30) comprises bearing discs (38) rotationally mounted in said bearing shells (34) , said bearing discs (38) each having a socket (60) receiving one of said trunnions (32) , and said sockets (60) are eccentrically arranged at said bearing discs ( 38 ) .

17. A system (100) comprising a first component (102) and a second component (104) movable relative to the first component (102) , wherein a linear unit (10) according to any one of the preceding claims is mounted to said first component (102) by means of the two mounting assemblies (30) , and said second component (104) is coupled to the nut (16) of said linear unit (10) .

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