WO2016078687A1

WO2016078687A1 - Screw nut assembly

Info

Publication number: WO2016078687A1
Application number: PCT/EP2014/074759
Authority: WO
Inventors: Jérôme DUBUS
Original assignee: Aktiebolaget Skf
Priority date: 2014-11-17
Filing date: 2014-11-17
Publication date: 2016-05-26

Abstract

The screw nut assembly comprises a screw (12) provided with an outer thread, a nut (16) surrounding and coaxial with said screw, said nut being provided with an inner thread, and at least one guide means (24) provided on the nut and protruding radially outwards with respect to said nut. Said assembly further comprises a guide member (22) comprising at least one guide portion (26) cooperating with said guide means and extending both axially and circumferentially on the guide member so as to guide combined axial and angular movements of the nut relative to said guide member.

Description

Screw nut assembly

The present invention relates to the field o f screw nut assemblies comprising screw mechanisms for transforming a rotary movement into a linear translation movement, and vice versa.

Ro ller and ball screw mechanisms are used in a variety o f industries for example to displace loads by transforming rotary action into linear motion. A ro ller screw mechanism is generally provided with a screw having an outer thread, with a nut arranged around the screw and having an inner thread, and with a plurality o f longitudinal rollers having an outer thread engaging the outer and inner threads of the screw and of the nut. The outer thread of each roller is extended axially at each end by gear teeth themselves extended axially by a cylindrical stud or pivot extending outwards.

In a ball screw mechanism, the rolling engagement between the screw and the nut is achieved by a plurality o f balls engaged into both of the threads provided on said screw and nut. Recirculating means may be provided on the nut or the screw to achieve the recirculation o f the balls. With regard to a ball screw mechanism, the main advantage of a roller screw mechanism is that their admissible static and dynamic lo ad capacities are higher.

Otherwise, it is also known other type of screw mechanism having a direct threaded connection. In this case, the outer thread of the screw engages directly the inner thread of the nut without interposition o f rolling elements.

With such screw mechanisms, when the rotating member is driven by a motor, either the screw or the nut, the resulting axial displacement of the non-rotating member is defined by the lead o f the screw.

It is a particular object of the present invention to provide a screw nut assembly wherein, with a fixed design o f the threads of the screw and nut and with a constant rotation speed applied to one of these members, the resulting axial displacement of the other member may be adjusted at the desired length per turn.

In one embodiment, the screw nut assembly comprises a screw provided with an outer thread, a nut surrounding and coaxial with said screw, said nut being provided with an inner thread, and at least one guide means provided on the nut, or the screw, and protruding radially outwards with respect to said nut or screw. Said assembly is further provided with a guide member comprising at least one guide portion cooperating with said guide means and extending both axially and circumferentially on the guide member so as to guide combined axial and angular movements of the nut, or the screw, relative to said guide member.

In one embodiment, said guide portion comprises facing guide surfaces extending parallel with each other and in contact with the guide means .

The guide member may further comprise a straight anti-rotation portion extending axially so as to prevent angular movement of the nut, or the screw, and connected to an end of said guide portion.

In one embodiment, said guide portion of the guide member has a helicoidal form.

Said guide portion o f the guide member may extend circumferentially in a direction identical to the thread direction o f the screw. Alternatively, said guide portion may extend circumferentially in a direction opposite to the thread direction o f the screw.

In one preferred embo diment, the guide member comprises at least one groove within which engage said guide means and delimiting said guide portion. The groove may be formed in the thickness of the guide member. Alternatively, the groove may be attached to the guide member.

In one embo diment, said guide means comprise a rolling means connected with the nut or the screw and rotatable about an axis lo cated at a fixed position on said nut or screw, said rolling means comprising an outer surface rollable along the guide portion. In one embodiment, the assembly comprises a plurality o f guide means provided on said nut or screw and a plurality o f guide portions provided on the guide member, each guide means cooperating with one associated guide portion.

In one embodiment, the assembly further comprises a plurality of ro lling elements radially disposed between the screw and the nut and engaged in both of the outer and inner threads of said screw and nut. Alternatively, the outer thread of the screw may engage directly the inner thread o f the nut.

The invention also relates to an actuator comprising a rotating means and a screw nut assembly as previously defined, the screw o f said assembly being coupled with the rotating means.

The present invention and its advantages will be better understood by studying the detailed description o f specific embodiments given by way o f non-limiting examp les and illustrated by the appended drawings on which:

Figure 1 is a cross-section o f a screw nut assemb ly according to a first example of the invention,

Figure 2 is a detail view o f Figure 1 ,

- Figure 3 is a front view o f the assembly o f Figure 1 ,

Figure 4 is a cross-section o f a screw nut assemb ly according to a second example of the invention,

Figure 5 is a cross-section o f a screw nut assemb ly according to a third example o f the invention,

- Figure 6 is a perspective view of the assembly o f

Figure 5 ,

Figure 7 is a cross-section o f a screw nut assemb ly according to a fourth example of the invention, and

Figure 8 is a perspective view of the assembly o f Figure 7.

A screw nut assembly 10 as shown on Figure 1 comprises a planetary roller screw mechanism 1 1 comprising a screw 12, with an axis 12, provided with an outer thread 14 , a nut 16 mounted coaxially about the screw 12 and provided with an inner thread 1 8 , the internal diameter of which is greater than the external diameter of the outer thread 14, and a plurality o f longitudinal rollers 20 arranged radially between the screw 12 and the nut 16. The screw 12 extends longitudinally through a cylindrical bore of the nut 16 on which the inner thread 1 8 is formed. The lead o f the outer thread 14 of the screw is constant. The screw 12 is adapted to be connected to a driving motor (not shown) . Only the screw 12 is connected to a motor.

As will be described later, the screw nut assembly 10 further comprises a fixed guide member 22 separate from the nut 16 and the screw 12, and guide means 24 connected to the nut and cooperating with a groove 26 formed on said guide member.

The ro llers 20 o f the screw mechanism are identical to each other and are distributed regularly around the screw 12. Each roller 20 extends along an axis 20a which is coaxial with the axis 12a o f the screw and comprises an outer thread 28 engaging the thread 14 of the screw and the thread 1 8 of the nut. The thread 28 o f each roller is extended axially at each end by outer gear teeth 30, 32 themselves extended axially by a cylindrical stud 34, 36 extending outwards .

As shown more clearly on Figure 2, the mechanism 1 1 also comprises two annular gear wheels 38 , 40 fixed in a non-threaded part of the bore o f the nut 16 and each comprising inner gear teeth (not referenced) meshing the gear teeth 30, 32 respectively o f the ro llers 20 for the synchronization thereof. Each gear wheel 38 , 40 axially bears against a radial surface o f the nut provided between the inner thread 1 8 and the associated non-threaded part of said nut. The gear wheels 38 , 40 are identical to one another.

The mechanism 1 1 further comprises two annular guides or spacer rings 42, 44 which are each mounted radially between the thread 14 o f the screw and the associated gear wheel 38 , 40. The spacer rings 42, 44 are identical to one another. Each spacer ring 42, 44 comprises a plurality o f cylindrical through-recesses (not referenced) which are distributed regularly in the circumferential direction and inside which the studs 34 , 36 o f the rollers 20 are housed. The spacer rings 42 , 44 enable the rollers 20 to be carried and the regular circumferential spacing thereof to be kept. The mechanism 1 1 further comprises elastic retainer rings 46, 48 each mounted in a groove (not referenced) formed on the bore of the associated spacer ring 42, 44 in order to radially ho ld the rollers into the recesses o f said spacer ring.

The guide means 24 are secured to the nut 16. The guide means 24 radially protrude outwards with respect to the outer surface of the nut 16. The guide means 24 are located at a fixed position on the nut 16. As will be described later, the guide means 24 are rollable along the groove 26 provided on the guide member 22 (Figure 1 ) .

The guide means 24 comprise a pin 50 connected with the nut 16 and a roller 52 rotatably coupled with the pin. The roller 52 is maintained on the pin 50 by a screw. In the illustrated example, the guide means 24 further comprise a sleeve 56 radially surrounding the part of the pin 50 which protrudes outwards . The sleeve 56 radially bears against the outer surface of the nut 16.

The pin 50 is connected to the nut 16. The pin 50 radially protrudes outwards with respect to the outer surface of the nut 16. The pin 50 comprises an inner mounting portion (not referenced) mounted into a ho le 54 o f the nut and an outer mounting portion (not referenced) onto which is rotatably mounted the roller 52. In the illustrated example, the inner mounting portion of the pin 50 comprises an external thread cooperating with an internal thread of the ho le 54 to secure the guide means 24 onto the nut 16. Alternatively, the pin 50 may be secured to the nut 16 by any other appropriate means, for instance by press-fitting, gluing, or welding, etc. The ho le 54 extends radially from the outer surface of the nut and open into the inner thread 1 8. The hole 54 extends through the thickness of the nut 16.

The ro ller 52 is radially spaced apart from the nut 16. The roller 52 is mounted into the groove 26 provided on the guide member 22 (Figure 1 ) and is rollable along said groove. The ro ller 52 engages within the groove 26. The roller 52 is rotatable about an axis 52 a lo cated at a fixed position on the nut 16. The axis 52a is perpendicular to the axis 12a of the screw. With regard to the axis 12a, the axis 52 a extends radially. The roller 52 comprises an outer rolling surface 52b . In the illustrated example, the rolling surface 52b has a cylindrical form. In another embodiment, the rolling surface may have a profile with curvatures or be a combination o f several surfaces.

Referring once again to Figure 1 , the guide member 22 radially surrounds the nut 1 6 while remaining radially distant from the later. In the illustrated example, the guide member 22 forms an annular sleeve or casing. The groove 26 is formed on the guide member 22. The groove 26 is made into the thickness o f the guide member. In the illustrated example, the groove 26 extends radially from the bore o f the guide member 22 and opens on its outer surface. Alternatively, the groove 26 may not open on the outer surface o f the guide member 22 with regard to the axis 12a.

In the illustrated example, the groove 26 extends helicoidally on the guide member 22 as clearly shown on Figure 3. The groove 26 extends both axially and circumferentially on the guide member 22. The groove 26 is delimited by two guide surfaces 26a, 26b facing each other. The guide surfaces 26a, 26b extend parallel to each other. The guide surfaces 26a, 26b extend both axially and circumferentially along the guide member 22. The roller 52 is disposed between the guide surfaces 26a, 26b of the groove. The guide surfaces 26a, 26b form guideways for the roller 52. The roller 52 rolls along the guide surfaces 26a, 26b o f the groove and comes into contact with said surfaces. The spacing distance between the surfaces 26a, 26b is slightly greater than the diameter of the roller 52 at least at the contact position.

When the screw 12 is driven by the associated motor and rotates with respect to the guide member 22, the rollers 20 rotate on themselves and roll about the screw 12 without moving axially inside the nut 16. The rollers 20 are rotationally guided parallel with the axis 20a by the gear wheels 38 , 40 and by the spacer rings 42, 44.

The rotation of the screw 12 is converted by the rollers 20 into a displacement of the nut 16. With the guide means 24 secured to the nut 16 and engaging into the helicoid groove 26 of the guide member 22, said nut is forced to rotate relative to the guide member 22. The roller 52 of the guide means rolls along the guide surfaces 26a, 26b o f the groove.

With the cooperation between the guide means 24 and the groove 26, the rotation o f the screw 12 is converted into an axial translation combined with an angular movement o f the nut 16 with respect to the guide member 22. The nut 16 moves simultaneously both in the axial and circumferential directions. The groove 26 guides the combined axial and angular movements of the nut 16 relative to the guide member 22. An angular displacement of the nut 16 relative to the guide member 22 and to the screw 12 is forced when said screw rotates .

The resulting displacement of the nut 16 is the combination o f two movements, i. e. the one generated by the lead o f the screw 12 under rotation and the other generated by the design o f the groove 26 of the guide member. The nut 16 is axially moveable simultaneously in the axial and circumferential directions with regard to the guide member 22. The groove 26 forms a guiding path for the guide means 24 and enables to obtain, during rotation of the screw 12, a translation movement of the nut combined to a rotation movement with regard to said screw and the guide member 22.

With the provision o f the guide means 24 on the nut 16 cooperating with the groove 26 of the member guide, the resulting axial displacement of the nut per turn is not equal to the lead o f the screw 12. If the thread 14 of the screw is a right-hand thread and if the groove 26 has also a right-hand angle direction as shown on the Figures, the axial travel per turn o f the nut 16 is higher than the lead of the screw 12. On the contrary, if the groove 26 of the guide member extends circumferentially in a direction opposite to the thread direction of the screw 12, the axial travel of the nut 16 per turn of the screw is smaller than the lead o f said screw. Therefore, the value o f the axial displacement of the nut 16 per turn of the screw 12 results from the lead o f the screw and the design of the groove 26. Accordingly, the value of this resulting axial displacement per turn may be easily adjusted at the desired length with a mo dification o f the groove 26.

In the first examp le, the groove 26 extends helicoidally on the guide member 22. Alternatively, the groove may also have different shape. For example, in the second example illustrated on Figure 4, in which identical parts are given identical references, the groove 26 is defined by a curve extending both axially and circumferentially on the guide member. In this example, the groove 26 has an increased pitch. Here, the guide member 26 is not annular. The groove 26 is formed radially towards the outside from the bore of the guide member 22 and do not open on the outer surface of said member.

In the two previous embodiments, the who le length o f the groove 26 extends both axially and circumferentially on the guide member 26 and forms a guide portion so as to guide combined axial and angular movements o f the nut 16 relative to said guide member when the screw 12 rotates .

In the third example illustrated on Figures 5 and 6, in which identical parts are given identical references, the guide member 22 o f the assembly comprises a groove 60 provided with a straight anti- rotation portion 62 extending parallel to the axis 12a o f the screw and with a curved guide portion 64 extending both axially and circumferentially on said guide member.

The guide portion 64 is connected to an end o f the straight portion 62. The guide portion 64 extends said end of the straight portion 62. The guide portion 64 extends both axially and circumferentially while the straight portion 62 extends purely axially. The groove 60 is formed radially towards the outside from the bore o f the guide member 22 and do not open on the outer surface of said member. The groove 60 is delimited by two guide surfaces 60a, 60b facing each other. The guide surfaces 60a, 60b extend parallel to each other. Along the straight portion 62 o f the groove the guide surfaces 60a, 60b extend axially on the guide member 22, i. e. parallel with the axis 12a o f the screw. Along the guide portion 64 o f the groove the guide surfaces 60a, 60b extend both axially and circumferentially on the guide member 22. The roller 52 of the guide means is disposed between the guide surfaces 60a, 60b of the groove.

When the screw 12 is driven about its axis 12a, the rotation o f said screw is converted into a displacement of the nut 16 relative to guide member 22. When the roller 52 of the guide means engages within the straight portion 62 o f the groove o f the guide member, the rotation o f said screw is converted into a linear translation o f the nut 20. Said linear guidance o f the nut 16 is parallel to the axis 12a of the screw. The nut 16 only has an axial movement relative to the screw 12 and the guide member 22. An angular movement of the nut 16 is prevented by the cooperation of the guide means 24 with the straight portion 62 of the groove. The ro ller 52 of the guide means rolls along the guide surfaces 60a, 60b of the straight portion 62 of the groove. The straight portion 62 o f the groove forms anti-rotation means for the nut 16 cooperating with associate anti-rotation means formed by the guide means 24.

When the roller 52 of the guide means reaches the guide portion 64 o f the groove, the rotation o f said screw is converted into combined movements o f translation along the axis 12a and rotation around said axis o f the nut 1 6 relative to the guide member 22 and to the screw. The nut 16 moves simultaneously both in the axial and circumferential directions.

Accordingly, under constant rotation speed of the screw 12, the assembly 10 has a variable lead. A first lead is obtained when the guide means 24 are engaged within the straight portion 62 of the groove o f the guide member 22 and a second lead is obtained when the guide means are engaged within the guide portion 64 of said groove. The lead o f the assembly 10 varies along the displacement of the guide means 24 into the groove of the guide member 22. In another embodiment, it could be possible to provide more than two leads for the assembly.

In the previous embodiments, the guide means 24 are connected to the nut 1 6 of the roller screw mechanism. Alternatively, the guide means 24 may be coupled to the screw 12 of said mechanism as shown in the fourth example illustrated on Figures 7 and 9, in which identical parts are given identical references. In this embodiment, the nut 12 may be connected to a driving motor (not shown) and not the screw 12.

The guide means 24 radially protrude outwards with respect to the outer surface of the screw 12. The guide means 24 are located at a fixed position at end o f the screw 12. The roller 52 o f the guide means is radially spaced apart from the screw 12. The roller 52 is mounted into a groove 70 provided on the guide member 22 and is ro llable along said groove. The roller 52 is rotatable about the axis 52a located at a fixed position on the screw 1 6. The groove 70 is similar to the one described in the third embodiment. The groove 70 comprises a straight anti-rotation portion 72 extending parallel to the axis 12a of the screw and a curved guide portion 74 extending said straight portion both axially and circumferentially on said guide member. The groove 70 is also delimited by two guide surfaces 70a, 70b facing and extending parallel with each other. In this embo diment, the guide member 22 is axially spaced apart from the nut 1 6.

When the nut 16 is driven about the axis 12a, the rotation o f said nut is converted into a linear translation of the screw 12 if the guide means 24 engage within the straight portion 72 of the groove o f the guide member, or in a linear translation combined with a rotation of said screw relative to the guide member 22 if the guide means 24 engage the guide portion 74 of the groove .

In the illustrated examples, the guide means are connected or coupled directly to the nut, or to the screw. Alternatively, it could be possible to foresee an indirect connection between the guide means and the nut, or the screw, in which one or more other components are interposed therebetween. In the disclosed examples, the guide means are provided with a rotatable roller in order to limit friction with the groove of the guide member. Alternatively, other component may be foreseen to engage within the groove, for example a rolling bearing, a plain bearing or a fixed pin sliding along the groove. Guide means may also have a combined movement o f rolling and sliding along the groove.

In another variant, it could also be possible to use several guide means connected to the nut or the screw. In this case, it is necessary to provide for each guide means one associated groove on the guide member. In the disclo sed examples, the groove is formed directly in the thickness of the guide member. Alternatively, the groove may be delimited by two separate plates attached directly to the guide member by any other appropriate means.

In another variant, it could also be possible to foresee other means on the guide member than a groove to cooperate with the guide means, for example two separate circular wires secured to the guide member. In this case, the guide means engage between the wires.

The invention has been illustrated on the basis of a screw nut assembly comprising a planetary roller screw mechanism. The invention can also be applied with other type of screw mechanism such as inverted roller screw mechanism, recirculating roller screw or ball screw mechanism. Otherwise, the invention can also be applied to a screw nut assembly having other type of transmission screw mechanism for instance a direct thread connection. In this case, the outer thread of the screw engages directly the inner thread of the nut without interposition o f rolling elements. However, such transmission mechanism requires large actuation torques.

Claims

1 . Screw nut assembly comprising a screw ( 12) provided with an outer thread, a nut ( 16) surrounding and coaxial with said screw, said nut being provided with an inner thread, and at least one guide means (24) provided on the nut, or the screw, and protruding radially outwards with respect to said nut or screw, characterized in that said assembly further comprises a guide member (22) comprising at least one guide portion (26; 64; 74) cooperating with said guide means and extending both axially and circumferentially on the guide member so as to guide combined axial and angular movements of the nut, or the screw, relative to said guide member.

2. Assembly according to claim 1 , wherein said guide portion comprises facing guide surfaces (26a, 26b ; 60a, 60b; 70a, 70b) extending parallel with each other and in contact with the guide means (24) .

3. Assembly according to claim 1 or 2, wherein said guide member (22) further comprises a straight anti-rotation portion (62; 72) extending axially so as to prevent angular movement of the nut, or the screw, and connected to an end of said guide portion (64; 74) .

4. Assembly according to any of the preceding claims, wherein said guide portion of the guide member has a helicoidal form.

5. Assembly according to any one of the preceding claims, wherein said guide portion (26 ; 64; 74) of the guide member extends circumferentially in a direction identical to the thread direction o f the screw.

6. Assembly according to any o f the preceding claims 1 to 4, wherein said guide portion (26 ; 64; 74) of the guide member extends circumferentially in a direction opposite to the thread direction o f the screw.

7. Assembly according to any of the preceding claims, wherein the guide member comprises at least one groove (26 ; 60; 70) within which engage said guide means (24) and delimiting said guide portion (26; 64; 74) .

8. Assembly according to claim 7, wherein the groove is formed in the thickness of the guide member (22) .

9. Assembly according to claim 7, the groove is attached to the guide member (22) .

10. Assembly according to any of the preceding claims, wherein said guide means (24) comprise a rolling means (52) connected with the nut or the screw and rotatable about an axis (52a) lo cated at a fixed position on said nut or screw, said rolling means comprising an outer surface (52b) rollable along the guide portion.

1 1 . Assembly according to any of the preceding claims, comprising a plurality o f guide means provided on said nut or screw and a p lurality o f guide portions provided on the guide member, each guide means cooperating with one associated guide portion.

12. Assembly according to any o f the preceding claims, further comprising a plurality o f rolling elements (20) radially disposed between the screw and the nut and engaged in both of outer and inner threads of said screw and nut.

13. Actuator comprising a rotating means and a screw nut assembly according to any of the preceding claims, the screw of said assembly being coupled with the rotating means.