The present invention is directed to a screw and nut linear drive assembly comprising a housing, a screw within the housing, an electric motor for rotating the screw, a nut which i s driven, when the screw is rotated, to move linearly in a longitudinal direction coinciding with the screw axis , an axial force trans fer arrangement to trans fer force from the nut to a push rod to be connected to a member to be actuated by the screw and nut linear drive assembly .

WO 2019/238224 Al discloses a screw and nut linear drive assembly according to the preamble of claim 1 . The linear drive assembly disclosed therein is an electric clutch actuator . This electric clutch actuator comprises a screw within a housing, an electric motor for rotating the screw, a nut which i s driven, when the screw is rotated by the electric motor, to move linearly in a longitudinal direction coinciding with the screw axis , and an axial force transfer arrangement to transfer movement of the nut to a push rod configured to trans fer force to a member to be actuated by the screw and nut linear drive assembly . The push rod is extending in a direction parallel to the screw axis , but is transversely of fset with respect to the screw axis . The push rod acts on an end portion of a clutch lever such that when the push rod is shi fted in longitudinal direction the clutch lever is driven to pivot to disengage the clutch when the linear screw and nut arrangement is activated and exerting a force on the push rod . A spring rests with one of its ends on the nut and is , with its opposite end, in abutment against a pressure piece which is guided in the housing of the drive assembly for linear movement along the longitudinal direction defined by the screw axis . The spring keeps a preload on the clutch when the screw and nut drive assembly is inactive , i . e . the clutch is engaged . The pressure piece is coupled with the push rod the axial force trans fer arrangement such that , when the nut is driven and moved in longitudinal direction a force is transmitted via the axial force trans fer arrangement of intermediate components in contact with each other to the pressure piece and further to the push rod so that the push rod in turn is moved in longitudinal direction to thereby pivot the clutch lever .

The pressure piece is guided for linear movement along the longitudinal screw axis . Since the push rod is laterally of fset with respect to the longitudinal screw axis a torque is acting on the pressure piece i f force is transmitted from the pressure piece to the push rod . Such torque can be further transmitted through the axial force trans fer arrangement to the nut . Such torque on the nut of a screw and nut drive arrangement results in increased friction when the nut is driven to move along the screw which eventually leads to increased tear and wear of the screw and nut linear drive assembly .

It is an obj ect of the present invention to arrange a screw and nut linear drive assembly such that any torque created by the force transmission of the screw and nut linear drive assembly on the member to be actuated is prevented from being trans ferred to the nut , or is at least substantially reduced when it is acting on the nut .

This obj ect is achieved by the screw and nut linear drive assembly comprising the features of claim 1 . Preferred embodiments of the invention are set out in the dependent claims .

According to the present invention the axial force trans fer arrangement which is configured to trans fer force from the nut to a push rod comprises a sequential arrangement of axial force trans ferring components , which arrangement of axial force trans ferring components compri ses , in the direction from push rod to the nut , a first and a second axial force trans- ferring component in force transmitting contact which each other . The first and second axial force trans ferring components are configured such that first contact points between them define a first axis in a transverse plane perpendicular to the longitudinal direction, about which first axi s the first axial force trans ferring component is capable of tilting with respect to the second axial force trans ferring component in order to reduce trans fer of any torque to the nut . The first axis is defined by a line connecting the first contact points . The contact points are not points in a mathematical sense , but can also have a certain extension . For example , the first contact points can also form a continuous line which defines the first axis ; such a contact line can for example be formed i f one of the components has a planar contact surface and the other one has a facing contact surface comprising a linear ridge which contacts the planar contact surface of the first component so that the components are capable of tilting about the axis defined by the ridge . Alternatively, there may be only two spaced apart first contact points , wherein the first axis is defined by the line connecting the two first contact points .

The first axis is , in order to be able to reduce trans fer o f any torque to the nut , oriented such that is not paral lel to a line connecting the screw axis with the contact point of the axial force trans fer arrangement with a component to be actuated by the axial force trans fer arrangement . Preferably the first axis is oriented perpendicular to this line connecting the central axis of the screw with the point where force is trans ferred from the axial force trans fer arrangements to the push rod which in turn drives the member to be actuated by the linear drive assembly . The latter arrangement achieves the most ef ficient torque absorption in the axial force trans fer arrangement , and thus most ef ficiently prevents that torque is transmitted to the nut . In a preferred embodiment the sequential arrangement of axial force trans ferring components comprises a third axial force trans ferring component which is disposed between the nut and the second force transmitting component and which is in force transmitting contact with the second force transmitting component . The second and third axial force trans ferring components are configured such that second contact points between them define a second axis in a transverse plane (perpendicular to the longitudinal axis ) about which second axis the second axial force trans ferring component is capable of tilting with respect to the third axial force trans ferring component to reduce trans fer of any torque to the nut , wherein the second axis is oriented perpendicular to the first axis . The capability of tilting in the axial force trans fer arrangement about the first axis has the main function to absorb torque exerted on the first force trans ferring component when force is transmitted to the push rod and further to a member to be actuated, wherein such torque may be absorbed by permitting a certain tilting movement of the first force trans ferring component with respect to the second force transmitting component which, therefore , is not subj ect to the torque acting on the first force trans ferring component . In other words , by allowing this tilting movement of the first force trans ferring component , the second force transmitting component does not take part in the tilting movement and there fore no torque is transmitted . The capability of tilting about the second axis in the axial force trans fer arrangement permits to absorb tolerances , and such tilting is normally of smaller magnitudes than the capability of the first force trans ferring component about the first axis to absorb any torque . For example , i f there is an angular misalignment or a small longitudinal of fset between two sides of the load trans ferring components this would result in side loads on the nut and screw i f there would be no compensation capability by tilting about the second axis , i . e . by allowing the second force transfer component to tilt about the second axis with respect to the third force trans ferring component which is fully supported on the nut and therefore does not take part in this tilting movement . Thus , in this embodiment the axial force trans fer arrangement permits ti lting movements about two perpendicular axes within the axial force trans fer arrangement which essentially decouples the nut from any force or torque transfer which is not a purely axial force trans fer in the axial ( longitudinal ) direction of the screw .

In a preferred embodiment the first axial force trans ferring component is a preload plunger guided for linear movement in the housing along the longitudinal direction, said preload plunger having a recess disposed in a front face thereof and configured to receive and to be connected to an end portion of the push rod, wherein the preload plunger comprises a central bore extending along the longitudinal direction and forming an opening in a back face opposite to the front face and having, opposite to the opening, an abutment surface formed by a circumferential shoulder in the central bore . The second axial force trans ferring component is a bushing tube which is at least partially received in the central bore . The bushing tube and the abutment surface of the central bore being configured to come into abutment with each other in first contact points only which define the first axis , wherein an outer wall portion of the bushing tube which is received in the central bore is provided with resilient surface features which permit tilting movements of the bushing tube with respect to the central bore of the preload plunger about the first axis defined by the first contact points . The resilient surface features of the bushing tube are in contact with the inner wall of the central bore . In case of an external torque acting on the preload plunger, the preload plunger may absorb such torque by tilting, wherein this tilting on the preload plunger is not trans ferred to the bushing tube which remains stationary within the central bore of the preload plunger and permits the tilting movement of the preload plunger by absorbing thi s relative movement in the elastic surface features of the bushing tube .

In a preferred embodiment the third axial force trans ferring component is an axial load carrier ring being guided for linear movement with the nut and being in driving contact with and fixed in longitudinal direction to the nut so that the load carrier ring moves axially with the nut as one component . Two opposite rotation stop arms extend from the axial load carrier ring and are received in recesses of the preload plunger configured to prevent rotational movement of the axial load carrier ring relative to the preload plunger . The end faces of the axial load carrier ring and the bushing tube facing each other are configured such that they come into abutment against each other in two diametrically opposite contact points only, wherein the two diametrically opposite contact points define the second transverse axis which is perpendicular to first axis .

In a preferred embodiment the resil ient surfaces features of the bushing tube comprise two bulges proj ecting from and extending circumferentially around the outer wall of the bushing tube and being spaced apart in axial direction of the bushing tube , wherein the bulges are made of elastic material and are dimensioned to be in contact with the inner wall of the central bore of the preload plunger when the bushing tube is received in the central bore of the preload plunger .

In preferred embodiments the screw and nut linear drive assembly is configured as a bal l screw and nut assembly or as a lead screw and nut assembly . In the following a preferred embodiment of the invention will be described with reference to the drawings in which:

Fig. 1 shows a perspective view of a screw and nut linear drive assembly in the form of an electric clutch actuator;

Fig. 2 shows a top view of the electric clutch actuator of

Fig. 1, with a central portion of a housing cut out and internal components being shown in cross-section;

Fig. 3 shows a side view of the electric clutch actuator of

Fig. 1, with a central portion of a housing cut out and internal components being shown in cross-section;

Fig. 4 is an enlarged partial view of the cross-section portion of Fig. 2;

Fig. 5 is an enlarged partial view of the cross-section portion of Fig. 3;

Fig. 6 is a perspective view of a part of the axial force transfer arrangement of Figs. 2 to 5;

Fig. 7 is a schematic plan view of the axial force transfer arrangement along a first axis which is disposed in a transverse plane perpendicular to an axial direction; and

Fig. 8 is a schematic plan view of the axial force transfer arrangement of Fig. 7 along a second transverse axis, which second axis is disposed perpendicular to the first axis.

Fig. 1 shows a perspective view of a linear screw and nut drive assembly, here embodied by an electric clutch actuator 1. The electric clutch actuator 1 comprises a housing 2 from which a moveable push rod 8 extends which is driven by the electric clutch actuator for linear movement . The push rod 8 is configured to be connected to a member to be actuated by the linear drive assembly, in thi s case to one end o f a clutch lever (not shown) which is pivotably mounted to be pivoted by the push rod 8 between an engaged and a disengaged position .

The electric clutch actuator 1 includes an electric motor and a transmission (both not shown) within the housing 2 . The electric motor drives , via the transmission, a screw ( spindle ) to rotate around the longitudinal direction defined by the central axis of rotation of screw . In this case the screw and the nut form a ball screw assembly . In such bal l screw assembly the helical grooves of the screw are in indirect engagement with internal helical grooves of a nut via balls which roll within these grooves and which are returned by a ball return system after they reached an end of engagement end of the nut . The nut is axially moveable within the housing but held in a manner such that it is prevented from rotation about the central screw axis .

Alternatively there may be a direct driving engagement between the screw and the nut ( lead screw and nut drive assembly) . However, as mentioned before in the embodiment shown in the Figures , the present invention is embodied by a linear drive utili zing a ball screw assembly consisting of a screw and a nut , each with matching helical grooves , and balls which roll between these grooves and which provide the only contact between the nut and the screw . When screw and nut rotate with respect to each other, the bal ls are returned by a ball return system back into the ball screw and nut thread raceways formed by the helical grooves .

With re ference to Figs . 2 and 3 a first overview o f the internal structure of the electric clutch actuator 1 will be given, wherein Fig . 2 shows a top view of the electric clutch actua- tor with a central portion of its housing cut out and the interior shown in cross-section, wherein Fig. 3 shows a similar view as a side view, i.e. the view axis is rotated by 90° around the longitudinal axis compared to Fig. 2. In Fig. 2 the push rod 8 is visible in the interior of the housing 2 in cross-section, and in a side view projecting from the housing. As can be seen in Fig. 2 the push rod 8 is transversely (in a plane perpendicular to the longitudinal axis (= rotational axis of the screw 6) offset with respect to the longitudinal axis of the screw 6. For this reason, if a force is transmitted to the push rod 8, as a reaction a torque is created which is acting between the push rod 8 and the nut 4 which is driven to provide the force for the push rod.

In Fig. 3 the push rod 8 is only visible with its projecting portion outside of the housing, whereas it is out of the plane of the cross-section in the cut out portion of the interior of the housing.

When the screw 6 is driven to rotate, the nut 4 linearly moves along the screw. The nut 4 is guided in the housing for linear movement along the longitudinal axis of the screw, but rotation around the longitudinal axis is prevented.

The axial force transfer arrangement transferring force from the nut 4 to the push rod 8 will now be described in more detail first with reference to Figs. 2 and 3, and then in more detail with reference to the enlarged cross-sectional views of Figs. 4 and 5, and the schematic views in Figs. 7 and 8.

With reference to Figs. 2 and 3 the axial force transfer arrangement comprises, in the direction from the push rod 8 to the nut 4 a first force transferring component 10, a second force transferring component 20, and a third force transferring component 30, the components of each pair of subsequent force trans ferring components being in force trans ferring contact with each other . In thi s manner the nut 4 is in force trans ferring contact with the push rod 8 through the axial force trans fer arrangement compri sing a train of force transferring components 10 , 20 , 30 .

With reference to Figs . 4 and 5 the first force trans ferring component 10 is a preload plunger guided for linear movement within the housing 2 . The preload plunger has in a front face thereo f a recess in which the end portion o f push rod 8 i s received and connected thereto . The preload plunger has a central bore which partially extends into the preload plunger from a back face opposite to the front face into and partially through the preload plunger . The central bore is continued through the remaining portion of the preload plunger by a smaller diameter hole so that the screw 6 extends through the central bore and further through the central hole , and so extends completely through the preload plunger 10 . The central bore of the preload plunger is provided with an abutment ring 12 resting on a shoulder end face of the central bore . A side surface of the abutment ring 12 provides an abutment surface 14 of the central bore which wil l be discussed in more detai l below .

The second force trans ferring component 20 is a bushing tube received in the central bore . The outer wall of the bushing tube 20 is provided with resilient features 22 , in the embodiment shown by two circumferentially extending ridges of elastic material .

As can be seen in the view of Fig . 5 the outer wall of the bushing tube 20 with its ridges 22 has contact with the inner wall of the central bore , whereas in the view of Fig . 4 the outer wall of the bushing tube has no contact with the inner wall of central bore of the preload plunger . This configura- tion is such that the preload plunger 10 is in principle capable of being tilted about an axis perpendicular to the Figure plane of Fig . 4 , without trans ferring such tilting movement to the second force trans ferring component 20 in the form of the bushing tube . This ability of relative tilting movement between the second force trans ferring components 10 and 20 i s also a result of the configuration of the first and second force trans ferring components 10 , 20 in such a manner that their contact surfaces define a first axis about which the first force trans ferring component 10 may pivot with respect to the second force trans ferring component 20 . In the embodiment shown the first contact points between the first and second force trans ferring components 10 , 20 are disposed diametrically opposite with respect to the longitudinal axi s . Thus , there are two diametrically oppos ite contact points , as seen in the view of Fig . 5 , where the first and second force transferring components 10 and 20 are in direct contact with each other, whereas in the remaining portion of the circumference between the diametrically oppos ite contact points there is a gap between the abutment surface 14 of abutment ring 12 of the preload plunger and the bushing tube , see Fig . 4 .

This configuration of the first contact points between the first and second force trans ferring components 10 , 20 is illustrated in a schematic way in Figs . 7 and 8 . As can be seen in Fig . 7 there is contact between the first and second force trans ferring components 10 , 20 at first contact points 42 located in close proximity to the intersection point of the first axis 40 which is the viewing axis of Fig . 7 . Fig . 8 shows a corresponding view with the view axis rotated by 90 ° . Now the first contact points 42 are visible as two diametrically opposite first contact points 42 between the first and second force trans ferring components 10 , 20 . With reference to Fig . 7 again, this configuration of first contact points 42 defining a first axis 40 (perpendicular to the Figure plane of Fig. 7) permits a certain pivotal movement of the first force transferring component 10 with respect to the second force transferring component 20. This tilting movement is indicated in Fig. 7 by the semi-circular arrow around the first axis 40.

Referring to Figs. 4 and 5 again the description of the axial force transfer arrangement will now be continued. The second force transferring component 20 cooperates with a third force transferring component 30 which in the embodiment shown is a load carrier ring 30 which is disposed between the bushing tube 20 and an outwardly projecting portion of the nut 4. The load carrier ring 30 is further provided with two opposite, longitudinally extending rotation stop arms 32 which extend into recesses formed in the preload plunger 10 to prevent rotational movement of the load carrier ring 30 about the longitudinal axis; as a consequence, rotational movements are also prevented for the nut 4 which is coupled to the load carrier ring 30.

As can be seen in Fig. 4, the second force transferring component 20 is, in the plane of the cross-section of Fig. 4, in direct contact with the adjacent load carrier ring as the third force transferring component 30. Again two diametrically opposite second contact points between the second and third force transferring components 20, 30 are formed in the plane of the cross-section of Fig. 4, whereas the contact surfaces are configured that there is no direct contact between the first and second force transferring components 20, 30 in the remaining portion of the circumference. As can be seen in view of Fig. 5 in which the plane of the cross-section is rotated by 90° with respect to Fig. 4, in Fig. 5 there is a gap between the load carrier ring as the third force transferring component 30 and the bushing tube as the second force transferring component 20. This arrangement of the second contact points between the second and third force transferring compo- nents 20, 30 can be better understood with reference to the schematic views of Figs. 6 - 8. As can be seen in the view of Fig. 7 there are two diametrically opposite contact points 52 formed by two small projections. These second contact points 52 are also visible in Fig. 6 which is a perspective view of the couple of load carrier ring 30 and bushing tube 20: In the circumferential area of the rotation stop arms 32 there are small projecting knobs on the inner side of the load carrier ring 30 which form the second contact points and which contact the end surface of the tube bushing 20.

Whereas in the view of Fig. 7 the two second, diametrically opposite contact points 52 are spaced apart, in the view rotated by 90° around the longitudinal direction as shown in Fig. 8 the two diametrically opposite second contact points 52 are now in line in the view axis of Fig. 8 (the view axis coinciding with the second axis 50) and are visible in the middle around the second axis 50 which is oriented perpendicular to the Figure plane of Fig. 8. This arrangement of two diametrically opposite contact points 52 between the second and third force transferring components 20, 30 defines this second axis 50 and allows a certain tilting movement between the first and second force transferring components 20 and 30 about the second axis 50.

As can also be seen in Figs. 7 and 8 the first axis 40 and the second axis 50 are oriented perpendicularly to each other in a plane transverse to the longitudinal axis. In this manner the axial force transfer arrangement formed by the first, second, and third axial force transferring components 10, 20, 30 forms a double joint with two perpendicular joint axes, which cooperate to absorb any torque created by a force input to the push rod (counter force generated by the member being actuated via the push rod) which is offset with respect to the longitudinal screw axis or by any tolerances of the components of the axial force trans ferring components or bearings or guiding components .