Patent Application
20050268736
Not applicable.
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The invention relates to the field of the mechanical linear actuators and, in particular, of the mechanical actuators driven by an electric motor (electromechanical actuators).
The development of the electromechanical linear actuators is related to the needs in fields such as robotics and home systems. Indeed, in these fields the electromechanical jacks compete with the traditional, hydraulic or pneumatic jacks, because they are more easily controllable, more accurate and do not require an external source of fluid.
These electromechanical actuators generally include a ball screw on which a nut is mounted. The nut is rotated by an external geared motor. The rotation of the nut drives the screw in translation.
The drawback of these electromechanical actuators is that they are relatively cumbersome.
Moreover, since the cost of the ball screws is generally high compared to the other mechanical parts they contain, these actuators remain relatively expensive.
An object of the invention is to provide a compact actuator structure and the manufacture of which would be simplified compared to the actuator structures of the prior art.
To this end, the invention provides an actuator including a first tubular body, a nut positioned inside the tubular body and having at least a generally helical ball-race, balls arranged between the ball-race and the tubular body, and driving means for rotating the nut, said driving means comprising a motor, the rotation of the nut driving the tubular body in translation with respect to the nut, characterized in that the motor is mounted fixed inside a second body capable of being displaced in translation with respect to the first tubular body.
The fact that the actuator comprises an internal nut allows to position the motor inside a second body. In addition, the re-circulation path can be integrated into the nut. This arrangement leads to a compact actuator structure the external appearance of which is similar to that of the pneumatic actuators. In particular, the actuator does not leave visible any external geared motor device. The actuator provided is thus particularly compact, compared to the effort which it is capable of generating.
In addition, the use of a tubular structure imparts to the actuator a better buckling strength than a traditional actuator having an external nut mounted about an internal screw.
In an implementation of the invention, the balls are fitted between the race and the first tubular body, with a determined radial prestressing.
The fact that the balls are fitted with prestressing allows to obtain a linear actuator capable of transmitting significant efforts, compared to its size.
In an implementation of the invention, the race includes a helical portion extending about the nut according to an angle of less than 360 degrees and a widened portion connecting the adjacent ends of the helical portion, said widened zone constituting a re-circulation zone for the balls.
This implementation has the advantage of not requiring the formation of an internal re-circulation race in the nut. The balls are automatically “recycled” as soon as they reach the re-circulation zone.
In addition, the inner surface of the first tubular body can advantageously have helical ball-races the function of which is to guide the balls. These ball-races reduce the risks of sliding of the balls on the inner surface of the first body when the actuator exerts a significant effort. The widened re-circulation zones allow the passing over of the balls from one ball-race to an adjacent race, over a race edge during their re-circulation.
In a preferred implementation of the invention, the nut includes several aligned elements, of a cylindrical general shape, each having at least a bevel forming a helical cam surface, the bevels forming, two by two, helical ball-races in which balls are positioned. Each element is formed from a cylindrical part with a straight cross-section, one circular edge of which is beveled, in order to form said helical cam surface inclined with respect to the axis of the cylindrical part, the ends of helical surface being joined by a setback surface with a preferably conical general shape.
Each element of the nut is formed from a cylindrical part with a straight cross-section, i.e. the cylindrical part is limited by two parallel planes orthogonal to its axis of rotation. This is a simple shape. The shape of the elements is therefore easier to be generated than in the prior art.
According to the technique for carrying out the bevel, the setback surface can also have a general shape that is convex, concave, planar, cylindrical, planar with conical connection or cylindrical connection or the like.
Advantageously, each helical cam surface forms a setback and two elements are so positioned with respect to each other that their setbacks are in front of each other, said setbacks forming the balls re-circulation zone.
Advantageously, the prestressing exerted on the balls is generated by tightening the elements with respect to each other.
To this end, the actuator can include an element adjusting nut for controlling the prestressing exerted onto the balls.
The effort which can be exerted by the actuator directly depends on the prestressing applied to the balls and adjusted by the adjusting nut.
Advantageously, the actuator includes elastic means interposed between the adjusting nut and the nut elements through which the adjusting nut exerts prestressing on the elements.
Preferably, the motor is an electric or hydraulic motor.
The invention also relates to a nut element aimed at being arranged in an actuator as defined above. The nut element is formed from a cylindrical part with a straight cross-section, one circular edge of which is beveled to form said helical cam surface inclined with respect to the axis of the cylinder, the ends of the helical surface being connected by a setback surface with a conical general shape.
The invention also relates to a process for obtaining a nut element aimed at being arranged in an actuator according to the invention. The process includes the steps consisting in machining a circular edge of a cylindrical part with a straight cross-section, in order to generate a bevel forming a helical cam surface inclined with respect to the axis of the cylinder, the ends of helical surface being connected by a setback surface with a conical general shape.
One understands that the process for obtaining the nut element is easy to be implemented with traditional machining means.
Further characteristics and advantages will clearly appear from the following description, which is purely illustrative and non-restrictive and must be read with reference to the attached figures.
In
To this end, the actuator includes a drive mechanism including a driving shaft 30 extending according to the longitudinal axis of the tubes 10 and 20. The shaft 30 is rotated by an electric motor 2 fixed at one of its ends and positioned in the inner tube 10. The motor 2 and the shaft 30 are maintained in the inner tube 10 through a cylindrical support 3 fixed to the inner tube.
Furthermore, the shaft 30 is guided in the inner tube 10 through two ball bearings 7 and 9 the inner ring of which is fitted on the shaft 30 and the outer ring rests against the inner surface 11 of the inner tube 10. Both bearings 7 and 9 are maintained at a distance by a spacer 8 in the form of a cylindrical sleeve resting on the inner rings of the bearings 7 and 9 as well as through a spacer 12 pinned in the inner tube 10 and resting on the outer rings of the bearings 7 and 9. The absorption of the axial forces exerted on the bearings can occur either through the spacer 12 or by any other equivalent means (for example circlips locking the bearing).
The shaft 30 supports in addition an adjusting nut 4, a set of Belleville washers 5, a first clamping washer 6 positioned between the support 3 of the motor and the bearing 7. The clamping washer 6 rests on the inner cage of the bearing 7. The shaft 30 also supports a second clamping washer 1 and ball nut 70, positioned between the bearing 9 and a thrust element 31 at the end of the shaft 30.
The nut 70 is formed of a succession of cams 40, 50 and 60 with cylindrical general shapes mounted aligned on the shaft 30 and locked in rotation with respect to the shaft by a key. The cams 40, 50, 60 have helical bevels 41, 51 and 52, 62, oriented at 45° with respect to the axis of the shaft 30. These bevels 41, 51, 52, 62 form, two by two, helical ball-races in which balls 22 are positioned. The balls 22 are into contact, on the one hand, with two surfaces with opposite bevels, 41 and 51, or 52 and 62 and, on the other hand, with the smooth inner surface 21 of the outer tube 20. The radial force applied to the balls 22 is controlled by tightening the nut 4. The adjusting nut 4 applies a compressive force to the Belleville washers 5 according to the longitudinal direction of the shaft 30. This compressive force is transmitted to the cams 40, 50, 60 through the clamping washer 6 which transmits and distributes the clamping force on the inner cages of the bearings 7 and 9 and on the clamping washer 1. The cams 40, 50, 60 are thus in compressed state between the clamping washer 1, the balls 22 and the thrust element 31 at the end of the shaft 30. By tightening the cams 40, 50, 60, the adjusting nut 4 advantageously allows to adjust the prestressing exerted on the balls 22.
The actuator of
The force that can be exerted by the actuator of
However, the prestressing force which can be applied to the balls 22 remains limited by the Hertz pressure which the surface of the cams 40, 50, 60 and the inner surface 21 of the outer tube 20 can be subjected to.
When the motor 2 of the actuator of
As shown in
The linear actuator of
As shown in
Traditional heat-treatment and rectifying operations can then be carried out on the helical surface 41 obtained (for example grinding of the helical surface).
As can be seen in
Of course, variants of the above-described embodiment can be contemplated. In particular, the shape of the setback can vary according to the path of the initial passage of the cutter. If the conical cutter penetrates into the part 400 according to a tangential passage start, the setback obtained will have a planar general shape. If the conical cutter penetrates into the part 400 according to an oblique passage start, the setback obtained will have a planar general shape with conical connection.
It is also possible to use a cylindrical cutter the axis of rotation of which would be inclined with respect to the axis of the cylindrical part and according to the path of the start of the initial passage, in order to obtain a setback with a cylindrical, planar or planar general shape with a cylindrical connection.
In addition, when the pitch of the race is large with respect to the diameter of the cams, the helical ball-race must be obtained by a different process. For example, a previous step of milling of the cylindrical part using a cylindrical cutter can be carried out, in order to obtain in the first place a helical surface oriented perpendicularly to the axis of the part. Then, a step of milling of the edge of the helical surface using a conical cutter is carried out, to make a helical bevel oriented at 45 degrees with respect to the axis of the part. The helical bevel thus obtained forms a circumferential surface with a constant width which is connected at its ends by a conical setback.
The conical setback surfaces 45 and 55 of both cams 40 and 50 advantageously form a widened zone 81 which accommodates the balls 22 and allows their re-circulation. When the shaft 30 of the actuator is rotated, the balls 22 roll on the ball race formed by the beveled surfaces 41 and 51. When a ball 22 arrives in the re-circulation zone 81 where the two beveled surfaces 41 and 51 have a maximum width, it is no longer into contact with the inner surface 21 of the outer tube 20, so that it does no longer roll. The ball 22 remains in the re-circulation zone until it is pushed by the arrival of a next ball and thus automatically re-inserted into the ball-race.
In
In the case of a nut including three roll-races which would have three re-circulation zones, the cams would be so oriented that the re-circulation zones are arranged at 120 degrees with respect to each other about the axis of the shaft 30.
In a general way, in the case of a nut including N ball-races (formed by N pairs of cams), the cams would be so oriented that the re-circulation zones are arranged at 360/N degrees with respect to each other about the axis of the shaft 30.
This feature allows to avoid a rotational movement of the inner tube 10 with respect to the outer tube 20 which can occur when the actuator comprises only one pair of cams (i.e. only one ball-race) or when the re-circulation zones are arranged aligned.
In a variant of the linear actuator of
In the event the outer tube 20 is made out of a light alloy, after having formed the ball-races, a ceramization treatment for hardening this surface in depth (0.1 to 0.2 mm) is applied to the surface 21 of the tube 20.
The constitution of the ball-races allows to apply compressive forces which a smooth cylindrical surface would not withstand. In addition, these races allow to apparently increase the external friction coefficient between the ball and the tube.
Alternatively, the ball-races allow not to apply too great a prestressing force to the balls. Since the balls are guided by the ball-races, they cannot slide with respect to the outer tubular body 20.
These ball-races have a helical pitch substantially equal to the helical pitch of the ball-race formed in the nut 70.
In this variant, the actuator includes, in combination, ball-races on interior surface 21 of the outer tube 20 and one nut 70 having re-circulation zones in the form of widened spaces. Thanks to this structure, when a ball arrives in a re-circulation zone, it penetrates radially towards the interior of the nut 70, so that it is no longer into contact with one of the races formed in the outer tube 20. Thus, when “recycled”, the ball passes from one ball-race onto an adjacent race, over a race edge, this passing over from one race to another one being possible thanks to the widened space forming the re-circulation zone.
In still another variant of the actuator of
This variant allows to use tubes made out of aluminum, KEVLAR©, carbon fibers or molded plastic, which guarantees the lightness of the final actuator structure obtained.
In an implementation shown in
The ball-races in the inner tube 93 are made as follows. For example, a burnishing or shaping machine is used, which includes a roller holder provided with three rollers arranged at 120 degrees with respect to each other and oriented according to the helix angle of the race to be obtained. The inner tube 93 is fixed on a chuck the shape of which is close to the inner profile to be achieved. The roller holder is rotated. At the same time, the tube 93 and the chuck are driven in translation. The speed of translation of the tube 93 is set so that the translation distance is equal to the pitch of the helix at each turn of the roller holder. The operation can be carried out in one single pass and the tube 93 is then highly cold hardened, which increases the rigidity and the hardness of the surface. Once shaped, the tube 93 is inserted into the outer tube 20.
In the event ball-races are formed on the inner surface of the outer tube 20, each cam 40, 50 or 60 has a bevel oriented according to an angle smaller than or equal to 45 degrees with respect to the axis 401 of the cam, preferably strictly smaller than 45 degrees and preferably of about 35 degrees. This feature allows to decrease the radial force which serves as a support for the reaction of the forces applied to the ball-race. Moreover, this feature facilitates the passing of the balls over the edges of the races during their re-circulation. Indeed, the component of the force which allows a ball to pass over a race edge (formed for example by a wire) passes above the edge of the race.
The linear actuator shown in
The tubes 10 and 300 are locked in rotation with respect to each other and are capable of being driven to slide with respect to each other in their longitudinal direction. The outer tube 20 is mounted floating, i.e. it is locked in rotation neither with respect to the inner tube 10 nor with respect to the third tube 300.
When the motor 2 of the actuator of
When the inner 10 and outer 20 tubes are in abutment against each other, the tubes 10 and 20 are then rotated simultaneously. The outer tube 20 then rotates the nut 370 including the cams 340 and 350. The balls 22 then roll between their ball-race formed by the cams 340 and 350 and the inner surface of the third tube 300. Since the tubes 10 and 300 are locked in rotation with respect to each other, the rotation of the nut 370 causes the unit comprised of the inner tube 10 and the outer tube 20 to be moved in translation with respect to the third tube 300.
This results into the thus produced telescopic actuator unfolding in two steps. In a first step, the inner tube 10 is displaced in translation with respect to the outer tube 20 and to the third tube 300, then, in a second step, the inner 10 and outer 20 tubes are displaced in translation with respect to the third tube 300. This unfolding in two steps is due to the fact that the couple necessary to rotate the nut 370 with respect to the third tube is larger than the couple necessary to rotate the nut 70 with respect to the outer tube.
The unfolding can also occur at random depending on the friction torques occurring in the mechanism.
Such a telescopic actuator has the advantage of being able to reach larger unfolding lengths than with a simple actuator as shown in
In the actuator shown in
The tubes 20 and 300 each have ball-races on their inner surfaces. These races preferably have the same pitch. Thus, the unfolding of the actuator will occur at a constant speed. In addition, it will be possible, by counting the number of revolutions of the motor, to know the exact position of the actuator.
If the races of the tubes 20 and 300 have different pitches, the unfolding speed of the actuator will vary according to the tube which will be moving at a given moment.
Generally, in a telescopic actuator including a plurality of tubes capable of being driven in translation with respect to each other, one can choose to establish different race pitches for the various tubes. One thus obtains a telescopic actuator which sequentially unfolds with programmable values of motor/movement reduction coefficient over the total travel distance of the actuator. This feature allows to adapt the evolution of the motor torque provided depending on the profile of the load the actuator has been subjected to during its unfolding, this profile being determined length by length.
If one wants the tubes to unfold in a given order, it is possible to add means for braking the rotation of the tubes with respect to each other (for example one or several O-ring(s) rubbing against the tubes, so that the latter unfold sequentially.
The preceding description relates to an example of linear actuator in which the means for driving the nut include an electric motor 2. It will be understood that it is of course possible to use other types of driving means: hydraulic motor or the like.
Now it will described more in detail the passing over of a ball from one ball-race to the next one in the case of an actuator including an outer tube the inner surface 20 of which has ball-races.
If α1=α2, we have FA=FB, so that the ball is in balance and the forces FC and FD are zero.
If α1>α2, we have FA+FB+FC=0 and the force FD exerted by the cam 50 is zero.
If α1<α2, we have FA+FB+FD=0 and the force FC exerted by the cam 40 is zero.
The cams 40 and 50 are rotated so that the ball 22 arrives at a widened re-circulation zone as shown in
The passing over of the ball 22 from one race to the next one can occur only if α1<α2, so that the resultant of the forces on the ball passes over the wire 94.
In addition, if one takes into consideration the frictions which are exerted on the ball 22 and which are designated by φ1 and φ2, the friction angles between the ball and the wire 94 and between the ball and the cam 40, a condition for the passing over of the ball from one race to the next one to occur is: α1+φ1+φ2<α2. When assuming by φ1 and φ2 in the range of 5 degrees (lubricated contact), and α1 in the range of 35 to 45 degrees, one deduces therefrom that α2 must be of more than 45 or 55 degrees.
In order to facilitate the passing of balls from one race to another and to maintain a good efficiency, α2 can be chosen between 50 and 60 degrees, preferably to be 55 degrees. When α2 is in the range of 55 degrees, the cam 40 has a helical beveled surface 41 oriented at 35 degrees with respect to the plane P. A cam having such a helical bevel can be achieved by machining a cylindrical part with a conical cutter having at the top half an angle of 55 degrees.
Furthermore,
The angle θ can be set in order to minimize the space of evolving of the balls in the re-circulation zone 81, in order to avoid the presence of several balls at the same time in this zone and to keep the largest possible number of “working” balls. The setting of the angle θ depends namely on the pitch of the ball-race, on the orientation of cam surfaces 41 and 51, on the diameter of the balls 22, on the diameter of the wires 92 and 94 used for making the races.
A way for determining this angle θ consists in determining the volumes in which the center O of a ball moves when the latter is resting against one of the cam surfaces, resting on the other cam surface and resting on the ball-races, respectively. The intersection of these volumes represents the space in which the ball is guided. This space can be modified by varying the angle θ. The space of intersection must both be large enough for a ball to be able to enter into the re-circulation zone and to move on the helical ball-race and sufficiently restricted to prevent several balls from being present simultaneously in the re-circulation zone 81. The shape of the space obtained depends on angle θ and also on the shape of the setback surfaces of the cams.
| Number | Date | Country | Kind |
|---|---|---|---|
| 02/10715 | Aug 2002 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR03/02607 | 8/29/2003 | WO | 7/28/2005 |
